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RE: HELP: DC at the input of an old MA1's.

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Thanks Lew!

I’m owning these amps since 2003 and use a dCS Elgar Plus as a Preamp! From the beginning on, I have some trouble with the Dac-Wadia combination when firing op the old MA1’s (from stand-by to filament warm-up). Not always, but sometimes, the Dac loses track with the Wadia drive and needs to be resetted (sometimes also the Wadia). There is nothing on the speakers at this moment, because I wait always 10 minutes or longer to start the High voltage. The system plays normal after that. I never give it a thought, but now I had some time and was curious and so I disconnect all items from the MA1’s. On the speaker terminals I did connect a 10 Ohm resistor and two Multimeters on “record” (Fluke 87III) to the XLR’s Ground-Hot and Ground-Cold. Everything ok with the AC. The DC however:
From Standby (first red on (1)) to filament heating (second led on(2))
After 10 seconds the meters give about 90 – 110 Vdc negative on both inputs!
This remains 1 or two seconds and goes rapidly to zero Vdc.
After switching to the third stage (three reds led on), there was nothing strange on the meters.
When pressing the switch to preheating (2), the voltage stays 0 Vdc, but after switching to standby (1), the voltage begins (after a few seconds and this time slower) peaking to 90-110 Vdc (-) again, stays for 1-2 seconds and drop to zero! Here is the Holland we have 230 VAC but that should have no influence!! I switched Tubes but the effect stays! And it concerns both amplifiers!!! I do have the older MA1’s with three driver tubes: 6DJ8 (E88CC), then 12AT7 (Sylvania 1960 gold brand) and at last the 6CG7 (Mazda).
I suspect that the B+/B- supplies as you mentioned, are not coming up, when switched to (2) because you can’t hear nothing from the speakers (Avalon Opus).
The input resistance is also a little on the high side (500K), it should be more like 600 Ohm I should think. To protect the driving device(s), I can always put some Coupling caps in, but should not be necessary.

Thanks in advance..
Regards Robert


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