ampeg sb12 very high voltages

[g1]

Do not disconnect the 100K resistor from the bias diode. This removes all bias from the power tubes and will kill them.
Have you used the bias probe in other amps? It sounds like the pins are the wrong size for standard 6L6 sockets. Regardless, if you have to jam it in the sockets, you will not get a good connection when you put the 6L6's back in. You will need to re-tension the sockets before you put the tubes back in.
Agree with DRH that bias should be sorted out before looking at other problems. Does the -48V at pin5 stay with that same socket if you swap the power tubes? Or does it stay with the tube?

[freeformfx]

Lot's of good ideas here guys! Yes I've used that bias probe for years in every amp from here to yesteryear never had a problem with it seating and the pins look the same as the tube ( to my eye)
yeah bypassing the tonestack would be a good way to troubleshoot that it's in that module, I'll do it if I got the instructions right. I had a bad head injury that limits some of my cognitive function so I can't do all that I used to. Or else I'd have already replaced that module with proprietary parts i'm sure

trying adding a 1 meg resistor to ground from pin 7 of V2 sounds like the easiest tip to try, it'd be nice if something that simple would cure all

I do have the bias issue resolved , they're matched 5881's at about 37ma each or so. But get this, turning back that treble pot from ten( to where all the bad stuff happens) changes the bias on the tubes! dropping to about 22ma, then full CCW bias comes back up. this is strange to me and I hope a good hint for one of you. I double checked this and it's happening

[g1]

That would probably be oscillation. It may be audible at times, at other times not. So perhaps the true bias is 22mA, and the other readings are during oscillation.

[TigerAmps]

"I do have the bias issue resolved , they're matched 5881's at about 37ma each or so. But get this, turning back that treble pot from ten( to where all the bad stuff happens) changes the bias on the tubes! dropping to about 22ma, then full CCW bias comes back up. this is strange to me and I hope a good hint for one of you. I double checked this and it's happening"

Do you still have a hum? If so, that could be why the current is still high. Preamp tube noise (hum) could also add to an increase in bias current.
I would try of get the idle current (bias) down below 30ma by increasing R25 to say around 68K. This would bump up the -48 volts a little
and be a little kinder to the 5881's (especially if they are not NOS or vintage - meaning Russian made).

Still sounds like something in the tone stack though; I would try to jump it out first (as described above).

[freeformfx]

Yeah i still have the hum for sure, and when the current reads 38ma is when the treble control is full up when there's no oscillation or bad noises except some mild distortion. I couldn't get the old treble pot to read 1M when I 'rebuilt' it again so I left the new one in. I also noticed the 500ma fuse is blown in my meter I don't know if that could cause me to get any funny readings but i'm suprised it was working. All i have is a 5amp fuse to put in there ; ) my resistance readings haven't been real stable sometimes. maybe time for a new meter it's an extech true rms w/ capacitance and freq

I do have identical bias voltages now -47v on each tube so that's took care of itself

and I tried the 1Meg(color code said at least) from v2 pin 7 to ground as someone suggested but it do anything for it.
time to try and jump the tone stack i guess.

[freeformfx]

I did the suggestion in #26 lifted the first leg of the tone module (R10) and pulled the lead from pin 7 v2. Then soldered a jumper from where R10 was which is C2, staight to pin 7 v2 and all I get is a big humming amp, no signal. Is there something else required to jumper out the tone stack or something I'm missing. I thought maybe C2 was leaking but replacing it doesn't matter in fact even with c2 removed and the wire going to pin 7 (which is like an antenna) the amp still hums and oscillates even some popping. but the tube bias stayed at 32ma while I observed it.
Someone else described a different way of bypassing the tone stack that is a bit more involved maybe I'll try that.

[freeformfx]

just an afterthought I notice my amp doesn't have that big 250ohm 10w resistor soldered to the ext. speaker jack, it looks factory like that, but from what I remember about that's purpose it wouldn't be causing any of the trouble. but I noticed it.

[J M Fahey]

I did the suggestion in #26 lifted the first leg of the tone module (R10) and pulled the lead from pin 7 v2. Then soldered a jumper from where R10 was which is C2, staight to pin 7 v2 and all I get is a big humming amp, no signal. Is there something else required to jumper out the tone stack or something I'm missing. I thought maybe C2 was leaking but replacing it doesn't matter in fact even with c2 removed and the wire going to pin 7 (which is like an antenna) the amp still hums and oscillates even some popping. but the tube bias stayed at 32ma while I observed it.
Someone else described a different way of bypassing the tone stack that is a bit more involved maybe I'll try that.

That is not what I suggested.
You did NOT supply a path to ground which what is needed.
And you are not attenuating the signal the same amount as the passive tone control does, so you are slamming ungrounded pin 7 with 10X the signal it expects ... not a good combination.

My full instructions were:

3) so to bypass that tone stack
a) set bass and treble controls to 5 , so they don't mess the test.
b) connect a 1M resistor from the node: C2/PEC to the node : R12/V2pin7(V2-7)/treble pot cursor(center leg) plus a 100k resistor from node R12/V2pin7(V2-7)/treble pot cursor to ground.
This is exactly the same as having a healthy tone stack with all pots set to 5 .

Not complex at all, you are halfway there, just replace the jumper from C2 to pin7 with a 1M resistor and add the 100K resistor from pin 7 to ground.

Since you did not fully remove the tone control PEC, just one leg, set tone controls to 5 (from 0/10) so they don't mess the test.

In a nutshell, we are replacing a suspect tone adjustable 10X attenuator (the full tone control, including pots) with a proper impedance 10X attenuator.
Flat, of course.

Then, we'll probably still have to repair the tone control, but at least we have solved a big enigma which so far is blocking us.

EDIT: that 250ohms resistor is a crude protection in case you forget to plug your speaker, it absorbs some opeaks.
I'd rather use a varistor there, voltage to be determined..

[J M Fahey]

JM, do you think that: 1. this will fix the bias problem? 2. this is more important to fix first, before the bias problem is addressed? Maybe I have my priorities off base. It seems to me that having an improper bias needs to be fixed first, otherwise you don't know what the bad pot fix really sounds like and also run the risk of further damage.

Yes, the power tube bias problem must be addressed first, because it's more dangerous.
The tone stack recovery triode is also poorly biased (I guess, it remains to be proven by testing) but is annoying, not dangerous, so it comes second in the list.

[freeformfx]

thanks , yeah i know , i hadn't tried your method yet I did what Tigeramps suggested in post 26, I just disconnected leg 1 of the module (r10) and jumpered the junction there from c2 as he suggested I recall to v2 pin 7 and the amp just went crazy hum pop oscillation but with no signal. maybe I missed something.
But I'll try your method now. which you didn't specify to pull pin 1 of the module (r10) I don't believe. Just a 1 meg resistor from that junction. So do I reconnect r10? as it's lifted now, and follow your instructions. that's my only confusion between your two guys suggestions.
thanks alot

[freeformfx]

btw two of the coupling caps should have - bias voltage on their low voltage end , this is correct right? just digging

[Enzo]

And when g1 suggested oscillation, it wasn't like some tone you would hear, it would occur at an RF frequency, like 100kHz or something. You can only see that on a scope. Well, or make an RF probe.

[freeformfx]

And when g1 suggested oscillation, it wasn't like some tone you would hear, it would occur at an RF frequency, like 100kHz or something. You can only see that on a scope. Well, or make an RF probe.

yeah I figured that after I spoke enzo thanks
I'm sorry J M but I tried every iteration of the instructions you gave I could think of (pulling the pin of r10 or not ,etc...) and the tone stack remained in circuit, it just got worse.
but before that I noticed my v1 input grid wire previously had came loose so I went back and tried what Tigeramps suggested (just lifting r10 then connecting a jumper from c2 to v2 pin 7 and this time this DID work to eliminate the tone stack. Though the volume can only be turned up to like 1 or 2 before it starts distorting, but before that it's a pretty ok tone.
so does this mean the problem is in the tone module or does the onset of distortion at such a low volume mean it's still hiding somewhere else.

Oh and I changed that 100k 2w resistor to one in spec and it changed the bias majorly, more than half as low thus raising my voltages. Making me need to go to 39k or lower on the bias resistor to achieve 30mA. Almost better off putting the off spec 100k 2w back in. but i can handle that
also forgot to ask is the hum I'm getting from the heaters being too high??

[DRH1958]

Do you have the model that has the external preamp out jack? It is marked "J3 EXT. AMP. on the schematic just before he phase inverter. If you do, do you have another amp that has an effects loop in it? This way you can use this J3 to feed your preamp signal into the power amp of your other amp. If the signal is distorted like you have now,, then your problem is in the Ampeg preamp circuit. If you have a nice signal and distorts normally, it is in the phase inverter or power amp of your Ampeg.. If your power tubes are biased correctly, I would suspect the preamp since there has been so much rework done on it lately. Have you checked the B+ and bias voltages on all the tubes in the preamp?
Do whatever you need to do to get your bias circuit operating correctly. Put in whatever resistor works the best.
One last thing, did your amp at one time or another sound normal after you replaced the output tranny? I'm wondering if you accidentally got the wrong one or hooked up a multi-tap tranny wrong. BTW, the way you bypassed the tone circuit should work as long as you have the 1Meg resistor to ground from pin 7,V2

[freeformfx]

I don't know why I've not tried the ext. amp out except that where it's at in the circuit I just expect the problem to be with it there too but I'll try it into my Blues Deluxe power amp in today.
I bought the correct replacment from Fliptops.net and followed the schematic. No the amp has never sounded right since replacing the OT that's why I knew it had more problems going on.

The way I bypassed the tonestack is exactly as Tigeramps suggested in #26 he didn't mention that so no I don't have the 1 meg to ground from v2 pin 7. IS that to mimic the treble control pot's resistance?

If you mean checking the cathode voltages on the 12ax7's i have and they seemed ok but I'll look again. unfortunately I don't have alot of 1/2w resistors stocked to experiment much