1968 Super Reverb Blowing Fuse

[MCreech]

Hi everyone - I'm new to the group. I'm a hobbyist - been working on a 1968 Super Reverb that's blowing fuses. Only does so with the power tubes installed, and it's fine until I switch the standby. I've found several problems including bad capacitor, cracked grid resistor, and cracked power tube socket...all now repaired. The filter caps are good. Any thoughts on likely culprits next? Is it possible that a bad output transformer could be causing this? I sure appreciate any input.

[Leo_Gnardo]

Any thoughts on likely culprits next? Is it possible that a bad output transformer could be causing this?

First thought is your bias supply is kaput, or for some reason bias is not reaching the output tube sockets. With output tubes not in their sockets what voltage do you read on pins 5 of each output tube socket?

Bad OT is possible but much less likely than a bias failure.

[Justin Thomas]

Couple ideas... one, are you sure the filter caps are good? Did you check them? Also, did you try a different set of tubes? I'm thinking shorted power tube before failed bias supply, just by virtue of it blowing immediately after switching to "Play." Wouldn't it take at least some time (even if a few seconds) for the tubes to overheat & blow the fuse if they were losing bias? As in, I've never seen an amp redplate instantly.

Were the bad resistor and cap also associated with the bad socket? If so, that would still make me think "shorted tube."

Also, with Leo: bad OT? That's the LAST thing I'd expect.

I admit, I've not seen near as many amps as Leo or anyone else here, so I'm kind of thinking out loud, too... So, not trying to challenge you on this, Leo!

Justin

[MCreech]

First thought is your bias supply is kaput, or for some reason bias is not reaching the output tube sockets. With output tubes not in their sockets what voltage do you read on pins 5 of each output tube socket?

Bad OT is possible but much less likely than a bias failure.

Thanks so much for the thoughtful reply. Bias is -48V on each power tube. Schematic indicates I should have -52V so that seems pretty good.

[MCreech]

Thanks for chiming in, Justin - much appreciated! The bad resistor was indeed the 470ohm resistor on one of the power tubes - split in two. The cap was the dual capacitor coming off of v1 and v2. I did indeed check the filter caps - I have a capacitor meter and was able to check each of them. I replaced the power tubes with brand new and same issue. I also used ones out of another amp that are working fine...same result!

[Justin Thomas]

Well then, seems the "easy stuff" has been eliminated...
Any chance a loose blob of solder, wire, chunk of whatever, chafed wire shorting to chassis or anything? Exhaust EVERY possible option before the OT or PT... I'd even lift the boards to check before ordering a tranny, unless you have one to sub in on hand.

Geofex.com (R.G. Keen' s) has a really good page for testing transformers. Worth trying out. In the top left, look for "Tube Amp Debug," and it'll be in there somewhere. Actually, that's a good read for anything.

Justin

[MCreech]

Thanks again, Justin. I'll make some more runs through the chassis. Also I'll check out the reference you mentioned - really appreciate that! Hope you have a great evening!

[nevetslab]

Measure the screen resistors (with the power off), usually connected between Pin 6 and Pin 4. 470 ohm 2W is typical. Screen supply comes in on pin 6 (not used on 6L6's). If these are ok, and you have -48VDC bias on the grids (pin 5), then it could be you have a bad rectifier tube. My AB568 schematic shows a 5U4GB while later re-issue Super Reverbs show a 5AR4 rectifier. I'd suspect the rectifier tube. They don't usually blow the fuse until you're pulling current thru them. If you have a replacement pair of power tubes, and it still blows the fuse, I'd replace the rectifier tube. With the power tubes removed, you should see around 480VDC at pin 3 of the power tube sockets (or lower on the later re-issue amps). Please be careful probing in this area, as this is lethal voltage, and mis-connection of the probe can make sparks that unnerves most folk! If you're going to replace the power tubes, buy them in matched pairs.

[g1]

So you have -48V at pin 5 with power tubes out?
Then you have to check that the pin5 voltage stays same when the power tubes are installed.

[Enzo]

Are we using the proper rated fuse?

[MCreech]

Many thanks to the kind and thoughtful input. Actually replacing the rectifier tube as suggested solved it. Thanks again everyone - great forum!!