5150 Block Letter with a strange Bias Mod?

[Norseman]

Hello all. I am a longtime lurker and finally decided to join so I could ask a question ops:

I bought this head brand new to replace an ailing Super Lead Marshall as a gigging musician in 1992. It was the very first one I has seen in the Boston area. At the time it seemed like a way to scratch my SLO100 itch without going broke. I immediately had Evan Cantor (of Bedrock Amps at the time) do a bias mod. He also installed EL34 power tubes without the 1K resister upgrade and it sounded OK for the most part. It cured the fizzy gain and tightened the lower mids like a Marshall (which was what I was used to at the time).

Fast forward to now. I decided to retube the old beast and while it was out of the chassis I did some Google research (not available to me in 1992...) and found that the bias mod Evan did was very different that what I see in every post I can find on this mod. It seems he added the pot on the diode side rather than eliminating the 15K resistor and jumping in the pot. Does it matter that he jumped it into the rectifier diode? Will it effect the sound compared to the resister side mod? Is this like the "choke" mod?

I have attached a pic to see what you guys think.

I ordered 6L6's this time as I think they will sound better than these shoehorned in EL34's (also no jumpers were installed in the sockets as I understand is common for the EL34 mod). I admittedly have extremely limited electronic knowledge (just in case it isn't obvious enough).

[defaced]

Your picture didn't come through.

This is nothing like the choke mod. The choke changes the high voltage power supply characteristic slightly. IMO, in high gain amps where the power stage isn't clipped, it matters little if there's a choke or a resistor.

For the bias mod we'd really need more details, but the role of the bias supply is just to supply a constant negative voltage to the grids of the power tubes. As long as that's present, how it's done is really not much of a concern. So maybe he did is an unorthodox way, but if it works there's no tonal benefit to changing it - in electronics there are often many ways to do something. Now, if we had a schematic of what he did there might be suggestions on how to do it better for other reasons (better range of adjustment, more fault tolerant, etc), but if it hasn't broken in 20 years, I'd be inclined to leave it alone.

[Norseman]

Your picture didn't come through.

This is nothing like the choke mod. The choke changes the high voltage power supply characteristic slightly. IMO, in high gain amps where the power stage isn't clipped, it matters little if there's a choke or a resistor.

For the bias mod we'd really need more details, but the role of the bias supply is just to supply a constant negative voltage to the grids of the power tubes. As long as that's present, how it's done is really not much of a concern. So maybe he did is an unorthodox way, but if it works there's no tonal benefit to changing it - in electronics there are often many ways to do something. Now, if we had a schematic of what he did there might be suggestions on how to do it better for other reasons (better range of adjustment, more fault tolerant, etc), but if it hasn't broken in 20 years, I'd be inclined to leave it alone.

I appreciate the advice. I was still trying to add a picture when you replied.

I posted a duplicate post on the Peavey site where the pic did load here:

http://forums.peavey.com:81/viewtopic.php?f=19&t=33244

[Jazz P Bass]

It looks o/k to me.
The bias voltage is a negative voltage.
That is why he came off of the anode.

[Norseman]

Thanks for the reply and for posting the pic. I have no idea why it wouldn't load, it just kept failing during upload (55 kb jpg?).

I was just curious because most mods show the resistor on the right removed and a pot with a different value resistor jumped to the right side blue cap. Mine is one the other side (the attached pic showing the newer style mod is upside down compared to my pic). I was wondering why the later bias mods do it that way? Granted this was probably the first 5150 to ever have a bias mod in the first place. It was dropped off for the mod on the way home from Daddy's Junky Music in mid-1992. Is there any advantage to the newer method?

[Enzo]

There are as many ways of doing it as there are technicians. The thing that matters is the ratio between the resistors, so it really doesn't matter where you stick the control.

[g1]

Here is the original circuit diagram:

Here is (near as I can tell) your amp with the bias mod, a variable resistor has been inserted in series between the anode of the diode and the neg. end of C43:

Here is how the mod was done in the other picture you supplied, R68 has been removed and replaced with a 10K variable resistor in series with a 5K resistor:


My concern with your amp is not the different variation of the mod, but the value of the trimmer. 10K seems like a large value, but it may be working just fine. It just seems to me like a small adjustment would cause quite a big change in the bias, and fine adjustments may not be possible. But that would be easy to check, if you can adjust the bias properly, then just leave it as is.