Peavey Classic 60/60 bias mod advice

[RIC]

Need someone to look at this mod, the instructions don't jive with the schem's nore the schematic's with the PCB layout. have this mod in zip, but it's to big, can I post a link? any help would be lovely. I don't see how one can set bias for each 6L6, with just one trim pot.

[RIC]

Here's the mod if anyone want's to take a peak.

http://www.geocities.com/duke_of_met..._6060_mods.zip

[SY]

It looks to me that all he's doing is increasing the value of the upper resistor in the divider string that provides the bias voltage. That allows the idle current through the output tubes to be increased. No need to wade through the text- just look at the schematic, put a pot in series with R1, then adjust the added pot to get you the idle current you want.

[RIC]

Thank you SY, I just wasn't sure, with the components not having the same location ID's/numbers, from pcb, know what I mean.
plus I've got lost on the part some-where, about setting each
tube. I had talked with this guy, but he had never went through with the mod, if I recall. and I didn't want to bug him about this.
really nice guy. He mention me getting a presence control put in.
got any help there? Thanks.

[SY]

He may have meant that you can check the idle current on each output tube (even though they're not individually adjustable) to make sure they're not too horribly mismatched.

Can't help on the presence control, sorry.

[Enzo]

I recently modded a 60/60 for adjustable bias, one for each channel. Ther is no way for one pot to adjust all four tubes individually of course. If you use a matched quad of tubes, hopefully they will stay close. I added controls for each channel, so he could use pairs rathre than a quad. I suppose I could have made all four tubes adjustable but that seemed overkill.

I don't like putting pots in the top resistor positiion because if the pot opens or gets noisy you lose bias. I make the lower resistor the variable so at worst the bias goes way up and the tubes run cold.

At 470 ohms, R68 does not allow much voltage division, so I make it a 3.3k. Then I replace R36 with a 10k pot and 6k resistor in series. That makes the whole thing adjustable.

To make separate bias adjustments, I run two resistors from the rectifier instead of just R68. They branch to the two supplies. Both are still my 3.3k. Of course now I need two C24 and two pot/resistor combinations. I mount all these things on a small pc board, with the two adjustmet pots on a small bracket. I thought about drilling holes adn mounting them for adjustmetn from the outside, but since you have to take the lid off to change tubes, why bother. RUn a wire from the small board back to the rectifier, and also wires to ground and the two bias feeds.

The tracery on the board expects one supply, so some trace cutting and jumpering is required to feed the separate bias supplies to the two channels. The PV parts layout shows the traces, so it is easy to plan. I soldered one supply feed to the end of R6,16 near the board edge, and cut the trace under C32. For channel 1 I removed the jumper between C13 and R23 and soldered the other feed to the pad near R15.

AS it turns out I did not actually remove R68 and C24, I just left all that, and ran a wire from the now vacant pad near R23 over to the two 3.3k on my little board. The drop across R68 doesn't affect me, and the extra filtration of the now redundant C24 can't hurt.

[roszatycki]

ENZO, I know this is an old post but I recently found it after looking for some info on doing a bias mod on my Peavey Classic 60 and 60/60. I did recieve the schematics for both units from Peavey and was going to follow your directions on this thread but, I was wondering if you made an error on the directions of replacing R68 with a 3.3K resistor and actually meant R38? If you read this please clarify. Thanks