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DId lightening strike twice? Amp s problem

OK, so I'm pretty frustrated 'cause I'm not sure I believe that two amps can apparently fail (same symptoms) minutes apart. Just got a pair of Hornings from Trelja and they sound great, but I had a bit of a low hum that was just barely audible at the listening position w/o any music playing. Amp is a VAC Ren 30/30 MKIII. I check cabling, try lifting ground on the amp, plug it into my Dodd power conditioner, then direct to the wall, swap out power cords, no help. So I sez to myself, we'll see how the Hornings sound w a little EL-34 magic, so I swap in my Doshi-modified Lectron JH-50. As soon as the start-up relay closes, I get a very loud buzz!! Not a hum but a loud buzz, increasing in volume for a few seconds after the relay closes until I get to the power switch. Figuring that maybe something happened while the amp has been sitting idle for about a year, I swallow about 6 advil, disconnect the pre and the source components, shut down all of the computers, sacrifice a goat and swap the VAC back in (man, I thought that the iron on the Lectron was heavy; the VAC should come w a crane!) Hook it back up, power up and SAME F'N BUZZ. W the VAC it's a little different; you begin to hear it at very low volume while the start-up sequence is finishing, gradually ramping up until the relay closes and then WHAM. Same in both channels. The VAC has variable feedback via a rotary switch on the rear apron and if I increase the feedback, the volume of the buzz does come down a bit, but its still so loud you could never listen to music and I'm afraid it will damage the speakers. I've tried running an extension from another outlet, I've tried running each amp from the conditioner and from the wall. I disconnected the cable box from the power and the cable (TV in same room, but not interconnected to main rig). I've swapped out i/cs. I've tried it w and w/o ics connected to the amp's input(yes w speakers hooked up...I made that mistake once, years ago and learned my lesson!). Same thing. HELP!!!!!!!!!!!!!!
128x128swampwalker
swampwalker's avatar
swampwalker OP
5,138 posts
 
Update- subbing in an old ss amp I have confirmed that it is an amplifier problem, or at least a tube amp problem.
almarg's avatar
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almarg
9,670 posts
 
Sorry to hear of that, Michael. And, yes, that is all pretty baffling. I have no particularly promising thoughts to offer, but fwiw:

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?

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).

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?

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.)?

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.

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.

Best regards,
-- Al
jmcgrogan2's avatar
jmcgrogan2
6,346 posts
 
Yes, do what Al says. I was surprised one time that simply loose locking barrel RCA's created a buzz in my system. I spent a couple of days changing tubes until I realized it was loose barrels. SS gear also did not display the buzz. I think tube equipment is more sensitive to buzz issues.
Good luck,
John
rhljazz
1,253 posts
 
I'd be inclined to separate the amp/amps from the system to test individually, basically the concept of divide and conquer. 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. I'm sure you would have at least another pair of speakers and spare cables to run the test. I'm betting it is not the amp/amps.
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