Guitar EL84 push-pull, tone control placement - diyAudio
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Old 10th August 2012, 03:04 PM   #1
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Default Guitar EL84 push-pull, tone control placement

Hello everyone!

I have this great tube amp from a Voice of Music 562 turntable. I would like to convert it to a guitar amp. It has a complex volume/tone control that I don't think would work for the guitar amp (yellow area).I will plug the guitar straight to pin 1 of V1 (with the usual 1M/33k guitar input wiring). Still, I would like to have some tone controls, like in the second attached schematics. I thought I could put such a tone/volume control block between V1 and V2 in the VM-562 schematics. Is that correct? Put it at the green arrow? Thank you for your help again!
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Old 10th August 2012, 03:16 PM   #2
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No, because the point you marked is live not also for audio, also for DC, and putting tone control there, you are changing the DC bias of the phase splitter causing severe distortion of audio signal.
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Old 10th August 2012, 04:48 PM   #3
AuroraB is offline AuroraB  Norway
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Look for schematics for Vox AC15 or AC30......
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Old 11th August 2012, 02:25 AM   #4
shanx is offline shanx  Canada
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The tone stack ''works'' as they are arranged to de-couple the dc component from the plate. All the capacitors running in the filter network are doing that. Problem being is you are going to attenuate the signal with a tone stack, and you only have a single preamp stage before hitting the phase splitter. OK for line level , maybe not enough level to boost guitar signal to get decent overall gain. If you had a dual triode , or another maybe FET boosted stage to kick the signal up you probably would be able to play with a tone stack.
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Old 11th August 2012, 02:26 PM   #5
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As shanx said this tone stack has a lot of loss, it also likes to see a reasonably low output impedance from the stage that drives it and the output of the pentode is high impedance. One option is to use a low loss tone control commonly called a Tweed tone control, you would also have your volume control at this point. Another option is to use a MOSFET as a cathode follower (source follower) on the pentode. This gives a low impedance for the tone stack and helps the pentode in the gain department.

It looks like you may have to modify the phase inverter as it is direct coupled to the first stage and throwing in the tone stack will change it's operation as Osval do de Banfield said. There are a number of ways to go here but I do not have time to go through them now. Hopefully someone here has a good suggestion.

You also have negative feedback around the whole amp on R2 which is not desired on a guitar amp, you can get rid of C6.

All and all it looks like a good basis for a guitar amp.
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Old 12th August 2012, 01:37 AM   #6
ivan H is offline ivan H  Australia
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Hi, I havent looked at the schematics, but is the power amp class A or A/B? If its class A ur best scource of inspiration for the PI & pre amp is as said b4, the Vox AC15 or 30 as they run in class A. If it running in A/B there is any number of amp designs u can draw from. Good luck with it, EL84's are sweet sounding tubes. Apparently they amplify voltage as well as current, & r pretty unique in this. Cheers
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Old 12th August 2012, 02:56 AM   #7
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Quote:
Originally Posted by ivan H View Post
Hi, I havent looked at the schematics, but is the power amp class A or A/B? If its class A ur best scource of inspiration for the PI & pre amp is as said b4, the Vox AC15 or 30 as they run in class A. If it running in A/B there is any number of amp designs u can draw from. Good luck with it, EL84's are sweet sounding tubes. Apparently they amplify voltage as well as current, & r pretty unique in this. Cheers
Sorry the Vox is not Class A.

Is the Vox AC-30 really class A?
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Old 12th August 2012, 03:08 AM   #8
ivan H is offline ivan H  Australia
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Thanks 4 the link, I'll have a look @, it'll b interesting & I dont doubt u, there's a lotta hype & misconception out there in guitar amp world, & I havent owned or been inside 1. I just asked as with class A the power tubes must b kept in the linear part of the load line & not pushed into cutoff or saturation so PI design is important. Cheers
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Old 12th August 2012, 03:24 AM   #9
ivan H is offline ivan H  Australia
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Again, thanks for the link, great reading & I learned a bunch, thanks. Cheers
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