worked on upgrading a rev B Blues Jr a while back. subbed the 3 12AX7's for a 12AY7 in V1, 5751 in V2, and then a balanced 12AY7 in the PI - cannot rave enough about the resulting tone, really opened the amp up. also bumped the screen stoppers up to 270R, and added a 10K pot between R31 and C- in the bias circuit to cool it way down, with a 2W 1R between the EL84 cathodes and ground to measure. reflowed suspect solder joints, cleaned contacts, jacks, pots - the works. amp sounded and tested just fine.
just got the amp back, apparently it shit the bed after only about 10 hours of play time, and i'm very frustrated especially because it screwed 2 of the lovely NOS preamp tubes and i have no clue why.
here's the damage i've found:
- CR10 R3000 protection diode has failed open, CR11 seems fine but i'm gonna replace both just to be safe. not sure if this is a new occurrence or not, don't think i tested it last time
- the PI 12AY7 has a busted filament, pin 5 no longer has continuity to 4 or 9 😭
- the 5751 in V2 is SUPER worn after only 10 hours! both sections used to test 90-95% of nominal plate current, now section 1 (which isn't even used in the circuit, plate grid and cathode are all grounded) is only about 70%, and section 2 is barely 75%! 😭😭😭 (tubes tested on an Etracer)
i haven't started it up because i'm waiting to install the new R3000's i ordered just to be safe, but i'm absolutely perplexed.
everything else seems to test fine. no bad solder joints or burning. the V1 preamp tube and both power tubes test exactly the same as they did before. i haven't removed the electrolytics to fully test them, but they all measure with plenty of capacitance in circuit, and resistances of at least several hundred Kohms steadily increasing, with no physical evidence of failure. and i don't suspect them in particular, because someone put 500V caps in the main HT supply and this amp maybe hits 350V at the worst. the caps in the bias supply and +/-15V supply are more suspect in general and i'll probably replace them just to be safe (especially C14 since i increased the bias voltage), but those don't really have much to do with V2 or the PI... tested every component around V2 and the PI in circuit, all seem fine, including plate and cathode resistors and coupling caps. OT reads about 55 ohms DCR per side.
i guess i can see the PI tube just randomly failing, even though it was a NOS RCA. but i have no guesses as to what would have caused V2 to become so stressed like that, especially considering its only got 242VDC supply voltage, and is very normally biased with 100K and 1.5K... and i can't find any logical link between the 3 things i found failed.
there is just one more variable. when i was originally installing the 2W 1R sensing resistor, i at first mistakenly installed one on the wrong ribbon cable. turns out i had put it between the junction of the two filament balancing resistors, R35 and R36, and ground. so i just left it there, figuring it wouldn't cause any issues, and installed another one in the right spot. could this have possibly caused problems?
interested in any thoughts as to what could have gone wrong here. my best guess at this point is that the somewhat higher heater voltage in this amp (6.6VAC, no clue why they designed it like that) stressed those NOS tubes out quite a bit, and the CR10 failure was unrelated. think i'm gonna use some 0.1R wirewounds in either leg to drop it down to like 6.1-6.2VAC. thanks for reading.
Topic: Blues Jr frying preamp tubes (Read 2450 times)