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In Reply to: RE: How reliable have your SET or SE tube amps been? posted by johnnycamp5 on April 18, 2016 at 18:55:57
Tube rectifiers have a minimum plate supply resistance (Rs) for a given voltage. The 5U4G at say 450~500V (as is typical for cathode biased 300B amps) needs to see about 70 to 80 Ohms ahead of it. That is the combined resistance of the transformer's primary and secondary given by the formula: Rs = Rsec + (N^2)xRpri + Ra. It's this this last term, the additional resistance, that must be added in order to bring the Rs to the desired value.
Depending on size, a typical transformer in an amp such as yours will have a source resistance of about 30 to 40 Ohms. You would probably need to add a ~30 Ohm resistor just before each plate of the rectifier (total of 2 resistors) to bring the source resistance (Ra in the equation above) to the required level as specified in the tube data sheets. I would use at least a 10W resistor.
Follow Ups:
I forgot to mention that with these added series resistors, you should be mindful of their voltage ratings. If for example the transformer secondary is at 400V, then a 500V resistor rating would not be ideal. Resistors need to be at or bellow 70% of their max voltage rating for a long and happy life. Either up the rating, or use two resistors in series.
I also forgot to mention in the previous post that the series resistors will also have the effect of lowering the B+. If your line voltage is on the high end of the scale as is the case in many places, there should be no issue. If you were to use the 5V3 which drops less voltage than the 5U4, you would probably end up at the original B+ level with the use of the series resistors. Hope this helps. Let us know if you resolve the issue with whatever means you might try.
Thanks sonicboom.
I wish I was electronically savy enough to try an make the changes you recommend, or to even fully understand them!
I am not.
I was only able to build the amp because of its kit design and build manual, and the fact that I have some soldering experience with RC cars.
Ill have to trust that this amps initial, intended design is a proven one, and hope that I have been unlucky with THREE different rectifier tubes (the stock kit supplied 274b, and the pair of 5v3's), which I find hard to believe.
Ill report back with my outcome on this, and if indeed it is or is not the rectifier tube/tubes. Then perhaps someone here can recomend an honest repairman for my amp.
Regards!
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