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Output transformer roasted?

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3/5/2004 10:23 PM

Output transformer roasted?

I have an Ampeg V4 from 1972 that I recently got. I cleaned it up, got new 7027's (and I also tried the stock 7027's as well) but I'm having a problem. It seems that when I'm playing (at around 5 or 6 on the volume) that one pair of the output tubes have more of a blue glow (I know that doesn't really matter in itself) than the other side, and that side starts to redplate, even though the other side has barely any blue and doesn't seem to be working as hard. I've checked the bias and idle current, and they seem to be the same. I measured the coupling caps from the phase inverter and they seem to be on spec. I replaced the PI tube. I checked the values of all the resistors on the PI and they seme to be ok. I got the same plate voltage (560V) on all plates and about 540V on all the screens. I got -64V bias on all 4 grids. Is it possible that my output transformer is shorted and making one side work harder than the other? I also noticed when I play through it you can hear the transformer physically vibrating along with the guitar (tested it with a hotplate set on 'load' and could hear it).

thanks for any help,
-aaron

3/6/2004 1:34 PM

Hey Aaron,

There's a simple tranny short checker - supplied by Doc Horner a few years ago - on RG Keene's site. But I don't think your tranny is bad. You checked the coupling caps (read: DC blocking caps) from the PI to the outputs but I'd probably just change these to make sure - oh, the way to "check" them is to pull the output tubes, fire the amp up, and see if there is DC on pin 5 of the 7027s. These are C11 and C13 - 0.33 uf/400VDC. And, while you've checked the bias and idle currents I suspect you've not measured them when the problem occured.

A 1972 V-4 was one of the Linden, NJ, made Ampegs and the trannies are pretty hardy - the TN/Magnovox Ampegs were a little weaker but still as good as current production. And lots of OTs vibrate under signal/load - lotsa stuff can loosen up in 32 years (hey, I'm 50 and you should hear me rattle!

). The factory spec on the tranny is 110 ohms DC P-to-P. FYI.

Oh, while you're replacing the PI coupling caps replace that big striped job that drives the reverb - for some reason these guys die often on V-4s (c205 - 0.47uf/400V) - never hurts to go over all of the solder joints with a magnifying glass and remelt any that are loose - again, 32 years shakes stuff around.

Rob

3/6/2004 2:04 PM
jeremy	maybe install seperate bias pots for each tube, or pair of tubes, per side? sounds like there is a bit of offset in the tranformer or your matched set of tubes. jeremy

3/7/2004 5:14 AM

Make sure to check the screen and plate resistors at each power tube. They may have drifted off value a bit, maybe not enough to give big changes in idling readings but enough to throw off the running current a bit. Do you get the same kind of results running only 2 tubes? You also said the coupling caps. from PI "seem to be on spec.". If you don't have others to try, at least try swapping them around and see if the problem switches sides.

3/7/2004 8:15 PM
aaron	Well, I tried just swapping those two capacitors and it seems to be acting the same. i checked the screen/plate voltages and they seem right on. I measured the resistance of the plate resistors and screen resistors and they seem right on. Hmmm.. I think I may have to borrow a scope from work and try and chase this down. -aaron

3/25/2004 2:24 PM

I haven't been able to get my hands on a scope, but here is what I've found as of late...

First, I replaced every part in the PI just to make sure, and I swapped the blocking caps to see if the problem would move from one pair to the other.. but it didn't, same problem, even after all parts were replaced in the PI.

Second, I noticed the voltage is maybe 50-60volts too high on the PI, but almost dead on on the output tube plates. I changed all of the tubes to new ones except for the 6k11 and 6cg7. Could the overly high voltage have something to do with this? Could the 6k11 and 6cg7 maybe not be pulling the current that they used to causing the voltage to rise?

-aaron

3/26/2004 4:59 AM
g1	PI has not been subbed with something other than 12AU7 has it? The 560V you mentioned on plates seems a little hot also, usually around 540-545, a bit less on screens. I'm inclined to think it's not that 2 ouput tubes are over conducting but rather 2 are underconducting, thus the high B+ readings. Maybe try reading B+ with 2 red plating tubes out, then with them in and the other 2 removed.

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