I did something stupid to my JTM45 build

[mort]

With recto and V1 only, no fuse blow.


With recto and V3 only, fuse blow.


With recto and V2 only, fuse blow.


Tried a different 12ax7 in V3 (to rule out the tube itself), and still the fuse blows. No power tubes installed in any of these tests.



Now I think I'm out of fuses again. Damn it.

[Jazz P Bass]

Put in a new rectifier tube.
Preferably NOS.

[mort]

Put in a new rectifier tube.
Preferably NOS.

I tried that. I pulled it from my old Princeton Reverb, which works just fine, and the symptoms didn't change a bit.



I don't get it. Everything seems to be working, but it ain't workin'

[Steve Conner]

It is possible that both of your rectifier tubes could be bad. The JTM45 circuit is a lot more demanding than the Princeton Reverb.

[dai h.]

look at the picture above and the 450V 10uF cap. Also, previous advice was to check the bias circuit. Was that carried out?

[mort]

It is possible that both of your rectifier tubes could be bad. The JTM45 circuit is a lot more demanding than the Princeton Reverb.

I thought rectifier tubes either work, or they don't work. And aren't they supposed to be alot more durable than other types of tubes?

look at the picture above and the 450V 10uF cap. Also, previous advice was to check the bias circuit. Was that carried out?

I had stated before that I "read" (with a meter) all the values of the parts in the bias circuit. And the bias voltage is in appropriate range.

I'm looking at the 450v 10uf cap. It's facing the right direction, it's within tolerance, and its leads are soldered in well.

[Steve Conner]

Yup, and one of the ways they "don't work" is by arcing back and blowing the mains fuse when they are hit by heavy peak currents.

[mort]

I have some unused NOS RCA 6Ax5GT's. Would that work as a test substitution?


edit: never mind. It's only rated for 125mA, whereas the 5AR4 is rated for 250 mA dc output current

[dwhutchens]

I've seen a lot of questions about the bias on the power tubes, have you checked to see if the plates are glowing before the fuse blows. Is the bias voltage dropping too low as the tubes heat up? Could the power tube sockets be loose on the bias pins? I think if the bias is too low on the power tubes they will draw a lot of current.

[isaac]

Have you tried messing with the bias pot? Could it be shorting somewhere, or possibly internally? And of course, try messing with cleaning and wiggling the pins on the power tubes and sockets, and the rect tube and socket. I don't remember seeing any photos, can you get us some HD photos on the power tubes and rect tubes? And I also dont remember once more... Is this a main 120v fuse, or a DC high voltage standby fuse?

[mort]

I highly doubt that smashing V1 had any electrical effect on the circuit.

The bad voltages on V3 grid are a red herring. The grids on the LTPI are very sensitive and a DVM can load them down and change the voltage.

The two schematics you posted show two different bias circuits. The original JTM45 got the bias off the main high voltage winding with a 100k resistor. The Weber kit uses a transformer with a bias tap. Which way did you do it? Getting the bias off the main winding can be problematic. For instance, if you connected the standby switch between the CT and ground, then it would turn the bias off too, and the result would be a surge of current when flipping from standby to play.

Note that if you just stress a fuse a little above its rating, it could take several surges to blow it. Correlation is not causation. Maybe the fuse blew on this particular flip of the standby switch, not because the surge was worse this time, but simply because it got fatigued by the last 2 or 5 flips.

Finally, even if there was nothing else wrong, this standby circuit naturally generates a current surge by dropping the rectifier straight onto a discharged capacitor. This can be stressful for a tube rectifier and could cause it to arc, which would certainly blow the fuse. Have you checked for a flash inside the rectifier when the standby is thrown? Have you tried swapping it out? Maybe it started arcing because it got damaged by the mechanical shock of dropping the amp.

OK, well to fix it I suggest you try a new rectifier tube, or simply leave the standby switch at "play" all the time

Put in a new rectifier tube.
Preferably NOS.

It is possible that both of your rectifier tubes could be bad. The JTM45 circuit is a lot more demanding than the Princeton Reverb.

Yup, and one of the ways they "don't work" is by arcing back and blowing the mains fuse when they are hit by heavy peak currents.

All this advice pointing me to the rectifier tube and ultimately I ignored it. Foolish me


I ended up driving near a somewhat local tube supplier and thought hell I'm just gonna buy a new tube like those guys on that forum told me to do and sure as shit, that was the problem all along. That's a lesson I won't soon forget...

But I must say, me being fooled by that second recto tube giving the same problem provoked me into a pretty intense study of tube amp operation and I learned quite a bit. I'm still foggy on preamp operation but I have a much better understanding of power supply and tube load.. lotta general type stuff that's hard to search on the net. Plus I learned the importance of datasheets. Alot of that stuff was greek to me until the last couple weeks.