Did JJ 6V6 tubes ruin my amp?

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Rockeraaj
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Did JJ 6V6 tubes ruin my amp?

Post by Rockeraaj »

I have built an amp based on the Fender Deluxe-ish circuit with no reverb or vibrato. It uses a cathodyne phase inverter and has three stages of gain. So that's two 12ax7 preamp tubes. It's pretty close to Leo's circuit with a few component value changes for taste. I had it sounding fantastic and was to the point where I just wanted to try some tube rolling. My preamp plates were all around 180-205vdc and nice. Then I put in a pair of JJ 6V6 tubes, fired it up, and nothing, just a teeny tiny hum out of the speakers. I looked and the only tube with heaters going is the GZ34 rectifier. The two 6V6's and the two 12ax7's heaters are dead. Here's where it gets weird. The pilot light goes on just fine and I show 3.4vac on each of the heaters. I now show 450vdc on all the preamp plates. Isn't that the whole uncut voltage? I put the other output tubes back in, and same thing. I changed nothing but the tubes. Any ideas before I dig in?
Now what should I build?
sluckey
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Re: Did JJ 6V6 tubes ruin my amp?

Post by sluckey »

Pull the chassis and look for a bad connection on the filament wires between the pilot lamp and the first tube.
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Deric
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Re: Did JJ 6V6 tubes ruin my amp?

Post by Deric »

The pilot light goes on just fine and I show 3.4vac on each of the heaters.
Is that measured at the tube sockets or at the pilot light? I would suspect a something other than the tubes.

Edit: What sluckey says.... ;)
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Rockeraaj
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Re: Did JJ 6V6 tubes ruin my amp?

Post by Rockeraaj »

I measured at each tube socket heater pin and also the pilot. All the same.
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Deric
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Re: Did JJ 6V6 tubes ruin my amp?

Post by Deric »

Have a schematic or pic?

Did you check the filament voltage with or without tubes installed?
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pompeiisneaks
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Re: Did JJ 6V6 tubes ruin my amp?

Post by pompeiisneaks »

if you have voltages but htey don't seem to glow, I once had a short from pin 3 to pin 2 on a power tube (EL34) and it blew out the heaters on all the tubes. Most tubes can't handle more than say 50 to 100V on the heaters as a maximum before blowing. Mine were JJ too, as I recall. Came from TubeDepot.com, but I won't get into that much :D
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sluckey
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Re: Did JJ 6V6 tubes ruin my amp?

Post by sluckey »

You cannot measure filament voltage accurately from each tube pin to chassis. Your meter will see 3.15VAC from each filament pin to chassis even though a filament wire is broken. To correctly measure filament voltage put one probe on pin 2 of a 6V6 and put the other probe on pin 7 of the 6V6. Then you will see there is really zero volts across the filament.

Now look for a bad connection (or broken wire) between the pilot lamp and the first tube in the filament string, probably a 6V6. Problem will most likely be at the pilot lamp or the first tube socket.
Rockeraaj
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Re: Did JJ 6V6 tubes ruin my amp?

Post by Rockeraaj »

Thanks guys! I will check on these things in a day or two. FYI: I did test those "dead" tubes in another amp and they are fine. Thank God, because the two preamp tubes are smooth plate telefunkens!

Any idea on why my preamp plates are showing 450ish volts? Is there something connecting the dead filaments and that?

(It's time like these I wish I lived next door to one of y'all!! I was a friend of Larry Rodgers and used to take my projects to his shop for critique, but sadly he has passed. RIP Larry.)
Now what should I build?
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xtian
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Re: Did JJ 6V6 tubes ruin my amp?

Post by xtian »

If there is no current flowing, there is no voltage drop. That's why 450v at preamp plates when tubes are absent or not operating.
I build and repair tube amps. http://amps.monkeymatic.com
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dorrisant
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Re: Did JJ 6V6 tubes ruin my amp?

Post by dorrisant »

FWIW... I don't think that the problem is due to JJ in particular. Maybe you do have a bad 6V6... something could have happened to it in shipping maybe...? As many JJs as I go through around here, I would say that the problems I've seen with them have usually been related to how they were handled or how they were abused in the circuit. Maybe others will give an opinion as well but I wouldn't blame JJ just yet. Just sayin'. :wink:
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ToneMerc
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Re: Did JJ 6V6 tubes ruin my amp?

Post by ToneMerc »

The JJ 6V6 is a pretty robust tube, but anything is possible. I ran a matched pair from Tube Depot in a test circuit at 580V plates/290 screens and they survived.

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Re: Did JJ 6V6 tubes ruin my amp?

Post by tubeswell »

More likely a broken wire between the end of the heater winding and the 1st socket or lamp (if you aren't measuring any VAC across the lamp)
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Rockeraaj
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Re: Did JJ 6V6 tubes ruin my amp?

Post by Rockeraaj »

UPDATE:
Upon acting on everyone's input, I found that I did indeed have an output tube socket failure of some sort. After replacing it, all is well! The JJ 6V6's are in and sound great. Thanks so much you guys!!!
Now what should I build?
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