"MM54" (mm54mk2)
12/01/2018 at 21:38 • Filed to: TUBELOPNIK
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"MM54" (mm54mk2)
12/01/2018 at 21:38 • Filed to: TUBELOPNIK | 4
| 3 |
Hi-fi tube amp came to me for a hum on the left channel; after some poking around it was pretty apparent that one of the cathode bypass caps let the smoke out.
Either a tube failed and shorted, or it was roasted to death from the resistor it’s piggybacking
Humorously, I had the exact same part on-hand.
Less humorously, this amp is set up for old KT77s rated at 30+ watts and is cathode-bias class A (push-pull) so it’s trying to push 35W/tube on current wall voltage. This isn’t jiving with the new-production JJ KT77s in there (rated at 25W) at all, so I’ve just ordered some new cathode resistors to bias it back to 25 or so watts so it doesn’t eat tubes weekly .
I’m also going to move the bypass caps further from those cathode resistors when I have it apart so they don’t get cooked again.
This is reminding me how I’ve been meaning to
build myself a hi-fi set for a long time but have never gotten around to it. I have a
design I did
years ago... somewhere.
MiniGTI - now with XJ6
> MM54
12/02/2018 at 04:40 |
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I have a Conrad-Johnson preamp in the closet. I was using it for digitizing records but hum (along with dodgy switch contacts) started to become a problem. Wish I knew enough and had a reason to fix it up
Future Heap Owner
> MM54
12/02/2018 at 04:41 |
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Man I wish I understood electricity beyond just pretending it’s binary.
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MM54
> MiniGTI - now with XJ6
12/02/2018 at 10:18 |
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That’s a pretty nice piece of gear; the switch contacts probably need cleaned if they’re noisy
(I like Caig Deoxit). The hum could be a few things, if it’s on both channels I’d look at the power supply (filter caps dry out with age) and if it’s just on one I would see if it follows the tubes (i.e. put the left channel’s tubes in the sockets for the right and vice versa). If it does, it’s a tube - if it doesn’t, it’s in the circuit and needs some real diagnosis.