Please check my layout for adding adj bias on peavey triumph 60

[jl2556]

Hi guys,
I've read quite a bit on this forum on how to implement the adjustable bias mod on my peavey triump 60. I think I have it figured out and I drew ump the mod in DIYLC and was wondering if you guys can check it. I used the schematic drawn out from another post from here as a guide (example A) (see next post for schematic) except i replaced the extra 22k resistor that was being used with a 10k pot. I know I might have to raise or lower the the value of the 470 ohm R65 resistor to get the bias in the right range.

[jl2556]

Again I used example A of this schematic from an older post on here for reference, I'm not interested in combining resistors to get the need bias, I want a pot and the
200k safety resistor

[g1]

Example B is using the pot as a rheostat, different from 'A'. The problem with B is that bias is taken off the wrong side of RV2.
'C' is the preferred arrangement. If range is insufficient, you can make adjustments. Have not checked math whether your range for 'C' is correct, seems off at first glance.

[mozz]

Your 220k have to connect directly to the tube grids, capacitor is in the way.

[jl2556]

Your 220k have to connect directly to the tube grids, capacitor is in the way.

Lol, thanks.... that's that's what happens when you try to draw a layout 15 min after you get up and no coffee, it corrected now.

[jl2556]

Example B is using the pot as a rheostat, different from 'A'. The problem with B is that bias is taken off the wrong side of RV2.
'C' is the preferred arrangement. If range is insufficient, you can make adjustments. Have not checked math whether your range for 'C' is correct, seems off at first glance.

I trying to use the pot and the 200k safety resistor in case the pot fails, that 220k resistor will still be able to keep the tubes in an acceptable bias range.
This is the older post on here where i got the drawing and info from, I'm still learning how to translate schematics to layouts:
https://music-electronics-forum.com/...as?view=thread

[g1]

Thanks for the refresher from that other thread. It seems to be a 'six of one, half-dozen of the other' type of scenario. Nicks point about hum with the rheostat type is valid, but Enzo's point about the rheostat type being easier to implement in the existing layout was also good.
So in your revised layout in post #1, I guess the PI coupling caps get attached between the 220K's and the 47K's.

Using a 10K pot instead of 22K will limit the range of adjustment somewhat. Can you post the original circuit?

[jl2556]

Thanks for the refresher from that other thread. It seems to be a 'six of one, half-dozen of the other' type of scenario. Nicks point about hum with the rheostat type is valid, but Enzo's point about the rheostat type being easier to implement in the existing layout was also good.
So in your revised layout in post #1, I guess the PI coupling caps get attached between the 220K's and the 47K's.

Using a 10K pot instead of 22K will limit the range of adjustment somewhat. Can you post the original circuit?

Thanks....
Here's the schematic... Triumph 60 schematic.pdf

[Chuck H]

I have to ask... Why are you wanting to change the stock arrangement? Will new tubes not bias sufficuently with a small resistor adjustment?

I had a Triumph 60 and it was a great amp (for what it was but see ahead).

This amp is not designed to "overdrive" the power tubes. Just drive them to full power. A cooler bias, and less critical, would be in order here since crossover distortion should never really happen. The amp should only ever reach full output without heavy power tube clipping. So I would think small adjutment to the stock circuit should be fine for this???

I will say that the one problem I did have with this amp was that it wasn't all that loud. The sound at full power was still quite scooped and didn't keep up in the mix on it's own. Which is pretty pathetic for a "60W" amp. So maybe your goal is to change this?

I don't think there's much improvement to be made with going from, say, "cool bias" to "hot bias". for this design.

What is your goal for the amp?

[jl2556]

I have to ask... Why are you wanting to change the stock arrangement? Will new tubes not bias sufficuently with a small resistor adjustment?

I had a Triumph 60 and it was a great amp (for what it was but see ahead).

This amp is not designed to "overdrive" the power tubes. Just drive them to full power. A cooler bias, and less critical, would be in order here since crossover distortion should never really happen. The amp should only ever reach full output without heavy power tube clipping. So I would think small adjutment to the stock circuit should be fine for this???

I will say that the one problem I did have with this amp was that it wasn't all that loud. The sound at full power was still quite scooped and didn't keep up in the mix on it's own. Which is pretty pathetic for a "60W" amp. So maybe your goal is to change this?

I don't think there's much improvement to be made with going from, say, "cool bias" to "hot bias". for this design.

What is your goal for the amp?

One reason the amp wasn't as loud as you thought a 60 watt amp should be is most likely the cold peavey bias. A hotter biased amp is louder than a cold biased amp.
Also, if you would have ran the amp through a closed back cab, even 1 1x12, you definitely would have gotten more volume. This thing roars throuhg one of my 4x12s. The small combo semi-open back enclosure is defiantly choking the volume.

As far as my goals on it, several. I HATED the way the tone stack came off the 1st gain stage before the distortion. From my understanding, that's reminiscent of early mesa boogie amps. That's why the distortion is muddy/woofy, yeah I know you can turn the bass knob all the way down to smooth it out and use a eq pedal in the loop to bring it back.

I've drastically changed the amp at this point. I've modded the cathode resistors and by pass cap values on every stage but the clean and reverb stages, even put a "cold clipper" on a stage.I'm trying to reduce the amount of gain and add a lot of asymmetrical/even order harmonics. The biggest change is I moved the tone stack which took care of that muddy distortion.The tone stack has a high pass filter on the mid pot that I removed as well.I also have 2 different sets of clipping diodes set in asymmetrical configuration on a 3-way switch.

As far as wanting to bias with a pot, 2 reasons. 1st, it's just plain easier than fooling with resistors. 2nd is I want the pwr tubes to run near 70% dispensation. Yes, it makes a difference in any amp. Even enzo (who I think is an ex peavey employee) in earlier posts yrs ago on this form said that peavey biased the pwr tubes on the cold. Cranking the bias up on my peavey VTM120 made a big difference. I wasn't into modding amps at that point and had the bias pot mod done.

Also, I'd like to get the phase inverter plate resistors offset like the 82k/100k marshall values to help with better harmonics(they're at 68k/68k now) which (if I'm correct) will change the output to the pwr tubes which means I need to re-bias and again, a pot makes it much easier

[tedmich]

Maybe take a look the JSX bias circuit, which is built to accommodate both EL34 and 6L6GC?

[g1]

One reason the amp wasn't as loud as you thought a 60 watt amp should be is most likely the cold peavey bias. A hotter biased amp is louder than a cold biased amp.

This is incorrect. It will only seem louder at any particular volume setting, as you are increasing the overall gain. The max. loudness does not change, you just need less signal to get there with hotter bias.