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Old 6th June 2013, 05:56 AM   #21
famousmockingbird is offline famousmockingbird  United States
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I have basically ruled out environment and cables etc.... The amp has been on two different speaker setups with the same results.

In regards to rfengineer2013 with the B+ I have (450) so the DC elevate arrangement you suggest I believe should give me around 75vdc correct?

(30,000/180000) x 450 = 75?

Anyway I do think this is a good step, it's just I am curious to what has changed to make this channel noisier than the other. Because the other channel has the same exact topology.

All good advice so thanks for all the help! This is the stuff that keeps me up at night :-)
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Old 6th June 2013, 06:38 AM   #22
rfengineer2013 is offline rfengineer2013
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I agree - it's odd that you are hearing the hum after years of service, but I suppose it's possible that the heater-cathode coupling has increased as the tube ages (or some other mechanism).

Which turns on a light upstairs... Have you tried swapping the 12AX7 input tubes channel-channel? I'm assuming it's possible, I think you mentioned that both triodes in each tube are connected in parallel which means you probably have a 12AX7 in each channel. (Sometimes, a single 12AX7 is used with one triode section providing the input stage for each channel)

See if the hum follows the input tube, if possible, and that will tell you if something has changed in the tube as it aged.

The input tube is the one that will cause the biggest problem if this is the hum mechanism - it has the most gain, and there is another gain stage after it (the power amp) so any hum introduced in this path will be amplified to some degree.

Changing tubes is an option, but the new tube might have the same problem - hum-coupling between heater and cathode is one of those parameters that is not characterized in the tube datasheet or in most (if not all) tube testers. No way to tell until you plug it in and listen.

Having said all that, your calculation is correct. The values I gave was a SWAG, 75V might be a bit high for this exercise - I would decrease the 30k to around 15k to get about 40V of bias. 75V is getting up near the 100V max Vhk (DC) for the 12AX7.

Without seeing the schematic, I'd hazard a guess that it won't matter if you elevate all of the heaters - which will happen since the 6.3VAC is common.

Fun hum problems! It's what keeps us _all_ up at night!
-- Just an average Man of Bronze --
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Old 6th June 2013, 02:01 PM   #23
20to20 is offline 20to20  United States
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You could also just pull the drivers to see if the hum is related to them or the outputs/B+/heaters.

One other possibility is that the power tranny has a bit more mechanical hum as it gets older that is coupling to a nearby tube. The mechanical hum may still be too low for you to hear it directly but you may be able to feel it with your hand.

EDIT: It may be a bit of new magnetic coupling. Try tightening the power transformer mounting/grounding bolts to see if that reduces the hum. It could change the mechanical hum coupling profile, too. If you can find a driver tube shield, slip it on for a test.

Can you induce any noise/hum into the amp just by placing your finger close to one of the drivers? I have an amp I can do that with.

Last edited by 20to20; 6th June 2013 at 02:11 PM.
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Old 12th June 2013, 12:16 PM   #24
20to20 is offline 20to20  United States
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Any progress?
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