Ampeg V4 B H Imstumped

[Smitty02]

Ive ran into a mind melter, well for me anyways I just got the schematics thank you Doctor. 90"s reissue Ampeg V4 BH came to me with blown cathode fuses n fly back 40007's I replaced these traced the schematic everything looks good PI voltages are high on 1 and 6.. but all else tests good ribbon cables and all I fired it up and checked bias ...Good -45 all power tubes B+ the 15v's and 10v's all are looking good amp worked well for about a half hour till it got up to temp and they get hot.... then a hum started kind of a buzzish hum that wasnt there before. I looked at the power tubes no red plating but the 2 right side from the front, power tubes cathode pins 1 & 8 on the only the 2 right tubes and Im am getting 250vdc on them. Ive been through all the circuit and cant find any issues other than the pi voltages being hi, it does it all the time now, even when you first lite it up... Could a filter cap cause this it looks to be power board issue I pulled the preamp ribbon and other jumpers and it still does it the left 2 power tubes have about 50v on pins 1 & 8.... Anybody have any issues like this before ?? Im kinda stumped at this point Thanks for any input,, and I can post schematics but I put them on my thumb drive and its not handy right this moment... 250v on the cathode pins ?? but only 2 tubes. Tubes all tested good and I swapped them out even. Im going to pull the tube board up again and take another look I chop stick'd it no bad joints I will do soem more checking and testing components...but so far a look good..

[g1]

Tough to guess without the schematic, but a couple thoughts. Did you check that the cathode voltages were good when you first installed the new fuses?
You have not replaced the output tubes? Are any of the cathode fuses now blown again?
Not sure how many cathode fuses there are but maybe the tubes are still bad and the fuses blew again?
An open (or blown fuse) in the cathode circuit is the only way I can see there being high DC voltage there.
Are there maybe cathode resistors for biasing that burnt?

[Jazz P Bass]

Yeah, try new power tubes.
For simple tests, you don't need to install all of them at once.

[Smitty02]

It was 2 of the fuses that went the left 2 showing 50v... I will try my own tubes this time though all voltages look good and no bad joints I did replace flyback diodes already they test ok which does seem like it is a tube or a socket issue ?? It did not blow the fuses till it was good and hot I was letting it run on the bench and testing voltages but it blew the fuses while I was NOT doing any voltage readings.. it just started humming like a bad cap, bais supply is good... Thanks guys Ill let ya's know how it goes