Help with Peavey Ultra Plus 120. Just makes a loud hum/buzz after 5 mins.

[Jazz P Bass]

Resistance on r98 is good but r99 constantly climbs until overload? Guess it was charging a cap???

Checking ac on either side of r98 seemed futile as it kept jumping around everywhere. I had the meter set to 20vac and as soon as I would ground the negative to the chassis I would start getting readings even without the positive touching anything, like it was picking up interference from sources around it.

That is perfectly normal for a meter to read Vac with one probe floating.

[mrmcsick]

Well I no longer have guitar signal coming out of the amp, sighhhhhhhhhhhhhhh. For some reason I plugged in and wanted to hear it play for the whole 2 mins before the hum showed up and I have nothing now. Started swapping out preamp tubes and the hum and pops come and go at random it seems. Maybe something is wrong in the phase invertor circuit?? v1 and v2 don't look like they are lighting up and feel cool to the touch. I took the board out and cleaned up all the connections super good and looked for broken solder joints but didn't see anything. I ohm tested each ribbon cable pin to pin and they are all fine. What kind of voltage am I supposed to be getting and on what pin of the preamp tubes? Guess that's next on the to do list before I smash this thing and set it on fireeee haha.

[mrmcsick]

Well Now that I no longer have guitar signal going through the amp and the pop and hums have seemed to disappear due to this and messing with the preamp board and tubes I decided to plug my guitar into the effects return jack and see if the power amp is working right. Works fine. No pops or hum or anything for about 10 mins. The preamp out was sending a good signal to my pc interface too. Obviously the problem lies between the input jack and where the signal gets fed into the power amp somewhere. Anybody have any ideas?

[mrmcsick]

Why are my replies treeing off of other comments and not just going to the bottom of the main thread?

[nashvillebill]

Change your Display option to Linear mode...

Try running a patch cable between you Preamp out and Poweramp In, again.

[mrmcsick]

It just has a preamp out and an effects send and return. I patched the effects send and return and plugged in both the low and high gain inputs but still nothing. I doubt they just both went bad without me touching them. Patched the preamp out and fx return and nothing. Only thing that works is when I plug my guitar into the fx return and then the signal comes out of the amp and the preamp out. I took the patch cable from the fx send (just like I did with the preamp out) into my pc interface like the preamp out but it didn't do anything.

At least the popping and hum is gone lol.

I tried all known good preamp tubes with no change.

[g1]

v1 and v2 don't look like they are lighting up and feel cool to the touch.

Sounds like you have lost the heater supply to V1 and V2. A connection was probably intermittent and has now quit completely, which could also explain the previous pops and crackling.
Hopefully when you solve the heater issue the pops will be gone.

[mrmcsick]

Thanks. Can someone please let me know what normal voltages on what pins I should be getting on the preamp tubes?

[Enzo]

Like most recent era PV tube amps, the preamp tube heaters run on DC. This amp has +24 and -24 supplies. Heaters for V1 and V2 are wired in series across +24v and ground with a couple resistors in series. Look at the schematic, between V2 pin 5 and V3 pin 4 is a connection to ground. So V1,2 are wired together and V3,4 are wired together, but other than both pairs having a grounded end, they are separate circuits.

Note on the schematic V1-4 has a connection to FIL, and also C301 to ground for a little decoupling. Now look on the page next to the phase inverter. There are parallel R301,302 at 22 ohms each, so 11 ohms in series with the heaters. The top end of the resistors is FIL, while the bottom is +24.

Your V1,2 are dark? The voltages are implicit. We have two 12v heaters in series across a 24v supply. Since V2-5 is at ground, I expect zero volts there. V2-4/V1-5 I expect about 12vDC there, and at V1-4/C301 about 24v. And the 11 ohm resistance? Heaters draw 0.15A, so Ohm's Law says: V=IxR or 11 x 0.15, which comes out to 1.6v. So my +24 at V1-4 should be roughly 1.6v less than the 24v supply itself.

So if they are dark, something is open. Or +24 is missing. If the relays work - if channels switch - then the +24 is there. If the reverb works, then +17 is there, which comes from +24. So that is a quick check for the basic +24. Then start at the resistors. Got +24 on the +24 end? NO? Then check the ribbon cable from the main board. Then got +23 on the other end? If not, check the two resistors for open. The resistors are nestled between pins 1 and 9 by the V4 socket. If the resistors are open, also check C301, it is on the main board, connecting back to V1 through that ribbon cable there. So by now we ought to find +24 (or 23 or something similar)on V1-4, so we check V-5. In a working circuit, we would see +12 there. If we see zero there, the tube is open or the socket bad. If we see +24 there, that means no voltage drop across V1 heater, so zero current, which implies that V2 is open or not connected. And finally to V2-4. It beter ve the same as V1-5, since they are wired together. If it is different, then ther is a broken connection between the V1 and V2 sockets. And finally V2-5 should be a connection to ground. And that ground is through another ribbon.

[mrmcsick]

Channel switching works with the pedal plugged in, I'm assuming that reverb does to as the led on the pedal turns on and off but I don't have the reverb pan hooked up, should I hook it up just to be sure?

R301,302 are open.

How can I test c301, check voltage across it? Or should it be sending 24vdc over the 3rd pin of the first jumper to v1?

As for pin voltages. v1-4 and 5 nothing. v2-4 and 5 nothing (ground). v3-4 nothing(ground) v3-5 -12vdc. v4-4 -12vdc v4-5 -25vdc

[Enzo]

Well as we discussed, with the resistors open, the 24v cannot get to the rest of the circuit, so all the zeroes make sense. And V3, V4 are a totally separate circuit.

If channel switching works, then we know +24 is being made.

The circuit is basic here, we have +24, through the resistors, then a cap to ground then the two heaters in series to ground. All C301 does is smooth out any ripples and helps isolate the high gain tubes from other influences. With the resistors open, there will be no voltage across it. MY only real concern about it is that it may have shorted or gone VERY leaky and might then have been the reason the resistors opened. So see if C301 is shorted, and if not, then replace the resistors and power up the amp. If C301 gets hot or the 24v gets loaded down on the C301 side of the resistors, then the caps is likely faulty. And if the cap is bulging, it is bad.

[mrmcsick]

YYYYYAAAAAAAAAAAAAYYYYYYYYYYYYYYYYYYYYYYYYYYYYY!!!!!!!!! !!!! Fixed.

Replaced the 2 resistors today and it came to life. Put the old Sovtek LPS back into the v4 and it started popping and did the hum acouple of the times but not all of the time. Bye bye sovtek lol. Threw in an old spare. Looks like it was an intermittent tube failure. Checked ac on the cap side of the resistors and ripple was almost non existent.

Going to order a couple new preamp tubes and should be good to go.

Thank you guys very very much for your help and patience. I'm am extremely grateful.

[mrmcsick]

I'm not touching anything else in the amp but have 2 questions out of curiosity.

Can r99 be replaced with a pot and a lower value resistor in series to make it an adjustable bias like the bias mods for the 5150?

I noticed an opamp on the main board. Was wondering if it is engaged and adds the extra gain stages for the crunch and ultra channels when the gain buttons are pushed in?

[Enzo]

No, you can't. R98 is way too small to allow it. You cannot make the bias any cooler in this circuit, but then no one ever wants that, they only want to run the amp red hot. That can be done, but you also need to change R98 to something like 3k give or take.

As far as I know there is only one IC on the board, and it is your reverb circuit.

[mrmcsick]

Ok thanks. I wouldn't try to run it cooler just a little hotter plus the option to adjust with different/new tubes. I forget what the bias ended up being. Lower 30 milliamps I think, aren't 6l6's happier around 38-42?