1978 SVT Power Problem

rhb · Newbie Alert Joined: 13 Jun 2007 Posts: 2

I recently acquired a 1978 SVT head in very nice condition. While experimenting with it, I was playing through the Ampeg into a 4 ohm EV 215-m cab and also using the pre amp out to my Fender amp. As I was playing, the sound from the EV cab started to fade and slowly died out over a few minutes. I was still getting good sound from the pre amp out to the Fender amp. I put my old power tubes back in and still had no output from the Ampeg power section(Speakers and cords are all good and tubes are new). I pulled the power amp out and looked it over to see if anything appeared burnt or corroded. Having found everything to look like new in there, I didn't do much more. I suppose I will have to meter all the resistors and caps but was wondering if there should be something specific to check out. Bias pots etc. seem to be working fine and Output tubes seem to power up coming out of stand-by. Anyone have a similar problem and solution? I could probably take it in but would rather explore it myself if possible. Thanks. RHB

rhb · Newbie Alert Joined: 13 Jun 2007 Posts: 2

Well, I found my problem. Turned out to be a bad 15Kohm, 1 W resistor (R-50) at 362 V power transformer tap supplying pins 1 and 6 of the 12DW7 control tube in the power section. It took a while to pin down because it always read 'good' when it was cold. Hopefully I'll be lucky enough that nothing else will go wrong for a while. I just want to play through it! This thing cranks! Good Luck to all you other SVT fans. RHB