Peavey 6505 Plus questions

[Randall]

Got a Peavey 6505 Plus 120 watt head that came in with bad sounds after changing all six pre and 4 output tubes. Customer wants me to check bias, but then tells me these Peavey amps are "plug and play" so they don't need to have the bias checked. Well, the clean channel sounds good, but the dirt channel sounds raspy, so I don't think it is the power section. But, I wonder about the bias points on the back panel (sorry have not found the schematic as of yet). The measurement is expressed in volts, and I read +54.6 volts and find the pot all the way to the extreme. I measure one tube at 25.5mA in this position. With the pot all the way to the opposite extreme I read 65v, and 9.5mA. Seems like kind of a useless measurement.

Then I read in the Peavey forums that Peavey put those test points there with a pot with a narrow sweep so guys couldn't damage their amps? And that they intend for these amps to run that cold because they are for metalheads. I confess, I did not know metal amps were intended to run at 20mA or so. That would explain the "plug and play" comment, since it is set so cold with the sweep range that you can't really do much with it anyway. I set it a bit towards the center.

Another question, I saw a video of a serving of this amp, and the guy found that he had 6.3vac at the power supply, but only 5.7vac on the filaments, so he ran a jumpered pair straight to the tubes to get full filament voltage. I checked the one I have, and same thing, 5.8vac. Now I wonder, should I do the same "repair", or is this a thing that Peavey did on purpose for some reason?

[Jazz P Bass]

For starters: 6505+ 112.zip

The distortion is mostly derived from the super high gain preamp.

As to bias, use your scope.
Set the bias for minimum crossover distortion & forget about it.
1K input/ clean 10Vsc output.

[Randall]

That schem is not exact, the amp I have has one more pre and two more power tubes. I found the ratty tone to be resolved by a thorough cleaning, especially the FX in/out jacks. I tried the crossover distortion method, it landed me right back where I started, about 20mA.

But what about the filament voltage "repair"? Do it or not?

[Jazz P Bass]

So you must have the 'head' instead of the combo.

Peavey_EVH_5150-II_Head_6505+.zip

[Enzo]

Unless the contacts on the ribbon connector are getting hot on the heater voltage pins, I;d ignore the low heater volts. it really is within 10%. For that matter pull off the ribbons to the tube boards and push them back on to refresh the contact surfaces. PV did not design this with half a volt low in mind, it is just a common result of running all that heater current through connectors, the connectors get resistive.

Adjust that bias by ear, it really doesn't matter to the amp. Factory spec is 55v of bias, there is no spec for current, as they don;t expect owners to measure current. If it doesn't sound much different end to end, then do exzactly as you did: set it in the middle.

I find 5150 amps generally are cool at something in the 11ma to 20 ma range normally. I'd call 15ma average for one.

When a customer suggests I check bias, I tell them I always do. And in fact, it is always a good policy to check for bias in case the bias circuit is compromised. The fact the adjustment range is extremely limited doesn't excuse us from making sure it is working. I don't expect more than 5-10 volts range from that control. So the customer is right, check the bias AND it is plug and play.

Just my opinion, but I think you have it backwards. it isn;t that metal amps are biased to 20ma, it is that the PV amp is designed to get its tone from the preamp, they are not expecting someone to play the power tubes flat out looking for sag. The power tube section is there to make the preamp loud, not to lend power tube dynamics. So there is no point in biasing it real hot. That 70% mantra so many chant is really putting tubes on the edge of red plating. By running the amp cool, the tube life is greatly expended and the amp is more reliable.