SOT: amp power supply

[brett]

Hi
This should be simple, but I'm not thinking too well today.
My amp usually has a nice smooth B+ of 500V.
The other day, the sound went very "warbly" and "Dalek".  It might be due to wobble an the B+ voltage, which has dropped to 460V with about 30V of AC on the line.
Is this problem more likely to linked to the plate and screen caps (2 x 47uF/550V) or a diode (it has the common 4 x 1N4007 bridge)?

All of these things are difficult to access in this amp, so I'd like to sort it out on the first attempt. 
(So maybe I should replace all of the electros, power and preamp, at once?).

Thanks for helping.

[R.G.]

Look at the frequency of the ripple on the DC. If it's the same as the power line, you have rectifier problems. If it's double the power line (from full wave rectification) you have loading problems.

And it's not necessarily either of the things you mention. Could be anything from one half of the PT going open to an open rectifier, to a failing filter cap or a failing insulation on the insides of the choke. Or a bum bias voltage letting your output tubes pull too much.

[brett]

Thanks RG.
I should have mentioned that there's no choke, the bias has gone low (with the voltage, I assume), and I've tried new tubes.
I hadn't thought about half the PT falling over. ($$$  )  At least it's easy to test.
Have a good day, sir.