How to get more bottom end out of this amp

[leftu2]

Morning fellas,
I've been messing with my peavey vtm  (for those not familiar with it, it's mostly a jcm 800 circuit with jose mods on dip switches).
I got this thing dialed in pretty good but need some more low end. All the coupling caps (best i can tell) are .022uf. I thought there might be a .0022uf cap in there as some marshalls have them. I don't want to sacrifice the tightness of the amp by changing the coupling cap values to let more lows in unless someone knows which one to change with out without loosing the tightness.  I've tried a 47k slope resistor (stk is 33k), which helped, but didn't get me to where I wanna be. I'm thinking I need to play with either the 470pf (c4 on the preamp board) in the treble peaker circuit or the .022uf cap that's part of the low control in the tone stack. Also, #s 4 & 5 are the 2 dip switches that boost low end. Maybe that's where I can increase the low end by changing the caps at c9 and c10 that are attached to those switches, though I don't know which  values to change them to as I don't understand how those caps add lows. I am using a ts-9 which I know cuts lows. I don't want to use a eq pedal. I'm not a huge fan of the effects loop on this thing. The cab and speakers are not the issue, I'm playing through a large 2x12 cab with dark voiced speakers.



EDIT sorry about the png pic not being full size ( i assumed it was) , here's a link to the schematic:
http://www.thetubestore.com/lib/thetubestore/schematics/Peavey/Peavey-VTM-120-Schematic.pdf

[SILVERGUN]

You could try a higher value in C5.
This is easy to do by just tacking over the existing value with a higher value. If you don't like it, just un-tack.

[Latole]

And remove treble bleed cap on master volume , if MV have one

[leftu2]

You could try a higher value in C5.
This is easy to do by just tacking over the existing value with a higher value. If you don't like it, just un-tack.

Thanks, I'll try that

[leftu2]

And remove treble bleed cap on master volume , if MV have one

If I'm reading that right, that .001 bright cap in C2 on the control board was for the low input only. I only use the high gain input.
If I'm wrong, someone please correct me.

[SILVERGUN]

And remove treble bleed cap on master volume , if MV have one

If I'm reading that right, that .001 bright cap in C2 on the control board was for the low input only. I only use the high gain input.
If I'm wrong, someone please correct me.

I traced it out and it looks like you are correct. The PRE control R1 is only in the circuit on the LOW GAIN input.
I think Latole was just giving you a general tip by sayin "if MV have one"
I wouldn't call R1 (PRE) a master volume, but the actual MV (R2) doesn't have a bright cap...so that is moot


And for the record, from my earlier tip. You can go as high as you want, ideally 10uf - 25uf (5V minimum rating for C5) will boost some lows that are currently attenuated with just a .68uf
And to give you some direction regarding your earlier questions, don't miss this:
https://ampbooks.com/mobile/classic-circuits/peavey/

[SILVERGUN]

Reading through that article myself I see where my suggestion to increase the bypass cap value is handled by the LOW 2 dip switch

[leftu2]

And remove treble bleed cap on master volume , if MV have one

If I'm reading that right, that .001 bright cap in C2 on the control board was for the low input only. I only use the high gain input.
If I'm wrong, someone please correct me.

I traced it out and it looks like you are correct. The PRE control R1 is only in the circuit on the LOW GAIN input.
I think Latole was just giving you a general tip by sayin "if MV have one"
I wouldn't call R1 (PRE) a master volume, but the actual MV (R2) doesn't have a bright cap...so that is moot


And for the record, from my earlier tip. You can go as high as you want, ideally 10uf - 25uf (5V minimum rating for C5) will boost some lows that are currently attenuated with just a .68uf
And to give you some direction regarding your earlier questions, don't miss this:
https://ampbooks.com/mobile/classic-circuits/peavey/

Thanks for the link on that, I never knew someone broke down the "response" system of that amp. It says the the transition frequency when gain 2 is boosted is 234hz, I wonder if raising the 68uf cap value to introduce more lows would help without introducing blocking distortion/ farting out etc.?
Though, instead of fooling with the preamp cathode values, maybe a safer idea would be changing the 10uf cap value on the on switch 5 (low 2) to bring the lows up even more. I'll still try to piggy back some caps as well on c5 as you suggested. 

I have 2 more questions. 1st, on the preamp board, what is that 22uf cap doing at C1 that's tied to the 100k plate resistor at R1?

2nd question is, on the preamp board, on v1b, what effect would it have to raise the 6.8k  cold clipper resistor on R5 to 39k like the slo or 10k like a jcm? better harmonic content ? lower distortion?  I don't mind loosing distortion to get better harmonics as I'll just use a tube screamer to boost the front end more to make up for it if I can get the feel I want. Looking for a slightly more liquid feel.

[SILVERGUN]

I traced this HIGH GAIN INPUT signal path out again and it now looks to me as though the signal comes off the plate of V1A, through C2, through ribbon cable connection #3, and back through the same path that the LOW input does. So that would mean we were wrong about R1. Does R1 function on both channels?...it looks like it should.

I have 2 more questions. 1st, on the preamp board, what is that 22uf cap doing at C1 that's tied to the 100k plate resistor at R1?

That's the filter cap for those first 2 stages. It's a must and changing value would have little to no effect.

2nd question is, on the preamp board, on v1b, what effect would it have to raise the 6.8k  cold clipper resistor on R5 to 39k like the slo or 10k like a jcm? better harmonic content ? lower distortion?  I don't mind loosing distortion to get better harmonics as I'll just use a tube screamer to boost the front end more to make up for it if I can get the feel I want. Looking for a slightly more liquid feel.

On the SLO that 39K Rk stage is later in the circuit.
Changing your 6.8K to a 10K like the Marshall isn't going to be a magic bullet.

The BIGGEST change you would hear would be to add a cathode bypass cap to V1b. And if you were to make it a large value (as suggested for V1a earlier), you'd be boosting lows. You'll want to keep the dip switch open in the 1/16 and 2/15 positions (if you're just tacking across the 6.8K).
OR
You can audition this by jumping out R15 and tacking a larger value on C8, with dip switch position 2/15 closed.

[leftu2]

I traced this HIGH GAIN INPUT signal path out again and it now looks to me as though the signal comes off the plate of V1A, through C2, through ribbon cable connection #3, and back through the same path that the LOW input does. So that would mean we were wrong about R1. Does R1 function on both channels?...it looks like it should.

I have 2 more questions. 1st, on the preamp board, what is that 22uf cap doing at C1 that's tied to the 100k plate resistor at R1?

That's the filter cap for those first 2 stages. It's a must and changing value would have little to no effect.

2nd question is, on the preamp board, on v1b, what effect would it have to raise the 6.8k  cold clipper resistor on R5 to 39k like the slo or 10k like a jcm? better harmonic content ? lower distortion?  I don't mind loosing distortion to get better harmonics as I'll just use a tube screamer to boost the front end more to make up for it if I can get the feel I want. Looking for a slightly more liquid feel.

On the SLO that 39K Rk stage is later in the circuit.
Changing your 6.8K to a 10K like the Marshall isn't going to be a magic bullet.

The BIGGEST change you would hear would be to add a cathode bypass cap to V1b. And if you were to make it a large value (as suggested for V1a earlier), you'd be boosting lows. You'll want to keep the dip switch open in the 1/16 and 2/15 positions (if you're just tacking across the 6.8K).
OR
You can audition this by jumping out R15 and tacking a larger value on C8, with dip switch position 2/15 closed.

Just a quick update, 1st, thanks soooo much for helping me......and thanks to all the others who took time to read my post..
Changing the .68 uf to a 1 uf made a pretty good difference. Using a online calculator it shows the transition frequency down to 159hz instead of the stk 234hz. It's a touch muddy with both gain boosts and compression boosts engaged but with a tube screamer it clears it right up. All I do is dime the bass control to make up for what the tube screamer takes away. I'm not sure I wanna lower that C8 cap value any more. The way it is now makes you have to really dial in the feel of the bottom end, which I love. It's worth the hassle, especially when going to a different cab and speaker.

[DummyLoad]

eye cld not resist... 

[shooter]

 
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