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Old 4th July 2015, 02:17 AM   #1
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Default Need help diagnosing no sound / blown fuse issue

Hi all,

I have a Tubelab SPP that I built about 4 years ago. All has been well until I moved cross country. I removed the tubes, and packed the amp very carefully with a lot of padding before putting it in a top box in my shipping container.
The box was not crushed, and there was no apparant damage to the amp. After putting the tubes back in, I powered it up and it worked perfectly for about an hour. Then, all of a sudden, no sound. No hum, hiss, nothing even at full volume.

I did some simple debugging and found that the R1, 150ohm resistor has totally open. I had already moved it off the board and bolted it to the inside of the case with a big washer years ago because ran so hot. In place of it, I ordered a 25W, 150ohm dale. Put the tubes back in, and before I had a chance to listen for sound, a fuse blew. I noticed a spark in the GZ34 rectifier, and figured it was shorting. So I removed all the other tubes, leaving just the rectifier tube, and it blew a fuse again. Put all tubes back in except the rectifier, and let it sit for several minutes, and it did not blow a fuse. So I thought I had it isolated.

It had been years, so I just replaced all the tubes, as follows:
GZ34 rectifier
EL84 power tubes
12AT7 pre tubes

The guy I bought the new tubes from even ran all the new tubes through a tube tester he had, just to be certain the weren't the issue (all Mesa's froma guitar store - I understand they're just rebranded "hand tested" mix and match).

So I put the new rectifier in, no other tubes, and it blew another fuse. WTF mate? So then, I measured the resistance from the case to each of the leads of the dale, and one of the leads was reading only a 20kohm resistance - obviously it was leaking to the case. So I removed it from the case, and isolated it floating about 1/2 inch above the board with some sturdy, fabric insulated wire scrap I have. Not a permanent solution, but I figured I could at least get it back up and rounning while I get it replaced. Powered it back up with just the rectifier, and it was fine! I figured I solved the problem. Put the pre amp tubes in, powered it up, still fine after about a minute. Then I put the power tubes in.. and after about 10 seconds it blew a fuse. Unfortunately, I didn't have time to do a sound check and I only have 2 fuses left (these are 2amp slow blows). I'll have to get some more this weekend before I try anything else.

I'm past my knowledge of what else could be shorting and drawing a current. Anyone have ideas of what else I could test for?

Just for more info, I have all Edcor transformers:
XPWR008-120, and XPP25-8CT-7.6K for the output transformers.

Thanks so much!
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Old 4th July 2015, 03:20 AM   #2
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Just a quick update. I've tested every resistor, and they are all reading the correct values. So no problems there. Seems like the only thing left could be the caps, or the transformers. Any tips on testing the caps or transformers would be much appreciated.
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Old 4th July 2015, 10:16 PM   #3
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Another update: I've removed every capacitor from the board, and tested it with the capacitance setting on my multimeter. Every single cap is reading the correct value. So there is nothing wrong with anything on the board.

I can safely put in the rectifier, and the pre tubes, without the fuse blowing. But now, once I put in the brand new EL84's I bought, I see an arc in the rectifier after about 10 seconds and the fuse blows. Does this mean the power tubes I just bought are bad?
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Old 5th July 2015, 12:20 AM   #4
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Quote:
Originally Posted by webspinner View Post
Another update: I've removed every capacitor from the board, and tested it with the capacitance setting on my multimeter. Every single cap is reading the correct value. So there is nothing wrong with anything on the board.

I can safely put in the rectifier, and the pre tubes, without the fuse blowing. But now, once I put in the brand new EL84's I bought, I see an arc in the rectifier after about 10 seconds and the fuse blows. Does this mean the power tubes I just bought are bad?
It would help to have a schematic, and some images.
here is a Quad 2 showing GZ34 quad2powerampmods

If all that has happened is moving from one location to another,and voltage is the same
perhaps incorrect polarity from a wrongly wired extension cord. Its a bit left field
but worth checking. Even Grand Funk Railroad famously documented incorrect polarity
on their Live Album.
Cheers / Chris
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Old 5th July 2015, 03:00 AM   #5
Fusor is offline Fusor  United States
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With the cross country move and jostling I would have looked for a small piece of metal or solder working loose and shorting, or a bad tube socket connection. Maybe one of the many board connectors getting loose.

You said your resistance reading from R1 to the case [ground] was 20kohm. When I try to measure this my digital meter goes a bit crazy as it tries to charge the two caps, C1 & C2. Either side of R1 should show the series & parallel resistances of R2,R1, R4 & R3 - about 81kohm to ground. If you could read it without the caps.

Without the big caps, doing ohms checks at the tube socket pins would also be fairly straight forward.

I don't have your particular output transformers but would guess unconnected each tap roughly measures 50 ohms DCR. [transformer plate, screen, B+, screen, plate - 50,50,50,50 ohms DCR approximately]

A multi-meter measures a cap at say 9 volts? It can't test for a high voltage leak/short.

Did you do voltage tests with only the rectifier tube installed? [at the 12at7 socket pins 1&6 and the 6bq5 sockets pins 7&9]

Did you try just using one channel with tubes during a power test? I.E. the left side 12at7 & two 6bq5 only and then only the right side tubes.
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Old 6th July 2015, 05:02 PM   #6
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The arcing in the rectifier happens independent of what side I put power tubes on. However, if I let it continue arcing for about 20 seconds, it stops. If I leave all the power tubes out, everything is fine. I've also tried different rectifiers and pre tubes.

The voltages on the 12at7 are identical on both side: pin 1: 280v, and 6: 125v

I added two 1n4007 diodes to the red leads out of the power transformer going into the rectifier, per a suggestion to help with the arcing from George. This signifigantly reduced the arcing, to the point where it finally stopped. Now when I turn the amp on, it doesn't arc at all and it works fine with all power tubes in!
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Old 6th July 2015, 05:35 PM   #7
kevinkr is offline kevinkr  United States
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You should now replace the rectifier tube, any 5AR4 that has arced is suspect.
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Old 7th July 2015, 02:09 PM   #8
DIYBras is offline DIYBras  Brazil
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Quote:
Originally Posted by kevinkr View Post
You should now replace the rectifier tube, any 5AR4 that has arced is suspect.
It also occurs with several indirect heating rectifiers. In my SE amplifier with 300B I use a pair of PY88 in the rectification. One day a supply capacitor shorted and did produce a lot of arcs in one PY88 before blowing the primary fuse. After this incident, this PY88 began arcing at startup, and I was forced to replace it.
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Old 7th July 2015, 11:29 PM   #9
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Originally Posted by kevinkr View Post
You should now replace the rectifier tube, any 5AR4 that has arced is suspect.
+1 - could just be a bad rectifier and that's all.... I would put some coin on a bet there. Once they arc they are bad and will be prone to do it more often.

I install CL-90 inline with the 5V going to the board - that will slow the current down enough to give the 5AR4 a fighting chance at start up.

Seems in your case there's actually a short inside the tube. Check all solder connections to the rectifier tube - arcing causes the wires and the connections to heat up which is one reason why your fuse blows. So every solder joint from the AC socket to the rectifier should be looked at closely to be sure all solder joints & wires look good/are good. Tighten up any wire clamps too if you used them.
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