Amp Troubles

[Structo]

Hi Guys,

Today I was measuring voltages inside my 100w ODS clone and specifically
I wanted to check the power tube plate voltage and the screen voltage.

As I was probing for pin 3 I slipped and a big spark arced, bang!
NOOOO!!!!

So now when I turn on the amp I get a loud 60Hz buzz.

I should know by now to clip on the test leads rather than probing those high voltages.

So on to the troubleshooting.
I first swapped out the tubes to eliminate those.

Still a loud buzz.

I found when I pulled V3 the P.I. tube that the noise went away, dead silent without that tube.
I measured a few more things but didn't see anything wrong yet.

So I decided to take a break and took the wife to the grocery store.

I got to thinking what could have been damaged when I shorted that plate to the heater.

This amp has 100R ohm resistors for an artificial center tap on the heater circuit.
So I put the ohmmeter on those and they are open.
No continuity, nada, fried.

So I will hunt through my parts drawers and see what I have.

I measured 6.8vac before which is a touch high.
It's within the 10% tolerance for heater supplies.
But the closer to 6.3vac you are the longer the tubes are going to last.

[Structo]

So I replaced the two 100R resistors and the amp is back to being glorious.

[CHIP]

Good to hear everything is fine.

[dcribbs1412]

Nice save Tom...
I gasp every time I slip checking live voltages...clips are safer.
Just had my 50 watt(dlite) blow a fuse(s) the other day...
checked the caps and 1 was bad (F&T 500V)...not sure what happen
but had a replacement handy and bang...back in business...feels good to
find a problem and fix it fairly quick.

Darin

[Bob-I]

Glad you found the problem. Probing around inside an amp can cause a ton of little big mistakes.

[Bob-I]

Glad you found the problem. Probing around inside an amp can cause a ton of little big mistakes.

[Gaz]

Wonder what would've happened if you had used the PT's center-tap... I don't think I wanna know. You're one lucky man.

[sonicmojo]

I've had a spark before on a pin3 slip but with no damage, luckily, but it zapped the test lead wire right out of the meter socket. I'll take your advice and clip to test from now on. Not worth the pain for sure. Thanks for the reminder and I'm glad it worked out for you fairly easily.

[talbany]

Try shorting pin's 2 and 3 for an instant then get out the 100 ohmers.. ..A perfect example of where the false CT on the filaments comes in handy..

Tony

[Structo]

Wonder what would've happened if you had used the PT's center-tap... I don't think I wanna know. You're one lucky man.

Well as luck would have it, the Magnetic Components Twin power transformer doesn't have a
center tap on the heater winding.
And I guess I was lucky that it doesn't.
I'm kind of surprised the fuse didn't blow.
But I guess it wasn't shorted long enough, but long enough for the resistors to blow.

So I had a couple small 100R connected to pins 2 and 7 on the middle power tubes like the #124 layout shows.
I can't remember what the wattage of them was but they were small.

When I first glanced at them I didn't see anything wrong.
But when I looked closer one had a tiny burnt spot.
When I metered them, both of them were open.
This was the first time I have seen these resistors in action.
Made a believer out of me.

Plus it is said that it is a better center tap because on most heater windings
the center tap isn't balanced that great.

The only better alternative is the Humdinger that Fender used on some of the amps
which is a 500R pot across the heater wires with the wiper connected to ground.
I'm not sure how well that would protect the amp with a shorted power tube.

[Structo]

Nice save Tom...
I gasp every time I slip checking live voltages...clips are safer.
Just had my 50 watt(dlite) blow a fuse(s) the other day...
checked the caps and 1 was bad (F&T 500V)...not sure what happen
but had a replacement handy and bang...back in business...feels good to
find a problem and fix it fairly quick.

Darin

Say Darin,
How did you discover the F&T cap was bad.
Did the cap actually short to ground?

When I was metering my amp I measured the AC content (ripple) on my filter caps and I was getting around 32mvac on the B+ nodes.

I wasn't sure how much ripple is accepted on the power supply.


Here are the two resistors that saved my bacon.

[gary sanders]

My shaky right hand is what "sparked" the birth of my 124.I was testing voltage in the 15v op amp supply for switching problems.I slipped and popped almost every IC in it.Then the Carvin became an ODS.

[butwhatif]

Make sure you clean the socket of carbon before using it, or it will keep on happening and get worse. Not a good idea to mess w/ hv unless something is indeed wrong, shit always happens. If u blew the bal resistors either the socket and/or the tube took a hit also.