Peavey 5150 MK1 screen resistors and restoration

[henter2017]

Hi Folks,

I managed to score for cheap a neglected 5150 MK1 Block letter, great amp.
The beast has been neglected for the last 10 years, It has missing knobs and broken pots, replacement pots with the wrong taper of them, making the control unusable... etc...
Not a biggie, as the important things are there and working.

Im sure this has been asked few other times, but since I will be working inside the amps, to fit the proper pots, fitting a bias Pot, etc, I was thinking to change the screen grid resistors which are 100R 5W, Im a big fan of, If aint broken dont fix it, also the power tube board is riveted to the chassis (not a biggie, can drill it out) and the resistors are behind that board... but in the other hand, I know few others recommend changing those and fit 470R as the standard 6L6 calls.

Im concerned this will change the sound of the amp, as I have been reading online, that bigger screen grids makes the amp feels looser and less tight, this in a 5150 will be a total deal breaker!

my questions are, shall I leave what is there if they are testing good? or swap them out and solder them on the acessible side of the board for the future, and if so shall I stick with the weird 100Ohm value or put 470Rs? Any experiences in sound changes jumping out on value on those?

[pdf64]

I think that 100 ohm screen grid resistors may have been chosen as the HT dropper to that node is 400 ohms R210 http://bmamps.com/Schematics/Peavey/...Schematics.pdf
470 ohm screen grid resistors are usually used when the HT to the screen grid node is supplied via a choke whose resistance is around 150 ohms.

[Enzo]

The 5150/6505 has been around for a long long time, and if it sounds great to you, why oh why would we want to change those resistors NOW? If it ain't broke, don't fix it. The 5150 is not a tube eater.

Fender liked 470 ohm screen resistors, others like 1k. But we need not change this amp just because somebody else uses a different value.

Part of this amp's design is the powr amp is much cooler biased than the typical hot setting everyone feels the need for. I find them typically idling at 11-15ma cathode current. Peavey does this for long tube life, reliable operation (not a tube eater, remember?), and the idea that the tone will come from the preamp. Most guys will not drive these beats into power tube distortion anyway. SO if you just have to make bias adjustable to run tubes at the mythical 70%, then maybe you ought to increase the screen resistors, because it will be harder on the tubes.

I almost never have to drill out the rivets. If I need to replace a screen resistor, I unsolder it from the side facing me, poke the leads down through the holes so the part falls loose down into the chassis. I fish out the old part, then install the new parts up on the solder side. One of them needs to have the leads kinked a little to clear the jacks.

The original 5150 knobs tended to fall off, get loose. Peavey fairly quickly made new knobs that look the same but have a good grip inside. SO if you order new knobs they won't fall off. The model is still in production, so the correct pots should be easily found at Peavey parts department.

[Enzo]

The 5150/6505 has been around for a long long time, and if it sounds great to you, why oh why would we want to change those resistors NOW? If it ain't broke, don't fix it. The 5150 is not a tube eater.

Fender liked 470 ohm screen resistors, others like 1k. But we need not change this amp just because somebody else uses a different value.

Part of this amp's design is the powr amp is much cooler biased than the typical hot setting everyone feels the need for. I find them typically idling at 11-15ma cathode current. Peavey does this for long tube life, reliable operation (not a tube eater, remember?), and the idea that the tone will come from the preamp. Most guys will not drive these beats into power tube distortion anyway. SO if you just have to make bias adjustable to run tubes at the mythical 70%, then maybe you ought to increase the screen resistors, because it will be harder on the tubes.

I almost never have to drill out the rivets. If I need to replace a screen resistor, I unsolder it from the side facing me, poke the leads down through the holes so the part falls loose down into the chassis. I fish out the old part, then install the new parts up on the solder side. One of them needs to have the leads kinked a little to clear the jacks.

The original 5150 knobs tended to fall off, get loose. Peavey fairly quickly made new knobs that look the same but have a good grip inside. SO if you order new knobs they won't fall off. The model is still in production, so the correct pots should be easily found at Peavey parts department.

[henter2017]

Thanks! the idea to install the bias pot was the be able to adjust the Bias without the need of buying a certain set of tubes in particular, and in the future be able to another set running at the exact same draw, but I would like to bias them pretty cold, definetly not at 70% disipation as I think it doesnt sounds good that way but enough to warm up a bit the clean channel which is all kinds of sterile. Have u ever scope the amp? will be good to Bias it using the crossover distortion method on the clean channel at max headroom? or simply go for a particular disipation %? Any recommendations?
I managed to get replacement pots from Tubeampdoctor in germany as well of the original knobs, a set of EH 6L6GCs (apparently the same as the sovteks WTX+ recommended for the amp), a sovtek LPS for the PI, TAD RT008 for V1 and the rest will put whatever others I got lying around...
The idea of swaping the resistors, was mainly for safety and future quick servicing if needed, but u are right, if aint broken, why swap them... unless Biasing a tad hotter introduces a new risk...

[Enzo]

You don't have to buy special tubes as it sits. That is another aspect of the cool running Peavey design - pretty much anything that says 6L6 on it will work fine installed. Setting a future set to the same current is no guarantee of them amplifying exactly the same.

I scope SS power amps for crossover distortion, I never use that method for guitar amps. Guitar amps are not hifi.

[henter2017]

gotcha! so u recommend leaving it fully stock? including the bias resistor? seems that crossover distortion is part of the VH sound!
Also, show u on another old threads regarding the 5150 cameron mods, have u tried them?

[Enzo]

I leave the mods to the amp fans. I have no substantive opinion there. I don;t know them by name.

There is no real clean channel on these amps. And I find it hard to separate out the sound of crossover distortion among all the other distortions.

My thought is that if I like something I don;t go changing it. If I want to modify an amp, I do so in pursuit of a specific goal.

[g1]

seems that crossover distortion is part of the VH sound!

I'm not sure about the VH part, but the cold bias is definitely part of the 5150 sound and some parts of the circuit depend on it. I believe Jazz P Bass posted an article about the designer once, where he explained that the resonance circuit needs the cold bias to be effective?

[Jazz P Bass]

YEP.

[henter2017]

Cool! any links to the article? Cant find it...

[g1]

The links JPB posted are dead and the site is gone.
But the mighty internet archive comes to the rescue! (I was wrong, the article is not by the amp designer, but it is a tech article about the 5150):

https://web.archive.org/web/20040209...overdist1.html

https://web.archive.org/web/20100401...overdist3.html

[henter2017]

amazing read and information regarding about how the crossover notch and resonance controls work on the 5150! thanks a lot!

[Jazz P Bass]

Good catch G1!

Here is a pdf. copy of the pages.

[Jazz P Bass]

Maybe it's time that someone contacted the designer, James Brown (No. Not the "I feel good" one) and asked him.

Who is James Brown? - Amptweaker.com - good tone comes from tweaking...