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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!!!!!!!!!!!!!!
swampwalker

Showing 6 responses by almarg

almarg's avatar
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
almarg's avatar
almarg
9,670 posts
 
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
almarg's avatar
almarg
9,670 posts
 
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
almarg's avatar
almarg
9,670 posts
 
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
almarg's avatar
almarg
9,670 posts
 
Hi Michael,

Glad to hear it is shaping up to not be as bad as first feared.

Re the hum: First, how much more sensitive are the new speakers than the old ones? And did you hear any hum at all, at any distance, with the old ones? Keep in mind, btw, that if a speaker has a 4 ohm impedance and its sensitivity is defined with respect to a 2.83 volt input, you have to subtract 3 db to get the sensitivity with respect to 1 watt. For an 8 ohm speaker the sensitivity number is the same regardless of whether it is defined with respect to 2.83 volts or 1 watt.

I've found that my VAC 70/70 will produce a slight hum with my 98 db/1W/1m speakers if the 6SN7's have small amounts of heater-to-cathode leakage. If you have some other 6SN7's, give them a try. Also, try interchanging the positions of the two 6SN7's that are within each channel.

Finally, I suggest purchasing some shorting plugs, to facilitate future troubleshooting.

Best regards,
-- Al
almarg's avatar
almarg
9,670 posts
 
Is heater to cathode leakage what is measured by when I check a tube for "shorts" w my Hickock tester?
Yes. I have a Hickok 800A. On that tester there is a leakage test switch which has 5 positions, in the lower right corner of the tester. The important reading with respect to this issue is position 1, iirc. The lowest increments (highest leakage resistances) that are marked on the meter dial are 10 megohms and 5 megohms. Tubes measuring in the vicinity of 3 to 10 megohms caused significant hum in my amp, with my speakers, especially when used in the inner two positions on the amp, iirc. A good tube should not cause the meter to deflect to the 10 megohm point, meaning that the leakage resistance is greater than 10 megohms.

Best,
-- Al
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