Silver Jube Clone Oscillation?

[Stevem]

What does the wave form look like at the effects send jack?

[ViperDoc]

I’m just back from a family reunion, now off to a wedding. I’ll gather the best set of scope photos I can when I return. I noticed that even with the “effects loop jack components” tacked in place and power tubes pulled, my entire preamp circuit passes signal until just on the back end of the master volume output/PI input filter cap. I either have a bad solder joint there or bad cap, would you agree? The waveform just goes dead flat. The pot’s halfway open and I get signal on the cap’s front end. Or perhaps a bad V3 tube? Pics forthcoming...

[ViperDoc]

All right, here are some scope shots of my preamp with location descriptions. If you'd recommend another location, please freely advise.

--Bulb limiter in place, 40W bulb; element barely conducting
--no power tubes
--preamp tubes in
--500HZ 2V signal from generator injected at the input jack

1) POST V1B PLATE COUPLING CAP


2) POST V2A PLATE COUPLING CAP


3) POST V2B PLATE COUPLING CAP


4) POST V3A (PI) PLATE COUPLING CAP


5) POST V3B (PI) PLATE COUPLING CAP


6) MASTER VOLUME OUTPUT, LEAD CHANNEL: MASTER 50%, LEAD GAIN 50%, LEAD VOL 50%


7) MASTER VOLUME OUTPUT, CLEAN CHANNEL: MASTER 50%, CLEAN GAIN 50%


MASTER VOLUME OUTPUT, LEAD CHANNEL: MASTER 50%, LEAD GAIN 100%, LEAD VOLUME 25%


9) POST PHASE INVERTER INPUT CAP, MASTER 50%, LEAD GAIN 100%, LEAD VOLUME 25% (SAME AS LAST ABOVE)


I'm using a Siglent scope that seems to be temperamental with wave height adjustment. It seems to want to relocate the shallower waves rather than alter the vertical axis units to inspect the waveform better. The last photo is a perfect example. But it sure looks different, DON"T SHE????

Brethren, I welcome all your recommendations. Thank you!

[NOTE: The last measurement after the PI input cap says the input frequency is about 10 Hz. It's 500 Hz on the other end of that cap. I'm going to hazard a PI input cap replacement is in my future. Or does that indicate a short? ]

[ViperDoc]

I looked for improper grounds on my board to chassis and I don't find any. All connections test intact. I also tested for shorts on my PI tube and didn't find any either. I pulled the *[heater CT] fuse and tested for heater shorts to chassis, and I don't have any.

[EDIT:
1) I replaced the .022 uF P.I. input cap and verified connections. No change.
2) I replaced the P.I. tube with another 12AX7EH. No change. ]

What is the waveform on a PI tube with no power tubes supposed to look like?

*I mistakenly said "HT fuse" earlier.

[ViperDoc]

Two more Scope shots:

1) Y: LEAD CH., MV OUT, ALL GAIN/VOL 50%; B: POST V3A PLATE COUPLING CAP



2) Y: LEAD CH., MV OUT, ALL GAIN/VOL 50%; B: POST V3B PLATE COUPLING CAP

[xtian]

Brian, please post the schematic for your amp here.

I reread the thread. Seems like now your signal is fine after the MV, but not good after the PI?

And earlier you saw a white flash--something may have burned up--did you figure that out?

[ViperDoc]

Brian, please post the schematic for your amp here.

I reread the thread. Seems like now your signal is fine after the MV, but not good after the PI?

And earlier you saw a white flash--something may have burned up--did you figure that out?

Here's the build SCH. DO NOT TRUST OR USE IT. I designed my board to simply parallelize components as much as I could, I didn't change the circuit at all. The only thing this amp does is add an individual gain pot for the Lead channel that switches in and out with the channel switch relay.

VIPERDOC 2550X SCH 1.2.pdf

I think the white flash was me blowing my HT fuse. I pulled the power tubes and put a new HT fuse in and on the limiter, my bulb is just about comatose after firing up. My power tubes do not show any shorts on the pins.

Interpreting scope shots is entirely uncharted territory for me, but the signal just in-between the MV output and P.I. input cap looks dandy, and then just on the other end of that cap, in between the P.I. input cap and the V3A grid looks like the waveform is getting clipped by Thor. I've changed PI tubes without any change. Now, I don't know if that's normal without power tubes in, or not? Doesn't seem like it would be.

My OT is [EDIT: not actually, but I hadn’t figured that out yet.] wired according to factory spec, I've just never seen a common secondary wire be brown. Maybe that means nothing.

The first time I fired it all up, I could play and hear everything with the MV just barely cracked. Maybe I'll go back to that and scope around for clues. No idea what to look for though.

[ViperDoc]

I just put the power tubes in and put the amp on the limiter. It powered on and the preamp tubes did not appear lit. The rectified DC measured quite a bit lower than without power tubes. The bulb lit brighter and decayed after a bit, so I took the amp off the limiter and turned it on and the HT fuse blew again after a short buzzsaw sound out the speaker as it powered up.

[xtian]

I just put the power tubes in and put the amp on the limiter. It powered on and the preamp tubes did not appear lit. The rectified DC measured quite a bit lower than without power tubes. The bulb lit brighter and decayed after a bit, so I took the amp off the limiter and turned it on and the HT fuse blew again after a short buzzsaw sound out the speaker as it powered up.

So many things!

I took the amp off the limiter and turned it on and the HT fuse blew again after a short buzzsaw sound out the speaker as it powered up.

Question: When you flipped the power on, did the buzzsaw sound happen immediately, or after 10-15 seconds or warmup?

If immediately, you have either a bad power tube or a bad OT, and this fault happens only when full amperage can be drawn from the wall (that is, with no lamp limiter).

[ViperDoc]

Question: When you flipped the power on, did the buzzsaw sound happen immediately, or after 10-15 seconds or warmup?

If immediately, you have either a bad power tube or a bad OT, and this fault happens only when full amperage can be drawn from the wall (that is, with no lamp limiter).

It took about 10-15 seconds, then it blew.

[xtian]

OK, so when the power tubes start conducting, they draw too much current. On page one, you show 425v plates and ~34mA each, which is fine (~60%). But maybe there's a runaway oscillation (suggested earlier) that's driving the tubes past 100%, and this burns the fuse.

I suggest repeating this experiment WITH THE PI TUBE PULLED OUT. That should eliminate any signal/oscillation on the control grids of the power tubes. See if amp runs steady without PI tube.

One of my helpful diagnostic tools is a power meter that goes between wall service and my amp, and reports consumption in watts, in real time. My 50w Marshall build draws ~60w while warming up, and close to 100w after warmed up at idle. But last month when I was trying to solve the runaway oscillation in a client's Marshall DSL100HR, I could see power draw ramp up to 180w when Master Volume was turned up. This is a clear indication of the problem caused by ultrasonic oscillation.

Here's what I use: https://www.ebay.com/sch/i.html?_from=R ... 0&_sacat=0

This is the only one I can find that has a display that can be separated from the plug, and moved to where you can easily read it.

[ViperDoc]

FACE PALM.

Look, I don't grovel, but I'm tempted. But I'm happy to say, at least for now, I FIXED MY PROBLEM.

After all my careful planning and triple-checking my work, I still managed to mis-wire the OT primary. The OTS CT isn't brown after all, it's white. The secondary COMMON is brown, but I must have somehow mis-labeled the CT/Common wire colors in my mind. I don't know. Anyway, I swapped the white for the brown and my amp now works completely! It's burning in right now, glowing like a hot ember. It just happens to be past 1:00 AM, so I can't test it at any appreciable volume, but everything works well. The presonance knob works great, too. I'm looking forward to hearing how it performs at louder volumes. And the amp is DEAD SILENT. No noise whatsoever.

Having never made this error before, I don't know how many lives my OT lost, but I welcome your advice as always. More to come as I learn how to scope-test the distortion on this amp.

[joeboo88]

That’s great that you found it, without losing too much hair.
I’ve been there too. I have Hammond 50 watt OT that has weird wiring colours as well and wanted to smash the amp until I put it aside for a few days, then calmly looked at it again, then realized that it was the pilot (me) that was at fault.

Again good stuff, it’s always rewarding to figure it out.
Joe.

[ViperDoc]

Some interesting findings that I hope to learn how to manage:

The Lead channel on this amp is a complete gain monster, it just wails. It chugs, it rocks, it squeals, it's everything you'd want a British rock amp to be. A major discovery is the gain character improves drastically as I turn the presonance all the way to the right, like it should stay there. I don't notice an appreciable difference when rotating left into "depth territory". I may change the presonance cap values or consider installing the stock presence and see where that gets me.

The Clean channel's distortion does not sound good. It has quite a "splat" to it. Engaging the clipping circuit improves it a bit, but it sounds like the channel you'd never use. I hope to figure that out. I dropped out the "effects loop components" as described before while tooling around yesterday, so I may put them back in to see if the attenuation makes much difference. The Lead channel has gain to spare like no other, so there. But right now, the clean tone I get rolling the guitar volume back on the Lead channel sounds better.

[xtian]

The Clean channel's distortion does not sound good. It has quite a "splat" to it.

I like my amps to have a smooth transition from clean to distorted. "Splat" sounds like this transition is rough. Is that what you're describing? I don't know what makes one amp awesome and one bad in this regard, but I suspect lead dress plays a part. Always looking for insight.