My supro is silent... dead silent

[EFK]

DEfinitely the million dollar issue. I did measure the OT impedance - I use a 9V wall wart and it works quite well. Doing the calculations, this OT is about 5.5K into an 8 ohm load which is good for 6L6. The other transformer I temporarily used (which made no diff.) was about 4K. There's no nfb here, but I've tried reversing the plate leads anyway. No difference.

Amp is cathode bias, very definitely well grounded.

Here are some readings from this morning:

Idle, no signal:

pin 3: 410VDC / 4.9 VAC
pin 4: 404 VDC / 0 VAC
pin 5: .008 vdc / 0 VAC
pins 1/8 (tied, cathode bias): 33.4 VDC / 0 VAC

Signal applied:

pin 3: 377 VDC / 24 VAC
pin 4: 367 VDC / 0 VAC
pins 5: -8.2 and -4.1 VDC, 63 VAC and 56.7 VAC
pins 1/8: 57.2 VDC / 1.1 VAC

[Jazz P Bass]

I do not have a clue.
With a 56Vac swing on the grid & a 24Vac swing on the plate, that tells me the tube is NOT amplifying.
Should you not see something like a 300Vac swing on the plates?
I would remove the cathode cap & try again.
Something is eating the signal.

[tubeswell]

Looks like you found your problem right there

[EFK]

Yep, AC swing on the plates is 5.1 VAC no signal, to alternating 20-24 VAC at full bore. It swings about every second, 20V up to 24V, then back to 20, then up to 24 etc. etc etc.

I hate to say this, but I did try it w. the cathode bypass cap removed. I remember reading something about that somewhere. Didn't make any difference. Good way to describe it - the signal is being "eaten." By what? In my limited understanding, it seems like a HF oscillation as I can even physically hear the tubes ringing at a high frequency when I crank the volume up on the signal being injected.

You know, I put in all new sprague filter caps. Any chance one could be bad? I measured AC on the first one at @ 5 volts, and down to practically nothing on the second, so they seem like they are doing what they are supposed to do. I did try paralleling the first filter cap with another, didnt make a difference and there is no voltage coming off the ground side of the cap. I used a new 5W 1K input resistor between the first two, which is how it was stock. Could possibly a bigger input resistor, or even a choke, help in any way?

Because I can run the whole preamp through a different power amp, I'm going to assume that the problem has to be confined to the power amp chassis. I even removed the connecting cable (power amp up to a socket on the preamp, which is a new socket) and temporarily ran new wires up to that socket just to test the cable - no difference.

I can;t see how it can be a ground issue, because you could not possibly find a more haphazrd, willy-nilly grounding scheme than a Supro and I've implemented a much better semi star system at this point (although originally I did try it out as-stock also, and no grounding changes have helped in any way).

This is very frustrating - I really appreciate you fellows hanging in there. I imagine it has to be entertaining anyway!

[Mark Black]

Just a thought - does this thing have a 1/4" output jack or is it straight-wired to the speaker? Reason I ask is I have encountered bad/shorted jacks from time to time (phenolic broken or shifted, allowing tip & sleeve elements to touch), and such a thing would make it very quiet indeed...

Best of luck anyway.

[EFK]

Thanks Mark. Originally they all have this weird little 2-prong cable that is hard wired to the speaker and plugs into the power amp chassis. The one on this was in pretty bad shape, and being as this is the only 15" speaker amp I've got, I set up a jack wired to the speaker, then ran a cable w. a 1/4" plug directly into the chassis so I can swap around different cabs/speakers. I've checked polarity more than once - I have that set up correctly and I can plug heads into the cabinet and it works fine.

[EFK]

FWIW, I measured DC current on the plates at idle and with vol. on 10 (w/signal injected). Is my DC current supposed to be negative? I got -61ma at idle, and -103ma with volume up. ???

[tubeswell]

What I meant was, it sounds like your OT is bad.

In regard to your question about the negative meter readings, which way around were the meter leads?

[EFK]

I got you. Problem here is, it does it with a new OT as well - I've bounced back and forth b/t the original OT and a new spare, and it didn't make any difference. Nor does it make any difference if I swap plate leads around, or swap grid leads from the PI.

Didn;t think about the meter leads! I had the black on the OT center tap and the red on the plates.

[Mark Black]

I got you. Problem here is, it does it with a new OT as well

This is what got me thinking about the output jack. It seems as though everything is working right up through the OT primaries, but nothing is making it out of the secondaries.

Could you try desoldering/disconnecting any jacks or tie points from the OT secondary and clip-lead a speaker directly to the secondary leads?

[Jazz P Bass]

The tubes are not amplifying.
Try new output tubes.

[HaroldBrooks]

EFK, did you ever sort this out ? Hope so ! Please let us know if you did, and what went wrong.

Thanks !