Peavey Deuce fluctuation in bias current

[JZRepair]

Ok, I've got this Deuce on my bench.  I had thought the tubes were bad, because V1, V2, V4 were reading about 24-26mA with 515V, and V3 was reading 32mA.  After replacing the tubes, I'm now getting the bais current fluctating pretty rapidly by milliamps, but, I'm still getting a higher bias current on V3 alone.  Voltage is steady at 507V.  What would cause the bias current to fluctuate like this?  I'm missing something here, but I've never seen this before.  It very well still could be a bad quad set of tubes, but, they were just shipped to me and I really haven't had a problem with this distributor in the past.

[shooter]

no schematic so i'm guessing fixed bias?
if so with ALL the PA tubes out, is the bias volts steady at the grid pins of the 4 tubes?
IF not, and it's self biased swap V3 with any other tube, does the weird stay or follow tube?

[JZRepair]

no schematic so i'm guessing fixed bias?
if so with ALL the PA tubes out, is the bias volts steady at the grid pins of the 4 tubes?
IF not, and it's self biased swap V3 with any other tube, does the weird stay or follow tube?

I've attached the schematic.  There is only the 4x 6L6GC output tubes, the rest is solid state.  The tubes that were in there fluctuated in the microvolt range, which isn't unusual, but millivolt fluctuations are odd.  It is an adjustable bias with a 50k pot for biasing.  As stated, the previous tubes didn't fluctuate like this, but the V3 tube was 32mA while the other 3 were about 22-24mA.  Original reason it was brought in, was random static on the output and volume spikes.  The first thing I did was look at the tube biasing and noticed the oddity of V3.  Throwing all that together, I concluded the tubes were at fault since the customer told me he replaced the tubes with a set from another deuce, because the even older tubes started flaking out and the fuse blew.  I can bias the new tubes, which were Apex burned and matched.  I was able to get them to V1/V2/V4 = 507V and ~29.5mA, but V3 was still 32-33mA, but again, this is a rough estimate from all the fluctuations taking place.

[shooter]

since it's a pcb, i'd be looking for bad solder points, cracked traces.
with all the PA tubes out is the voltage on pin 5 of the PA tube sockets solid?

[JZRepair]

since it's a pcb, i'd be looking for bad solder points, cracked traces.
with all the PA tubes out is the voltage on pin 5 of the PA tube sockets solid?

Yes, pin 5 is solid at -57.6V to -57.7V  slight fluctuation could just be from my meter's resolution, but it's pretty stable and no AC present.

[pdf64]

Screen grid voltages?
Notice how 1 of each pair doesn’t have a dedicated resistor?

[JZRepair]

Screen grid voltages?
Notice how 1 of each pair doesn’t have a dedicated resistor?

I did notice that and thought it was a bit odd, however, as previously stated, the old tubes didn't produce this same fluctuation, they acted normal with the typical "slight" variation of current in the uV range rather than the mV range.

The distributor accepted the tubes and has issued a new burned in, matched quad for a replacement.  I got the new tubes boxed up and shipped out last night, so I can't test with them at the moment.  I'm wondering if there was someone new on the process of apex burning and matching, and/or some sort of damage from the shipping.  The last set I got from them also had an issue with static/noise present and the tubes wouldn't bias correctly (for a completely different amp).  I asked about these tubes and they ensured me that there was indeed a problem with them, but no other information was provided.  I'm hoping once they receive these tubes I just sent back, I can gather some more information.  The only thing to change is the tubes, and that variation leads me to believe the tubes are at fault, but it seems like someone would have seen something similar on other tubes for other amps at some point.  That was more reason of reaching out on this issue.  I just don't have enough experience with tubes, nor do I have access to a tester to verify (something I need to eventually get my hands on is a tester, but I just haven't been able to afford one yet).

[JZRepair]

Ok, update time.  New tubes arrived and the bouncing current still seems to be a problem.  So unless it's another batch of bad tubes (highly unlikely), I'm wondering if a cap or two is bad.  Measuring AC on the plates, grids, and the 300V lines, I am getting AC through and I'm questioning those filter caps.  There really shouldn't be any AC on any of those if the filter caps were doing their job.

[shooter]

typically you might find ~4VAC on the 1st tap, <2Vac the second, and milliamps on the 3rd


what frequency is the AC that's wigglin the tubes?

[JZRepair]

typically you might find ~4VAC on the 1st tap, <2Vac the second, and milliamps on the 3rd

it's fluctuating quite fast.  My meter can't get a lock on it.

what frequency is the AC that's wigglin the tubes?

I don't have a way of testing that.  My scope can't handle the high voltage on the plates (it's just a cheap one, unfortunately).  However, I pulled all the filter caps and tested them.  All tested just fine and surprisingly good quality.  I don't trust my cap tester, so, I swapped them out anyways.  Problem was still there.  Pulled the rectifier diodes and the cap in-between.  These all tested just fine, too.  What I'm now curious of is these Overvoltage protection diodes, the SR2873's.  Reading up on them, they shouldn't conduct unless there's a problem.  However, if they conduct, and reset quickly, they would show an AC presence on the B+.  With tubes out and standby in the OFF position, the AC is present, so it's narrowed down to these 2 diodes, or OT.  Primary OT windings are about 60 Ohms, 30 on each side.  Secondary side had about 0.1 Ohms, which I don't know if that's right, but I believe it is.  I've ohmed out the speakers to make sure they aren't causing an issue and it is 4 Ohms.

If it's the diodes, I can't get them in time but testing them, they are close to 3V for forward voltage, but I can't test them in reverse voltage.  I can't even find a datasheet on them.  My gut tells me it's them, because I've eliminated everything else that could be inducing an AC signal, at least, I think I have.  I'm open to suggestions though.

[shooter]

what's the amp sound like when playing?


If you have a smartfhone, there are apps to measure frequency at the speaker. NO input signal(guitar) just amp making it's own sound.

Screen grid voltages?

still need to measure these with meter and post reading

[JZRepair]

what's the amp sound like when playing?

it makes me a bit nervous to play with current bouncing around like that.  Clearly there's an AC signal on the plates, so that AC is going to cancel out some if not all of the signal.

If you have a smartfhone, there are apps to measure frequency at the speaker. NO input signal(guitar) just amp making it's own sound.

what's the name of the app?

Screen grid voltages?

still need to measure these with meter and post reading

With no tubes, 537.  Unfortunately, I didn't check with the tubes installed yet, but I will.  Plates, with no tubes was 542.  Plates with tubes is 507, so grids should be about 500, but I'll verify this when I can and update it on here.

[JZRepair]

Ok, I've looked over everything.  I can see Voltages are good throughout the amp.  I hate that this PCB has no identifiers, because it's making this whole project a nightmare, constantly having to remove the board so I can find the traces and connect the dots.  I've never appreciated a good silk screen more than right now.  This whole thing pisses me off, because this amp was working great a few months ago and something blew which caused all of this.

Here's the issues I've got.  Reverb isn't working.  Tubes still have a fluctuation of +/- 3mA bias current.  Bias voltage is 503V.  I don't remember the exact voltage on pin 4 of the tubes, but it was right around the 490V (I think it was 491-493).  Tremolo oscillation wasn't working, replaced the 576 FET since signal was going in the gate, but not coming out the Drain like it should have.  I've replaced the 3904s on the reverb drive amp as both originals proved faulty in the tester.  However, I have no signal getting to the tank, could be another bad transistor, or, what I just noticed is that I'm getting 60Hz through the +24V and +48V supplies with nothing on the input. 

I think this is what's causing my bias fluctuation, but for the life of me, I can't seem to figure out where it's coming from.  My rectifier diodes are good, and even still, once they go through the diodes, that 60Hz from the mains, becomes 120Hz in a full wave bridge rectifier.  I had a missing ground connection, but was able to resolve that fairly easily (the grounding wires for the reverb tanks were cut, patched in some wire to the shields and attached to the chassis).  Still, I can't seem to find where this 60 Hz would be coming from.  Could a ground loop create that 60Hz?  Or, could I somehow be picking it up from a wire too close to one of the secondary wires?  The Red and R/Y wires for the secondary are twisted together with the violet wire for the bias/preamp rail.  I doubt that's it, but I'm quite literally about to lose my mind on this amp.

[sluckey]

Bias voltage is 503V.

  May want to recheck that.

[JZRepair]

Bias voltage is 503V.

  May want to recheck that.

Schematic calls for 500V, so 503 is definitely within +/- 20%.

[sluckey]

Bias voltage is 503V.

  May want to recheck that.

Schematic calls for 500V, so 503 is definitely within +/- 20%.

The bias voltage is -55V according to the schematic you posted.

[JZRepair]

The bias voltage is -55V according to the schematic you posted.

  Sorry, not the bias voltage, the plate voltage.  Bias voltage is -57V.

[JZRepair]

Ok, solved the issue with the fluctuating bias current.  The 2x 64278 transistors (tube drivers) were bad, even though in my tester, they tested good.  Apparently, they didn't like working within the circuit.  I've replaced them with 2x 2N3439 per the Peavey Semiconductor Transistor Cross Reference list.  Immediately, the bias current stopped fluctuating and has been holding incredibly steady. 

However, now I have no signal reaching the Mixer Amp (final stage in preamp).  I've got signal on the F.S. 5 connection (at the 10k), for the Effects channel, and I have signal getting through to the F.S. 2 connection for the Normal channel.  However, both channels dissapear as they go through their respective 10k resistor.  Logic would state that the .1uF or the .01uF caps were bad.  I replaced them, even though they tested fine.  Still no difference.  I'm just not getting any signal through to base of that last transistor, but, with no signal, I can see the tremolo pulses coming through.  I'm curious if this canceling out any signal from either channel, but failing to see how that tremelo would be getting through with the depth and rate pots full CCW.  I'm starting to not trust my tester, but the 576 in the tremolo circuit was bad, I replaced that with an NTE2937, because I wasn't getting anything from the tremolo previously.  I've tested this 2937 a few times, to make sure it was still good, but again, I'm beginning to not trust my tester.  This 2937 is highly susceptible to ESD, so I'm wondering if maybe it crossed something or I did, on the PCB and shorted it after testing. 

Has anyone else seen anything similar?

[PRR]

> both channels dissapear as they go through their respective 10k resistor.

Or..... the input to the power amp is a DEAD short.

There's not too many ways this can happen. Solder-blob tops my list.

[JZRepair]

Or..... the input to the power amp is a DEAD short.

There's not too many ways this can happen. Solder-blob tops my list.

Signal is lost before it enters the power amp.  If there was a dead short on the input of the power amp, I should still see signal on the coupling capacitor, should I not?  I can guarantee it's not a solder blob.  I'm IPC trained with soldering and the board has been cleaned after soldering to remove any solder splatters while soldering or any solder balls that could create a small short.  I'll double-check, but I don't think this is the issue.

[PRR]

I wish you luck .

[JZRepair]

Yeah, solder blobs were not the issue.  I went back through and cleaned the underside of the PCB again, just to be safe.  I cleaned up anything that looked like it could cause an issue, clipping a few leads shorter that were a hair longer than they needed to be, but no signs of anything suspicious, as far as any shorts from solder balls, protruding leads, etc.  Retested and it's still the same. 

Went through the board and I'm seeing an extra coupling capacitor that I'm not finding on the schematic.  It's a 100pF disc cap that is parallel to the master gain pot.  The gain in this circuit is setup in a way that the feedback from the mixer amp will cancel out any signal when the pot is full CCW (low volume).  Then, as resistance increases, less signal get's through, which should increase the output of the mixer amp, by not cancelling the input.  It seems to feedback at the point I'm losing signal, so I'm almost curious if the pot has gone bad.  I cleaned out the pots with F5, but this didn't seem to do anything.  I've tested the pot in circuit (since pins 2/3 are tied together), I can see the pot adjusting in resistance, but, I don't have any way of testing this under power, other than watching the signal on the scope, which I can see signal going in, but again, because of where it comes out, that's the point I'm losing signal, so I can't verify that it's actually getting fed back.  I touched up the solder joint on pin 1 previously, because it appeared to be a broken joint.

The Effects input, I can see more signal getting through and even see a slight signal appear on the output tube's grid, through the coupling cap, just not what I would expect to see under normal conditions.  On the normal input, I can't see any signal getting through to the mixer amp, but on the gain pot, I can see signal being fed in and increasing as I increase the resistance of the pot.  Again, not nearly what I would expect to see.  Logic would say the transistor is bad.  That has been replaced with an NTE47, because previously, I wasn't getting any signal out of the SPS 953 that was originally in there.  But I am not seeing any signal at the bases of the tube drivers.