I did something stupid to my JTM45 build

[mort]

Decided to pull this one out again and see if I could get somewhere with it.

Still blowing fuses without a limiter when amp is warm and stand by is switched to playing mode (if amp is cool, does not blow fuse when switched to play mode). Also, the fuse blows with only power and recto tubes in place.

When amp is cold, bulb momentarily glows very brightly and then goes to dim when mains is switched on. If I immediately switch to play mode, there is no effect on the bulb. If I wait until it's warm there is a slightly brighter bulb when in play mode vs stand by mode.


There seems to consensus that there is a problem with the bias circuit. I looked into what I installed, and was reminded that I used a bias pot, unlike in the schematic above. Now remember, the amp worked fine for months before it got that tube bent over. The bias circuit is this one, with an 8 uf going from the diode/resitor connection to ground and the 15k was replaced with a 22k. https://taweber.powweb.com/store/6m45_layout.jpg

With a 75 watt bulb in the limiter, all tubes installed, these were my voltages today on the power tubes

V4
1. .001
3. 241.4
4. 242.2
5. -30.3
6. 242.2
8. .001

V5
1. .001
3. 242.0
4. 242.0
5. -30.23
6. 242.8
8. .001

I've read the components in the bias circuit and all are in the ballpark and also subbed in a new diode. Still no cure.

Any thoughts?

[mort]

V1 through V3, read at the same time as the power tubes.

V1
1. 139
2. .001
3. .792
6. 140.9
7. .002
8. .792

V2
1. 126.6
2. .003
3. .433
6. 181.6
7. 126.9
8. 126.6

V3
1. 153.5
2. 10.4
3. 17.3
6. 151.3
7. 10.85
8. 17.3

[isaac]

I don't remember all your problems here, or what you did, but did you try new tubes? and did you check all the wires in the area of the rect and power tubes? Or maybe one of your filter caps is bad. BTW, the bias cap should be 100uf, not 8uf, so that might help, but also check all those components to see if they are shorted out. You say you read all the components in the bias circuit. Check them with meters.

[Steve Conner]

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.

[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.

Finally, 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.

It's got a bias tap on the PT, so no dropping resistor before the diode. 36vac into the diode and -48vdc out.

[Steve Conner]

I added a note about the rectifier. Are you running a tube rectifier and did you look to see if it's arcing when the standby switch is thrown?

[mort]

Here's a photo of the bias circuit. It looks a little sloppy at the moment because I've been lifting/reading components. And when I say "read", I do mean read with a meter, not just reading the values as printed.

It's sort of a cross between both bias circuits, but did work just fine and the amp sounded great.

[mort]

I added a note about the rectifier. Are you running a tube rectifier and did you look to see if it's arcing when the standby switch is thrown?

It is a tube recto and it does arc when the fuse blows as the standby is thrown (only once the tubes are good an warm)

[Steve Conner]

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

[mort]

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

Just tried with no limiter and the standby switch at 'play' from the time the mains was switched. The fuse did eventually blow but I was able to play it for about a minute before it blew(arc in recto tube was still present). Could that still be a recto tube problem? I've only ever had one go bad and it just went from completely working, to completely dead.

I only have some oddball rectifiers on hand. Don't know if any of thses would work...

6x5gt
6ax5gt


I thought I ad a couple more types but that's it.

[mort]

Just remebered I had an identical recto tube in another amp that is working fine. Just tried it and still no cure. The arc was still present in the alternate tube as well.

[isaac]

Are you sure you are using the correct value fuse? Maybe its just wrong fuses. Check that. Wouldn't that be funny...

[mort]

Yesterday I bought two new packs of 2a slo blo, 250v fuses. I remembered you mentioning that and I made sure to have the correct fuses on hand.

I've thought about just replacing the tube socket that got the tube bent over in it just to rule it out.. doesn't sound like that's my problem though. How frustrating!

[mort]

The V1 socket is now ruled out as problematic. Still no cure

[mort]

Just tried it with all tubes in besides power tubes and it still blew the fuse. Tried with only the recto installed and it made a slight arc in the recto tube at about the time the fuse would normally blow, but the fuse didn't blow. Now I'm putting in V1 through V3 one at a time, independantly, and then I will start it up with multiple preamp tubes in and see if I can narrow down when the fuse will/won't blow.