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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.


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

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



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)


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.


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.


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.


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 .


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.


Radio Wrangler 8th Jul 2020 11:03 pm

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


bikerhifinut 8th Jul 2020 11:07 pm

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

Originally Posted by Radio Wrangler (Post 1267968)


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


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).


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.


peter_sol 10th Jul 2020 6:52 pm

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


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.


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.


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?


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.


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.

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