Peavey Triumph 120 adjustable bias mod

[beedoola]

I picked one of these up and the clean channel is really nice. Don't care for the gain channels but given I paid $100 for the amp I'll use the clean channel with pedals.

Looking at the schematic, I was thinking making R81 a trimmer would allow for adjustment of the bias? I've been unable to find any confirmation of info on how to add a bias adjustment to this amp despite seeing people saying they have.

Schematic:

https://www.thetubestore.com/lib/thetub ... ematic.pdf

[sluckey]

I would replace R81 with a 50K pot and 22K (maybe 27K) connected in series. This will allow you to take the bias voltage above and below the orig fixed position.

[beedoola]

Do I need to tie the wiper to one of the outer lugs of the trimmer? I have a 22k resistor on hand and had purchased a Bourns 50k trimmer.

[sluckey]

Do I need to tie the wiper to one of the outer lugs of the trimmer? I have a 22k resistor on hand and had purchased a Bourns 50k trimmer.

It's not "necessary" but I would.

[norburybrook]

$100 that's a bargain. The transformers are great in these amps. Dumble used them to make a few of the ODS amps in the 80's.

I've built a BLuesmaster ODS with these transformers and my JM/SSS and they're great amps.

There's a recent thread on the Dumble forum with another build with Peavey transformers.


Worth a though



M

[martin manning]

This adjustable bias plan is not going to work. By varying R81 you are varying the current through R65 to get more or less voltage drop, and there isn't much to work with. Right now there is 56V/47k=1.2mA flowing, so only about a half a volt is dropped across 470Ω R65. You can't get any more negative voltage (cooler bias) than that, and you will get more ripple if you reduce R65. If hotter bias is what you want, you could put a 47k in series with a 10k trimmer across C47, and take the bias from the wiper. If you want fully adjustable bias, you'll need to add a voltage doubler circuit.

[beedoola]

Would be I putting the R81 47k back into the circuit and then adding an additional 47k with the 10k trimmer?

Looks here like someone added a 10k trimmer in series with R65:

https://el34world.com/Forum/index.php?topic=9385.0

[martin manning]

A 10k pot in series with R65 will work, and is easier, as you can see. It will also be a "hotter only" adjustment. Where does it run with the fixed bias setup now?

[beedoola]

I’ll have to put the 47k R81 back in and read it.

I put the bias mod in as initially suggested and was only getting 18MA. That was with a 22k in series with 50k trimmer in place of R81.

I measured 480v off the plates so I was looking for bias it at 30ma or so.

[beedoola]

For the adjustable bias, I was going to add cathode resistors - which given the schematic means adding the 1 ohm resistor between Pin 8 and ground, right? I need to cut the traces for Pin 8.

[xtian]

For the adjustable bias, I was going to add cathode resistors - which given the schematic means adding the 1 ohm resistor between Pin 8 and ground, right? I need to cut the traces for Pin 8.

Correct. To avoid confusion with "cathode biased" amps, I prefer to call them "bias sense" resistors.

[beedoola]

Thanks. Worked great!