Horrible Hum Problem - Heathkit W4 Rebuild - jacksonwest

Tubes Asylum

Questions about tubes and gear that glows. FAQ

Horrible Hum Problem - Heathkit W4 Rebuild

63.201.59.164

Okay, so I restored two Heathkit W4's (20w power-amp, push-pull 6L6). The first one worked when I got it, and after replacing all the capacitors and tubes works even better.
The second one, I wasn't so lucky. First, it didn't work when it showed up. No output signal. Plus someone else had already made some modifications. After trying to fix the original problem unsuccessfully, I went ahead and did all the work that I did on the other - changed all the capacitors and tubes (in an effort to get it working, and to achieve a matched pair of mono power amps). Now I get sound out of it - it's just as loud as the other amp. The problem is the hum...
...when I turn it on with the 1/4" input in place, I get a low level of hum. More than the other amp, certainly (which does produce a little hum). When I take the input out the hum gets very loud. In the first amp, I can remove the input without an apparent change in the level of hum (except maybe even quieter).
I have tried:
-Moving the amps to opposite circuits - doesn't seem to matter which circuit the amp is on.
-Disconnecting the ground wire from the chassis - I replaced the old power cord with a three-pronged grounded cable (like in the other amp). Removing this connection did not improve either aspect of the problem.
-Shunting the input ground - I figured maybe if I grounded the input jack's ground tab directly to the chassis, it might solve the problem when the input is removed. No luck.
-Reconnecting the output transformer/speaker ground - Just to make sure I didn't botch the connection the first time, I re-soldered the ground joint from the output to the speaker post.
-Fiddling with a hot circuit - connected a test lead to ground and probed all the known ground points to shunt them to the chassis. No improvement.
Also, sometimes I'll notice a slight scratchiness in the sound from the humming amp as I increase the input level, though it's not consistent. At one point, when driving the input level full-on (2.5V RMS), the amp stopped working - it just started hissing loudly, like really bad tape hiss. This only happenned once or twice, and I can't always repeat it. Could this point to a ground fault with one of the driver tubes, maybe? Any help would be greatly appreciated! Thanks!
Jackson West

This post is made possible by the generous support of people like you and our sponsors:

Atma-Sphere Music Systems, Inc.

Topic - Horrible Hum Problem - Heathkit W4 Rebuild - jacksonwest 16:55:27 11/08/02 (2)

Re: Horrible Hum Problem - Heathkit W4 Rebuild - steven oda 20:30:54 11/08/02 (1)

Re: Horrible Hum Problem - Heathkit W4 Rebuild - jacksonwest 10:20:32 11/11/02 (0)

You can not post to an archived thread.