Hum issues in a EH-150/185 build

[Jerry garrcia]

Please redo the test with the tubes in.

Ok. Will do tomorrow.

So now tomorrow. Measured with tubes in and on tha las filter node (B+ 273VDC) started at 0.91VAC and slowly increased upp to 1.2V
On speaker jack 0.029 VAC.

Did some experiments also. When touching the chassis the hum decreases a bit.
Since I removed the volume pot on the instrument channel that one is always on. When plugging a guitar in the jack the hum is a bit lower but still annoyingly there. When increasing volume on mic channel the hum volume increased and some hiss. Hissing and a bit of crackling when turning the tone control. The lowest level of hum is when the tone control is maxed out, guitar in instrument channel and mic volume is turned down.

When pulling out the 6SQ7 (preamp stage of mic channel) no difference in hum. When pulling out the 6SC7 (preamp of the instrument channel and the other half the gain stage of the microphone channel. A common cathode in that tube so have a high value bypass cap so I don’t get any motor boating) the amp is totally quiet and no hum or mis-sound at all. Tried 5 different (non microphonic) 6SC7 and when inserted no hum at first but when the volume was turned it all started again.

[Jerry garrcia]

Checked the frequency of the hum. 100Hz. Redid all groundings including grounded both OT at the bus close to first filter cap. Resoldered most of the connections and redrew some of the wires that carried the signal.
Hum went away. Happy

[Stevem]

You sent it packing, good job!

[pompeiisneaks]

Checked the frequency of the hum. 100Hz. Redid all groundings including grounded both OT at the bus close to first filter cap. Resoldered most of the connections and redrew some of the wires that carried the signal.
Hum went away. Happy

Great to hear!

I do recall the same thing with mine, it's a very sensitive amp to noise and making ever effort to move the wires about etc does help a ton.

~Phil