HUM! Need some help...

[MHProd]

The PT works fine.

The choke measures 108 Ohms.

will check voltage soon.

[Littlewyan]

At the same time as doing what Steve said can you tell us what ppimv you used? Also can we have a wider gut shot that shows the rest of the wiring for the output stage and the PI?

[MHProd]

Regular LarMar PPIMV.

The fuses only blow when the power tubes and PI are in. If it's just the power tubes, it's fine. If it's just the preamp tubes, it's fine. Both, and fuses blow.

The choke measures 108 Ohms.

With the preamp tubes only. I get 470V on both sides of the choke.

With only the KT66 in biased at 37ma, I get 435V on both sides.

[matt h]

(deleted)

[MHProd]

I checked both side of the coupling caps with V3 in. I get +V going to the plates and -V to the grid leaks. So the caps seem fine.

I went over the schematics about 12 times and checked the circuit every time and I really don't see where the problem is. It's driving me nuts!!!!!

[matt h]

(deleted)

[MHProd]

Right! I'm getting confused now... It's late here... I'll check with the preamp tubes in and report back!

[67plexi]

Please bend the 620R on V-5 away from pin 3 and pin 7

[Littlewyan]

Why is there two wires connected to the anode on each of the output valves? Whats that for? Also what fuses are you using?

[matt h]

(deleted)

[MHProd]

There's a conjunctive filter between the OT's primary. It tames the top end and make it sound more like my originals. Regular fuses. 3A and 500mA.

[Littlewyan]

Well this is a tough one................So with just either the PI or Power Tubes its fine, with both it blows a fuse and obv hums. Now we could be dealing with two seperate issues here so lets just concentrate on the fuse blowing first. With all of the valves in the amp can you measure the B+, then the plate voltage on both power tubes and the B+ after the choke. I think I recall you saying if you lower the voltage using a variac you can keep the amp on without blowing the fuse. Don't leave it on for too long though we want to be quick so we don't damage anything. Perhaps clip your meter in place, turn amp on, get reading then turn off and repeat for all other areas.

This should hopefully tell us what is drawing enough current to blow the fuse.

[MHProd]

I tried that at first. I can read the B+ to the plates using a Bias probe. As soon as the power tubes are warm enough, the B+ starts to oscillate. I don't think I will be able to measure anything stable.

[matt h]

(deleted)

[Littlewyan]

Matt H of course! I have heard of this happening before. I'm willing to bet thats what it is.