Ampeg M12-Q grid problem?

[drewl]

So, this M12 I had repaired a couple years ago has the left 6V6 red plating.

After it warms up it starts getting voltage on the input grid pin 5.
replace tube and it's okay BUT- there seems to be a around 2v on pin 5 of both output tubes if you turn the tremolo intensity pot up.

With the Intensity down there's maybe .2v on the grids. Ground that line and there is zero volts on the grids.

With the tremolo swinging away, my meter reads DC voltage on the grids, but that is just the AC trem signal swing, right?
Not really DC voltage?

The phase inverter caps were replaced, pull either 6V6 and there is no voltage on either pin 5.

So where could this DC voltage be coming from, or am I crazy?

[Leo_Gnardo]

Good that you replaced the caps from the PI. I'm suspecting the 0.1 uF cap from the plate of the vibrato tube is leaky. Swap in a new one & see if things settle down.

[drewl]

Already did that, still the same.

Thanks.

[Leo_Gnardo]

pull either 6V6 and there is no voltage on either pin 5.

That's what's left, an output tube that's leaky making its grid voltage drifts upwards. As you dial up the intensity knob, less resistive load on the circuit feeding the grids, voltage increases & the suspect tube goes red. Swap in another & monitor grid voltage whilst it warms up. So you can shut it down quick in case there's some other not yet identified problem.

[drewl]

But even on "good" tubes there's a couple hundred millivolts to about to volts , unless they are slightly "leaky"

I was wondering if the board or sockets we're slightly conductive.
The owner is known to spray stuff with DeOxit to "fix" things.

So the DC voltage my meter reads when the tremolo is on I'd not really DC, just the swinging tremolo signal, right?

The scope shows no DC offset on the tremolo signal.

[Enzo]

The scope shows no DC offset on the tremolo signal.

There you go.

[drewl]

Yeah, just checking my sanity and to see if I was missing something in my paranoia.

This owner has a ton of old amps he wants to keep as close to stock as possible for playing and selling which makes repairing them a royal pain.

Normally you would replace parts that typically go bad.
No, I have to check them and try and keep whatever is original as much as possible.

At least he understands and I'm paid for the extra effort.

Thanks again all, and have a wonderful Thanksgiving!

[drewl]

Okay, so this same amp is claimed to have the same problem, eating 6V6's.

I feel bad for the guy, he tinkers with this stuff and wonders why he has problems- giving me chassis in a box with a box of tubes and wonders why a different amp is intermittent.

Back to the problem at hand-

I put in an old pair of 6V6's into this Ampeg and have been slamming the shit out of it with an IPod into a load resistor. Surprisingly it is still working fine.
I was hoping to blow something up, but it ain't happening.

The only thing I can report is with it cranked the F- up I'm getting plenty of negative voltage on the input grids almost like a fixed bias amp's bias voltage.

The Cathode bias voltage is around 25-30 volts across a 250 ohm resistor.

Am I still missing something or is this guy still putting in old bad 6V6's?

Inquiring minds want to know.

I'm tempted to keep blasting this thing until it does blow the hell up.

[Enzo]

SO does he bring you the failed tubes he was using? You have god tubes, but when you fix the amp and return it, does that include a set of tubes YOU installed? )or is it "here the amp works, you install your own tubes."?

It keeps blowing up tubes", are they always tubes you sold him or are they always tubes he stuck in it? And are they new? (what brand) Or are they old stock? new or used old stock?

How fast does the keeps blowing happen? After 10 minutes? 10 hours? Ten days? 10 weeks? 10 months?


If I fix an amp, I put tubes in it I know are good, and the whole thing is working. If someone wants me to fix it, but go home and stick their own mystery tubes in it, I will let them, but then I can't put a shop warranty on my work.

[drewl]

None of my tubes, he has his own mostly old tubes.

I hear what you're saying, just wondering if I'm missing something.
He is trying to have the amp be reliable to sell.

[Enzo]

He wants to sell it? Your new tubes are working fine, SELL him the new tubes. This is the price to pay to play with the big boys.

His crappy old box of tubes, or some hidden weird thing you are missing? When you hear hoofbeats, think horses, not zebras.


Have faith in your repair ability.

[drewl]

Okay, so when I tested the amp I was using an old pair of RCA 6V6's I had.
Popped in a pair if EH's and sure enough one then the other red-plated.
With a meter I was reading a few Meg Ohms between pin 5 and 7 when they were hot.

Would a loose socket pin cause the tube to fail? Because I re-tensioned the sockets and the EH's are hanging now.

Although I am still getting DC voltage on pin 5 on both creeping up.

There was no DC on the input grids using the old black plate RCA's.

[drewl]

Oh, and when I crank a signal into the amp, the + DC on the input grids goes to around negative 24 volts.

[The Dude]

Same thing with trem intensity all the way down?

[drewl]

With the tremolo pot down it lowers the voltage but it still creeps up.