I did something stupid to my JTM45 build

[g1]

Looking a little more, I'm hoping the low bias voltage isn't just because you are on the limiter bulb.

[mort]

14 volts across it

and -45.2 on the diode side, -31.3 on the other

[mort]

Looking a little more, I'm hoping the low bias voltage isn't just because you are on the limiter bulb.

That's what I had assumed, but it did go way down when putting the power tubes in (both on the limiter bulb), but I had assumed that because all the voltages go down once it's under load, that the bias voltage would move too...

[g1]

So is there -31 at the 15K but -25 at the actual grids?

[mort]

So is there -31 at the 15K but -25 at the actual grids?

There's only about a 1v drop at the 220k resistor before it hits p5. I may have taken one reading with the standby on and one with it off, by accident.


It's getting late and I'm getting a little frustrated. Better get on it with a fresh head tomorrow....

[mort]

I did find another voltage chart that shows voltage on the V3 grids. Now I'm really confused

[g1]

Grid voltages on preamp tubes can be difficult to measure with respect to ground. The best way is to measure from cathode to grid and do the math.
Yours seem ok. I'm pretty sure if you remove V3 only you will still have the problem with the bulb lighting up.

[g1]

When you get back into it, I would suggest you try removing the power tubes and the bulb limiter. If it doesn't blow the fuse when you come out of standby (with no power tubes), you can be fairly sure the problem is the bias circuit (or power tubes).

[mort]

One last question before logging off...


Is there anything that can be damaged by P3 of the V1 tube being open or highly resistive and being powered on? Is there some capacitor somewhere that can be easily fried by this scenario?

[mort]

When you get back into it, I would suggest you try removing the power tubes and the bulb limiter. If it doesn't blow the fuse when you come out of standby (with no power tubes), you can be fairly sure the problem is the bias circuit (or power tubes).

It's a quick test so will do...


The fuse does not blow with the power tubes removed and all other tubes in.

It also does not blow with the preamp tubes removed and all the other tubes in.

[Jazz P Bass]

If pin 3 (cathode) of V1 is not connected then the tube simply will not conduct.

[g1]

Like Jazz said, nothing will be damaged from pin3 of V1 being open.

The fuse does not blow with the power tubes removed and all other tubes in.

It also does not blow with the preamp tubes removed and all the other tubes in.

Ok, with preamp tubes removed and no bulb limiter, recheck the power tube voltages and post them.
What wattage bulb is in your limiter?
I'm thinking maybe the original power tubes were the problem, and the bulb may be distracting us.
Were the new power tubes tried with the fuse (with all tubes in) or just with the limiter?

Edit: Just to check, you are using slow-blow fuses, and it is the mains fuse (2amp slow) that was blowing, is this correct?

[thehoj]

Sort of off topic question here.. but I didn't realize it's okay to turn a tube amp on with no tubes installed.
There's no risk of damage to the output transformer in doing this for troubleshooting purposes?

I have a Marshall DSL-401 that blows the mains fuse immediately when powered on.. I wanted to try it with no tubes installed, but wasn't sure what the effect on the OT would be.

[Jazz P Bass]

No the output transformer could care less.
The main item stressed is the capacitors as they really do want to see a loaded down power supply (ie: all of the tubes installed)
It is highly unlikely that the preamp section is blowing the fuse, so remove just the output tubes for the initial test.

[isaac]

It sounds almost like you should rebuild your bias supply completely, and possibly the wiring around the rectifier tube... Sometimes rebuilding whole things, even if you don't know that's the problem, just fixes it and makes it easy. + you get back to playing faster. ;-)