5150 combo - New GT-6L6C Power Tubes

[zzlong55]

Can anybody help? I just pulled two stock Chinese 6L6GC tubes and obtained two new Groove Tubes 6L6C. The tech support guy at Peavey said I should check the flyback diodes (SR2873) @ CR13, CR15 on the schematic (5150 Combo). Q: Will these new GT 6L6C's work okay, or should I find 6L6GC's? Q: How do I go about "checking" the flyback diodes? Any help will be greatly appreciated!

[52 Bill]

The tech support guy at Peavey said I should check the flyback diodes (SR2873) @ CR13, CR15 on the schematic (5150 Combo).
Why did he suggest this? Are you blowing fuses?

Q: Will these new GT 6L6C's work okay, or should I find 6L6GC's?
They should work fine in your amp. Does your amp use two or four output tubes? I personally never recommend mixing brands or new and old output tubes unless they have been current matched.

Q: How do I go about "checking" the flyback diodes?
You need a multimeter with a diode check setting or at least an ohmmeter. Place the leads of your meter across the diode leads and check the reading. It should either read open or some small value. Now reverse the leads and if the original reading was low, with the leads reversed, the reading should be high. If the original reading was high, with the leads reversed, the reading should be low.

[zzlong55]

Hi Bill, Yeah I was blowing fuses. So I took the Power Tranny apart and saw no evidence of meltdown (whew!). I turned the amp on and a power tube flashed, then the fuse tripped. So I bought the matched duet of 6L6Cs. But I didn't want to install them without doing some research first. The Peavey tech support dude told me I should "check" the flyback diodes before installing the new power tubes. The amp is a 60w 212 combo 5150, and it has two (2) power tubes and five (5) 12AX7 preamp tubes. I only bought 6L6C power tubes; I think the preamp tubes are okay. Thanks for thye killer advice. You're awesome!

[Amp Kat]

You will need a multimeter with a diode test on it. Make sure there is no high voltage on pin 3 of the 6L6 socket with your DC volt meter and the amp off. With the tubes out put the meter on diode test and put the black lead in pin socket # 3 and the red lead to a good chassis ground. You should get a reading of about .550 v . If the diode is bad you will get around .100 or below or a continuos continuity beep if you have that feature. It sure sounds like you had a bad tube but the only problem is if it shorted bad it could have taken out a screen resistor as well so the diodes may not be the only problem if there is one. Try powering the amp up without the 2 power tubes and see if the fuse blows. If it does the diodes are most likely shorted or you have another problem. Make sure the speaker is attached or the volume pots at zero.

[Enzo]

If the amp does not blow the high voltage fuse when the power tubes are removed, then the diodes are OK. If those diodes short, they blow fuses. There is no way for those diodes to cause any tube failure. SO if they are not blowing the fuse, then the tubes are.

The amp is not picky about what 6L6 you install. I wouldn't mix and match them like that either, but the amp does not require any special tube selection.

If the amp blows the fuse without power tubes, then we are looking at the diodes, or potentially the output transformer. Those would only blow fuses when the standby switch is on. If the high voltage fuse blows even when standby is off, then look for a shorted rectifier or filter cap.

Transformers don't necessarily look any different when shorted. A winfing shorted to frame will do it, and a primary to secondary short will kill it.

[zzlong55]

Thanks guys! You are all awesome. I replaced the power tubes and everything seems fine, the amp cranks and is a real bastard! It was just a funky 6L6 tube and now all is good. But the noise this amp makes when the distortion (Lead Pre) is maxed makes me feel uneasy. Other than going through a noise suppression pedal, what can I do to get rid of this extraneous noise? I've heard many 5150 owners complain of this "buzzing" or "humming", which would make for awkward and/or embarassing moments on stage.

[Enzo]

This is a screaming high gain amp, the peramp will make hiss and noise, especially when maxed. I cannot imagine the need to turn the preamp all the way up. But if you must, try a noise gate pedal in the loop.

[zzlong55]

Yeah, I max the pre but keep the volume low in my room 'cause I like the heavy distortion and I don't want to blast my neighbors into oblivion. I'm getting a noise suppression pedal and some other toys soon! Thanks, Enzo.