DId lightening strike twice? Amp s problem

Update- subbing in an old ss amp I have confirmed that it is an amplifier problem, or at least a tube amp problem.

OK, tried it w the Lightspeed passive. Hum there but no buzz. Checked outlet and it is wired correctly. Any advise on the hum?

Thx, guys. Some good ideas, w my responses below:

Almarg sez:
1)Do the interconnect cables fit so tightly that you might have damaged the RCA connections on the amps when removing or reconnecting the cables, especially the connection of the RCA ground shells?

That is a possibility. Any way to check?

2)Did you try all three positions of the ground switch on the VAC amp? (Don't change the setting of that switch while the amp is powered up).

Yes, although I did do it while powered up. No significant difference.

3)Are both the original outlet and the other outlet you tried, as well as the extension cord you used to connect to the other outlet, all 3-prong types?

All are three-prong except the cheater of course, but house is ca. 1936 so I cannot be sure that the new 3 pins were wired correctly. Boxes are metal and the exposed wiring that I can see in the basement is metal armored. Of course I cannot be sure what kind of wire feeds the outlet I am using.

4)Do you have any shorting plugs handy that you can connect to the inputs of the amps? If not, connect RCA cables to the inputs and stuff aluminum foil into the unconnected other ends of the cables, to short out the inputs. Of course, have the amps turned off when you do that, and make sure that when you turn them on nothing and no one comes in contact with the unconnected RCA plug. See if the noise still occurs when the amp inputs are shorted.

5)Just to make sure that nothing simple is being overlooked, are you sure that the amp-to-speaker connections were made properly (no shorts; connected to a proper tap and common terminal on the amps, etc.)?

Yep.

I suspect that the slight hum you originally heard with the VAC was unrelated to this problem, probably the result of either a ground loop, or heater-to-cathode leakage in one of the 6SN7's in the amp, or aging filter capacitors in the amp, or something like that that would have been brought out by the (I assume) higher efficiency of the new speakers.

I was thinking it was most likely a ground loop and when some quick tinkering (cheater plug, cable dressing, etc.) did not help that, then I tried the other amp.

Also, as you may realize, the reason for the reduction in the level of the noise that occurred as you increased the feedback setting on the VAC is undoubtedly that its gain goes down as the amount of feedback is increased.

I figured as much but as many of you know, my technical knowledge is quite limited.

Jmcgrogan sez:
I was surprised one time that simply loose locking barrel RCA's created a buzz in my system.

I've had that problem before and so may be guilty of over-tightening the RCAs, which Al points out may have contributed to the problem.

RHLjazz sez:
Pull out the amp, connect a different set of speakers with different speaker cables, and without any inputs connected to the amp listen for the buzz.

Old SS amp, same speakers, different cables, no buzz. Same amp, different speaker cables, same speakers, BUZZ!

I'm sure you would have at least another pair of speakers and spare cables to run the test.

Yep, but my back strongly suggests I exhaust other options before lugging 80 lb amp downstairs or 100 lb speakers (each) upstairs.

VAC does not buzz w inputs shorted; the same low as before but no buzz.
The Doshi still buzzing like a madman.

Not sure what that means but there it is. Can't find my multi-meter..yet.

Cables originally were Jade Audio Vermeil. Changed to an unlabeled pair; I forget the brand but not G0ertz.

Hifitime- Yes, I have a full loom of Jade Vermeil. Good deduction and good idea. When swapped in the ss amp, I used the i/c that was already connected to it. I will change the i/c and report back.

Changed 1/cs but as far as I can tell, that is not the issue. What seems to make the difference is changing the position on the ground switch on the Doshi Alazap pre-amp when paired w the VAC. The switch is not labeled, but toggled one way, loud buzz, toggled the other way just back to the moderate hummmm this whole episode started with. Maybe I'll swap my Lightspeed passive back in and what that does. I certainly don't need any pre-amp gain w the 96 dB Hornings.

Hi Al-
The Hornings are 4-7 dB more sensitive than the Ref 3As and merlins respectively so I understand that a bit more hum would be present. I'll try swapping the 6SN7 position (or more accurately, the CV-181 or whatever designation Shuguang BT uses for their 6SN7 equivalent). I'll also try the VAC label 6SN7s (presumably Chinese) again.

Finally, is heater to cathode leakage what is measured by when I check a tube for "shorts" w my Hickock tester? Is so, I can measure that directly and compare tubes.