UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Reply
 
Thread Tools
Old 11th Jul 2020, 9:40 am   #21
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 3,136
Default 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
__________________
http://www.ampregen.com
GrimJosef is offline   Reply With Quote
Old 11th Jul 2020, 10:19 am   #22
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 15,282
Default 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
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now   Reply With Quote
Old 11th Jul 2020, 10:24 am   #23
AdrianH
Hexode
 
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 366
Default 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.
__________________
Ask questions and learn, or try to!
AdrianH is online now   Reply With Quote
Old 13th Jul 2020, 7:35 pm   #24
AdrianH
Hexode
 
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 366
Default 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
__________________
Ask questions and learn, or try to!
AdrianH is online now   Reply With Quote
Old 14th Jul 2020, 1:20 pm   #25
AdrianH
Hexode
 
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 366
Default Re: Hope to start an audio amp with CV415/CV4046/TT15

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
Attached Thumbnails
Click image for larger version

Name:	psu6.jpg
Views:	57
Size:	63.0 KB
ID:	210976   Click image for larger version

Name:	psu7.jpg
Views:	56
Size:	70.0 KB
ID:	210977   Click image for larger version

Name:	psu load line.png
Views:	31
Size:	25.3 KB
ID:	210978  
__________________
Ask questions and learn, or try to!
AdrianH is online now   Reply With Quote
Old 14th Jul 2020, 10:32 pm   #26
AdrianH
Hexode
 
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 366
Default 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
__________________
Ask questions and learn, or try to!
AdrianH is online now   Reply With Quote
Old 18th Jul 2020, 9:52 am   #27
AdrianH
Hexode
 
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 366
Default Re: Hope to start an audio amp with CV415/CV4046/TT15

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
Attached Thumbnails
Click image for larger version

Name:	Dryfit1.jpg
Views:	59
Size:	47.8 KB
ID:	211237   Click image for larger version

Name:	buildup1.JPG
Views:	71
Size:	61.3 KB
ID:	211238  
__________________
Ask questions and learn, or try to!
AdrianH is online now   Reply With Quote
Old 18th Jul 2020, 8:51 pm   #28
g4wim_tim
Pentode
 
Join Date: Nov 2018
Location: Freckleton, Lancashire, UK.
Posts: 122
Default 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.
__________________
Building and tinkering for over 50 years, from 807's to digital amateur TV.
g4wim_tim is offline   Reply With Quote
Old 18th Jul 2020, 9:41 pm   #29
AdrianH
Hexode
 
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 366
Default 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
__________________
Ask questions and learn, or try to!
AdrianH is online now   Reply With Quote
Old 18th Jul 2020, 10:17 pm   #30
Top Cap
Octode
 
Top Cap's Avatar
 
Join Date: Mar 2006
Location: Watford, Hertfordshire, UK.
Posts: 1,225
Default Re: Hope to start an audio amp with CV415/CV4046/TT15

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
Attached Files
File Type: pdf 2N3585.pdf (63.3 KB, 20 views)
__________________
Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way!
Top Cap is offline   Reply With Quote
Old 18th Jul 2020, 10:32 pm   #31
AdrianH
Hexode
 
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 366
Default 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
__________________
Ask questions and learn, or try to!
AdrianH is online now   Reply With Quote
Old 20th Jul 2020, 6:04 am   #32
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 5,473
Default 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.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline   Reply With Quote
Old 20th Jul 2020, 9:06 am   #33
AdrianH
Hexode
 
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 366
Default 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
__________________
Ask questions and learn, or try to!
AdrianH is online now   Reply With Quote
Old 22nd Aug 2020, 8:55 pm   #34
AdrianH
Hexode
 
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 366
Default Re: Hope to start an audio amp with CV415/CV4046/TT15

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
Attached Thumbnails
Click image for larger version

Name:	amp1.jpg
Views:	66
Size:	63.3 KB
ID:	214108   Click image for larger version

Name:	EF86x3.jpg
Views:	58
Size:	64.2 KB
ID:	214109  
__________________
Ask questions and learn, or try to!
AdrianH is online now   Reply With Quote
Old 23rd Aug 2020, 5:39 pm   #35
AdrianH
Hexode
 
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 366
Default 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
__________________
Ask questions and learn, or try to!
AdrianH is online now   Reply With Quote
Old 24th Aug 2020, 7:46 am   #36
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 5,473
Default Re: Hope to start an audio amp with CV415/CV4046/TT15

That's a pretty good result, nice neat little amp, well done.

Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline   Reply With Quote
Old 24th Aug 2020, 11:06 am   #37
AdrianH
Hexode
 
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 366
Default 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
__________________
Ask questions and learn, or try to!
AdrianH is online now   Reply With Quote
Old 24th Aug 2020, 11:33 am   #38
Aub
Octode
 
Aub's Avatar
 
Join Date: Mar 2005
Location: Nuneaton, Warwickshire, UK.
Posts: 1,082
Default 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
__________________
Ignorance of The Law is No Excuse - but I didnt know that!
Aub is offline   Reply With Quote
Old 24th Aug 2020, 11:56 am   #39
AdrianH
Hexode
 
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 366
Default 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

Adrian
__________________
Ask questions and learn, or try to!
AdrianH is online now   Reply With Quote
Old 26th Aug 2020, 8:59 am   #40
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 5,473
Default 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.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 1:57 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Copyright ©2002 - 2020, Paul Stenning.