I hope you can solve a confusing problem with my Mark III rebuilds. Your records will show I bought two SDS cap boards from you, and I bought two Poseidon boards direct from DIYtube (their forum has not solved my problem). I built one Mark III with original Mark II transformers, the output being the A-430 with 8 and 16 ohm outputs, with your SDS and a Poseidon board. It works fine. The second amp I used the A-431 transformer I bought from you, and a new P-782 that I bought from Dynakit Parts, using the same boards. On startup, as soon as the tubes have warmed, I get a very loud motorboating. I have checked and rechecked my wiring and both amps are exactly the same. The first one is dead quiet with no hum, so I don't think the problem is grounding. One answer from the DIYtube forum was that the SDS board is at fault (this person seems to be very much against cap boards).
Any help and/or suggestions will be very welcome!
Please help if you can!