Troubleshooting a set of Triode mkIIIs

for Dynaco Mark II/III/IV and DIY PP monoblocks

Troubleshooting a set of Triode mkIIIs

Postby highflyin9 » Mon Mar 19, 2012 5:19 pm

I've finished a build for a set of Dynaco mkIIIs using the SDS Labs cap board and the Triode Electronics board (I know, I know, I kinda just wanted a one stop shop for all the pieces, I've bought three boards from Shannon so far so I'm still a DIYTuber ;)

Anyway, I followed all the directions as accurately as possible, checked all the resistors on the meter before installing, re-walked through all the steps 3-4 times, but they just don't work. Once power is applied, the tubes warm up, and after about 10 seconds a sputtering sound goes to the speaker (loud, then nothing, then loud, then nothing). Both amps do exactly the same thing. I've tried new driver tubes and Chad and I have been going back and forth at Triode but no luck.

I was hoping someone here familiar with the mkIII might be able to take a look and see if something sticks out to them.

Many thanks in advance, I'm really at a loss.

Initial Top-down photo before the ground wires on the SDS board and the front light were wired up.
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Amp 1
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Amp 2
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Postby dcgillespie » Mon Mar 19, 2012 8:11 pm

High -- Remove the driver tubes and see if the noise is still there with them removed. If not, reinstall them and disconnect the NFB connection from the OPT. If the amplifier is again stable, it is likely the phase of the push-pull signal has been reversed somewhere along the line. You can correct this by swapping both OPT leads of each output tube with the other -- or by swapping the to drive signals to the output stage from the driver board.

If this doesn't correct the issue, it will at least remove this condition from the list of possibilities.

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Postby highflyin9 » Tue Mar 20, 2012 5:19 pm

You were absolutely correct Dave, the two power tubes were reversed on pin 1 and 3, flip-flopping the wires did the trick. Thanks for your insight!!
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