HELP fixing an old 100W Plexi clone

[w437776]

So, I got this amp 2 years ago, traded a ProCo Rat clone and 6000 dinars (about 70$) for it. Great plexi tone and very loud. I think it was made somewhere in Yugoslavia, probably in the '70s. It has some weird tubes in it - 3xECC83, 2xEL503 and one ECL82. Had no problems until now... Turned it on the other day and there was no guitar sound, just a moderate hum that got a bit louder by turning the volume knob. I tried changing the preamp tubes but no good. Any ideas?

Pics:
http://i43.tinypic.com/n6rl8o.jpg
http://i39.tinypic.com/10oegrn.jpg
http://i44.tinypic.com/m79yls.jpg

[Gtr_tech]

EL503....strange choice. According to the datasheet they'll do 40w at 265v cathode biased. Not too bad.

Anyway...something is amiss in the signal path. You get output, just not signal from the preamp. Anyone who is well versed in tube amp servicing should have no problem getting it going again.