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I just completed rebuilding two Mark IIIs with the Poseidon boards and the SDS cap boards. The first one works fine - no hum, etc. When I turned on the second one, there was a small amount of hum. I found a loose solder joint in a ground connection and repaired it. Turned on and as it warmed up, I got very loud "motorboating". It's been too many years for me to remember what causes motorboating other than it has to do with the power supply. All help will be very welcome!

There probably is a ground loop somewhere. In my experience, what happens when a ground wire is not connected properly, is nasty hum and at worst, a motorboating type oscillation.

Well, I have checked and double checked the grounding and have compared it wire by wire with the working Mark III. Any other ideas? I am really stumped.

I just read on another forum that occasionally different speakers can cause motorboating. I am using one original A-430 output transformer from a Mark II with 8 & 16 ohm outputs (this is the amp that is working) and a new A-431 from Triode on the motorboating amp. Any clues here?

Each transformer is different. One due to inductive properties may cause a motorboat condition and the other may not. I do not know about the cap boards but the original MKIII had called for wiring a ground loop because the main filter cap shared a common ground since they were all in a multi-section can. To eliminate the resulting low level hum, I was forced to install a separate main filter cap.

The cap before the choke must have its own path to ground, or at the very least, you will have a hum. At the worst, you will get a motorboat condition. I never bought the cap boards because I do not trust them (proper grounding was my main concern). I would check the PCB construction to see if the designers routed it properly. If they did not, demand a refund and that they revise the circuit.

Therefore in all my new designs, I never use multi-section caps due to cost and ground loop potential. When I do PCBs, I put the main star point on a large copper pour on the board or even better, route each ground to a terminal strip lug that connects to the chassis.

Thanks for the informative response! So you are saying that the difference in output transformers, although electrically similar, could be the cause, even though the first amp's SDS board is completely quiet? What would my easiest and least expensive solution - use a new quad cap in the motorboating amp? If so, I assume it would be best to use one of the new, higher capacitance quad caps? Thanks again for your help!

1) Double-check the input RCA connector wiring (e.g. see if + and - are swapped on the motorboating one).
2) Double-check all the solder joints on the SDS board. Verify all voltages.
3) Disconnect feedback on the Poseidon board. Verify all voltages.

Hope this helps, and good luck!

Shannon

Does the motorboating amp have a cap board, a multisection cap, or both? That cap board is supposed to take the place of the multisection. If you have both hooked up, do not use the quad cap. Judging from the writing, it seems like you may have both hooked up.

Problem solved! Roy with Tubes4hifi suggested that I might have the polarity reversed on the input side of the OPT - that was the case and now all is well!
Thanks to all of your help!

I might have the polarity reversed on the input side of the OPT

Oops

I do that all the time (freshman mistakes made by seniors ). Before I connect the feedback I always check with the scope to see if the phasing is proper. When the polarity is reversed, the output is 180 degrees out of phase and the feedback becomes positive. Not good.

Glad you found the error.

Classic mistake. Congrats and a big thanks to Roy Mottram, who has rehabbed more than a few MKIIIs in his day. Now enjoy some music.

Shannon

OK, all is not well. When I solved the previous problem, I was using test speakers with the inputs shorted. Today, I connected them to my system, and the first one (with the original transformers) sounds great. BUT, the second one. that I had the reversed OPT leads, is very low volume and distorted. I swapped tubes between them just in case, with no improvement.
Again, any and all suggestions/solutions will be very much appreciated.

Well, I am a little red faced, but the solution is so bizarre, that I will share. One tab on the slide power switch had too much solder and created a tiny gap between the tab and the metal body of the switch which is of course grounded. Now, all really is well and sounding great!

If there was a little gap there, I'm surprised it did not arc. The excessive solder seemed to act more like a resistor than a spark gap.

Well, I am back again - my ears must be playing tricks on me. The second one is NOT right - still very low, distorted output with no high frequencies. No hum, no motorboating, just low, distorted output. I am a kit builder, not an engineer, so I am stumped. Again, tubes swapped between amps with no change. I cannot see any difference in the Poseidon boards, and have checked the voltages (at least some of them) but not every pin of every tube. Where do I go from here?