Hot Rod Deluxe III bright switch causing hum...?

[acorkos]

https://www.thetubestore.com/lib/the...atic-Rev-B.pdf

I installed new power supply electrolytic caps and all new tubes, everything seems fine and sounds normal until the bright switch is engaged...then a loud hum kicks in. I replaced the caps in the bright circuit (C4 and C8), no effect on the hum. Resoldered the bright switch contacts, and checked the soldering on the power supply cap feeding V2, no change. Tried different tubes in all positions, no change. I measure 1.8VDC on TP9. I welcome any suggestions on what to check out next. Thanks for your help!

Added: This may be of significance...when I first reassembled the amp after swapping in the 2 small caps, the amp played normally with bright switch on for 30-40 seconds before the hum started. Now it hums right immediately after power on/standy switch on.

[Chuck H]

Does engaging the bright switch cause hum in all channel selections?

[acorkos]

No, only on normal channel (non-master)

[Chuck H]

The bright switch in that amp acts on the cathode circuit of V2a and is switched out for the other "channels" (as far as I can tell). There looks to be a pin connection for that circuit if I'm reading the schematic right. Maybe check that connection or any possibility of a fatigued or broken board trace associated with that circuit.

[acorkos]

Well, it turns out the hum was a grounding problem caused by the shielded input and output cables on preamp tube V1, they were zip-tied to the ribbon connector on that tube with its separate PCB. While it was humming, I started poking around and when I pushed on those cables, the hum stopped. I resoldered all the PCB connections on that small V1 PCB, and tightened the zip-tie holding the shielded cables still...all is well again.