Ok RG lets give this a try,
Normally in the electronics repair game I use the divide and conquer approach at any point, at anywhere in the circuit and continually do so throughout the process of investigation, this approach provides the fastest analysis when applied, your gonna have to slug it out here with us here so here we go.
Please see the new attachment I have posted. We shall start fron the power side first ok, then we'll move on, be very careful as we are working on live voltage.
First of all, Pull out 12ax7 V4 from the socket.
Secondly, attach your black multimeter probe to chassis ground/earth somewhere and make sure its secure, now you are only using the red probe for safety.
Test 1. Set your multimeter to the highest AC range, usually 500/600V AC on your meter depending on what make it is. (You may have Auto range function built in to your meter, but still make sure your on AC).
With the amp turned on and everything turned down to zero, test the voltage on the F1 fuse, any side will do and tell me what you read.
Test 2. Set your multimeter to the highest DC range, away up as high as it goes (or else Autorange DC), again, it should be 500/600VDC. Now measure the voltages on pin 1 of the V4 socket. (Looking at the socket from the rear side we count the pins in the normal clockwise direction otherwise from the top side where we push the valve in we count in the counter/anti clockwise direction). So tell us what DC voltage you get on pin 1.
Test 3. Set your meter to a lower DC range and tell me what you get on pins 4 and 5, (they are not drawn in on the schem, but you know the deal how to do it now) normally you should see about 12.6 V on either one of those pins with the valve pulled out.
Tell me what you get for test 1, 2 and 3.
It shouldn't take long at all, and from there I'll tell you what to do ok.
Rusty.