First Post! Hi Chaps, As a first tube amp project, I recapped a Heathkit AA-151 amp. It seems to work but I'm puzzled by the difference in individual cathode readings. As you may know, this is a 2-channel push pull amp that uses four 6BQ5/EL84 tubes sharing a common cathode resistor. I put bias probes on the individual tubes and read
Channel 1, tube 1, 31.6 mA Daystrom tube
Channel 1, tube 2, 32.p mA Daystrom tube
Channel 2, tube 1, 24.4 mA Daystrom tube
Channel 2, tube 2, 45.5 mA Westinghouse tube
The last tube is also visibly hotter. Is this just a characteristic of the Westinghouse tube, or a bad tube, or just normal variation?
Thanks -Kent
Channel 1, tube 1, 31.6 mA Daystrom tube
Channel 1, tube 2, 32.p mA Daystrom tube
Channel 2, tube 1, 24.4 mA Daystrom tube
Channel 2, tube 2, 45.5 mA Westinghouse tube
The last tube is also visibly hotter. Is this just a characteristic of the Westinghouse tube, or a bad tube, or just normal variation?
Thanks -Kent
Channel 2 tube 1 appears to be weak. Channel 2 tube 2 may have a leaky coupling capacitor associated with it, or could be gassy or could have been mismatched relative to the others from the beginning.
Presumably these tubes all matched fairly well when the amp was new.
If I recall it is one cathode resistor for all 4 tubes. You may want to consider separate resistors and caps for each output tube so they bias independently of each other.. (4X the resistance of the original single cathode resistor and ~ 1/4 the value of the original cathode bypass cap. for each tube.) Make sure to use 50V caps, 47uF per is probably adequate, and use 3W resistors minimum for the cathode resistors.
Presumably these tubes all matched fairly well when the amp was new.
If I recall it is one cathode resistor for all 4 tubes. You may want to consider separate resistors and caps for each output tube so they bias independently of each other.. (4X the resistance of the original single cathode resistor and ~ 1/4 the value of the original cathode bypass cap. for each tube.) Make sure to use 50V caps, 47uF per is probably adequate, and use 3W resistors minimum for the cathode resistors.
Kevin,
IMO/IME, a shared RC bias network in each channel works well. Space and money are saved. Also, PP pair matching requirements are reasonably relaxed. with minor differences in cathode current being acceptable. Naturally, close matching of each pair's gm is essential.
IMO/IME, a shared RC bias network in each channel works well. Space and money are saved. Also, PP pair matching requirements are reasonably relaxed. with minor differences in cathode current being acceptable. Naturally, close matching of each pair's gm is essential.
Kevin: I replaced the coupling caps. I'll buy a pair of new tubes and give them a try. I don't suppose you would try to bring the hot tube current down with a small additional resistor?
Eli: Been reading about the self-adjusting characteristics of shared-cathode resistor PP amps. Seem to recall that you can get a slight advantage by splitting the channels and using a shared resistor for two tubes vs four. Yes?
Eli: Been reading about the self-adjusting characteristics of shared-cathode resistor PP amps. Seem to recall that you can get a slight advantage by splitting the channels and using a shared resistor for two tubes vs four. Yes?
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