Clueless Idiots and High Voltage Vacuum Tubes Really Do Mix!

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Debugging the Medalist – F#%$!!

March 31st, 2011 · No Comments · Amp Repair, Medalist Debug

Ok. When last we met (do I say that every time?), we had decided that the power caps were bad. At least C4A and since those crap cans go bad all the time, I decided to go for them all. First lets have a go at the schematic with test points marked in. One thing is that we didn’t get any voltages from Gibson. That’s not normal as most of the schematics I have seen have a bit of voltage reference.

Blue for a reason!

Ok.. so I took some readings after the caps were in. Sorry to do this as a picture, but I can’t for the life of me get a decent looking table out of wordpress.

Voltage Measurements after recapping

Voltage Measurements after recapping

Basically, everything looks reasonable. Max ratings for this tube are 300V on Plate and Screen with normal Class A operation at 250. The cathode is 7.3 V. No real surprises in the other tubes. So lets see if we have actually cured our little ripple problem. First, please recall that we could not even scope the ripple at the first filter cap before. It was way out of the range of my scope and I didn’t want to blow the damn thing. Here is the new picture at TP3 (right off the rectifier)

10 V AC Ripple after the first filter cap

Oh.. that’s nice. Just like the one we saw when I alligator clipped the cap in place. That’s 10 V (p-p) just like before and it’s at 2 mSec ( ~ 120 hz, double the standard house frequency of 60 hz, just as expected). Ok, great, moving on to TP5 which has the second filter cap (this time a 10μF).

0.3 Volts ripple at TP5

Had to dial it way back to find this ripple. 0.2 V per division makes that about .3 V ripple. On a 262V line, that’s essentially nothing, unless of course you can hear it. Recall that this was large and weird looking previously. Not sure exactly why this looks more curvy than the sawtooth above, but I think that’s the right curve, I saw it on the internets.

Finally, on to TP9 (I skipped 7 because it should be exactly the same as 9). TP9 is the most sensitive part of the amp, the first preamp stage getting tiny signals from the guitar. Any interference here will be amplified a hundredfold.

TP9 AC Ripple 0.01 or less

So.. that’s 0.01 or less ripple on a 173 V DC rail. I think we can say we successfully cleaned up the power supply, so we button her up and see if we can make some music.

And we can! Sweet sweet music. Much louder than I expected. Very clean, remarkably clear note definition. Bass is good, and the amp is much brighter than the Crestline and the current GA20RVT. The tremolo is outstanding! Couldn’t dime it at 12:30 pm, so I’m not sure, but I don’t think this thing breaks up very early.

Well.. let me amend that statement.. it broke up all right.. after about 10 minutes of noodling the power tube failed spectacularly. Arcing and flashing, horrible noises..

F#%$!! It could well be the original tube, which would make it 44 years old, oh well, back to the bench.


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