OT? vibrochamp..newb help please?

[Ben]

heres a youtube vid of the pedal that killed my amp.
prepare yourself, brace yourself, for its tonal awesomeness

YouTube - THING PLAYS THE dod BUZZ BOX

[distorto]

Well there's your problem. Any self respecting vintage tube amp would blow up on principal with one of those plugged in. Those are meant for a Bandit 65.

[Ben]

I disconnected the pink wire from the speaker jack, retubed and plugged in and turned on. no sound.

I did not get a chance to check the voltages yet, I'll do that tonight.

I forgot to mention something that may be important. I changed the speaker that was in this amp, an Oamu 1965 ? to a new weber sig series 4 ohm 15 watt speaker. Im sure I wired it correctly, so not sure if this is pertinent info or not.

anyway I'll report back with the voltages tonight.

Thanks for the help.
-Ben

[MWJB]

As long as one speaker wire went to one speaker terminal, the other speaker wire to the other terminal, with a sound connection, then you can't have done anything wrong there.

[Ben]

voltage readings are in

Here are the voltage readings taken with the amp on and tubes in the sockets. there are two 12ax7's in a vibrochamp. i took my readings off the first 12AX7 in V1.

12AX7
pin 1: 238, this reading started higher (350) and dropped steadily to 238.
pin 3: 1-2 volts
pin 6: 320
pin 8: 1-2 volts

6V6
Pin 3: 0
Pin 4: 410
Pin 8: 12

I hope this is useful info. im looking at the schematic and tho i cant really read these things it looks like pin 8 on that 6v6 should be more like 214 volts? everything else seems close but pin 6 n the 12AX7 which should be 260 instead of 320?

Thanks for your help.
-Ben

[MWJB]

12AX7 pin 6 - 320v seems high, voltages at pins 1 & 6 should be similar, make sure that there is a good connection between pin 8 and the 1500ohm resistor and a good connection to ground from the 47ohm resistor at the other end of the 1500.

Pin 8 of the 6V6 might read 25v-ish, or more.

0v at 6V6 pin 3 is bad - shot tube/socket/OT (we know it's the OT from your earlier resistance measurements from red to blu wires.

410 at 6V6 pin 6 sounds OK, in reality the amps work on much higher voltages than the schematics show, maybe up to 430vdc on the plate of the 6V6.

[Ben]

Okay thanks. I need to figure out why pin 6 is so high.
Schem says 260, my reading is 320 at pin 6 and 410 at the other side of the 100K resistor.

The connections you asked me to check seem solid, ook ad feel solid, and the resistors all check out close to expected value. Do i need to reheat these joints to be certain?

Would a blown OT affect my reading at pin 6 of the V1 12AX7?
as in perhaps the OT when working sucks off a little of that current. Im fishing here...thats probably the newbiest of newb questions....

I stuck a new jj 12AX& in that tube socket. te reading was even higher but then settled back down to 320 again. still ...60 to high.

Hey i wanna say thanks to you MWJB for your help with this. Even if the amp never works again (and it will work again!) its been worth the $300 in the lessons Im getting.

I dunno, whats next? That higher current must be originating earlier in the circuit if its 410 behind that 100K resistor??

[MWJB]

The schem is WRONG, forget it! The voltages that you have are what you need to deal with.

With 410 at the 100K I'd expect more like 270-280 at pin 6...maybe a little more (10v-ish). Clip your black meter lead to the chassis, measure ohms with the red lead from the grounded eyelet of the 47ohm resistor (0ohms), then again at the juction of the 47 & 1500ohm resistors (47ohms), then again at where the wire from pin 8 meets the 1500ohm (1547ohms).

Either way, another 20-30v on pin 6 is not going to stop your amp from working, it looks borderline odd, but without measuring all plate & cathode resistors at V1 it's hard to say whether it falls into reasonable perameters.

The blown OT shouldn't make the voltage at pin 6 different to that at pin 1, but a lack of current draw at the 6V6 will push up voltages for both equally (or almost equally, voltage on pin 6 might be a tad higher because of the extra 47ohms after the cathode resistor).

Sort the OT first so that you can check voltages with the proper current load.

[MWJB]

Thinking about it, voltages at pin 1 are a little low, so do the same checks at pin 3/1500ohm as suggested for pin 8, you may have a leaky coupling cap at the tone stack, easiest thing to do is sub the 250pf/0.047/0.1 caps for new parts and see if voltages even up.

[Ben]

That voltage at pin 1 of the 12AX7 DID start out about the same as pin 3, it just dropped quickly and steadily down to that 238 from 350. whats the dropping voltage mean? anything?

I'll check pin 3 tonight as you suggest and order the replacement caps you suggested also when i decide on an OT, which Im trying to decide upon right now. Allen has a nice looking one but theres a $50 minimum order and I'm not sure m gonna make that ..aw who am i kidding, i cant turn around without spending $50.
thanks much!
-Ben

[Ben]

Allen turned out to be impossible to order from. called three days in a row no one there to take my web order which I had to call to finalize for some odd reason...annoying!, then the next week he puts up a notice on his webiste..on vacation for another week. lame. add the $50 minimum and allen was out and a full week wasted on him waiting for some response on my order

Weber was 2-3 weeks for a heyboer, so I just ordered the mojo (which is a heyboer) and had it two days later.

Amp now has sound. I dont think it sounds as good as it did, but its hard to remember exactly since I only had the amp for a few days before it went belly up. voltages at 1 and 6 of the first 12ax7 are now similar at 250 and 251.

I still get zero at pin 3 of the 6v6 but that has got to be a mistaken reading right? the amps all back in its cabinet so I cant really re-test. with zero at pin three i wouldnt get sound tho would i?

anyway, shes done unless i decide to slow down the trem or blow this new OT.
Thanks for all your help.

[RWood]

I still get zero at pin 3 of the 6v6 but that has got to be a mistaken reading right?

Pin 3 is connected directly to the OT that was recently wired-in, right? Unless it goes to a terminal strip first, that should have been one of the ones you recently resoldered. I would reflow that joint. Then make extra sure your black probe has a good ground connection and with the amp warmed up hit pin 3 with your red probe. Sometimes you need to use the point of that probe to scrape beneath any flux or other scum that might be on the top layer of the solder connection.

But yes, that connection is the output from the power tube so to get sound, something gotta be conducting there. T'would be nice to know what that voltage reading is, so you may then properly bias that power tube.

RWood

[Chuck H]

Your 0 on pin three reading has to be a mistake if the amp is working. And as unlikely as it sounds it's probably just a coincidence that your erroneous reading happened after you replaced the OT. Sometimes the solder joint will be encapsulated in flux such that your meter probe isn't actually making contact. And meters without "auto adjust" will not read unless set correctly. A step I miss all the time. Point is, if the amp is working properly there is definitely voltage on pin three.

Chuck