Peavey 6505+ bias

[guitardad]

Seems like a simple circuit, but I am reading -74vdc. For bias v ps that would be excessive. These are supposed to run about -55 iirc. I can get the tubes around 40ma-46ma on the bias probe. Still seems kind of high.
Plate= 487vdc

Thoughts?

[Enzo]

OK, first, do you have an actual 6505+ or do you have a 6505+ Combo?

You have test points? What is the range of voltage you get when you sweep the bias control end to end?

Are you reporting with 74v bias you get 40-46ma per tube?

[guitardad]

Yes. 6505+ head. Unit came in for just a retube, clean/maint and bias adj. I haven't seen any 6505+ but have worked on some 5150ii's which are basically the same amp. I just thought that high a bias voltage was odd. Doesn't look like the bias pcb was modded either.

There was no spec on the schem about the actual bias voltage from that bias PS.

Yes the range reading on the bias current is 40-46ma.

I am going to test the bias current using the OPT method. I had been using the Bias Pro with LCD from Eurotubes.

Something is definitely off. The amp works ok. Unless the meter/probe is fubarred.

But on my Fluke 77 DMM I get 71vdc at the bias pins on the rear panel. I still use the probe for any of these amps like this or the Bugera I had in. I don't trust the readings, but yeah that one seems rather high.

[Enzo]

trust the readings? The test points show the bias VOLTAGE, so if they say 74v, I will assume there are 74v on the grids. You may not trust it to be a desired result, but I'd trust the numbers.

Question remains, what voltage range do you get on the test pins? I calculate roughly 55-61v.


Or just power down and discharge and see if the resistance across the test points runs from roughly 12k-22k.

[Jazz P Bass]

You do not want to run the tubes hot at idle.
Peavey prefers to run them cold as it helps with the 'Resonance' control.
Crate BV120H bias? - Page 2

[Enzo]

he says he's getting 40ma or more, while I expect more like 15-20ma in a 5150.

[Jazz P Bass]

If the old tubes that where in the amp 'work', I would measure them in comparison.

The new tubes may simply run hot.

[guitardad]

Bias probe was faulty. I have used that rig for a while now, but something went south on it.

That's why I posted. Readings seemed really really odd. When I removed the bias probe and just checked the dc at the pins it went from 71 with probe to 54v w/out. So for giggles I hooked the probe back in. TP voltage went up to 71v again. Bad probe. Why the 20v or so change? IDK.

Ended up using more traditional methods. haha

unit is fine ! That test equip failure just took a simple repair and turned it into something else.

Thanks for that link. Was a good read.

Thanks all for the assist.