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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
almarg's avatar
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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
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swampwalker OP
5,138 posts
 
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.
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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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