Hum problem on one channel

I have a hum that started quite suddenly. I'm not going to gloss over the fact that I have played around with this board and some of the tracings could be the problem.

Here's what I've tried so far for the hum on ONLY the left (facing the amp)


Rotated all the tubes both on the driver and outputs - no change

Hum goes away when I pull either the phase tube associated with that channel OR the gain tube.

Checked every connection along the heater circuit about 200 times. Reads 6.2vAC across the heater wires (seems like a 120hz hum)

Checked the plate voltages - seem normal

Checked as many connections as I could and reflowed, I believe, every connection on the board associated with that channel

Changed out the LM334 associated with that channel (I was getting 13.5v, now I'm getting 10.?v)

Checks sources in and speakers out

PLEASE help, this is starting to drive me nuts

thanks
charles

Is the hum still there when the inputs are shorted? Have you checked all possible ground connections?

Yes, the hum is there with inputs connected.

I've checked all ground points on both the gain and phase tubes effected side.

The hum diminishes just a bit with a little pressure on the heater pins 4/5 on the effected channel side. I have reflowed those connections a FEW times. Measured the heater voltages - all 6.2vAC. There is 2vDC on pin 1 of the heaters effected side. But again, the hum stops with the gain OR phase tube removed.

Pin 1 is the plate. Pin 9 is the other side of the heater.

Just to be clear on shorting the inputs, you should have the pin of the RCA connected to ground of the same jack. Not connected to something else.

Can you explain what you mean by playing around with the board? Did you happen to solder the tube sockets with tubes in them? Have you tried different tubes? Any other changes to the circuit?

I always remove tubes when soldering. Years ago I think I trashed a tube doing that.

Yes, I've rotated and tried a fun set of different driver tubes.

The circuit is as it came.

What I meant by playing around was I did try different value plate resistors. And at one time I did reverse the CCS 12au7 tubes to the LTP 12AT7 version. It was all reversed and working. Then suddenly the hum kicked in and haven't been able to get rid of it since.

It's something physical like a crapped out joint. But damn, I can't find it.

The hum is one channel and goes away with pulling the gain tube. I've checked the usual tubes, sources, etc..

So I'm thinking by a process of elimination that the the problem should be associated with something from the input, grid or cathode on the effected side of the gain tube.

If pulling the gain tube stops the hum then I'm assuming that it's not a problem in the plate circuit of the gain tube.

It could be anything from a bad solder joint, grounding point issue or short.

Does this make sense?

Email Roy at info@tubes4hifi.com