Homebrew Audio Amp - UK Vintage Radio Repair and Restoration Discussion Forum

Sean Williams · 28th Nov 2006, 12:23 am

Ok, time I looked into building a proper amplifier.

I have a huge abundance of Pen45 bottles, so figured that using these might be a reasonable idea.

First stumbling block..... 4v Heaters at 1.75A each!

Ok, no real issue, have a nice chunky transformer that has several 6.3v tappings at lots of amps, along with HT....

Apply Ohms law, and come up with .66R resistance in the heater leads (paired heaters) and problem solved....

Now, second problem.....

HT windings are not centretapped - screws up rectifier idea a fair bit, do not want to use semiconductor diodes, even if I do the rectified DC is 408v!

Max ratings for the pen45 is 250v on the anode, and 40ma anode current (I am looking at stereo, and push pull, so there looks to be 160ma of ht current before the preamp and splitter stage is considered.....)

So, any bright ideas, or do I start looking for another transformer?

I want to use indirectly heated rectifiers (GZ37 - I have a fair few)

Over to the assembled throng.....

Cheers
Sean

Radio_Dave · 28th Nov 2006, 2:00 am

Quote:

Originally Posted by m1ecy

HT windings are not centretapped - screws up rectifier idea a fair bit, do not want to use semiconductor diodes

Hi Sean,

I always say that I have far more enthusiasm than real knowledge, so I've probably got this totally wrong, but I can't see why 4 half wave rectifiers can't be used to make a bridge rectifier

David

6AL5W-Martin · 28th Nov 2006, 8:00 am

buy output transformers specially designed for your tubes. This is expensive but the only way to a fine sound. The Power Transformer can be rewindet, or buy this also exactly. There is some Companys who make very fine output transformers for every thing an every tube. Calculate less 100 Euro per one of them.

Sean Williams · 28th Nov 2006, 5:02 pm

Hi Dave,

Sure I could use 4 gz37 rectifiers, but this will cause its own problems - Masses of heater current, and more space!

Looks like I might have to re evaluate my ideas here....

Pity, as the pen45 is a smart looking valve, and I have lots of them........

mickjjo · 28th Nov 2006, 5:33 pm

I believe Gerry Wells uses PEN45s in his WADAR £500 stereo valve amplifiers

, Maybe he could advise you on suitable power supplies.......

.

Regards, Mick.

Skywave · 28th Nov 2006, 6:43 pm

Hi Sean,

Now here's a quick thought that just might be worth investigating . . .

If you took your non-centre tapped sec'y. HT transformer and made a bridge rectifier and then - since you have got lots of PEN 45's - a push-pull bridge for the AF O/P stage (4 bottles per channel) - should look - and sound - pretty impressive!

Since the HT is across 2 valves at any one time, each valve should have about the correct HT.
Finding a suitable O/P transformer (Qty. = 2) might be a problem, though . . .

As I say - just a thought "off the wall".

Al / G8DLH

Ed_Dinning · 28th Nov 2006, 7:50 pm

Hi Sean, don't give up, I'll talk you through the transformer wind if you like. Note that a bridge output would use a pair of op transformers per channel.
There used to be a transformer winder in Hitchen who could supply the lams etc.
As Adi would say: It would glow impressively if you had a stereo bridge pair.

Ed

Skywave · 28th Nov 2006, 8:28 pm

Quote:

Originally Posted by Ed_Dinning

Note that a bridge output would use a pair of op transformers per channel.
Ed

Hi Ed.
Why a pair of output transformers, per channel?

Al / G8DLH

Kat Manton · 29th Nov 2006, 12:11 pm

Hi,

Quote:

Originally Posted by G8DLH

Why a pair of output transformers, per channel?

"Bridged output", to my mind at least, means that the speaker is bridged between two outputs driven in antiphase. Hence one push-pull pair of valves and one o/p transformer driving each side of the speaker. Hence four valves and two o/p transformers needed - per channel.

I've never encountered this done with a valve amp before, only solid-state; so I'm intrigued. The common way to crank up the power seems to be to stick to straight push-pull operation but parallel up the bottles each side as needed.

I've also had bridged solid-state amps decide to be oscillators; is this configuration one which is prone to instability or is it just my "luck" with semiconductor circuits striking again..?

Cheers, Kat

ppppenguin · 29th Nov 2006, 12:25 pm

Dont see why you can't use a single output tranny hung between 2 pairs of valves, each pair of valves effectively in series across the supply. At least there's no standing DC in the primary.

I can't see why a bridged amp should be inherently prone to instability. Normal rules of gain and phase margin apply, just as to any other feedback circuit. I can see a problem if you want overall feedback. Where on earth do you take it from? I suppose you treat each half amplifier as a separate entity and have a phase split at the input.

The amps which do have potential stability problems are electronically balanced (transformerless) line outputs where you have some very strange looking feedback arrangments to ensure that if you ground one leg, the other leg doubles in amplitude. I'm sure they use something pretty close to positive feedback to do this.

Sean Williams · 29th Nov 2006, 1:03 pm

Whoa!

Slow down a bit eh?

I do like the idea of this style of amplifier, but at the moment a simple push pull pair is enough - I really do not have the room for a monster 4 valve per channel beast!

I think I need to source a more suited transformer - the one i have here is potted, so a rewind will ruin it.....

Time to go Iron shopping....

Ed, I am unsure if there are any companies that do transformers in Hitchin now - there used to be one called Tibbles, but they had gone before I walked the earth!

More research needed.

Cheers
Sean

hotbulb · 29th Nov 2006, 4:46 pm

Hi Sean

If you are going to have a mains tranny wound, you might consider using a pair of efficiency dides as rectifiers. A pair of PY500's looks very impressive and will provide more than enough current for your needs

A pair of Pen45's ought to be good for about 12 watts in class AB1, and if memory serves they need a load of 7.5k. You could probably use an output transformer for a pair of EL84's, these are normally 8k and the power rating should be ideal.
Just a few thoughts to play with.......

Sean Williams · 29th Nov 2006, 5:10 pm

Hi Phil,

My recent rummaging has found another potted transformer - lots of useful tappings now.

Still have only got 6.3v heater supplies, but a couple of well placed resistors will sort that out.

A far more sensible 250-0-250 winding, that is currently sypplying a couple of 60 watt bulbs on soak test - so far so good, no drop in voltage, so the tranny seems good for a couple of hundered mills of anode current.....

It also has a 750-0-750 winding - I didnt realise it was going to be that high, so its time to purchase multimeter no 3

So, far more promising - I might get a lump of ply out now, and knock up a quick breadboard - see what these are capable of!

Ok, next question - Phase splitters - any preferences - i like the look of the simple concertina system, or possibly a long tail pair... - I am planning to use 6SN7GT for the phase splitter, driving the pen 45 directly, then another 6SN7GT to drive the phase splitter - any comments?

Ok, off to the drawing board - time to sketch a circuit diagram, and possibly a chassis layout.....

Cheers
Sean

Kat Manton · 29th Nov 2006, 8:26 pm

Hi Sean,

Quote:

Originally Posted by m1ecy

Ok, next question - Phase splitters - any preferences

I'd been meaning to play with the 'floating paraphase' one (I think it was called) in my Radio Designer's Handbook (quite a few other phase splitters in that mighty tome, too.)

...no real reason other than it looked interesting...

Cheers, Kat

Sean Williams · 29th Nov 2006, 8:32 pm

Have looked at many different types - i like the idea of the one that is floating too - might give good results.....

Hmm, the breadboard might well get a pasting....

Cheers
Sean

Kat Manton · 29th Nov 2006, 8:39 pm

Hi Sean,

Quote:

Originally Posted by m1ecy

Have looked at many different types - i like the idea of the one that is floating too - might give good results.....

I'll have to dig The Book out and have a look; I think it was recommended as a high-performance one for high-class radios and radiograms. I recall it uses 4 triodes hence would cost more to implement in a commercial design, but in a homebrew project this is largely irrelevant.

Good performance and an excuse to have more bottles on the chassis...

Cheers, Kat

Ed_Dinning · 29th Nov 2006, 9:04 pm

Thanks for adding the explanation on bridge outputs folks. This appeares to be turning into an interesting project.
I have in the past wound small auto transformers for 4/5/6.3 volt conversion and to run strange heater voltages. They can be pretty compact.

Ed

Skywave · 30th Nov 2006, 1:14 am

Quote:

Originally Posted by Kat Manton

Hi,

"Bridged output", to my mind at least, means that the speaker is bridged between two outputs driven in antiphase. Hence one push-pull pair of valves and one o/p transformer driving each side of the speaker. Hence four valves and two o/p transformers needed - per channel.

Cheers, Kat

Hi Kat.

OK, perhaps I used the wrong terminology.

I'll try to describe in text form what I had in mind - I don't have the kit here to attach a drawing.

Take two valves, V1 and V2, and common their anodes to HT+. Take the cathode of V1 and join it to the anode of a 3rd. valve - say V3. Take the cathode of V2 and join it to the anode of a 4th. valve - say V4. Join the cathode of V3 and the cathode of V4 to gnd. Now connect the primary of the O/P transformer from the cathode of V1 / anode of V3 to the cathode of V2 / anode of V4. Voila! One O/P transformer!

Note the resemblance of the config. to a bridge rectifier . . .

The driving arrangement will be V1 / V4 grids driven in-phase and V2 / V3 grids 180 degrees out-of-phase. It will be obviously be necessary to introduce a dc offset to the grid feeds of V1 and V2 since their cathodes are way above ground. Plus well-insulated heater supplies (V1, V2) for same reason.

I have met this concept using power FETs in high-power SMPUs, but not in audio work.

However, as I said / implied in my original post, this was only a conceptual "idea" - since Sean did say he had plenty of these valves. It's all a bit irrelevant now anyway - reading Sean's later Post.

However, I can think of one Forum Member who might just be prepared to investigate this idea . . . . . .

Al / G8DLH

Merlin · 1st Dec 2006, 2:18 pm

You don't need a centre tap to use a valve recitifer. Just use solid state then add the rectifier in series with the DC to drop the voltage as normal. Then you can use a cathode bias output stage to further reduce the anode-cathode voltage.
You can drop heater voltage by placing diodes or a motor run capacitor in series with the heater supply.

Is this for hifi? If so the long tailed pair is the phase inverter of choice, that or the Isodyne.

28th Nov 2006, 12:23 am	#1
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,517	Homebrew Audio Amp Ok, time I looked into building a proper amplifier. I have a huge abundance of Pen45 bottles, so figured that using these might be a reasonable idea. First stumbling block..... 4v Heaters at 1.75A each! Ok, no real issue, have a nice chunky transformer that has several 6.3v tappings at lots of amps, along with HT.... Apply Ohms law, and come up with .66R resistance in the heater leads (paired heaters) and problem solved.... Now, second problem..... HT windings are not centretapped - screws up rectifier idea a fair bit, do not want to use semiconductor diodes, even if I do the rectified DC is 408v! Max ratings for the pen45 is 250v on the anode, and 40ma anode current (I am looking at stereo, and push pull, so there looks to be 160ma of ht current before the preamp and splitter stage is considered.....) So, any bright ideas, or do I start looking for another transformer? I want to use indirectly heated rectifiers (GZ37 - I have a fair few) Over to the assembled throng..... Cheers Sean __________________ Engineers make things work and have spare bits when finished

28th Nov 2006, 8:00 am	#3
6AL5W-Martin Hexode Join Date: Nov 2006 Location: Biedenkopf, [Hessen], Germany. Posts: 425	Re: Homebrew Audio Amp buy output transformers specially designed for your tubes. This is expensive but the only way to a fine sound. The Power Transformer can be rewindet, or buy this also exactly. There is some Companys who make very fine output transformers for every thing an every tube. Calculate less 100 Euro per one of them.

28th Nov 2006, 5:02 pm	#4
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,517	Re: Homebrew Audio Amp Hi Dave, Sure I could use 4 gz37 rectifiers, but this will cause its own problems - Masses of heater current, and more space! Looks like I might have to re evaluate my ideas here.... Pity, as the pen45 is a smart looking valve, and I have lots of them........ __________________ Engineers make things work and have spare bits when finished

28th Nov 2006, 5:33 pm	#5
mickjjo Rest in Peace Join Date: Jul 2004 Location: Dartford, Kent, UK. Posts: 1,661	Re: Homebrew Audio Amp I believe Gerry Wells uses PEN45s in his WADAR £500 stereo valve amplifiers , Maybe he could advise you on suitable power supplies....... . Regards, Mick.

28th Nov 2006, 6:43 pm	#6
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Homebrew Audio Amp Hi Sean, Now here's a quick thought that just might be worth investigating . . . If you took your non-centre tapped sec'y. HT transformer and made a bridge rectifier and then - since you have got lots of PEN 45's - a push-pull bridge for the AF O/P stage (4 bottles per channel) - should look - and sound - pretty impressive! Since the HT is across 2 valves at any one time, each valve should have about the correct HT. Finding a suitable O/P transformer (Qty. = 2) might be a problem, though . . . As I say - just a thought "off the wall". Al / G8DLH

28th Nov 2006, 7:50 pm	#7
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Homebrew Audio Amp Hi Sean, don't give up, I'll talk you through the transformer wind if you like. Note that a bridge output would use a pair of op transformers per channel. There used to be a transformer winder in Hitchen who could supply the lams etc. As Adi would say: It would glow impressively if you had a stereo bridge pair. Ed

29th Nov 2006, 12:25 pm	#10
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Homebrew Audio Amp Dont see why you can't use a single output tranny hung between 2 pairs of valves, each pair of valves effectively in series across the supply. At least there's no standing DC in the primary. I can't see why a bridged amp should be inherently prone to instability. Normal rules of gain and phase margin apply, just as to any other feedback circuit. I can see a problem if you want overall feedback. Where on earth do you take it from? I suppose you treat each half amplifier as a separate entity and have a phase split at the input. The amps which do have potential stability problems are electronically balanced (transformerless) line outputs where you have some very strange looking feedback arrangments to ensure that if you ground one leg, the other leg doubles in amplitude. I'm sure they use something pretty close to positive feedback to do this.

29th Nov 2006, 1:03 pm	#11
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,517	Re: Homebrew Audio Amp Whoa! Slow down a bit eh? I do like the idea of this style of amplifier, but at the moment a simple push pull pair is enough - I really do not have the room for a monster 4 valve per channel beast! I think I need to source a more suited transformer - the one i have here is potted, so a rewind will ruin it..... Time to go Iron shopping.... Ed, I am unsure if there are any companies that do transformers in Hitchin now - there used to be one called Tibbles, but they had gone before I walked the earth! More research needed. Cheers Sean __________________ Engineers make things work and have spare bits when finished

29th Nov 2006, 4:46 pm	#12
hotbulb Tetrode Join Date: Apr 2003 Location: Wantage, Oxfordshire, UK. Posts: 93	Re: Homebrew Audio Amp Hi Sean If you are going to have a mains tranny wound, you might consider using a pair of efficiency dides as rectifiers. A pair of PY500's looks very impressive and will provide more than enough current for your needs A pair of Pen45's ought to be good for about 12 watts in class AB1, and if memory serves they need a load of 7.5k. You could probably use an output transformer for a pair of EL84's, these are normally 8k and the power rating should be ideal. Just a few thoughts to play with.......

29th Nov 2006, 5:10 pm	#13
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,517	Re: Homebrew Audio Amp Hi Phil, My recent rummaging has found another potted transformer - lots of useful tappings now. Still have only got 6.3v heater supplies, but a couple of well placed resistors will sort that out. A far more sensible 250-0-250 winding, that is currently sypplying a couple of 60 watt bulbs on soak test - so far so good, no drop in voltage, so the tranny seems good for a couple of hundered mills of anode current..... It also has a 750-0-750 winding - I didnt realise it was going to be that high, so its time to purchase multimeter no 3 So, far more promising - I might get a lump of ply out now, and knock up a quick breadboard - see what these are capable of! Ok, next question - Phase splitters - any preferences - i like the look of the simple concertina system, or possibly a long tail pair... - I am planning to use 6SN7GT for the phase splitter, driving the pen 45 directly, then another 6SN7GT to drive the phase splitter - any comments? Ok, off to the drawing board - time to sketch a circuit diagram, and possibly a chassis layout..... Cheers Sean __________________ Engineers make things work and have spare bits when finished

29th Nov 2006, 8:32 pm	#15
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,517	Re: Homebrew Audio Amp Have looked at many different types - i like the idea of the one that is floating too - might give good results..... Hmm, the breadboard might well get a pasting.... Cheers Sean __________________ Engineers make things work and have spare bits when finished

29th Nov 2006, 9:04 pm	#17
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Homebrew Audio Amp Thanks for adding the explanation on bridge outputs folks. This appeares to be turning into an interesting project. I have in the past wound small auto transformers for 4/5/6.3 volt conversion and to run strange heater voltages. They can be pretty compact. Ed

1st Dec 2006, 2:18 pm	#19
Merlin Retired Dormant Member Join Date: Apr 2006 Location: Shrewsbury, Shropshire, UK. Posts: 67	Re: Homebrew Audio Amp You don't need a centre tap to use a valve recitifer. Just use solid state then add the rectifier in series with the DC to drop the voltage as normal. Then you can use a cathode bias output stage to further reduce the anode-cathode voltage. You can drop heater voltage by placing diodes or a motor run capacitor in series with the heater supply. Is this for hifi? If so the long tailed pair is the phase inverter of choice, that or the Isodyne.