Help, blowing 100 ohm resistors in heater supply

[Stanz]

I was oh so close, then crap, now it seems like 5 steps back.

ODS HRM built into a Quad Reverb Head. I had the amp up and running, sounded fantastic. The one last item was to install one of KOC's power scaling circuits. Had a few issues with that but got the first part which is the scaling part, no attenuation installed though at that time (master volume). Scales fine, but of course over drives the power section too much, which is why I need the master volume to compensate. Was planning on installing a bootstrapped MV, basically replacing the 1M resistors in the PI with a dual ganged 1M pot and adding grid resistors (15K) into PI tube.

So that was the last step before completing everything and the "pinnicle" of my progress. It has been all down hill from there. The last step was the MV, but it produced no sound at all when fired up. Here is where it gets weird. Without tubes in the amp, all voltages are good, scaling reduces B1 to OT, screen, and bias voltages as it should. When tubes are put in, the two 100 ohm resistors tied from the light to chassis blow. (man they stink, these were the original carbon comps). Fuse does NOT blow. Remove tubes, install two 2W 100 ohm resistors because that is what I have, recheck everything, no shorts that I can see (doesn't mean there aren't any), all voltages fine, including scaling and heater supply. I also double check MV wiring, checks out fine. Put tubes back in, same thing, it blows the two 100 ohm resistors. Remove tubes, voltages are fine and no fuse blown. CRAP!

Making changes and firing it up to see whether or not the resistors blow is not what I would consider a good troubleshooting path. Big concern is transformers. The scaling worked, but it screwed up after putting in the MV. Hard to say that caused a problem with the heater supply, but, it was the change before it had problems.

1) Any ideas what the cause would be or things to consider? Tubes? Another higher voltage supply getting into the heater supply, but why not when the tubes are in?

2) What is an easy way to check both transformers?

[heisthl]

You either have a bad output tube or you may be off a pin, Some output tube sockets (usually the plastic ones) even though they have a keyway will still let you plug a tube in wrong. If you are off by one pin the resistors burn.

[Funkalicousgroove]

It is very possible that you have a carbon trace between pins 2 and 3 on one of your sockets, sometimes they are actually down inside the hole and through the wall of the socket-

You may have a teeny strand of wire, or perhaps even steel wool swarf between 2 and 3 on one of your sockets. Since the voltage will choose the path of least resistance to ground, sometimes that means through the air(or a wall of bakelite plastic) and through the 100 ohm resistors to ground.

[Funkalicousgroove]

One more thought, the short between pin 2 and 3 could be one of your tubes- have you tried other sets?

[Stanz]

Thanks Henry and Brandon.

Tubes was the first thought. One rule I have when trouble shooting is being able to predict or make the problem happen on purpose. If the problem happens only when the tubes are in, well, hmmmm. That is also why I suspect it is not something loose causing a short, at least outside of the pin holes, like a wire touching, because it looks good when there are no tubes and power is applied.

I will recheck the orientation of the tubes as well as for possible junk in the pin holes. Somethings you take for granted. These are the tubes that came with the amp and I do have a fresh set just for this amp.

I was pretty much ready to start tossing things last night, so it helps to get a few opinions to get back on track. Sounds like everyone is sure it is a voltage from somewhere else invading the heater supply. My main concern is proceeding with troubleshooting and messing something else up, so thanks guys.

[Funkalicousgroove]

If the problem is a carbon trace It may not arc until it's under load, fire it up with the lights off and see if you can tell what's arcing-

[Stanz]

If the problem is a carbon trace It may not arc until it's under load, fire it up with the lights off and see if you can tell what's arcing-

Good to know.

Preliminary findings, found one tube with about 500k between pins 2 & 3, then 1.5R with a little pressure on it. AH HA! I kept seeing pins 2 & 3 referenced so I started there. So it is off to the local parts store for a handful of 100 ohm resistors, a recheck of the other items on the list, install resistors and new tubes, and hopefully my wife will is telling me to turn it down.


Brian

[Funkalicousgroove]

if you have anything but an infiniti reading between 2&3 replace the socket before you burn up any more resistors-

[Stanz]

The short between pins 2 & 3 was in one the tubes, not the socket. Double checked everything, put in the new set of tubes and it fired up fine.

Way cool.

Thanks.