Harp amp design advice

[sluckey]

I've had amps that would give crazy readings on the plates of the output tubes. Even using a Fluke 87V! Touch a probe to the plate and see a small arcing between the pin and probe and the meter just goes nuts with impossible readings. I suppose the arcing was causing some oscillations. I could usually resolve the issue by putting the amp in STBY mode, connecting the probe, then putting the amp back in operate mode. This does not happen on all amps and when it does happen the other tube usually reads fine.

I suspect something similar may be happening to you, or as Martin suggests, maybe a meter issue.

[martin manning]

Or the long meter leads are kicking the power stage into oscillation. Try measuring the cathode voltage. There is minimal chance that that will cause an issue, and you can see if the power tube current is reasonable.

[TUBEDUDE]

Once that is straightened out, you might find things a little bassy. If so, cut the value of the input Rk from 20uF to 4.7uF. And the coupling caps from 100nF to 22nF-47nF. At least that's what mine needed.

[SeddoniaPhil]

How are the batteries in your meter? Sometimes low battery state can cause bizarre readings.

Well, that was a good steer - 9v battery measured 5v!
With new battery and tubes in place, the new values measured are alot more sensible ( https://drive.google.com/file/d/1asZqOt ... sp=sharing )- 356 pin 3 inboard 6v6 and 363 outboard pin 3 6v6 are more as expected.

Now, with that out of the way, the amp is not going.

I'm not getting any speaker hiss, but I get a sort of clicking in the circuit when I touch a guitar cable tip in input 2.

I'm not getting any response from input 1, but as it's more an experiment in grid leak bias, I think I'll remove it from the circuit to focus on input 1 and getting the amp working first.

[SeddoniaPhil]

Once that is straightened out, you might find things a little bassy. If so, cut the value of the input Rk from 20uF to 4.7uF. And the coupling caps from 100nF to 22nF-47nF. At least that's what mine needed.

As it's a harp amp, the "bassiness" shouldn't be as much a of an issue. I built a 5f2H previously and it makes a guitar sound really dead. When I can get it going I'll have a better idea anyway

[Lynxtrap]

At the speaker.
Amp chassis ground to + on speaker(s)

I don't get it. That makes it out of phase with what? I don't see how it would make a difference unless the speaker is out of phase with the microphone?
I'm curious about this as a bandmate has started to play harp.

[SeddoniaPhil]

Ok had some time today to progress this. Voltages were all over the place. Swapped out the rectifier tube and it settled down (5U4 tube; 290BEX has current of 3A, so should be fine). Put a radio signal through and i can hear some sounds - possibly from the OT, but not the speaker. Back to Robinette's trouble shooter for the next installment

[SeddoniaPhil]

I put a radio signal through and found the OT is making noise, but not the speaker. What do you make of this? https://drive.google.com/file/d/1DLqZ8X ... C-pQ0lYBFQ

[Stevem]

If you have driven high level audio signal thru a fully working amp, but with the secondary side of the OT feeding into no load ( as in a open circuit ) then there's a real good chance you have caused a arc over of the OTs primary and now it's shorted out and may not work at all, or only output very very low distorted sound.

It's ok and no harm will come to a OT if you have the secondary side driving into a short, but when driving into a open it's like a car's spark coil producing a high voltage with no way to get to ground, as in having no spark plug connection.

This then causes the high voltage to arc across the primary winding in search of a resistance closer to a ground potential.

Hearing sound eminating from the OT means it indeed is getting driven with audio and if it's been driven hard enough it may arc over and harm itself.

On the secondary side of the OT the start of the winding must be grounded to the chassis, either directly or by means of being solder to the shell terminal of the speaker jack which should be at ground.

[SeddoniaPhil]

Thanks for that, appreciate your knowledge. Hopefully it's not as dire as it sounds.

If you have driven high level audio signal thru a fully working amp, but with the secondary side of the OT feeding into no load ( as in a open circuit ) then there's a real good chance you have caused a arc over of the OTs primary and now it's shorted out and may not work at all, or only output very very low distorted sound.

It's ok and no harm will come to a OT if you have the secondary side driving into a short, but when driving into a open it's like a car's spark coil producing a high voltage with no way to get to ground, as in having no spark plug connection.

This then causes the high voltage to arc across the primary winding in search of a resistance closer to a ground potential.

Hearing sound eminating from the OT means it indeed is getting driven with audio and if it's been driven hard enough it may arc over and harm itself.

On the secondary side of the OT the start of the winding must be grounded to the chassis, either directly or by means of being solder to the shell terminal of the speaker jack which should be at ground.

So far in this process, the amp has not emitted anything via the speaker. No hiss, no reaction to any signal, however touching the tip of a guitar lead would elicit a buzz from the OT.

Before the test, the black OT wire was soldered to ground terminal of the speaker jack, green OT wire is soldered to the tip. Speaker was hooked up and I had verified it was working.

Following the test I drained the caps and after reading some more trouble shooting tips I checked the continuity of the OT secondaries and there's around 9 ohms resistance.

Your thoughts from here?

[Stevem]

That’s a very high resistance if there is no speaker hooked up!

This is a ball park for what a good OT should test at resistance wise on the secondary side.

OT tap four .5 ohm

Eight. .71 ohm

Sixteen 1 ohm