ampeg sb12 very high voltages

[freeformfx]

ok a little more info, and i am crossing my fingers that the problem isn't in the ampeg tone module, but turning the treble pot through it's paces has a wacky effect on the buzz gating distortion turning it all the way down nearly sends the amp into runaway. The bass pot effects the sound this way a bit too.
I found that although i do have -49v on pin 5 of V3 BUT have -59.8 on V4, what could be causing this?
also if i pull one end of the 100k 2w resistor going to the diode in the bias supply I get a much stronger fuller full volume signal, though a bit distorted but much better. This resistor is carbon comp and has drifted to 125k , could it be the problem.
I've been working on this amp on and off for over 4 years so i am pretty out of ideas any help is sooo appreciated!

[The Dude]

no not really. I just used the method resistance across red and blue OT leads and then voltage across them. Then divided voltage by resistance to get the current draw. I got about 39ma...

Did you account for 2 tubes in your calculation? In other words is the 39ma per tube or both together?

[DRH1958]

You're going to either find a way to get that bias probe installed or solder a 1 ohm precision resistor in series with pin 8 of the 6L6s. You have to be able to get the current measured on them. Do you know how that works? Your bias probe does just that. With a 1 ohm resistor, you measure the voltage across it and since it is a 1 ohm value, the voltage translates directly to current. So a 50mV reading is 50mA of current. This might be advisable to do because then all you have to do to get cathode current is measure voltage across these resistors. And you can go back and forth to measure both tubes without removing it and putting the tube in the bias probe socket. In the meantime, measure voltage for plate and screen for both tubes(6L6s).
When you say "pull one end of the 100k 2W resistor" what do you mean, like pull ON it or disconnect it?

[Enzo]

You have mentioned the tone module a couple times. If you mean the flat thing with a bunch of wires coming out the edge, it is just resistors and caps inside, and the schematic of it is readily available, you can duplicate it with a few parts.

Google Ampeg tone module, lots of stuff comes up. here is a guy built his own.

http://www.vintageampeg.com/ampeg-to...-modification/

Flip Tops sells them for $49, screw that.

[freeformfx]

yeah screw that but I don't really wanna perf one together when I found a used one reasonable. The question though does it sound at all like it could be the 7 pin module causing this problem.
and What about the fact a pull one end of the 100k 2w resistor feeding the bias diode and get the problem to pretty much go away ( still distorted some though)
also the drastic difference in each tubes bias voltage?

[Enzo]

Oh I wouldn't make one on a perf board either. it is only a few parts, and most of them would mount right on the control's legs with the others point to point. But all the parts in it are accessible by the leads, so look at the schematic and you can measure the internal resistors and caps if suspect.

That module wil have no effect on power tubes, if you have a different voltage on the two grids, then check the bias supply. If you have 59v on one grid and 49 on the other, I'd have to expect the bias suply to be about 59v too, so the other one being low might indicate a leaky coupling cap from the phase inverter stage.

I don't know what your buzz tone issue is, my first suspect would be the treble control itself.

[freeformfx]

I guess I could attempt building the module point to point like that but anyway , Right on ENZO. I narrowed it down to the treble control. If it's full on 10 the signal is pretty healthy and distortion free. Treble control turned anywhere between 1-9 and the signal gets all buzzy gated and very weak. I took the pot apart looking for broken carbon and it looks good so I tried a new 1MA pot in there to be sure and the symptom remained. the only part connected to it other than the module is a 120k resistor which I forgot to test. Tired
if that resistor tests within spec does it sound like the tone module then?

I strangely had to open up the tube socket 'pins' to accomodate my bias probe, only got it to fit in V4 so far where the bias is -59v and one tube tested 58ma and one was 22ma so i see I'm using some very mismatched old tubes. Could that be my hum?
I checked everthing in the bias circuit and tested good except that 100k 2w resistor is a bit out of spec
getting there I hope

[freeformfx]

oh I'll get that bias probe to fit both sockets alright they just need loosened I don't get it. But I can't install 1ohm r's because this is a pcb amp with pcb mounted sockets. Not without cuttings traces , no way
btw can that bias diode be replaced wih a standard 1N4xxx device? I think it's good though

[Enzo]

Wouldn't be the first time I ever cut traces for that.

You seem to have a good bias supply, not sure why that diode needs replacing, but like most all bias supplies, it has almost no current demand, so a common 1N4007 would be fine.

Just a thought, have you looked closely at the bias probe male pins? The pins are hollow, and they stick wires down them and then solder them. Sometimes the solder blobs a little so the end of the pin comes off as a little fat. Any chance of that?

[freeformfx]

no I made the probe myself the pins are all flush the sockets were just too tight I guess.
I put the old treble pot back in I never should've taken it apart I think I buggered it up a little bit I should've just measured it! so If I cant fix it I'll have to use the replacement.
Now even at full treble I have distortion , backed off a bit the signal cuts way down and lots of buzzy distortion ensues. I think I've checked all my coupling caps enough times but maybe check em again

any thoughts that this IS in the tone module yet?
I hate to pay $$ for one and it not be the issue. argh

[TigerAmps]

Don't give up, you are close! I would change the 100K 2W resistor as well as the bias cap 100uF/100V.
They are not you problem, but I would change them for reliability purposes.
Did you try inserting signal into the External Amp jack? This would inject signal into the V2 (12AX7) and power tubes only,
hopping over the preamp section. You might need a preamp or a pedal to boost the signal going into the Ext Amp input.
This would confirm the problem is in the preamp.

Next I would try to lift or remove R10 and jumper from junction of R10 and C2 directly to pin 7 of V2.
(remove the wire at pin 7 and jumper soldered to pin 7). This would "remove" the tone control from the circuit.
Then insert signal into the Normal/Bright inputs. It should have normal volume and the volume control should work.
If this work, the problem is definitely somewhere on the tone control. If this is the case, I would go with Enzo's
recommendation on post number 19 above. It would definitely work.

[TigerAmps]

Also - What tubes are you using. The bias at 59ma will definitely cause some hum. You really need to get down.
I would try to get a good set of matched tubes. I have an SB-12, but I have the one with the 7868 tubes.
They are nice little amps, rugged as hell (and heavy for their size). I ask because the different Sovtek tubes.
bias very, very differently (6L6WXT+ vs 5881WXT).

[DRH1958]

Yes, that diode can be just a garden variety 1N400X. It's bothersome to me that if you lift that 100K 2W resistor, your amp sounds better. This resistor is the dropping resistor from the high voltage secondary to your bias circuit. That should stay in place. I suppose you could replace it, 125k is a little high. I think it is very important to get those 6L6s biased correctly first. This is where you did the work to replace the OT and also the place where a malfunction can do the most damage. Yes, the treble pot needs to be addressed, but in my book, the correct biasing of the power tubes should come first..This is for sure known to be incorrect. You don't know for sure about the other issues.
I didn't realize the sockets were PCB mounted. The problem could be your bias probe has squared off pins rather than the nice taper that older tubes have. I have noticed some newer tubes don't have the taper at the end of the pins. It's probably hitting on the top edge of the metal connector inside the socket. Look to see if your bias probe has pins that aren't tapered and try to file them to a nice taper.

[J M Fahey]

turning the treble pot through it's paces has a wacky effect on the buzz gating distortion turning it all the way down nearly sends the amp into runaway. The bass pot effects the sound this way a bit too.

I narrowed it down to the treble control. If it's full on 10 the signal is pretty healthy and distortion free. Treble control turned anywhere between 1-9 and the signal gets all buzzy gated and very weak.

I put the old treble pot back in I never should've taken it apart I think I buggered it up a little bit I should've just measured it! so If I cant fix it I'll have to use the replacement.
Now even at full treble I have distortion , backed off a bit the signal cuts way down and lots of buzzy distortion ensues.

All this matches, and STINKS of bad pot, or to be more precise, V2b pin 7 losing ground reference.

1) Please do me a favor and solder a 1M resistor from pin7 (grid) to ground.
I bet your problem goes away.
That said, a bad pot (or bad tone control block) may kill your signal level , but whatever you hear should be cleaner and buzzless.

2) or even better, for testing purposes, you can bypass the tone stack completely.

Definitions:
a node is *any* copper connecting the parts I'll mention, touching/measuring any leg or wire connected to it "is the same".
A PEC is what they used to call that "tone module", an array of resistors and capacitors, I guess it meant Printed Electronic Circuit .

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 .

How does it sound?

[DRH1958]

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. He replaced the OT, so I would suspect this area needs attention. He also stated he has -49v on one 6L6 and -59.8 volts bias on the other one in post #17. It seems as though he has at least two separate problems or even three. Remember, this post started with very high voltages on all PT secondary windings. He corrected part of this by increasing the series resistor coming off the output B+ of the 5AR4 but this didn't help the heater voltage. It is still too high.