Oops!Shorted something!

[ajeffcote]

While setting the bias on my Fender DRRI,with a brand new set of JJ 6V6S power tubes,I let the two leads on my bias probe get together,while it was powered up.Before that happened I read 450V on pin 3 of the tube I had connected to(I'm using a single probe).Afterwards I read a negative voltage,I think it was -24 or -25V on the same pin.I thought maybe it was only the tube I had trashed,but the original tubes read the same.That pin is fed by the output transformer.So did I short a winding?The forth pin was unaffected and still reads around 450V.The other tube/transfomer feed was unaffected.I'm going to put the old OT back in later today(yes,this was a brand new OT)and try it out.If that does not work where do I look next?I put it back together and played through it for an hour last night,and it really doesn't sound very different,maybe a little harsher.But that was only low volume after my wife and son were in bed.
Thanks,Aubrey

[Don Moose]

Sounds like the new OT is toast. You could verify that by measuring the resistance from the primary center tap (where the B+ voltage goes into the OT) to that plate lead. I expect it to be high compared to the resistance from CT to the other plate lead.

If that is the case, you have a new paperweight. Sorry.

Hope this helps!

[ajeffcote]

I don't know what is going on,but I just fired it up and checked voltage on all the pins of the rectifier and both output tubes and everything looks OK.The rec has 343VAC coming in and 437VDC going out.The putput tubes read 434VDC on both 3 + 4.#2 reads -1.1,#5 reads -37.8,#7 is -1.1,#8 is -.2.#'s 1 + 6 are both fluctuating between 2 to 6V.
The PT lead that I thought I had shorted is reading a lower impedance,about 117 Ohms than the other lead which is showing 134 Ohms.I better leave well enough alone.Thanks

[Don Moose]

Well, to be safe, make sure you still have a VERY high resistance betwen the suspect primary plate lead and ALL of the secondary speaker leads.

Hope this helps!

[ajeffcote]

I will check that.This is going to drive me nuts trying to figure out why I read the negative voltage on that third pin.I reset the bias current to 22.5ma and it sounds pretty good now.And the harshness I had been hearing was from the treble control that had gotten turned up without me noticing.It was late and dark,and I was tired.
I now have a brand new noise to troubleshoot also.When it's cranked up into distortion,there is a low frequency rattling sound,almost like a cone rattle but not a cone rattle.Maybe one of the larger caps vibrating?

[Don Moose]

almost like a cone rattle but not a cone rattle.Maybe one of the larger caps vibrating?

Or a tube, socket, tranny, choke, speaker frame not tight, ...

Hope this helps!

[bbartilson]

While setting the bias on my Fender DRRI,with a brand new set of JJ 6V6S power tubes,I let the two leads on my bias probe get together,while it was powered up.

Unless I'm missing something REALLY basic here, you shorted the leads on your bias probe for a moment?

Every bias probe I'm aware of inserts a 1 ohm resistor between the cathode pin of the tube socket and ground, giving you a known resistance across which to measure millvolts which translate to milliamps of bias. Shorting the leads would take the 1 ohm resistor out of the circuit. Which would mean you couldn't measure little voltages across the leads. That's all.

It's possible you've got a bias probe that does something different. But even if you shorted the plate of the tube to ground for a moment, I wouldn't expect any damage. If you left it that way a long time, I would expect the fuse to blow and protect the circuit instead.

In either case, a new OT could easily handle either scenario. At least, they darn well ought to. The circuitry here is robust and forgiving. This ain't an old RAM that's so sensitive to static it will fail if you look at it funny.

By all means, make sure there isn't a shorted turn between the primary and secondary. I suspect there isn't.

As for your small negative voltages, me wonders where your ground lead is attached when making these measurements? Sounds more like bad test lead placement to me.

Overall I'm saying to relax. The 'harshness' you're talking about is likely the extra treble response as a result of having new tubes. Put the old ones back in and see if it goes away again. That's an easy test.

-Bill

[ajeffcote]

You were right.The 1 ohm resistors in the probe are the only casualties I can find.
I'm sure I shortened the life span of the tube I was checking,but it's still functioning.
Thank you both for the help.