What's a good way of making a homebrew valve amplifier louder - UK Vintage Radio Repair and Restoration Discussion Forum

Neil Purling · 29th Feb 2012, 4:59 pm

Possibly three ways as I see it. The present amplifier had a single 6V6 as originally built.
1. A higher HT rail voltage. (280V with no signal input & dummy load resistor plugged into spkr socket).
2. Convert the output stage to push-pull.
3. Keep a single o/p valve, but one with a higher gm, (EL84). Not a big audible difference.

Lucien Nunes · 29th Feb 2012, 5:31 pm

You need to define what you mean by louder...

Do you want more gain (i.e. for a given input signal the output is louder) or do you want more power (i.e. you get the same output for a given input but you can have more before the onset of distortion).

As a single 6V6 doesn't give you more than a few watts I'm guessing you want more power, but naturally you can't achieve this by changing the gain of any particular stage. If you want more gain then it's probably fairly simple to increase at or before the driver. If you have NFB you still can't just change the gain of one stage though.

A 6V6 won't like you putting the HT up much over 300V, and if you do you will get more power only at the expense of more distortion. To use that power you will need to change the ratio of the output transformer. In a homebrew amp, there is a possibility that the transformer is not a perfect match, i.e. is not presenting the optimum load under the chosen operating conditions anyway, so that might be a path to explore. Changing output valve type usually requires changing the output transformer and in my book that makes it a new amplifier built on an old chassis.

Lucien

GrimJosef · 29th Feb 2012, 5:36 pm

If all you want is louder then raising the HT may well help. However I'll make two points:

1. The human ear's response is logarithmic so if you find the amp is currently seriously under-powered then you probably need to increase the power by a factor of several - three or four perhaps. There is no straightforward way of doing this without a complete redesign and rebuild.

2. If you only increase the HT you could find that you add a substantial amount of distortion a) because you've moved one or more of the valves away from their ideal operating points and b) because you've overloaded the output transformer.

Obviously it's worth checking that the amp is currently working to spec. But if it is and it's still much too quiet then I'd suggest looking at a different amp. Or a more efficient speaker.

Cheers,

GJ

murphyv310 · 29th Feb 2012, 6:52 pm

Hi.
I agree with the above totally. You could try a 6L6 with the correct O/P transformer and higher HT and the correct cathode bias resistor, have a look at the drive and make changes if necessary, whether you notice any difference, only you can tell!

kalee20 · 29th Feb 2012, 7:05 pm

You definitely need to know what's limiting the volume now.

If it's quiet but undistorted, even at max volume setting, then the 6V6 is not being fully driven and you need more gain in front of the 6V6.

If it is distorted, then assuming the output transformer is correct, raising HT voltage will allow you to wind up the volume control farther before distortion sets in. However, the 6V6 may not like it long term - it's not good practice to exceed ratings.

Neil Purling · 29th Feb 2012, 7:30 pm

The HT rail is 28-289V under no-signal input & a dummy load resistor plugged into the speaker socket according to a DMM. My AVO appears to show a whisker under 300V, but a finer judgement is not easy on the 500V range. This figure does not vary if I inject a signal & use a actual speaker.
The output transformer was made for me by Ed Dinning. It was made specifically for a 6V6.
The Primary.... I believe it is split, or centre-tapped. Thus there is the capability of a push-pull o/p being created by adding another 6V6 and using the 'spare' triode in the first valve (a 6SL7) as a phase-splitter.
The choice of phase-splitter: As simple as possible! Maths has never been my forte.

I have not got an AF signal generator to make proper tests. I was told that a 6V6 needs to see a 7V input for full output. Is that right? By rights I ought to test by injecting a signal at the input & onto the grid of the 6V6 to see if it is being properly driven before I convert to p-p operation.

val33vo · 29th Feb 2012, 9:34 pm

Why not use transistors instead if you want high power at a reasonable cost ? much cheaper than huge mains and audio output transformers not to mention smoothing chokes

If you are honest you will not know any difference re audio sound unless you drive the amp into distortion ( guitar enthusiasts )

regards

val33vo

Neil Purling · 29th Feb 2012, 10:42 pm

Those cold-cathode triodes? Yes, I have heard of them.
Just because I know they exist doesn't mean I want to use them if I can avoid it!

I made the amplifier myself. It works well with a radio feeder, which is a fairly strong signal.
I didn't realise the amplifier was inadequate for that reason.
I want more power. I have a transformer fit for the new role and the components.

I need a phase splitter though......... I was thinking of a Cathodyne. Will the unused half of the 6SL7 be suitable?

glowinganode · 29th Feb 2012, 11:45 pm

Hi Neil, I can recommend the circuit in post #13 of this thread.
I have built a stereo pair of these using 6V6's pentode connected, giving around 10W per channel.
Minimal component count and sensitive, will give full output directly from a line out.
Rob.

Neil Purling · 1st Mar 2012, 7:32 am

I will have to punch two more holes. One for the phase-splitter & then the other 6V6.
I would have separate cathode resistors of 330R 3W bypassed with 22uf @ 63V.
The closest thing I have to a matched pair of 6V6's is two of the same envelope type
For visual impact you can't beat the 'G' envelope.

HG MICKE · 1st Mar 2012, 10:08 am

I don't know if I have missed something, was the output transformer that Ed made for Push Pull or for Single Ended use.
If it was made for Push Pull it would saturate when used for Single ended, this would result in very low output.

Colin

1st Mar 2012, 11:17 am

This https://www.vintage-radio.net/forum/...8&postcount=34 is quite loud enough.

peter_sol · 1st Mar 2012, 4:07 pm

A 6v6 gives 5 watts which is quite loud enough for any radio
I suspect it is not getting enough signal into its grid
Maybe try an EL33 which is a high slope pentode and change the cathode bias resistor and see if that is louder.

beamcurrent · 1st Mar 2012, 10:47 pm

One thing that has not been mentioned is loudspeaker efficiency, the modern loudspeaker have a very low efficiency often in the 1 to 2% range (due to the ease that transistor amplifiers can have 50+ watts output). Your amplifier would sound much louder by using a loudspeaker of the horn type, yes they do tend to be a bit big and are inconvenient in a small room, but with efficiencies approaching 25-30% will be much much louder. It should be noted that, in the past, when watts were hard to generate with valves cinemas used big horn speakers behind the screen to great effect.

regards Brian

Neil Purling · 1st Mar 2012, 11:27 pm

I have a little test square-wave generator that I set to deliver 100mv @ 1Khz.
I injected this into the amplifier and looked at what the 6SL7 is putting into the o/p stage: 5V p-p.
Here is the Brimar data sheet for the 6SL7: http://www.r-type.org/pdfs/6sl7gt.pdf .
I am getting a gain of 50 as per the data sheet.

There is currently a 6CH6 as o/p. I have no idea what amplification it is supposed to deliver as it isn't really a audio o/p valve. Across the Secondary of the o/p transformer, measured with a dummy load plugged into the speaker socket I see 10V p-p.
Probably an appalling mis-match to this transformer.
Before I remove the Noval base and replace the IO base & 6V6 i'll try a EL84.

Herald1360 · 2nd Mar 2012, 1:34 am

You should be able to get 3W or more from a 6CH6 running with about 5K load impedance with 250V anode and grid supply. Biassing will be different from EL84, you'll need about -4.5V rather than -7V or so for the EL84.

Apart from the standing bias level, the two valves characteristics are fairly similar with both running around 11mA/V gm.

No idea how the distortion levels would compare, but I wouldn't expect a vast difference from the 9% or so of the EL84.

tri-comp · 2nd Mar 2012, 9:09 pm

Quote:

Originally Posted by HG MICKE

I don't know if I have missed something, was the output transformer that Ed made for Push Pull or for Single Ended use.
If it was made for Push Pull it would saturate when used for Single ended, this would result in very low output.

Colin

Actually this is an issue that should be addressed first and foremost.
You can't use a SE-transformer for Push-Pull and likewise not a PP-transformer for Single-Ended.
Exactly WHAT do you have?
Look at the core. Is it E-I interleaved or are the E's and I's separate?
Interleaved = PP; Separate (with a gap) = SE

rgds,

/tri-comp

Neil Purling · 3rd Mar 2012, 2:54 pm

The transformer core is gapped, with a paper shim inserted. Adjustment of the gap with different shims is possible, but I can't remember what effect this would have. I have never had it apart. Ed mentioned the split Primary made push-pull a possibility, but didn't express an opinion on whether the transformer would be operating inefficiently in transferring the signal to the Secondary.

The material of the core laminations is either GOSS or Unisil. This I was told is better than Stalloy.

Herald1360 · 3rd Mar 2012, 3:08 pm

If it's gapped it's fine for single ended. Being gapped per se doesn't rule out p-p use, it's just not optimum from the materials used point of view. The other way round is not likely to work at all well, though.

Do you know what the winding ratio is? The 6CH6 may simply be seeing a completely mismatched load.......

TrevorG3VLF · 3rd Mar 2012, 5:46 pm

Hi Neil,

10 p-p is 3.54 RMS. Into 8 ohms this gives 2.5W. Not bad for a diddy valve. You may be able to double this but will not notice much difference in sound level.

With regard to horn loudspeakers, my uncle had a cinema which had a PX4 in the amplifier and there was enough sound to reproduce the whoops of the injuns and the drumming of the hooves of the stampeding buffalo.

Trevor

29th Feb 2012, 5:31 pm	#2
Lucien Nunes Rest in Peace Join Date: Apr 2008 Location: London, UK. Posts: 2,508	Re: What's a good way of making a homebrew valve amplifier louder You need to define what you mean by louder... Do you want more gain (i.e. for a given input signal the output is louder) or do you want more power (i.e. you get the same output for a given input but you can have more before the onset of distortion). As a single 6V6 doesn't give you more than a few watts I'm guessing you want more power, but naturally you can't achieve this by changing the gain of any particular stage. If you want more gain then it's probably fairly simple to increase at or before the driver. If you have NFB you still can't just change the gain of one stage though. A 6V6 won't like you putting the HT up much over 300V, and if you do you will get more power only at the expense of more distortion. To use that power you will need to change the ratio of the output transformer. In a homebrew amp, there is a possibility that the transformer is not a perfect match, i.e. is not presenting the optimum load under the chosen operating conditions anyway, so that might be a path to explore. Changing output valve type usually requires changing the output transformer and in my book that makes it a new amplifier built on an old chassis. Lucien

29th Feb 2012, 5:36 pm	#3
GrimJosef Dekatron Join Date: Sep 2007 Location: Oxfordshire, UK. Posts: 4,311	Re: What's a good way of making a homebrew valve amplifier louder If all you want is louder then raising the HT may well help. However I'll make two points: 1. The human ear's response is logarithmic so if you find the amp is currently seriously under-powered then you probably need to increase the power by a factor of several - three or four perhaps. There is no straightforward way of doing this without a complete redesign and rebuild. 2. If you only increase the HT you could find that you add a substantial amount of distortion a) because you've moved one or more of the valves away from their ideal operating points and b) because you've overloaded the output transformer. Obviously it's worth checking that the amp is currently working to spec. But if it is and it's still much too quiet then I'd suggest looking at a different amp. Or a more efficient speaker. Cheers, GJ

29th Feb 2012, 6:52 pm	#4
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,422	Re: What's a good way of making a homebrew valve amplifier louder Hi. I agree with the above totally. You could try a 6L6 with the correct O/P transformer and higher HT and the correct cathode bias resistor, have a look at the drive and make changes if necessary, whether you notice any difference, only you can tell! __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

29th Feb 2012, 7:05 pm	#5
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: What's a good way of making a homebrew valve amplifier louder You definitely need to know what's limiting the volume now. If it's quiet but undistorted, even at max volume setting, then the 6V6 is not being fully driven and you need more gain in front of the 6V6. If it is distorted, then assuming the output transformer is correct, raising HT voltage will allow you to wind up the volume control farther before distortion sets in. However, the 6V6 may not like it long term - it's not good practice to exceed ratings.

29th Feb 2012, 7:30 pm	#6
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: What's a good way of making a homebrew valve amplifier louder The HT rail is 28-289V under no-signal input & a dummy load resistor plugged into the speaker socket according to a DMM. My AVO appears to show a whisker under 300V, but a finer judgement is not easy on the 500V range. This figure does not vary if I inject a signal & use a actual speaker. The output transformer was made for me by Ed Dinning. It was made specifically for a 6V6. The Primary.... I believe it is split, or centre-tapped. Thus there is the capability of a push-pull o/p being created by adding another 6V6 and using the 'spare' triode in the first valve (a 6SL7) as a phase-splitter. The choice of phase-splitter: As simple as possible! Maths has never been my forte. I have not got an AF signal generator to make proper tests. I was told that a 6V6 needs to see a 7V input for full output. Is that right? By rights I ought to test by injecting a signal at the input & onto the grid of the 6V6 to see if it is being properly driven before I convert to p-p operation. Last edited by Neil Purling; 29th Feb 2012 at 7:33 pm. Reason: extra text.

29th Feb 2012, 4:59 pm	#1
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	What's a good way of making a homebrew valve amplifier louder Possibly three ways as I see it. The present amplifier had a single 6V6 as originally built. 1. A higher HT rail voltage. (280V with no signal input & dummy load resistor plugged into spkr socket). 2. Convert the output stage to push-pull. 3. Keep a single o/p valve, but one with a higher gm, (EL84). Not a big audible difference.

29th Feb 2012, 9:34 pm	#7
val33vo Retired Dormant Member Join Date: Apr 2008 Location: Banffshire, Scotland, UK. Posts: 191	Re: What's a good way of making a homebrew valve amplifier louder Why not use transistors instead if you want high power at a reasonable cost ? much cheaper than huge mains and audio output transformers not to mention smoothing chokes If you are honest you will not know any difference re audio sound unless you drive the amp into distortion ( guitar enthusiasts ) regards val33vo

29th Feb 2012, 10:42 pm	#8
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: What's a good way of making a homebrew valve amplifier louder Those cold-cathode triodes? Yes, I have heard of them. Just because I know they exist doesn't mean I want to use them if I can avoid it! I made the amplifier myself. It works well with a radio feeder, which is a fairly strong signal. I didn't realise the amplifier was inadequate for that reason. I want more power. I have a transformer fit for the new role and the components. I need a phase splitter though......... I was thinking of a Cathodyne. Will the unused half of the 6SL7 be suitable?

29th Feb 2012, 11:45 pm	#9
glowinganode Octode Join Date: Apr 2009 Location: Aylesbury, Buckinghamshire, UK. Posts: 1,522	Re: What's a good way of making a homebrew valve amplifier louder Hi Neil, I can recommend the circuit in post #13 of this thread. I have built a stereo pair of these using 6V6's pentode connected, giving around 10W per channel. Minimal component count and sensitive, will give full output directly from a line out. Rob. __________________ We have done so much, for so long, with so little, we are now qualified to do anything with nothing.

1st Mar 2012, 7:32 am	#10
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: What's a good way of making a homebrew valve amplifier louder I will have to punch two more holes. One for the phase-splitter & then the other 6V6. I would have separate cathode resistors of 330R 3W bypassed with 22uf @ 63V. The closest thing I have to a matched pair of 6V6's is two of the same envelope type For visual impact you can't beat the 'G' envelope.

1st Mar 2012, 10:08 am	#11
HG MICKE Pentode Join Date: Jul 2007 Location: London, UK. Posts: 165	Re: What's a good way of making a homebrew valve amplifier louder I don't know if I have missed something, was the output transformer that Ed made for Push Pull or for Single Ended use. If it was made for Push Pull it would saturate when used for Single ended, this would result in very low output. Colin

1st Mar 2012, 11:17 am	#12
Guest Guest Posts: n/a	Re: What's a good way of making a homebrew valve amplifier louder This https://www.vintage-radio.net/forum/...8&postcount=34 is quite loud enough.

1st Mar 2012, 4:07 pm	#13
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: What's a good way of making a homebrew valve amplifier louder A 6v6 gives 5 watts which is quite loud enough for any radio I suspect it is not getting enough signal into its grid Maybe try an EL33 which is a high slope pentode and change the cathode bias resistor and see if that is louder.

1st Mar 2012, 10:47 pm	#14
beamcurrent Heptode Join Date: Mar 2008 Location: Camberley, Surrey, UK. Posts: 805	Re: What's a good way of making a homebrew valve amplifier louder One thing that has not been mentioned is loudspeaker efficiency, the modern loudspeaker have a very low efficiency often in the 1 to 2% range (due to the ease that transistor amplifiers can have 50+ watts output). Your amplifier would sound much louder by using a loudspeaker of the horn type, yes they do tend to be a bit big and are inconvenient in a small room, but with efficiencies approaching 25-30% will be much much louder. It should be noted that, in the past, when watts were hard to generate with valves cinemas used big horn speakers behind the screen to great effect. regards Brian

1st Mar 2012, 11:27 pm	#15
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: What's a good way of making a homebrew valve amplifier louder I have a little test square-wave generator that I set to deliver 100mv @ 1Khz. I injected this into the amplifier and looked at what the 6SL7 is putting into the o/p stage: 5V p-p. Here is the Brimar data sheet for the 6SL7: http://www.r-type.org/pdfs/6sl7gt.pdf . I am getting a gain of 50 as per the data sheet. There is currently a 6CH6 as o/p. I have no idea what amplification it is supposed to deliver as it isn't really a audio o/p valve. Across the Secondary of the o/p transformer, measured with a dummy load plugged into the speaker socket I see 10V p-p. Probably an appalling mis-match to this transformer. Before I remove the Noval base and replace the IO base & 6V6 i'll try a EL84.

2nd Mar 2012, 1:34 am	#16
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: What's a good way of making a homebrew valve amplifier louder You should be able to get 3W or more from a 6CH6 running with about 5K load impedance with 250V anode and grid supply. Biassing will be different from EL84, you'll need about -4.5V rather than -7V or so for the EL84. Apart from the standing bias level, the two valves characteristics are fairly similar with both running around 11mA/V gm. No idea how the distortion levels would compare, but I wouldn't expect a vast difference from the 9% or so of the EL84. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

3rd Mar 2012, 2:54 pm	#18
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: What's a good way of making a homebrew valve amplifier louder The transformer core is gapped, with a paper shim inserted. Adjustment of the gap with different shims is possible, but I can't remember what effect this would have. I have never had it apart. Ed mentioned the split Primary made push-pull a possibility, but didn't express an opinion on whether the transformer would be operating inefficiently in transferring the signal to the Secondary. The material of the core laminations is either GOSS or Unisil. This I was told is better than Stalloy.

3rd Mar 2012, 3:08 pm	#19
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: What's a good way of making a homebrew valve amplifier louder If it's gapped it's fine for single ended. Being gapped per se doesn't rule out p-p use, it's just not optimum from the materials used point of view. The other way round is not likely to work at all well, though. Do you know what the winding ratio is? The 6CH6 may simply be seeing a completely mismatched load....... __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

3rd Mar 2012, 5:46 pm	#20
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: What's a good way of making a homebrew valve amplifier louder Hi Neil, 10 p-p is 3.54 RMS. Into 8 ohms this gives 2.5W. Not bad for a diddy valve. You may be able to double this but will not notice much difference in sound level. With regard to horn loudspeakers, my uncle had a cinema which had a PX4 in the amplifier and there was enough sound to reproduce the whoops of the injuns and the drumming of the hooves of the stampeding buffalo. Trevor