Author Topic: Difficult model T questions - mystery resistors - any experts? EdBass...anyone?  (Read 1760 times)

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Offline thehornedone

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I have two model Ts now, both of which I've recapped, neither of which I've had for more than a month.  One of them I recapped more recently, and yesterday was the second time I had played it since recapping it. So, I'm playing, hear a pop, and one of the power tubes rapidly gets brighter than the sun. I dive to turn the thing off.  Get the chassis out of the head shell, and the problem is pretty clear.  The resistor attached to position 4 on that power tube literally exploded (pics below).  Upon further investigation, the resistors in position 4 on every tube don't seem to be right.  If you look at the highlighted part of the schematic below, position 4 should have a 1.5k 5w resistor.  The resistor in this spot is a 47 ohm resistor, and is definitely not 5w from the look of it...it's a simple film resistor, which I don't believe are ever good for more than 1/2w or maybe 1w.  For power resistors, they're typically the wirewound variety that look like an elongated sugar cube.  I open up my other model t for comparison, and it indeed has the 1.5k resistor in position 4, but in this amp, position 3 has a 50 ohm resistor, rather than a 47 like the schematic says. At 10% tolerance, this doesn't make a difference, but it's still kind of weird. Was sunn really this sloppy?  Could they really have installed a 47ohm 1/2w resistor in place of a 1.5k 5w resistor?!  Is there any reason there would ever be a 47ohm resistor in this spot? Maybe someone replaced them for a 6L6s or something and didn't know what the hell they were doing...?

Anyway, I replaced the blown resistor with a 50ohm 10w resistor from radioshack, with more resistors in the mail to get the power tube section back to spec.  In the meantime, the thing works again with 6550s just fine, but should it be safe with ~50 ohm resistors in position 4? Or should I get it back to spec with the seemingly "correct" 1.5k resistors?


blown resistor:


schematic:


power tube wiring with the mystery 47ohm resistor:


power tube on other model t with correct value resistor:

Offline thehornedone

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Hmm, after thinking through it while posting this, I'm thinking maybe it really was a manufacturing error.  As you can see, the pin labels on these tube sockets are confusing as each number could refer to the pin on either side of it.  Since there's a 47ohm resistor in position 3, maybe the person in the factory read it wrong, and installed a 47ohm resistor on lug 4....

I just can't believe the amp has held up this long with such a wrong and weak resistor there! these are seriously tough-ass amps.

Offline loudthud

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The 2000S used a 47 ohm 5W to pin 3 and a 47 ohm 2W to pin 4. (Note sometimes 50 ohm resistors were used.) KT88s from the 60s work fine with these values but 6550s tend to not live as long. Your amp may have been a very early model or someone could have changed the parts. To extend tube life, you should install the 1.5K resistors to pin 4, 47 or 50 ohm resistors to pin 3. These resistors tend to burn up when the tubes die so they should be checked when tubes are replaced and the amp rebiased.

Offline thehornedone

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Thank you very much for that info!  Yeah, the tubes on this amp when I got it were ancient mismatched 6550s, no two alike, and made all kinds of weird noises when powered up.  Maybe those tubes dying weakened this particular resistor, and then replacing the caps, and putting new tubes in was enough to shock it towards destruction.

I will be replacing them with the 1.5k 5w resistors then.  I'm just happy nothing else broke after this happened.