Any ideas how to put a master volume on an EL34 power amp section??

[nedhogan]

I was going to just put the same kind of master volume in as in modern Vox AC30s, that phases out the two sides of the power section with each other. I like this idea as you can still get phase inverter distortion. But EL34's seem to be wired up slightly differently and Im worried it wouldnt work the same with these as it does with el84's. Any ideas..... ? Cheers

[big_teee]

I was going to just put the same kind of master volume in as in modern Vox AC30s, that phases out the two sides of the power section with each other. I like this idea as you can still get phase inverter distortion. But EL34's seem to be wired up slightly differently and Im worried it wouldnt work the same with these as it does with el84's. Any ideas..... ? Cheers

I guess that would be a AC50?
It had EL34s.
And, It looks like the power is layed out like a JCM 800.
I think the Lar/Mar PPIMV would fit fine.
Lar/Mar PPI-MV - MetroAmp Wiki
Good Luck,
T

[nedhogan]

It a very old Linear Conchord amp 2x el34's at moderate output, about 30 watts. They where sold as a kit or pre-made in the 60's. They where designed for music rather than instruments but the circuit is very similar to the first AC30's so many people who couldnt afford an ac30 in the 60's bought one of these and customized them. The one I've just repaired sounds awsome on full volume, really bluesy and warm. I'd just like to have that tone at lower volumes though.

Thanks for the link I'll check it out...

[nedhogan]

This is its schematic....

[Chuck H]

Replace the 330k bias resistors with a dual ganged pot. Left lugs to ground, right lugs to EL34 grid resistors and PI feeds to the wipers. Like this:

[nedhogan]

Thanks Chuck, that looks like a good idea. I can never tell whether these mods will effect the high frequencies, or general tone, until I've tried them though.
Below is the way I was thinking about doing it. Its the way they do it on the modern vox ac30's with el84's and it sounds good, just dont know if it would work with the el34's. I can't decide whether to leave the bias resistors at 330K and just use a 1M pot though. Or would none of this work...

[Steve Conner]

I don't see any reason why that wouldn't work. I'd be wary of increasing the grid resistors to 1M though. EL34s could leak more grid current than EL84s.

[R.G.]

I speculate that one could replace that 1M grid resistor on the second half of the PI tube with a 1M pot. The wiper would go to the grid of the second tube. Full volume would be turning the pot to the cap to ground, min volume would be turning the pot to the input side. The PI is a differential amplifier.

Oh, wait. That would work, but has some unintended side effects. It would still turn the output tubes on and off, but in synchronism, not in opposition. They'd be running full out, but little or no sound coming out.

Never mind.

[nedhogan]

I've read a few other threads and today decided to put an experimental 500K CLMV Cross Line Master Volume in, which is what I have described above. Aparently you dont need to do anything more than just put the pot in, but it doesnt work on fixed bias amps, and some just plain dont like it whereas some do. Mine just plain didnt like it. It was on nearly full volume or off, nowhere inbetween. I think I'll put a Lar/Mar PPI-MV in instead.
http://metroamp.com/wiki/index.php/Lar/Mar_PPI-MV

[Chuck H]

The Lar Mar type is much like the schem I posted, but the power tube grids go on the wiper and the PI outputs are on the outside lug opposite the bias input. I posted my version of it some time ago for a guy who wanted to use a bias modulation trem AND a PPIMV. I've never actually tried my version but it was noted by another poster that there may be some sonic oddities since the AC load of the PI plate can be set to 0. The person I posted it for said it works fine! A lot of tone controls and even some volume controls do this too and it's never been a problem. I'm actually going to suggest that you try my version first only because my version doesn't have the power tube bias relying on a pot wiper. That just seems like a bad idea to me. If you don't like how it sounds you can always change a couple of leads and make it the normal Lar Mar circuit. The 2.2M resistors in the Lar Mar circuit will probably stop a catastrophy if/when a pot wiper fails.

[Tage]

Chuck with 1Meg grid leak resistors those power tubes have ~4 times there rated grid leak resistance. To me that seems like asking for trouble, I know 1M is ok with el84, but el34's should only run around 250K max

[Chuck H]

My schem has 250k grid resistance. The OP's schem has 1M grid leaks.

[Tage]

Gotcha, didn't notice that... I think I may test this out in an exsisting amp, I'll report on how it goes.

[Chuck H]

Thanks. Since the circuit was niggled above I've been interested in how well behaved it will be. Please re post when you've tried it. I'll probably try it myself the next time I have an amp opened up.

[nedhogan]

I put the Lar Mar mv in but used a total resistace of 330k because thats what grid leak resistance mine had. It didnt like it at all, for some reason the total resistance through to ground changed when it was on, from 330k to around 500k which the tubes did not like one bit! I dont know why but it did, I used 500k stereo pot with two 1M resistors in parallel. I think I'll try the exact same setup as LarMar this time, well I'll just put a 220K pot in and let that be that as mines cathode biased.

It did work very nicely up to half of the turn of the pot so it should work out ok this time....