DId lightening strike twice? Amp s problem

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.

Any way to check?

If you have a multimeter, set the ground switch on the VAC to the DC position, and with the amp's inputs and AC power disconnected, check for continuity between each of the RCA ground sleeves and the chassis. The AC safety ground pin on the IEC connector (or on the plug at the end of the power cord, if it is connected to the amp) can be used as a connection point to the chassis, as can any exposed chassis screws.

Then set the ground switch to either of the other two positions, and verify that continuity is no longer present.

Also, of course, make sure that the RCA jacks, and the nuts that secure them to the rear panel, don't feel loose. And check that the nuts are centered within the white circle that immediately surrounds them, and that the jacks have not slipped out of position to the point that the nuts contact any of the metal that is adjacent to the white circle.

Best regards,
-- Al

Another question: By any chance have any or all of the speaker cables you have tried been types having ultra-high capacitance, such as Goertz?

Best,
-- Al

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.

Try unplugging the TV and everything else in sight that could conceivably be generating and radiating RFI/EMI into the amps. The fact that the VAC buzzes when the inputs are unconnected, but does not buzz when the inputs are shorted, may be an indication that the unconnected inputs are picking up some kind of interference that is radiating from some other electronic device. And perhaps the Doshi is picking up the same RFI/EMI at an internal circuit point that is not affected by having its inputs shorted.

Also, if you have a portable battery powered AM radio, set it to an unused frequency near the bottom of the band (e.g., 540 kHz), and if it picks up any audible interference that gets louder and lower at various places in the room, use it to track down the source.

Also, when you find the multimeter run some checks with it on the AC outlets. Set it to measure AC volts, on one of the higher voltage scales. You should measure approximately 120 volts between the two vertical slots in the outlet. You should also measure approximately 120 volts between the smaller of those slots and the screw that holds the wallplate on, and between that smaller slot and the safety ground connection in the receptacle. You should measure close to zero volts between the larger of those slots and the screw that holds the wallplate on, and between that larger slot and the safety ground connection in the receptacle.

Best,
-- Al