Hey guys, back to tinkering. So I have cruised the web a while and I have seen that most people talking about the mod are referring to the head, and that the schematic for the head and the 112 have different part designations for the resistor. I have a few questions:
1. Which resistor am I looking at here to remove
2. I have heard some conjecture over what value, so would you guys think a 4.7k ohm resister would work? (also, some said to use a one watt resistor, but does that really matter?)

Thanks guys, feel free to give me other hints, tips, and other things to show me that I don't know what I'm doing.
I'm just a kickin' and a gougin' in the mud and the blood and the beer.

Peavey 5150 II and 6505+

The 5150 II and 6505+ are the same amps and fall into the love it or hate it category. I get calls and emails all the time from players who sold or traded their original 5150's off for the 5150 II or 6505+ and the question is always, "how do I get my 5150 II or 6505+ to have the balls my 5150 had?" Unfortunately the answer is you can't get there from here... The voicing on these amps is totally different than the original 5150 and Peavey choose to basically neuter the 5150 II. It simply does not have the low end grunt and raw sound that the original had. This is not bad, its just different and you will find plenty of players who love the 5150 II.

I'm questioned a lot as to the bias adjustment on the 5150 II and 6505+. This amp does have an adjustable bias but the available sweep is pretty narrow and the only way to get the power tubes out of crossover distortion is to lower the resistance to the circuit and once again this is a very easy mod and well worth it.

Inside the amp there is a small circuit board that has the bias trim pot mounted on it and the final bias resister which is a 12K 1/4 watt resister. All that's necessary is to replace that resister with a 6.8K 1/4 watt resister which will increase the window of available bias adjustment. A final bias setting of between 36 to 40mA of plate current per tube measured with a bias probe is a good setting. You only need to probe one tube, set the bias for it and you’re done.

The bias pot is located right next to the power tubes so if you use a short bias probe you don’t even have to remove the amp from the chassis. We have our own Eurotubes bias probes which can be used with any multimeter that will do this for 25.00

And there you have it. Google works.
Last edited by R45VT at Feb 22, 2012,
Yes, but my amp does not have the bias trim pot. It is the 112 combo, not the head. Thus, I am at a loss to locate said resistor. Thank you for showing me that, though.
I'm just a kickin' and a gougin' in the mud and the blood and the beer.
Sorry- wasn't aware the 112 didn't have the trim pot.

What you really need to understand is the bias current. You don't even know where your amp is at. I've read multiple places that they are VERY cold on the 112's, like less then 20mA.

If I remember correctly 34mA seems to be the sweet spot for tone vs. tube life. Any hotter and you are risking shortening tube life, however it may sound more to your liking.

Do you know what your current bias is? If not then really you should before trying to change resistors. I don't want to sound like a dick, but this is your starting point. If you don't have a means of doing this then you shouldn't be messing with it, otherwise you are shooting in the dark. If you do have a bias probe and meter then you can tweak it yourself.

Another edit- if you don't have a bias probe- I have 6L6 probe and meter, and live in southern OC. Problem is I am screwed for time during the week and playing catch up on weekends. If you don't have a probe and you wanted to to get it checked I'd be happy to help time permitting. That way you actually know what ballpark you are in(this is assuming you are in Orange County CA, and not New York).
Last edited by R45VT at Feb 22, 2012,
Well while I appreciate the offer, I'm in neither Cali or NY, I'm in North Carolina

I do have a probe, last time I checked (early december) it was around 18 mA, so yeah, a little chilly. I will check it again tomorrow, but I can't imagine it swayed that much. I know it is kinda like shooting in the dark, but that seems to be about the only way I can go about this, being that there is no trim pot. Thanks again
I'm just a kickin' and a gougin' in the mud and the blood and the beer.
Last edited by dementiacaptain at Feb 22, 2012,
Wow. 18 is low. Recheck it. You can probably add a trim pot(variable resistor if nothing else) that way you don't have to change resistors to get it dialed in every time.
Alright, I'll report back tomorrow after another check. so could I just put a variable resistor in place of the one in there now? That'd be convenient.
I'm just a kickin' and a gougin' in the mud and the blood and the beer.
^^I can't offer any help on this matter, but if you are able to do this mod, please post your results on the "Mother of all 5150/6505 threads." I'd be very interested in what you find.

Just out of curiousity, what kind of change to the sound could you expect with a "hotter" bias?
Well, mostly its to stop the notorious cross over distortion you get from the rhythm channel, but in addition to that, it warms up the sound a little bit. I love my 6505+, don't get me wrong, but it is pretty sterile out of the box. I'm doing this because my old 6260 sounded so much better once I got the bias up a bit, and I am betting that this amp will as well.
I'm just a kickin' and a gougin' in the mud and the blood and the beer.
Thanks for the link. I will look at it when I get home. Hopefully one of the other guys can shed some light before then.
Looks like it would be R14. Let me do a bit more research. I'm surprised no one else has chimed in.
yeah man I know. Well I measured the bias again and it was about 19, so yeah, bout the same.
I'm just a kickin' and a gougin' in the mud and the blood and the beer.
That looks like hell. Thanks for the bump
I'm just a kickin' and a gougin' in the mud and the blood and the beer.
I guess you have no love if you have a 112 6505+.

I started to compare to some other 60w 6l6 circuits last night. Most of them are non-adjustable bias when the 4 tube head versions are. I still think it is R14 and you would want to split the resistance in half or 3/4 with a variable resistor/potentiometer for the remaining.

I'll try to dig some more up this weekend. There are few different forums in which the guys have modified the wholly hell out of them, just no detail for the bias portion.