JTM45 Clone Blowing Fuses

[AtomicMassUnit]

Hi Guys,

This amp is a JTM45 clone I put together about 8 years ago. It's been going strong all that time, until last year the OT went out, and I replaced it. It was working fine until it started blowing fuses again (6 months or so later).

I put it on the current limiter and have been trying to figure out what the problem is.

With no power tubes in, plugged into a limiter with a 75w bulb, everything powers up normally, I get 427 on pin 3, 425 on pin 4, and -28 on pin 5... and this remains stable.

The amp had KT66s in it, and unfortunately I don't have an extra set handy, but I do have EL34s, and if I put them in, the light bulb starts getting brighter and brighter until I turn it off...

Only thing I've done so far is replace the filter caps and bias cap, just in case these got stressed last time when the OT went out.

Any ideas?

[big_teee]

Your bias voltage sounds too low.
Can it be turned up?
My 2204 JCM800 with EL34s, runs between -35 to -45 bias voltage.
Depiending on the tubes used.
T

[AtomicMassUnit]

Your bias voltage sounds too low.
Can it be turned up?
My 2204 JCM800 with EL34s, runs between -35 to -45 bias voltage.
Depiending on the tubes used.
T

Yes, it's adjustable...

[g1]

I'm with big T thinking the bias sounds quite low. You get that same reading for both tubes?
Try your bulb set up but remove one output tube at a time to see if it is just one tube causing the problem. If so, you may have a bad coupling cap. If it does it with either tube, the bias may be dropping off. Measure it while the bulb is going from dim to bright, does the grid voltage (pin 5) stay stable or does it drop when the bulb gets bright?

[soundguruman]

Your bias voltage is wrong...that would explain it.

Also make sure your screen grid resistors are installed.

If you were using an old schematic, it may not show the screen grid resistors.