Hello everyone.
I managed to complete the restoration of a Heathkit A7-E. I recovered many components from the original amp(metal work, wiring etc. were terrible and not original heatkit parts) and assembled them in a new hammond steel chassis. The only problem I encountered is a too loud 60 cycle hum - please consider the following:
Please also consider the following;
Does anyone have an idea why there is so much hum?
In addition, I started thinking - everything considered - that there is one ground wire (the only one grounded to the volume pot case rather than directly to the chassis) which runs close to the heater supply wires and this could pick up all the noise and injecting it directly to the volume pot - please look at the pictures below (specifically the fourth one with a red circle and arrow). Is this plausible?
I managed to complete the restoration of a Heathkit A7-E. I recovered many components from the original amp(metal work, wiring etc. were terrible and not original heatkit parts) and assembled them in a new hammond steel chassis. The only problem I encountered is a too loud 60 cycle hum - please consider the following:
- the hum responds to the volume control
- the hum is present whether one uses the high gain input or normal gain input
Please also consider the following;
- adding a virtual center tap (using two 100ohm resistors) to the 6,3V OPT heater supply slightly reduced the hum but it is still too loud
- covering the 6SL7, 6SN7, 6SQ7 tubes with a shielding does not alter the hum
- tapping the glass of the 6SL7, 6SN7, 6SQ7 tubes with a wooden stick does not alter the hum (so probably none of the tubes are microphonic)
- multiple star grounding points have been used to avoid grounding loops (I will double check again as soon as possible)
- all heater wirings have been twisted closely and run at the edges of the chassis
- OPT and PT are a little bit close to each other but the OPT is a heavy potted one and the PT is shifted 90°
- shielded wires have been extensively used (grounding only one end of the shields)
Does anyone have an idea why there is so much hum?
In addition, I started thinking - everything considered - that there is one ground wire (the only one grounded to the volume pot case rather than directly to the chassis) which runs close to the heater supply wires and this could pick up all the noise and injecting it directly to the volume pot - please look at the pictures below (specifically the fourth one with a red circle and arrow). Is this plausible?
Attachments
Disconnect the pot's case ground wire from the pot's audio ground terminal. Run a separate case ground wire to a chassis ground nearby. Also use a separate ground wire for the pot's audio ground terminal,to the B tube's local common (if it is not already connected there).
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That's how the tube is labeled on the schematic. It's the tube just after the volume control. Even if this doesn't get rid of all of the hum, it's the way the wiring should be done. Also, the wiring to and from the volume control should both be either twisted or shielded.
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I just disconnected the pot's case ground wire form the pot's audio ground terminal and grounded the pot's audio terminal to one of the ground point directly to the chassis (not sure if with "B tube's local common you mean pin 1 of it). The hum has been slightly reduced but still too high.
I will try use shielded wire also for the volume control. Idk why is the only one I wired without shielded cable
Here is a short video (YouTube).
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No the hum is still present with the volume at zero. And what you mean with "shorted inputs"?
Seems that there are two different noises one which depends by the volume pot (it has been discretely reduced I think by rewiring all the filaments and it start increasing at almost the maximum volume) and a second one which does not depend by the volume pot.
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