Need Help - Ran Amp with No Load

[SBax]

Thanks so much for your help guys. This happpened 4 years ago and I couldn't bear to even open it up since that time.

All tubes out (except Weber copper cap rect) - fuse blows.
Rect out - fuse ok.
Screen resist both measure very close to the 470 ohms.
No charring on tube socket.
Pin 3 to chassis measures approx 3M on both.

[Leo_Gnardo]

<This happpened 4 years ago and I couldn't bear to even open it up since that time.>

How well I know that feeling...

<All tubes out (except Weber copper cap rect) - fuse blows.
Rect out - fuse ok.
Screen resist both measure very close to the 470 ohms.
No charring on tube socket.
Pin 3 to chassis measures approx 3M on both.>

This leaves 2 major suspects. CopperCap rectifier may be fried. You could substitute any common rectifier tube, or tack in some 1n4007 rectifiers on the rectifier socket in place of a tube rect. With amp plugged into your "Light-Bulb Limiter" start it up. If the lamp lights and stays lit, then the fickle finger of fate is pointed at your OT.

Sometimes an arc, whether on a tube socket, inside a transformer, or anywhere else, leaves an incomplete carbon track. You may measure "open circuit" with an ohm meter, but when high voltage is applied, current follows that track and jumps where you don't want it to go. If your OT is suspect, you can tack in a known working one as a substitute. It doesn't have to be the exact same. If your amp starts to work at all after that, without blowing a fuse or other distress, you'll be shopping for a replacement OT. And you'll be doing your best to not make that same mistake again, right?

[SBax]

It looks like the bias cap is fried.

[Leo_Gnardo]

<It looks like the bias cap is fried.>

Yes it looks like one of those caps is coated with smoke residue. It may be a good idea to replace both in case they were damaged in this event, cheap enough. Double check the values of resistors in the bias circuit & you might even replace the bias rectifier, another inexpensive part. BUT it still doesn't answer the musical question "why does the fuse blow only when the rectifier 'CopperCap' is in place, but output tubes aren't?" Once you are sure you have a working bias circuit, measure the voltage it sends to pin 5 of each output tube. Should be -50VDC more or less. Then let's see what happens with a substitute rectifier, with output tubes removed.

[SBax]

Update: I took off the power supply cap cover and found the cover charred and a strong burnt smell. I will replace all power caps and bias circuit.

[Chuck H]

Be certain to inspect the board for charring. Charring on circuit boards can act like a resistor that isn't suppose to be there! If your the type that never cuts corners you'll need to replace a charred board. If you DO cut corners where logical you can sometimes cut out the charred portion and jumper wire the contacts as needed. This could be considered a hack, pretty much. But it can make for a perfectly functional repair in many cases.

[SBax]

Update: I replaced the whole bias circuit (except pot) and replaced all the caps and resistors in the doghouse. The amps works perfectly now. I tested the parts and except for the first 100 uF, 350 V cap, everything tested ok on my Radio Shack 22-181A meter.

True RMS Multimeter

The first cap in question was a Nippon Chemical cap. There were two 100 uF, 350 V in series and the first one on the B+ line charged slower than the second cap. These were very, very large caps compared to the cheap 100-350V replacements I just put in. See photo in post 20 for the original black cap. The lead going into this first B+ cap in question looked...off. Right where the lead enters the cap looked grainy...like there was some welding going on... heh, heh.

Since I may have replaced a bunch of good parts for no reason, I will guess as to what really happened. The B+ line massively overheated. The burnt bias cap in post 18 is sitting right on the B+ connection on the board. The board uses hand rolled eyelets so there's a good blob of solder right there next to the apparently fried bias cap (that tested good too). Maybe the outside plastic casing charred from the B+ line connection there?

See red circle for 2 B+ connections on card

And, there was a lot of charring (doghouse photo in post 20) everywhere around where the B+ to first cap solder connection eyelet.

See red square

It even fried down to the chassis where I saw a weld mark into the chassis. I think there was a B+ short onto the chassis from carbon arcing at that spot that was instantly blowing the fuses. I re-worked the area, replaced grommets, clean all the carbon off the circuit card (using a Magic Eraser) and doghouse.

I really pushed the wife's patience and closed it up. I should have added on a separate B+ fuse in hindsight. I need to work on the preamp voicing sometime in the future and may add the fuse then.

Thanks for all your comments guys.

[big_teee]

Great Thread, I just read it end to end.
If you or anyone else is still needing the Dim Bulb tester, here's some to look at.
I love mine and use it for many things. Mine has 2 bulbs, wired in a parallel series design, if that makes sense.
You can use it with one or both bulbs.
dim bulb tester - Google Search

[Justin Thomas]

Thanks Terry!

Bookmarked - "BUILD THIS!!!"

Justin