Help me troubleshoot. Blowing tubes and fuses

lgehrig4 · Post by **lgehrig4** » Sat Apr 12, 2014 3:02 pm

This is a Concorde build. The voltages seem fine (see below) and it sounds good but I blew 3 pairs of EL34s and 2 HT fuses. I originally tubed it with JJ EL34Ls and everything was fine. It was initially noisy when fired up but it quieted down and really sounded good.

I wanted to try Winged Cs and when I put the volume to 12:00 the tubes flashed and the fuse blew. I figured the tubes were on their way out anyway so I put the JJs back in and the same thing eventually happened but without the big flashing in the tubes. So I put in a 3rd set and after some time being on I hear a faint ticking and as the ticking was getting louder I noticed a blue glow flashing in the center of the tubes in time with the ticking.

When I initially turned it on and it was noisy someone suggested that I just needed to reverse the OT leads to the power tubes. I didn't do this because it quieted down but if they were reversed would the amp even sound good, which it does? Would these leads be the cause of the tubes and fuses blowing?

Where should I start checking?

Please note
The ticking is a low, deep ticking if that matters
The amp is actually Concorde trannys and power section with K60 preamp
From OT the blue lead is on V5 and brown on V4

V1
Pin1=333
Pin3=25
Pin6=285
Pin8=1.99

V2
Pin6=403
Pin8=4.37

V3
Pin1=313
Pin2=37
Pin3=58
Pin6=292
Pin7=36
Pin8=51

V4 & V5
Pin3&4=487
Pin5=-48

B+ from the three leads going from the power to preamp board= 481V, 452V, 429V

M Fowler · Post by **M Fowler** » Sat Apr 12, 2014 3:07 pm

Right now all I can suggest is to walk away from it if you been at it a long time and then come back all fresh and retrace the entire circuit to make sure it is wired correctly and checking each solder joint as well as looking for any wire that may be touching the chassis or something it should not be. This happens to all of us.

Mark

Phil_S · Post by **Phil_S** » Sat Apr 12, 2014 5:09 pm

I have found that it is useful to check the circuit to the schematic at least twice. One time starting at the input jacks and the second time starting at the speaker output. You'd be surprised what you see when you are working backwards.

lgehrig4 · Post by **lgehrig4** » Sat Apr 12, 2014 7:40 pm

I've done this many times. During the build I do that with every section that I build and then another 3-4 times before I fire it up.

I haven't looked at it since last week so time off won't help. If something is wired incorrectly wouldn't that show with voltages that are off or weird sounds?

What about my questions about the OT leads possibly being reversed? Any technical advice that you can give me? If this were your project and you founds that it was wired correctly, what would you check given the issues described?

Banzini · Post by **Banzini** » Sat Apr 12, 2014 8:22 pm

I had an issue like this years ago w/ an old bf super reverb. IIRC the problem was a bad choke. Sounds like your problem is in your HV supply somewhere.

lgehrig4 · Post by **lgehrig4** » Sat Apr 12, 2014 9:24 pm

Thanks. I'll point my research in that direction and see what I can find.

lgehrig4 · Post by **lgehrig4** » Fri Apr 18, 2014 3:07 am

Ok, I really don't know where to start researching information that would help me diagnose this. I did a search on "chokes" and none of the threads really spoke of this.

If my voltages are correct (see above) how can the tubes burn out? If there is a short would't that reflect in the voltage readings on the tube sockets?

Can someone start me off on something specific to measure or check? I went over the layout 2 more times and I cannot find anything wired incorrectly, based on the layout and photos I have. If there's a cold solder joint, again wouldn't that reflect in the voltages?

When I first fired this amp up I played it for a while and had no problems. Had is cranked to about 2-3:00. I put in a pair of SEDs and played low volume for a while. When I turned the amp to 12:00 and hit a couple chords they flashed and the fuse went. I put the JJs back in and then the same happen to them (without the bright flash). I tried another pair of JJs and while playing I heard the low ticking and saw a blue glow in the center of the tubes pulsing with the tics.

I'm at a complete loss and just need something to get me started.

M Fowler · Post by **M Fowler** » Fri Apr 18, 2014 4:19 am

Post photos of your build and we will see what we can find.

tubeswell · Post by **tubeswell** » Fri Apr 18, 2014 5:45 am

And we need to know the g2 voltages for the output tubes (as well as the Plate and grid voltages). You also should test Pr:Sec VAC ratio of the Output Transformer (both sides of the primary should be the same)

armillary · Post by **armillary** » Fri Apr 18, 2014 12:36 pm

Flashover from bad tube socket? Check for tracing on the sockets surfaces?

johnnyreece · Post by **johnnyreece** » Fri Apr 18, 2014 1:56 pm

Personally, I'd disconnect the NFB if you aren't sure about the OT primaries. Then you can see if you have a problem elsewhere. It's quick and easy, and takes that completely out of the equation.

lgehrig4 · Post by **lgehrig4** » Sat Apr 19, 2014 1:38 am

Banzini · Post by **Banzini** » Sat Apr 19, 2014 4:02 am

Did you check all of your HV supply and your bias supply? The blue flash is most likely caused by the screens losing voltage - caused by a short in the choke or a bad filter cap. Is the flash in both outputs or just one tube?

lgehrig4 · Post by **lgehrig4** » Sat Apr 19, 2014 4:07 am

I checked the voltages on all the filters and the wiring scheme. To my knowledge everything checks out.

The flash occurred only once when I had the Winged Cs in and it was both tubes. Before I tried those tubes I had JJs and the amp was working fine. Since that first incident I've been having the issues described.

Banzini · Post by **Banzini** » Sat Apr 19, 2014 4:14 am

Well, not much more to say other than you've got a bad component somewhere in your power supply. Bypass the cap at the node that feeds the screens w/ an equivalent cap and see what that does for you.

The Amp Garage

Help me troubleshoot. Blowing tubes and fuses

Help me troubleshoot. Blowing tubes and fuses

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Re: Help me troubleshoot. Blowing tubes and fuses

Re: Help me troubleshoot. Blowing tubes and fuses

Re: Help me troubleshoot. Blowing tubes and fuses

Re: Help me troubleshoot. Blowing tubes and fuses

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Re: Help me troubleshoot. Blowing tubes and fuses

Re: Help me troubleshoot. Blowing tubes and fuses

Re: Help me troubleshoot. Blowing tubes and fuses

Re: Help me troubleshoot. Blowing tubes and fuses

Re: Help me troubleshoot. Blowing tubes and fuses

Re: Help me troubleshoot. Blowing tubes and fuses

Re: Help me troubleshoot. Blowing tubes and fuses