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Pulled the Tramp out after months of playing with other amps, and found the volume to be extremely low, even with all controls dimed. Put in brand new tubes (12AX7 and 6L6) with no improvement. What should I check next ?

Here are the voltages (tubes in, all controls dimed)

Mains 121.7 VAC

V1
Pin 1 : 33.1 VDC
Pin 2 : -460 mVDC
Pin 3 : 97 mVDC
Pin 4 : 3.5 VAC
Pin 5 : 3.5 VAC
Pin 6 : 31.5mVDC
Pin 7 : 412 mVDC
Pin 8 : 97 mVDC
Pin 9 : 3.5 VAC

V2
Pin 1 : 0
Pin 2 : 3.5 VAC
Pin 3 : 45.5 VDC
Pin 4 : 44.4 VDC
Pin 5 : 1.1 mVDC
Pin 6 : 1.1 mVDC
Pin 7 : 3.5 VAC
Pin 8 : 2.6 VDC

B+ (at first 50mF cap) is only 45V

VRM is not working, thought even while not working I was getting full volume earlier.

Your voltages are all super low. So yeah my guess is VRM isn't working.

Check your first filter cap before the VRM, it should have 400V or so.

Check all the component values (to make sure they are in spec and not blown) for the VRM.

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http://pdfelectronics.com

THE VRM was not working earlier, but volume was at max (would not drop when the VRM was turned down).
Any theory on why suddenly the volume could have dropped?

Most likely, this is directly related to the original issue with the VRM, IMHO. If there was a faulty component, that component may have either totally failed or it may have caused another component to fail. Similar story if the VRM wasn't set up correctly or if there was a weak connection or ground.....the connection or a component may have failed, either independently or as a result.
First check would definitely be to do as kurtlives suggested...check the voltage at the first filter cap (C13). After that, I'd check my ground connections as well as look for any possible shorts like stray wire strands or a bent lead. Next I'd try taking the VRM out of the circuit. You should be able to bypass the VRM by running a jumper between R23 and the + side of C12. <----AMP GURUS PLEASE CHIME IN HERE IN CASE I'M OFF BASE. If that fixes it, the problem's all in the VRM. If not, there's something else further downstream.

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You can short out the VRM (back side of R23 to + side of C12 or try this:

If the VRM is doing nothing it's good to check the Zener diode out. Use a continuity tester and see if it conducts. If it does conduct it's bad.
If the VRM wont pass voltage check the 5W 100R resistor. You can measure it in the circuit to see if it reads correctly or it has opened up.
Other than triple checking all your wiring, jumpers and component values then suspect the MOSFET.

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Stephen
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