TubeLab SE blowing fuse on B+ ramp up

[yasofnos]

Please help!

I recently finished assembling a TubeLab SE (will run 300b's), using a Hammond 376x power transformer. The transformer is putting out 700 V with no load (as George warned on other posts this is a known issue with some of the Hammonds). The B- is -280 volts.

When I powered the unit up with only the rectifier tube installed, the B+ went up to 400 volts (I think, I was hiding behind a couch some of the time during the first power up!), then the 2 amp fuse blew.

Now when I power up with just the recitifier tube, the B+ climbs only to 35 volts or so and then the fuse blows.

I am thinking that I had some bad filter caps, or they were damaged during initial power up? They are 100uf 450v Nichicon low impedence.

Am I heading in the right direction? Other thoughts on things to check before swapping the filter caps? Any help would be appreciated.

[rknize]

Are you using a choke instead of a resistor in the B+ supply? Try disconnecting it. Something is shorted...could be a cap. Usually they smoke or leak or to something nasty when they go, but not always. Also try disconnecting the output transformers,

[whitelabrat]

What rectifier are you using? I think your cap ratings are fine if the B+ stabilizes at 400v.

Photos?

[yasofnos]

Thanks for the responses. Yes, I am using a Triad choke, 150 Ohms. I measured it at 150 Ohms while in place in the board.

The rectifier is a new JJ GZ34. After about 10 seconds the power starts coming up, then "pop".

Am I correct in thinking that it would be the filter cap's that are most suspect?

I will upload pictures tonight.

[rknize]

Try disconnecting the choke. That will only leave one cap and a resistor on the rectifier. If it still goes, look closely at your resistor values and the orientation of that capacitor. Also note that a lot of people have been having issues with JJ GZ34s lately.

[yasofnos]

I removed the choke completely, and before putting in a resistor in place for R4, gave it power. Well, the fuse didn't blow, but of course no B+.

I put a 150 ohm resistor in place of choke for R4, and the fuse blew as before.

I then replaced R7 with another 100k ohm resistore, fuse blew as before.

My next step was going to be removing output transformers from the board, but I ran out of fuses for tonight.

I noticed one thing that I think might be a problem, though not sure if it is related. I measured the orange output tranformer leads (while connected to the board), and there is continuity between the left and right OPT orange leads. Its late, so perhaps I am not thinking straight, but I did not expect continuity there. Any thoughts?

P.S. I need to downsize photos a bit before it will allow upload.

[rknize]

R7 is in the bias supply, which uses the FREDs. It's R30 that should be scrutinized as it shunts B+ to ground. Also check that the CCSs and FETs are installed the right way and that the bias pots are turned all the way counter-clockwise.

Not sure what OPTs you are using, so I don't know what the orange leads are. Are these in the secondary? If so and the other side of the secondaries are grounded, you would measure continuity between both secondaries via ground.

[nic6paul]

Check you wiring for your opts. I did something stupid and my symptoms were the same as yours. Are you doing an under mount chassis? If so make sure you are paying attention to where the wires go. Sounds dumb but is easy to mix up.

[oldmanStrat]

If there is a short - and it sounds like there is one, you should be able to find it with an ohm meter and not blow fuses after every guess

You should have a non-zero resistance from B+ to gnd - and if not, isolate one component at a time that uses B+ until the short goes away...

[yasofnos]

I went through the entire circuit this evening with my meter. I followed every single path from the transformers and inputs back out to the speakers. All diodes test fine, all resistors (that are measurable on the board) test out fine. Everything on the circuit diagram that should have a path to ground, does.

There are no shorts, I was mistaken regarding the orange output transformer leads being shorted, there is exactly 20ohm between them as expected with the pair of 10 ohm resistors that separate them.

At this point I am very suspicious of the rectifier tube, given that others have had issues with the JJ GZ34, and that no other issues stand out.

I have ordered a Sovtek 5AR4, will pop that in Tuesday night, if that does not fix it, I am really at a loss. IC chips perhaps?