blowing fuses

[jetpack]

Still blowing fuses (now the 3A) and I'm still unable to measure any DC on V2 pin8 to ground. When turned on, with the 6ax5gt in place, AC seems to run away, going up and up and up until I turn it off or the mains fuse blows. Maybe there is something obvious that I'm missing. Maybe I've already compounded the problem by destroying my only 2 6AX5GT during this. Maybe I've blown some capacitors. I don't know. I do have a single good 6AX5. I think this is a direct sub, but I don't want to try until I'm sure I won't kill it.
Any help from the kind-hearted experts out there would be appreciated. I've read most of the forum and I know there is a huge knowledge pool!
Thanks.
Bill

[Ty_Bower]

with all tubes out:

V2 p8-ground: 434AC
but 0DC on my meter..?

You should expect to see zero volts DC with the rectifier tube pulled. That's completely normal. What isn't normal is the extremely large AC reading you are getting at pin 8. It should not have any AC present on it at all.

Are you certain your step down transformer is wired properly?

[Shannon Parks]

A stomach virus sidelined me, but I'm back.

Sorry about your rectifiers. I would solder in two 1N4007 diodes for further tests: one from V2-3 to V2-8 and another from V2-5 to V2-8. The cathodes (or banded ends) will be towards V2-8. This higher voltage probably isn't good for your 6A3s (so leave them out until you get another 6AX5GT), but it will get the power supply sorted out.

I agree with Ty that something weird is happening with the transformer arrangement. In particular, I would go over the mains hookup. Do you have a datasheet for your IEC connector? Verify safety ground goes to the chassis. Also, ground J6-2 or J6-3 to the chassis at the same spot.

You might be able to test for capacitor failure in circuit, using the cap function of your meter (if it has one). In particular, C7 (most likely blown cap if any) can be isolated for this test with the choke connections removed.

Shannon

[jetpack]

Sorry about the miscommunication, Ty. Yes, AC measurement on V8 is with the rectifier in place.

I assume the step-down transformer is fine, as well as the power transformer, as the secondary voltages are all okay.

I have J6-p2 running to the chassis ground (which is also mains grounded). Is it correct that I have the transformer's center tap running to J6-P3 (as in photo)?

Thanks. I'm looking forward to getting this running...

[Ty_Bower]

Yes, AC measurement on V8 is with the rectifier in place. .

You shouldn't have AC present at the cathode (output) of the rectifier, regardless of whether the tube is installed or not. Unless, of course, there is some horrible sort of misalignment with the tube that results in a dead short from plate to cathode.

Depending on how your meter is built, it may show an "AC" reading when metered across DC. If that is the case here, maybe things are OK. 330 volts AC should give 466 volts DC after rectification, less whatever losses are inherent in the rectifier tube. The number you report is in range.

[Ty_Bower]

Is it correct that I have the transformer's center tap running to J6-P3 (as in photo)?

Yes.

I don't know if it helps, but here is a photo from the underside of mine.

http://s69.photobucket.com/albums/i43/T ... 120109.jpg

I hadn't yet completed the line side of the wiring when I took the photo. Maybe I'll see if I can take a more recent shot.

[jetpack]

Thanks Ty.
Pictures always help. It seems to me, in my very limited understanding, that it must be a problem with my pa-774. I've ordered the 240V primary transformer from triodeel and a 10 pack of rectifiers. I was planning on buying the transformer on my next trip to the U.S. anyway, so I'm only out the extra shipping cost to get it sent here.
I'll post an update after I receive it and drop it in. It will probably take a couple of weeks.
Cheers.
Bill

[Shannon Parks]

I have J6-p2 running to the chassis ground (which is also mains grounded). Is it correct that I have the transformer's center tap running to J6-P3 (as in photo)?

Yes, I had closely looked at this photo, but it looks like safety ground is run straight to the PCB, and no assurance that chassis is grounded anywhere. Would you confirm zero ohms resistance between your safety ground plug, the chassis and J6-2,3?

The new tranny from Triode sounds like a great idea. It's a shame it costs so much to ship! Be sure to verify your caps are OK in the meantime.

Shannon

[Shannon Parks]

I have J6-p2 running to the chassis ground (which is also mains grounded). Is it correct that I have the transformer's center tap running to J6-P3 (as in photo)?

Yes, I had closely looked at this photo, but it looks like safety ground is run straight to the PCB, and no assurance that chassis is grounded anywhere. Would you confirm zero ohms resistance between your safety ground plug, the chassis and J6-2,3?

The new tranny from Triode sounds like a great idea. It's a shame it costs so much to ship! Be sure to verify your caps are OK in the meantime.

Shannon

[jetpack]

Yes, zero ohm path between the ground pin of my IEC socket - through the chassis (with a star washer) to J6-p3.
Bill

[jetpack]

With a new transformer, everything works like it should with both channels sounding good. Thanks again for the previous input. Bill

[Shannon Parks]

Awesome news, Bill. Time to enjoy some music.

Shannon