Amp Techs: Help Needed

[Derelict]

this amp is fixed bias

my power tubes (6L6) are getting EXTREMELY hot after just 1-2 minutes, glowing like the sun. after 20 minutes or so of playing, the amp gets mushy, which i attribute to the power tubes nearing meltdown. looking at the schematic, the bias supply hits (per tube) a 220k resistor, then in series a 1.5k resistor , then hits the grid. the screen resistor is 330 (not 330k, just 330). my other amps' tubes don't get near this hot to the touch, neither did my old boogie.

[brad347]

have you tried adjusting the bias? What kind of current are the tubes drawing?

[Derelict]

have you tried adjusting the bias? What kind of current are the tubes drawing?

i haven't measured it, but adjusting the bias means different resistor values ("fixed" bias). i have done mods on several amps thus far (classic 30, valve JR, jcm800 clone), but have stayed away from the power section. measuring it would mean opening her up and probing the power section. i know the shock potential, but where do i probe to measure what my power tubes are drawing, and what resistor (grid or screen) adjusts this, in what direction?

i know.. i know... (newb)

[tonezoneonline]

You definetly need to check the bias.If you don't feel comfortable doing it,take it to a tech.It's worth a few bucks.

[brad347]

"fixed" bias does not mean the bias is non-adjustable.

It means that the bias is not constantly varied by the cathode. The opposite of "fixed bias" is not "non-adjustable." It is "cathode biased."

The correct term for what your amp probably is (mesa?) is "non-adjustable fixed bias," just so you know!

Also, when posting for help it really helps to be as complete with your description of possible. Every amp is different, and if you post at least the make, model, and symptom you will probably get far more helpful replies, faster.

So, is it a Mesa?

If you invested in an inexpensive "bias probe" type device, you could measure the current draw without even opening the amp. But if you don't want to do that, posting the make and model of amp would help someone be able to tell you where to measure. In general terms, the search function would help also.

[John Phillips]

this amp is fixed bias

my power tubes (6L6) are getting EXTREMELY hot after just 1-2 minutes, glowing like the sun. after 20 minutes or so of playing, the amp gets mushy, which i attribute to the power tubes nearing meltdown. looking at the schematic, the bias supply hits (per tube) a 220k resistor, then in series a 1.5k resistor , then hits the grid. the screen resistor is 330 (not 330k, just 330). my other amps' tubes don't get near this hot to the touch, neither did my old boogie.
STOP USING IT RIGHT NOW.

If you're operating it for anything like 20 minutes under those conditions, you've already probably fried the tubes and may cause serious damage to the amp. The tubes glowing like that is a warning that something is VERY wrong and to shut off IMMEDIATELY, not keep trying to use it.

It sounds like a bias supply failure. You should be able to find which component has failed by metering round the bias circuit - do it with the power tubes pulled. If you aren't familiar with how to do this, take it to a tech.

[Derelict]

thanks guys.

its a peavey valveking, and it appears its the heaters burning hot; the plate isn't getting hot. the tubes are sovtek 6L6WXT.

i don't mind noodling around in the preamp, but in the poweramp...well, that's different. i just watched national geographic explorer about lightning strikes and how they affect the human body..... (as mentioned by others) might be a good idea to let a tech look at it. :rotflmao

on a side note,
i like the clean channel, but i would like to improve the crunch channel by making it more crunchy, and less fizzy. could someone point me in the right direction? i think i'll start a separate thread on that one.

[brad347]

on a side note,
i like the clean channel, but i would like to improve the crunch channel by making it more crunchy, and less fizzy. could someone point me in the right direction? i think i'll start a separate thread on that one.

Try a 5751 instead of a 12AX7 in the overdrive tube slot. You might even try a 12AY7 or even see what a 12AT7 sounds like.

[Derelict]

Try a 5751 instead of a 12AX7 in the overdrive tube slot. You might even try a 12AY7 or even see what a 12AT7 sounds like.



thanks!

[brad347]

no problem! John Phillips, for the record, is a good person to listen to. He is among the more knowledgeable group of people on this board (of which I certainly am not).

If your filaments are indeed burning hotter than normal (which, i think, is rather unlikely) then you might start with metering the voltage coming out of your wall. Have you measured it?

Also, the tap on the power transformer that steps down from ~120VAC to 6.3VAC... I'm not a tech (I just work on my own stuff in my spare time) but maybe the techs could chime in... if that winding was somehow shorted, could it deliver too high a voltage to the power tubes? Don't know for sure.

The 12AX7s heaters would be powered by that same tap, however, and if they are fine then it's likely the tubes' fault. Or another problem entirely.

Some tubes' filaments just burn brighter than others. Are these new tubes? Have they changed over time, or was this a sudden thing? Power tubes also normally get hotter than the other ones. I say this, of course, feeling pretty sure that it's probably not the case-- you probably know what is normal for a power tube and something obviously has you concerned.

So... hmm. Maybe tech time! Good luck. :)