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-   -   Hope to start an audio amp with CV415/CV4046/TT15 (https://www.vintage-radio.net/forum/showthread.php?t=168795)

AdrianH 7th Jul 2020 9:04 pm

Hope to start an audio amp with CV415/CV4046/TT15
 
3 Attachment(s)
Well that is the plan anyway, as if I do not have enough projects on the go, but after playing with an old aircraft transceiver I have a valve to play with, it was the reason I picked the TR1998 radio.

The valve is a twin beam Tetrode meant for VHF RF power output, but within the data sheet for the TT15 it has an interesting detail for using the valve as a push-pull class AB1 audio output. Well I hope I am reading it correctly. So this is where I will be looking to your assistance in checking things out for me?
The basic specs given for what seems to be best operating points are:

Va = 300 V
Vg2 = 175 V
Vg1 = -21 V
RL (a to a') 7K Ohm
Max input 41 V p-p
Max power out 16.8 Watts and
Max distortion 2.25%

I have attached images of part of the spec sheet showing this and the curves for the valve. Now I think this is with fixed g2 voltage, which would simplify making a transformer for it. I doubt very much I would end up running the valve to full output so would like around 10 watts maximum out of the amp when finished and if I manage to make one work then I will add another amp to make stereo!

The idea is to base it on the circuits of the Radford Valve amps. I have N.O.S. ECF82 valves to drive the output valve, thinking that if these can drive EL84's and EL34's then this should not be an issue, although I note that the TT15 requires a 100K grid to cathode resistor, this may be a bit lower than normal.

Driving the ECF82 could well be the EF86 if I can find a couple cheap enough, although I may also try the EF91, just because I have a handful of them out of this transceiver as well as other common 7 pin devices, (EC90, EL91).

Will it be Hi-Fi, I doubt it, would it be OK for my hearing, probably if I can get it to work without self oscillation.

I have picked up some imported valve holders to start with, but the valve will not fit! It looks as though the central locating fin is too wide for the holder, so now wondering if I try to file out the holder or file down the fin.

Anyway thoughts always appreciated, apart from derogatory of course.

Adrian

p.s. Just noted that as the g2's for both Tetrodes are common on the same pin that the g2 voltage must be fixed.

Diabolical Artificer 8th Jul 2020 5:58 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Cute little devils, I've some QQV03-10 and other double beam tets lying around I've been meaning to to play with for a while. First off you'll need more than 300v to get 10w with g2 sitting at 175v, I really can't see you getting 16w+, that's based on gut feeling and a bit of experience.

i found an OPT with a Z pri of 10k worked best for for the QQV etc, I'd wire them up, fixed bias, use any old OPT and load it till you get the best results. Use grid stoppers, start off with 2k2 g1, 220r g2. G2 supplied from resistive divider, don't forget to bypass the lower R.

If you get oscillations bung a 4n7 cap and 1k ( 5w) from both anodes to ground. Also put three reverse biased 1n4007's from each anode to ground reversed biased in case of F up's.

You could try a 6AU6 triode strapped as the front end, they're cheap, with a cathodyne PS, something like an ECC82. Design things so you have extra gain to through away on NFB.

Some last thoughts, try and find some more TT15's as spares, Andy.

Aub 8th Jul 2020 7:31 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Hi,

I've used a QQV03-10 as a push pull modulator in my ham radio transmitter. It worked very well using a mains transformer as a modulation transformer.

Never tried one in a hi-fi application though.

Let us know how you get on

Cheers

Aub

Radio Wrangler 8th Jul 2020 8:07 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
There are also QQV03-20 and QQV06-40
These should be a lot easier to find than TT15. They were the basis of PMR car-mounted transceivers and their base stations. Because a lot of radios of this era were AM, it was common to rate power devices for duty as both an RF PA and as a modulator.

Because they are all designed to give appreciable power (and hence gain) up to VHF, they will all need great care stopping RF oscillation. Some of the later and larger RF tetrodes have even nastier habits with Ig2 swinging into negative current! To keep 4CX250B and the like tame required special sockets with g2 decoupling capacitors built into them. (I once built an audio amplifier with four 813 as the final stage, parallel pairs in push-pull)

David

AdrianH 8th Jul 2020 8:50 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
I can remember the QQV0 series, 3-10 was in the Pye Cambridge. I think the 3-20 in the AM25B? Vanguard and the 6-40 in the FM vanguard, also If memory is still working the F27 Base station which was AM and F30 FM base stations of the day. There were also quick heat versions of the QQZ series that was used in the Westminster boot mounted radios, that takes me back to the early 80's working at Pye in Ardwick, Manchester.

As to availability, Langrex seem to have a few hundred of the TT15 in stock, so I doubt it is an issue unless I start a trend I think they will be there for years to come.

The next thing is to get the valve to fit the ceramic holder and try to fabricate something to use with my tester to check each Tetrode.
Then start building a chassis for the PSU which will be separate to the amp, that way it can always be used with other projects if this one is a dud.

As to power out, not sure what it will provide but if it can swing a few hundred volts in push pull at peak 50mA then it should be enough.

Time to get busy.

Adrian

AdrianH 8th Jul 2020 7:41 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
2 Attachment(s)
Started on the PSU today with a bit of metalwork, mapped out the smallest area I thought I could get away with and a bit of cutting welding and drilling later I end up with the start of the chassis., second picture is the general layout, nothing is connected up as I have to make holes for mains entry and supply out before I start to build it up.

Used 1.5mm mild steel as easier for me to weld as compared to aluminium and the transformer and choke weigh a bit.

Will be using silicon diodes at 1KV PIV and 3Amps max, to minimize Volt drop, but as the caps are 100 and 180 uF thinking of putting a couple of 100 Ohm 5 watt resistors in series with them to prevent the diodes seeing a dead short on switch on.

A hobby that can combine metalwork with electronics and Valves, not bad.

Adrian

bikerhifinut 8th Jul 2020 10:03 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
That's nice work.
What lets my DIY down is my cack handedness with matters metalwork and woodwork.
I always think if you start with the right chassis and plenty of room for the internal layout and keeping valves cool then you have a fighting chance.

I shall watch this one with interest.

Andy.

bikerhifinut 8th Jul 2020 10:11 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Just a note on EF86, they are horribly expensive, even the Russian ones.
And many of the EF86 out there either "NOS" or "Used tested" aren't much cop in terms of noise either because they are already on their way out or were rejects that made their way back out...……..
I have found that its predecessor the B8A EF40 rimlock can be found for a reasonable sum if you dig around on the web or once we are allowed out, at swapmeets. It's electrically identical so a straight swap in given a different base. (that fits in a B9A hole).
You might consider something like an EF184, not identical and the heater isn't helically wound so it can be prone to hum induced from the heater, but if you use regulated DC then its an option.
Of course you may have a stock of EF86 in which case …………………….

A.

AdrianH 8th Jul 2020 10:36 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Thanks for the comment on the chassis, all is not what it seems with my builds, I can not say I actually plan layouts before hand, much of it is dependant on what 'stock' I have. The steel I have left now is all offcuts from big sheets as I also from time to time play with plasma cutting. I had an off cut of a size this morning and got the transformer and choke and just sat them on there to see if things would just about fit. Same will probably happen with the valves, leave enough space between to get my fingers in to aid removal, put the three valves in a line and take it from there?

OK on the EF86, I do not have any and yes I noted they are getting to be pricey. Which is why I hinted at the EF91, I do have 3 EF184's as well but they will need testing to see if any good or not.

The reason I was considering the EF91, was I have several CV138 or Cv4014's in the old aircraft transceiver. I also went searching for valve circuits for them and found in one of the magazines an amp using the EF91 as the first valve, may not be perfect but at least I have some.

Adrian

Radio Wrangler 8th Jul 2020 11:03 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
6SJ7?

David

bikerhifinut 8th Jul 2020 11:07 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Quote:

Originally Posted by Radio Wrangler (Post 1267968)
6SJ7?

David

And cheap as chips loads out there, also has the advantage of being identical to a 6j7 ( half a 6SN7) wired as a triode.

:thumbsup:

AdrianH 10th Jul 2020 5:26 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
2 Attachment(s)
Little bit of progress, the PSU is bolted together and there is HT and 6.3 internal to be used. I need to gain a grid bias supply so thinking of using a 5 Volt winding on the transformer through a couple of Cockcroft-Walton Voltage multiplier stages, to get around -30 to -40 Volts Bias line (Yes I have been reading a book again).

Adrian

Radio Wrangler 10th Jul 2020 6:37 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
5volt RMS winding less a diode drop or two, times N is going to take rather a high number of voltage multiplier stages. I think you may be pushing it a bit too far.

David

peter_sol 10th Jul 2020 6:52 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Hello
I've recently built some amplifiers both single ended and push pull and attempted to use an EF91 in the input stage. Having a few new and used, every one was microphonic and rang like a bell so I would use ECC83 0r ECC81. If you only need one half the other can be left unused or you could paralell them.
Just my thoughts and good luck with the project.

AdrianH 10th Jul 2020 6:59 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
It will either work of not, I am not sure how much current I will need for the grid supply probably in the 5 to 10 mA range to stabilize it down to -21 to -25 Volt, so I will try 6 sections and see what I get. It's all suck and see.

Adrian

AdrianH 10th Jul 2020 9:29 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
OK David you are looking correct on the voltage multiplier, it does 42 Volts with no load. Stick on a resistive load and it drops quite a bit. When drawing 5 mA it is at 29 Volts and think I would like to have more volts and mA to play with to keep the bias steady. Will have another think on it, I really do not want to add another transformer, even if a miniature one.

OK Peter on your amps with the EF91, I was planning on a line up of EF91, ECF82, TT15 in push pull, the ECF82 as the phase splitter and the EF91 was I guess a pre-amp stage and part of the negative feedback (Well that's the plan).
I was picking them basically because I have them in the old TR1998 aircraft transceiver. Just checking over the circuit that actually uses an EF92 as the microphone amp so wonder if that would be any better?
How did you test the valve for microphony, guessing pinging it with a pencil of similar, but you may have a way better method. I note with the EF86 which is the preferred valve depending on application like tape recorders the spec sheet says to use an isolated valve holder or mounted in a floating chassis.

ECC83 are getting to be stupid costs to buy as are the EF86's.

Adrian

bikerhifinut 10th Jul 2020 10:03 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Do you need some ECC83?
I can spare a couple and I wont charge a stupid amount. You are right about the silly price and if a ECC81 does the job, its a good valve (tame its HF with grid stoppers etc as its really an RF valve it didn't stop HJ leak though)

As to the bias supply, is this really necesarry? At our end we don't need to squeeze the output valves so hard and cathode bias will produce a true HighFidelity result at the cost of a few watts that you ever knew you didn't need.
And an amp that doesn't need tweaking through the valve useful life.

And

AdrianH 10th Jul 2020 11:18 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Hello Andy,

No I will be fine without the ECC83, they seem to be typically used for the phase splitter in the likes of the Mullard 5-10 amps etc and for some reason the audiophiles seem to rave over them, or some types of them at least. Perhaps it is just the sellers trying to get the biggest profit from guidable people. I am going down the Radford amp route that used the ECF82 Triode/Pentode as the phase shifter, I have some of these, but both the Mullard and Radford designs used the EF86 Pentode for the first stage, I was just planning on using what ever Pentodes I have to hand, but keep being told they are carp!
The whole amp could be the carp! as I am looking to build it with a TT15 final in push pull rather than EL84 or EL34 bottles.

As to the bias I could go cathode bias resistors, but the spec sheet says fixed bias which I am assuming is a negative applied voltage. I am just not sure, the only way I can learn a bit more about how things operate is to try it and see, if it works, great, if it fails I'll just try another way.

I think! I can appreciate that when using cathode bias resistors I loose a bit of the HT Voltage, that overhead would drop by what ever amount I increase the cathode to, but I am also guessing I would have to also lift the screen voltage by the same amount to get back to the same screen operating point?

Adrian

Radio Wrangler 11th Jul 2020 2:38 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
The ECC83 is a fairly low current valve, and so the stray and grid capacitance of the following stage is going to introduce more phase shift than had something running lower impedance in its anode circuit been used. The phase shift puts more limitation on how much feedback you can use, or how much stability margin you get for a given amount of feedback.

The Radford ECF82 phase splitter was invented by Arthur Bailey. Quite a smart cookie. It's rather elegant and has sufficient advantage that I can't understand why anyone does it differently. Arthur Radford was an excellent transformer designer, and together the results really were synergistic.

Using fixed bias will condemn you to keeping checking quiescent currents and twiddling them. Cathode 'automatic' bias loses a little potential output power, but frees you from worrying about the amplifier so you can get on with listening to the music.

Unfortunately, some valves and some variants have been decided to have certain special properties which only true believers can discern. But those true believers assure the rest of us that the differences are actually very large and dramatic. In order to fail to notice them, the hearing of the rest of us must be dreadful or our powers of perception must have fled.

As evidence of the magnitude of these differences, the believers are prepared to pay fortunes for specific valve variants. It's definitely a case of putting their money where their mouth is, and rather a lot of money at that. Odd that the rest of us hear so little.

There are trends which surprise me: No small differences are ever described, no dead heats. You'd expect small differences to outnumber larger ones. There is a mathematical law involved that small things outnumber large things in general. Rare/expensive/hard to find/exclusive things always seem to be rated better than the mundane.

I have a couple of ECC83 in stock because I have a couple of receivers which use them. I can't tell you their makes, or the colour of their markings because I'm not bothered. They tested OK when I checked them, so they'll work just fine if the sets ever need them.

EF86 are well known that some can be very microphonic. Not specifically any manufacturer, shape of getter or whatever, just some individuals. You wind up having to try them out and tap them with a pencil. It's something to be wary about if you use them. This is a real difference. Repeatable, audible by people irrespective of belief-system, measurable even!

The cathode potential running autobias shouldn't cause you to have to up the screen voltage. It should be still within tolerable bounds.

Avoiding microphony is one reason for ECC83 input stages, but you would lose open loop gain. So maybe an ECC83 cascode? But many power amps of the era were excessively sensitive. maybe you'd be better designing for a larger input from the preamp. This would let you use an ECC82 cascode.

David

peter_sol 11th Jul 2020 6:54 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Hi again Adrian.
My experience with the EF91 in the input stage was that you only had to go near them for them to ring or howl. Plenty of gain can be had from an ECC83 using a 100K anode load and 2.2K cathode resistor.

GrimJosef 11th Jul 2020 9:40 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
One option for the bias supply would be to generate a negative as well as a positive output from the existing HT winding. It would just need a couple more diodes, connected the other way round. This gives you way too many volts of course, but compared with the 5V winding, which gave you way too few, it's not much more 'wrong'. You'd then just drop down to the voltage you want with a resistive divider. 300V at 5-10mA across the divider would only see you having to dissipate 1.5-3W. You wouldn't be working the mains transformer much harder if it's got a centre-tapped HT winding anyway. You'd ideally want a reservoir cap immediately after the diodes, to deal with the worst of the ripple, but it could be low value given the small current draw. You'd do most of the smoothing across the lower leg of the divider, where the voltage would be much lower.

Cheers,

GJ

Radio Wrangler 11th Jul 2020 10:19 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
For audio, the EF91 wasn't designed for low microphony, and you'll find the available gain somewhat less than an EF86.

They were designed as an RF/IF amplifier as a later generation replacement for the EF50. But in the 7-pin bottle it lacked multiple cathode pins to keep the inductance down for the best high frequency performance, so the EF80 took over and ran away with the TV IF market. The EF91 can be a bit unstable with the bandwidth of its gain and the barely adequate cathode connection. If one was in use at audio it would not be too hard to have RF oscillation going on and that could be noticed by the effects of moving things near it. As a youngster I bought a very big box full of ex-equipment EF91/6AM6/Z77 and used them for a few years playing with building oscilloscopes. I learned the hard way how to keep them stable. I also learned that lower impedance circuits were needed to go fast, and that If I still needed the same voltage swing, the current became higher. I wound up with 5763s driving deflection plates with lowish value big fat hot anode resistors. I hadn't made the break through to understanding active loads, still less distributed amplifiers at that time.

Anyway, the input stage of a power amplifier really shouldn't need anything as sensitive as an EF86. It's good practice to not have very small signals in places they don't have to be.

David

AdrianH 11th Jul 2020 10:24 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
GJ That is an idea, basically a bridge rectifier to a centre tap point on the transformer.

What I need to do is start loading the transformer and workout the regulation/load curve, it is an old transformer bought from this site.

But not now, I have just received orders I have to do something in the garden by SWMBO.

Play time will probably only return Monday.:(

Adrian.

AdrianH 13th Jul 2020 7:35 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Well no advance today, I was hoping to build a variable current load for the PSU just to map how it performed Volts against Current, I have a few devices such as BU508 or a S2000AF which should deal with up to 50 Watts on a decent heat-sink and cope with the HT voltages, but as the hfe of the devices are typically very low I need something else to drive them with that could also cope with several hundred volts and in that I seem to be lacking in the spares box. I continue to look.

So for now I have been having fun with my CV4014 or EF91 Valves for most of the day. I could not find any curves with a screen voltage less than 250 Volts so have been mapping them out myself. Then picking an operating point based on and selecting the anode load and grid values etc.

For small signal inputs to the grid, I have picked 250 Volts, 47K Ohm anode load, 200 Volt on g2 and control grid around -2.3/2.4 Volts gives around 3mA Ia and 1mA Ig2. So a 600 Ohm Cathode resistor probably called for.

This would have been an excellent task for one of the valve tracer projects!

Anyway at the end with the settings I had selected I mapped out Va against Vg and plotted it in a spreadsheet to provide what I have below.

It is all a bit of fun for me as it was something to do and helped me understand things a bit better than just reading about Pentode curves. I get Vout/Vin of 175 but would imagine that would be loaded with the input resistance of any following stage and capacitance.

Adrian

AdrianH 14th Jul 2020 1:20 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
3 Attachment(s)
This is a do not try this at home really, but I managed to add a variable load to the psu using a small Halogen lamp to dissipate some of the load and therefore heat. S200AF driven by a 2N3439 a Zener and a few resistors.
This give me a clue as to the output Voltage I can expect under what every load I use, looks like the full 300 Volts i require is out of my reach at 50mA+ draw but for a fun build it will do me for a single amp.

Time to crack on with another chassis.

Adrian

AdrianH 14th Jul 2020 10:32 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Here is hoping I have not done something daft?

Just ordered some push-pull output transformers suitable for the Mullard 5-10 EL84's etc. Decided not to wind my own transformers as I do not have two suitable identical cores, not much difference between 6K6 A-A and the 7K the spec sheet suggests. At least if performance is poor I can build a more standard amp.

Adrian

AdrianH 18th Jul 2020 9:52 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
2 Attachment(s)
I have been progressing slowly, made another small chassis to do one mono amp while I await transformer to arrive. One of the issues is to get the ground connection to the chassis, I can not cut and bend the solder tags out of it as in typical radio chassis, using 1.5mm steel makes this a no go for me and I would also need the punch, perhaps one day when I can get brass to make them!
So I am doing a dead bug style circuit using some copper clad, which is bolted to the chassis allowing me to solder to the board.

Anyway have wired the components to the EF and ECF valves, may not look very nice, as I am finding it difficult in the confined area. Have changed a few values from the original Radford circuit to suit my PSU.

The next stage is another piece of copper clad for connecting to the TT15 and transformer. I am hoping that if/when it all works I can sit down in Kicad do the circuit with final component values and complete a PCB layout, that is some time off in the future.

Initially I am going down the Cathode bias resistor route even though it will drop the HT to around 220 Volts I really wanted closer to 300, but the mains transformer is not suitable, soI am on the hunt for a better unit.


Adrian

g4wim_tim 18th Jul 2020 8:51 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Looking good Adrian.

Any reason you've not opted to use tag strips to support the components and provide ground connections where needed ?

Using tag strips (single or double) can look very neat and makes it fairly easy to change values etc.

At the audio power levels you're aiming for I think cathode self bias is a good option.

AdrianH 18th Jul 2020 9:41 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Hi Tim;
At the moment things are in the air, I am using an EF91 which many say will be carp! This due to micro-phonics, noise, lack of gain etc. If it really is, (One has to consider my hearing over 8-9K is practically none existent) then I will have to go for a larger hole to fit a EF86 (9 pin rather than 7). But the end game will be to produce a PCB board if I get it working as mentioned in my previous post.

So for now it's a get it going project anyway I can, like my previous projects get it working on strip board and then use Kicad.

The transformers are on order from a Co. called Primary Windings. Again I have done a compromise as they are meant for the Mullard 5-10 so slightly lower impedance then required for the TT15, this gives me another option in switching for the standard EL84's. In which case it will require another chassis anyway.

Have fun be safe.

Adrian

Top Cap 18th Jul 2020 10:17 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
1 Attachment(s)
I have a load of 2N3585 transistors if you're interested, FOC and I think were kept as spares for either HP or TEK scopes, present stock is over 50 :)

AdrianH 18th Jul 2020 10:32 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Hello Top Cap, it took me a little while to figure out why the 2N3585 transistor at first, but I assume you are meaning for loading the power supply?

They are something to consider either for a proper build load or for a regulated HV power supply.

You have reminded me I need to ask a question about smoothing!

I will send you a pm.

Adrian

Diabolical Artificer 20th Jul 2020 6:04 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Why not try a 6AU6? I've used em triode strapped, great little valves. I have a few if you want to give one a whirl. If you don't have enough gain you can use a bit of positive FB or bootstrapping to get a few more 10's of volt OP.

Andy.

AdrianH 20th Jul 2020 9:06 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
6AU6 or EF94, it's just I have the 91's to hand. I am not sure on gain, until I get the output transformer in place and test final voltage swing then it is difficult to say how I am doing. I have wired onto the grid resistors of the TT15 and with a 60mV p-p signal I am getting a 50 Volt p - p signal on the grids. But there will be some negative feedback added when the transformer is in place.

I am just playing try it and see but making a note of what others say as potential solutions.

Thanks for the offer.

Adrian

AdrianH 22nd Aug 2020 8:55 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
2 Attachment(s)
It has been a while on this project. I received the transformers over a week ago, but was playing with other things and this project tends to make noise, (this annoys the wife) so I was going to wait until she went back to work, but I got bored.

I think I have to make another chassis as the transformer is really too close to the output, but that will wait for a little while. Very please with the transformers, look nice and shiny.

Drilled the chassis and mounted the transformer, added the screen supply and bolted things together. Not using the g2 taps on this set-up as the valve has a single g2 pin for both tetrodes.

I did have a lot of initial issues:

Without any negative feedback the amp was noisy, picked up hum, hissed, whined, pinged etc. If I tapped the EF91 it would ring like a bell out of the speakers. I added some negative feedback and that quietened things down somewhat, but still hiss and a micro-phonic EF81. Out with the scope and found that the whole amp was singing at around 1.25MHz. It turned out the EF81 was oscillating and with very little space to move things around was difficult to determine what was causing it apart from my untidy wiring. A 100pf cap across the anode resistor has solved it for now.

Now with the oscillation gone, the amp is quiet, the EF81 no longer a problem in ringing out of the speaker when tapped and the amp seems to sound reasonable, up to loud! All subjective I know but it is good when something starts to work.

The HT is a bit low for the amp it is at 275 Volts with 21 Volts on the cathodes it could do with around the 320 Volt mark.

Now trying to build a 4 Ohm 10 load with a dozen 12 Ohm resistors.

One fortunate thing recently that did occur was a purchase of a batch of valves from Ebay around 26/27 Valves plus screening cans for £13.50.

I was please to find three EF86's in there and two Mazda valves look to be new with non tarnished pins.

Adrian

AdrianH 23rd Aug 2020 5:39 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
I have tried using my radio test set to measure the amps output as it allows settings for measuring dBm's into 4 Ohms.

At 1KHz the amp manages to get to 38.6dBm (5.4 Vrms) into 4 Ohm which is 7.24 Watts and at that point the anode starts to fast approach the cathode potential and the distortion hits 0.7%. So I am enquiring about a better mains transformer to get a higher HT.

I backed it down to 5 Watts @ 0.4% distortion and went through a range of frequencies to check on the response of the amp.

30Hz = 36.1 dBm
40Hz = 36.5 dBm
50Hz = 36.8 dBm
250Hz = 37.1 dBm
flat up to : -
4000Hz = 37.2 dBm
6000Hz = 37.3 dBm
flat up to : -
18000Hz = 37.2 dBm
20000Hz = 37.2 dBm

The test set does not go above 20KHz so could not test above that and I can not get any noise floor measurements better than -36dBm which would mean at present 74.6dB down on peak output.

All in all I am pleased with how the amp is working using the EF91 and the TT15, so think I will try and make a better job of it, may even try the EF86's now I have some.

Adrian

Diabolical Artificer 24th Aug 2020 7:46 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
That's a pretty good result, nice neat little amp, well done.

Andy.

AdrianH 24th Aug 2020 11:06 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Thank you Andy, the next step is to use KiCad and do a pcb layout for myself, first is to build up the libraries for the valves, it is one of them things where I do not do enough so end up re-learning every time.

Then I sit on it for a little while and decide if I want to get the pcb's made for myself and actually build two amps better.

Adrian

Aub 24th Aug 2020 11:33 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Nice work. Thanks for all the updates. It will be interesting to see what you think of the sound when running music through it.

Cheers
Aub

AdrianH 24th Aug 2020 11:56 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
I have been using it as a mono audio amp for the around 10 hours now whilst sat at my desk/bench/PC and it sounds OK to me, I like a bit of bass boost so have around 1dB at 67 Hz.

To be honest I am not into hi-fi. I would not know a expensive amp to an Amstrad by listening to it, go past 8KHz and I loose it, I had to be pushing full output at 12KHz to hear a faint hiss in the ears.

To such an extent is my hi-fi use is I use my Linux PC to play mp3 files so think the weakest link for me after my ears is the inbuilt sound card and software equalizer.

So I doubt I would ever spend lots of dosh on a system;D

Adrian

Diabolical Artificer 26th Aug 2020 8:59 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Try increasing the IP stage cathode resistor to get extra bass, I'm mucking about with my 807 amp at present trying to find the sweet spot where bass is juicy but not overwhelming, 100u is a tad too much, 10u not enough so next I'll try 25u and 47u. Often if you look at a lot of old amp designs they used 47u.

Keep up the good work, Andy.

AdrianH 26th Aug 2020 9:26 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Hello Andy;

You confused me to start with mentioning cathode resistor.
I am using a 47uF cap on the circuit, the radford diagram I am basing this lot on has a 50uF.

But suspect my like for a bit more base is down to the book-shelf speakers in use they can not be that good at the lower end. Simple Panasonic SB-14 boxes, probably supplied with a small system. But at something like £18 delivered from the Bay they were cheap.

I find it difficult to see lower powered speakers, all seem to want over 100 Watts drive which would be a waste to me, or at least I think so.

Adrian

AdrianH 26th Aug 2020 1:08 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
1 Attachment(s)
Here is my attempt in Kicad at the drawing some of the resistor values will depend on the final PSU voltage, but I am awaiting a transformer person to answer a query.
I need to start asking understanding the NFB better.

But I should be able to at least start on a PCB layout.

Adrian

Diabolical Artificer 27th Aug 2020 2:31 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Sometimes re-positioning speakers can deliver more bass, but as mentioned increasing C3 will also do that, as will increasing C4 and increasing C5. C5 as is, forms a LPF to make the stable at LF @ 33hz, increasing it to 47n will allow more low end through. R2/C2 & R8/8 & C7 all roll off HF and might need tweaking, it all depends on the OPT you use.

Having said all that, your frequency response is pretty good as is, so no changes to the amp are probably needed, but maybe better speakers. Technics did a brilliant series of mini hifi separates, their little speakers were brilliant, oodles of bass for their size, worth keeping an eye out for a pair.

Andy.

AdrianH 27th Aug 2020 3:04 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Hi Andy;

Now I would have said that C5 was a high pass or HF boost from the 1st stage to the Pentode of the ECF82, it goes between the anode of V1 to the grid so to me more signal would get though the higher the frequency?

For instance I worked out that at 1KHz the 4n7 was Xc ~33K but at 20KHz the Xc was 1K6 so to an a.c. signal was expecting it to boost higher frequencies.

C8 I will admit to adding in there, Xc for that being around 300K at 20,000 Hz and 4K at 1.5 Mhz where I had an issue with self osc at 1.5MHz.

The transformers are from Primary Windings dot com, they have the taps at 20% although I am not using them on this obviously, but they should be good, there were designed for the Mullard 5-10.

Kicad is doing my head in so about to go for a drive to CPC.

Adrian

Diabolical Artificer 28th Aug 2020 8:18 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
AFAIK C5 & R10 are a LF gain shelving network, but I could be wrong. I use them in my amps to stop any shennanigans at LF.

Sorry to hear about Kicad, i just installed on my Mint PC after being impressed with your schematic, but it sounds like I'll be sticking with a pencil and ruler having a low threshold for PC software idiocy.

Andy.

AdrianH 28th Aug 2020 9:39 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
1 Attachment(s)
My problems with Kicad are the footprints for the components, that and I am not sure about the method to include things like transformers and speakers but to not include them on the PCB. So I started to just have connectors in the diagram and miss out the transformer and speaker or any controls thinking these would all be external to the PCB anyway. I use Kicad 5 and most of the documentation I had is 4.

I found a good series of videos on Youtube on this latest version this would get you started, it did me.
https://www.youtube.com/results?sear...Basement+Kicad

Will post more in a few minutes but need to get something completed.

Adrian

Just to add a bit more here attached I have added my latest diagram that I am working from, one of the difficulties in Kicad is valves are not that well catered for, you will find EL34's and ECC83 in there but few others, so for the TT15 EF81 ECF82 I needed to add these, then you also find you have to add the footprints for the valve holders and if you use the existing valve holder footprints, as I found out when trying to lay out the circuits they are done with the valves on the same side as the rest of the components as I had it on my 6021 milliwatt amp. But on this I want the valve holders on the back of the PCB to the rest of the components so that the valves can be above chassis board under the chassis. This is my sticking point as to haw I do it, do I reverse the pin directions or find another way. There is a video i have to re-study again.

It is frustrating but it keeps my brain active, the frustration is my inability to keep the information in my head, my learning process has slowed considerably from my youth.

AdrianH 28th Aug 2020 11:33 am

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
3 Attachment(s)
Long post and this was done just for my interest.

I hope this works, the spreadsheet I use is LibreOffice so saved as ods, plus a save as xlsx.

In my years as a service engineer, I never had to worry about designing circuits. I had diagrams to follow when repairing something, I could fault find, replace components to get signals back etc. Yes I could do simple circuits and had a range of standard values I would use when doing them. But the idea is to progress beyond that, put theories into practice and start building things with older tech, Valves.

This means dealing with high impedance circuits were capacitors values really do make a difference. I have no spice, or circuit simulation software and would not know how to use it.

So with Andy's comment about LPF, me thinking high pass etc, I was not sure what is what. So last night I tried to sit down and work things out on paper for just the first stage of the amp. I gave up with paper and tried a spreadsheet.

Assuming a.c sine inputs at various frequencies.

I have made assumptions listed here and this is where I would like advice as to these being correct?

1) Assuming a.c sine inputs at various frequencies.

2) From the anode of the U1 there is a potential divider to the grid of U2 this is made up of C5 & R14 in parallel and R15,R17,C10 and any grid capacitance in parallel, going to ground. The R17 C11 junction taken as a.c. ground.

So in the spreadsheet C5 parallel with R14 = value Rb
R15||R17 = 500K, 500K parallel with C10 = value Rc (add grid capacitance into C10)
The potential at U2 grid becomes a.c. volts at Anode * Rc/(Rb+Rc).

3) Dynamically the anode load Ra of the valve will be it's anode resistor, in parallel with Rd = (R2 and C2 in series) and in parallel with the potential divider (Rb+Rc).

4) So voltage gain will approximate gm * Ra and the ac voltage at the grid of U2 will will be dependant on input volts * u * Rc/(Rb+Rc).


Anyway I stuck it all in multiple columns and worked out in graph form the gain of the first stage and what I think would be the response of the gain and voltage on U2 grid. It allows me to visualise things with changes in values. What I can not even attempt to do is to continue with this adding other stages and then adding negative feedback to the system, I would not know how.

Anyway would appreciate guidance.

Adrian

Red to black 28th Aug 2020 2:53 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Just touching on a point to do with NFB, Negative being the operative word here, so say you want to boost the bass ? cutting the treble is sort of equivalent yes ?
So having a high pass filter within the NFB loop allows more high frequency through said filter, ok so far so good, however as this is in the negative feedback loop this will in turn reduce the higher frequency gain overall, remembering this is Negative feedback here and so it works backwards.

Radio Wrangler 28th Aug 2020 3:41 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Having a highpass function (a zero) in the feedback branch tends to add a bit of phase lead to the overall loop gain. As the loop gain and bandwidth get somewhat limited by the need to restrict phase lag around the loop, the added highpass effect is actually a good thing from the point of stability.

David

Guest 28th Aug 2020 5:00 pm

Re: Hope to start an audio amp with CV415/CV4046/TT15
 
Quote:

I find it difficult to see lower powered speakers, all seem to want over 100 Watts drive which would be a waste to me
It's all marketing c**p, you don't have to drive them that hard, or even want to. My little Tannoy DC4s are rated for 50/200W I did measure the power supplied for comfortable listening, it was in the mW level.


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