I never thought this would happen to me! (PT failure)

[eschertron]

definitely ask and ... Definitely ask

Yeah, and well, no because I'm too nice of a person
I didn't take a reader's poll, but the consensus seems to be that the PT was underrated. I have placed an order for a bigger one. My fault as a designer.

[eschertron]

Update:
new PT is in, tested with tubes out, and then again with tubes in at idle. So far measurements match expected design. At idle, bias tester measures about 325v plate-cathode, 26mA cathode current. Matches up with Nick B's tube model nicely, about 60% dissipation. Measuring primary side power, I get .34A * 122vac (line voltage at that moment) = 41.48VA. Subtracting about 19.5VA for the filaments, I get about 11W idle dissipation per tube. OK so far.
I'm thinking about tacking in a 1R/1W resistor on the cathode network to better see how the tubes behave when I put a signal through them. But my primary data point will be primary current, monitoring to prevent exceeding the PT's rating.

[Steve A.]

I'm thinking about tacking in a 1R/1W resistor on the cathode network to better see how the tubes behave when I put a signal through them.

By all means do that, especially since we suspect that excessive current through the output tubes was the cause of the PT failure. Definitely old school but what works works...

But my primary data point will be primary current, monitoring to prevent exceeding the PT's rating.

I would have thought that the amp panel fuse would have offered protection but it obviously didn't.

I am a bit surprised that the PT internal thermal fuse didn't open before the windings were damaged. I bought a Pignose 40GV in 1998 which was the first tube amp I had seen that was made in China. There was a strange smell whenever it was turned on and within a month or two it died... there was no continuity in the PT primary. At that time there were no parts available for it so I was on my own (I had already started doing mods to it which voided the warranty.)

In any case I eventually tracked down the underlying problem to be the amateurish-looking power supply pcb which had not been properly etched so there was current leakage between the high voltage traces. I had to manually remove the unetched copper between the traces. As a temporary measure I bypassed the internal thermal fuse in the PT until I could track down an acceptable replacement and the amp worked fine after that and was the test bed for all of the mods documented in the attached PDF file...

g40v_mod.pdf

http://blueguitar.org/new/articles/b...s/g40v_mod.pdf

Steve A.

[eschertron]

1R resistor in place. Idle voltage/current measured up in post #47, here's max sine signal and square wave (test bench, dummy load):
note specs on PT 86VA rating primary, so in the vicinity of .73A max draw. 104mA at 550VCT for the HV secondary.

sine wave test
.40A primary side measure, .4*118 = 47.2VA
335vdc B+
27vdc at cathode
1R measures 89mv (ma)
OT secondary voltage gives me about 7.5W

sine driven to square output
.50A primary side measure, .5*118 = 59VA
329vdc B+
29vdc at cathode (zener voltage drawn up that far?)
1R measures 124mv (ma)
OT secondary gives me about 15.6W

The other thing I stumbled upon when doing my replacement transformer research is that Hammond provides TWO current ratings. One for the secondary AC current, and the other for the rectified DC current. In my ignorance, when I saw the two different numbers I chalked it up to publishing error. The numbers always appear on different places, one on the catalog index(and individual cut sheet), and the other on the transformer selection table. Never together. Some 3rd-grade arithmetic and I realized the posted value for DC current is always (very near to) 87% of the posted value for AC current. Does this represent transformer efficiency (87%)?

So THIS transformer is still only rated to 90ma draw on the B+. The old one? 65ma. I've just about convicted myself to believing that I could have subjected the old PT to an overcurrent condition approaching 200%. Might just have been enough to kill it. Maybe. I won't be running the amp even with the new PT into sustained, max volume square waves again. Ever. At least until the missus is out of the house again, or I take the amp to an open stage somewhere. Future designs will be more conservatively attuned to the numbers that matter, and less on my - previously - uninformed assumptions.

I have played it a bit, no issues so far, primary current draw seems to peak out in the low 0.4xA range (hitting the input hard, but no fuzz/OD).

The bright side of all this is that I learned a bunch about power transformers, including that the measured primary current (190mA with all tubes out) exactly matches the published magnetizing current on the spec sheet. I don't think I could have taken a class and learned as much for the cost of the blown PT.