Not entirely sure. It is my assumption that leaving the secondaries unloaded is verboten. Obviously, there must have been some kind of runaway condition, as you mentioned the rectifier tube glowing and arcing. Doesn't sound too promising.
Hopefully, someone more knowledgeable than me will chime in.
Jeff
well, the bad opt is what is what is making the rectifier tube glow and arch, it was still glowing and arching with that amp the second time i tried to turn it on.
i've got a question tho. in the transformer specs it says that the transformer is rated at 75mA, is this current the bias current? if it is, the amp is running pretty close to that. bias current reads 73mA, could that have possibly at least contributed to the transformer going out? maybe that high current and a speaker switch together made it pop?
i've got a question tho. in the transformer specs it says that the transformer is rated at 75mA, is this current the bias current? if it is, the amp is running pretty close to that. bias current reads 73mA, could that have possibly at least contributed to the transformer going out? maybe that high current and a speaker switch together made it pop?
The transformers would be rated for max. current, not zero signal bias current which is what you are measuring. Being that your amp is designed to run class AB1, it is my understanding that the tubes will swing higher than the zero signal bias setting under normal operation.
It would appear that you are also exceeding the original power dissipation rating for this amp, which was designed to run 5881's. This could also be a contributing factor in the loss of the OPT. The other amp's OPT may not be far behind.
Jeff
well i contacted Jack Elliano at Electra-Print Audio earlier tonight looking for some replacement suggestions and he pretty much agrees with what you just said, 73mA at idle, higher in operation most likely being what caused my transformer to go. so i've really got to get on this bias and bring it back down to between 50 and 60mA. i dont think that the higher dissipation of the new tubes is what caused my problems, the amp was playing pretty quietly when the tranny went. but definately something to keep in mind. i'll be working on the other amp very cautiously untill i get the bias down to where it should be. but it looks like i'll be getting two brand new transformers from Electra-Print so that i have two matching amps.
i'm trying to figure out what resistance i need to change the 250ohm resistor to to bring the current down to what it needs to be. at pin 8 with 5881 tubes i get 39vdc and with 6p3s-3 i get 41vdc. the heathkit book says that it should be 36vdc. i need to bring 73mA down to 50-60mA. i know i need to increase the 250ohm, but i cant remember the calculations.
Ok, here goes:
I'm pretty sure you can just use straight old Ohms Law for this. Solving for R, we get R = V/I. You know the voltage drop (41) and you know what the current should be for two tubes (2 x 50ma lets say). R = 41/(2 x .05) = 410. 400 ohms seems to be the closest value.
If we go by the heathkit book values, 36/(2 x .05) = 360 ohms. Plate dissipation would then be approx. 460V x .05ma = 23 watts. Coincidentally, that is the max. dissipation on the 5881 data sheet at 400V B+. Why they chose a 250 ohm cathode resistor is strange, obviously the 5881's will be operating well beyond their max. recommended dissipation.
If this looks wrong, then someone please correct me.
Jeff
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