shorted GZ34?

[87MJ]

I'm troubleshooting my AB763 build (first build).  Worked fine with (some minor tweaking) for about 2 weeks on the bench (probably 3 hours of total playing time).  So I finally buttoned things up and put it into the cabinet.  And of course, the first time I turn it on with the chassis in the cab, the fuse blows.  I'm guessing I must have shorted something when tightening all the bolts down and installing it in the cab.  I think it was a power rail turret on the board hitting a chassis bolt.

I fixed that, checked for other possible shorts, and then pulled all the tubes out and did another start up check.  Turned on to STANDBY.  Voltages were normal out of the PT, lamp lights up, no fuses blow.  So next I installed the GZ34 rectifier, and powered up to ON.  I had enough time check the B+ voltages (which were good), then the pilot light flickered, and the fuse went.  Probably 30 seconds elapsed from ON to dead fuse.  Replaced the fuse.  The next time I just put it on STANDBY with the GZ34 in, and let it sit.  After about 30 sec the light flickered, and fuse is shot again.  I then traced and checked as much as I could without starting to unsoldering alot of things.

So next, I checked the resistance across the GZ34 pins (tube was pulled out of the socket for measurements).  Pins 2 and 8 read 0 ohms, which makes sense.  Read an open circuit between pins 4->2 and pins 4->8.  But then I was reading 70-130ish ohms between pins 6->2 and pins 6->8 (resistance jumped around, esp. if I sort of shook the tube).  I'm guessing this is a shorted tube?  I don't have a spare to check against.

Also if it is a shorted tube, could a short from the power rail to ground have caused it?  Or should I keep poking around looking for problems?

Sorry, I got a little long winded there...

Thanks for any input or advice.

[Jack1962]

Short answer: yes it could

                                          Rock On

[tubeswell]

FWIW, I do all my tube-rectified amps with SS diode protection on the HT winding (after R.G. Keen's 'immortal amp' mods #1- or was it #2? can't remember which). It stops you having to worry so much about shorted rectifier tubes destroying your filter caps (which it will do if it shorts and dumps high VAC onto the filter caps  - because they ain't designed to cope with High VAC) and then in-turn destroying your PT (because the HT then shorts to ground through the shorted filter cap(s)).

So I'd start with replacing your rectifier and then your filter caps, because that's quite possibily why your fuse keeps blowing. JM2CW

[Willabe]

Are you using a lamp limiter on first power up? Easy to make, easy to use and could save you a lot of $ if you do have a short in your amp.

               Brad       

[Geezer]

Are you using a lamp limiter on first power up? Easy to make, easy to use and could save you a lot of $ if you do have a short in your amp.

               Brad        

A limiter will save you $$ on fuses, time AND possibly your PT (if the fuse doesn't do it's job in time) as you can quickly see if the amp is drawing too much current & shut down to find the problem, & keep doing this until the problem is corrected.
The parts can be purchased at any hardware or "big box" home supply store & should cost ~~ the same as a few fuses.

This thread has info on the Lamp Current Limiter>

http://www.el34world.com/Forum/index.php?topic=646.0



Here's a diagram Doug drew up several years ago........

[jeff]

FWIW, I do all my tube-rectified amps with SS diode protection on the HT winding

Is this just using diodes in series between the HT taps and the rectifier tube's plates?
Do you just run the AC to unused pins of the rectifier socket then a diode between those pins and the plate's pins?
What diodes do you use?
Do you have a schematic?

[Willabe]

That drawing is from a GW book. Notice the last sentance, "prewired in parallel" I never could figure out why he wrote that. Although, yes they come prewired in parallel,  there are 2 tabs, 1 on each sides +/- inbetween the 2 wire hook up screws that snap off. Snap those off and now they are 2 independent seperate outlets. There made that way so you can always keep 1 hot and the other on a wall switch, say for a lamp you can turn on when you walk into a room or for your bedside light and clock radio.
I used a standard duplex outlet in my lamp limiter, just snaped off the 2 little tabs, works fine.
 
               Brad                

[jeff]

Notice the last sentance, "prewired in parallel" I never could figure out why he wrote that.       

I think he wrote that so you'd know you have to modify a standard outlet. I didn't realize sockets were prewired in parallel so I didn't know enough to break the tabs. I found out the hard way what he meant. BZZZZT!

[tubeswell]

FWIW, I do all my tube-rectified amps with SS diode protection on the HT winding

Is this just using diodes in series between the HT taps and the rectifier tube's plates?
Do you just run the AC to unused pins of the rectifier socket then a diode between those pins and the plate's pins?
What diodes do you use?
Do you have a schematic?

Yep Yep 1N4007 Yep (- see attachment)

[whittele]

Is it a JJ 5AR4?

[87MJ]

Thanks for the input, and the info on the lamp limiter and diodes. 

I'm going to pick up an new tube this evening, and see if they have those diodes at RS.  In the mean time, I'm going to scavenge for some electrical parts in the basement for the lamp limiter...

Also, RG Keen's "Immortal Amp" articles that Tubeswell referenced:

http://www.premierguitar.com/Magazine/Issue/2008/Jul/The_Immortal_Amplifier_Mod.aspx

http://www.premierguitar.com/Magazine/Issue/2008/Aug/The_Immortal_Amp_Mods_Pt_2.aspx

http://www.premierguitar.com/Magazine/Issue/2008/Sep/The_Immortal_Amp_Mods_Pt_3.aspx

http://www.premierguitar.com/Magazine/Issue/2008/Oct/The_Immortal_Amp_Mods_Pt_4.aspx

[87MJ]

Is it a JJ 5AR4?

Its a JJ, box says GZ34, but I think its the same as the 5AR4.  Why do ask?

[Tone Junkie]

If it helps to have a picture like me look at the way ceriatone does it on there layout.
http://ceriatone.com/images/layoutPic/marshallLayout/JTM45ceriatone.jpg
 Bill

[bluesbear]

I was really sad when my Mullard GZ34 shorted. It had cost $95!! Then I tried a $20 Sovtek. Sounds the same and has lasted longer.
Dave

[tubeswell]

Then I tried a $20 Sovtek. Sounds the same and has lasted longer.

+1. Nothin' wrong with Sovtek (in terms of modern tubes) reliable work-horses that they tend to be.

[jeff]

Then I tried a $20 Sovtek. Sounds the same and has lasted longer.

+1. Nothin' wrong with Sovtek (in terms of modern tubes) reliable work-horses that they tend to be.

Just watch out for thier 5Y3s. They don't drop as much. Got 420v B+ with Sovtek, 360V B+ with NOS in a Champ.

[tubeswell]

Then I tried a $20 Sovtek. Sounds the same and has lasted longer.

+1. Nothin' wrong with Sovtek (in terms of modern tubes) reliable work-horses that they tend to be.

Just watch out for thier 5Y3s. They don't drop as much. Got 420v B+ with Sovtek, 360V B+ with NOS in a Champ.

A Sovtek 5Y3 is not like any other 5Y3. Its more like a 5R4 or 5V4 or somethin (can't remember which off the cuff). It will still work tho' - albeit at a higher B+.

[GuitarPilot65]

Great discussion, folks.  Helps me with my forthcoming conversion.  Definitely having my current limiter connected upon first throw of the Standby switch!

[87MJ]

Back in business!  It was the rectifier tube.  I installed the diodes coming off the PT first, then walked through the startup with a newly fabricated lamp limiter.  Pretty cool to watch how the current changes as it starts up.  Anyway, thanks again for the help.

[jeff]

 I get the diodes protect if the tube shorts but what about this? If the tube does short then it's ok because of the diodes so it's still making sound. You might think "something sounds different" but you might not think you've blown the tube and check/change it? Now you have 460V instead of 400V because you're not dropping voltage with the tube rect. The bias is set expecting 400V instead of 460V.

Problem?

[whittele]

I have had a nearly 90% failure rate with the JJ's 5AR4 (12) on amps that use more than 20uF for B+.  In the same amp with no work I put a Sovtek and no issues.  I quit using the JJ's 5AR4.  You can google similar issue with many folks.  I do love JJ's 6V6 so no JJ hating going on here.

[Willabe]

Back in business!  with a newly fabricated lamp limiter  Pretty cool to watch how the current changes as it starts up. 

Alright!          How's she sound?

Yeah, it is a pretty cool little tool. Guys have been using them for decads. Simple but work great.       


                  Brad         

[87MJ]

Alright!          How's she sound?

Haven't had a chance to crank it up yet - hopefully this weekend I can put it through the paces...

I sort of "tweedified" the normal channel - that is probably open to some tweaks, I'm not crazy about how it sounds so far; not bad, just ok.  Might sound better at louder volumes.  I'll probably post looking for suggestions on that in the near future.  Vibrato channel so far sounds pretty much like a DR, which is good.  I have been playing through a bandmate's blackface'd SFDR - I'm curious to do an A/B between the two next time we practice.

[tubeswell]

I get the diodes protect if the tube shorts but what about this? If the tube does short then it's ok because of the diodes so it's still making sound. You might think "something sounds different" but you might not think you've blown the tube and check/change it? Now you have 460V instead of 400V because you're not dropping voltage with the tube rect. The bias is set expecting 400V instead of 460V.

Problem?

Possibly, but the voltage difference depends on the type of tube rectifier. With a 5Y3GT it could mean 90V difference, but in a cathode biased stage bias won't be much of a problem. Other rectifier tubes will make for less of a difference. Here we are talking about a GZ34, which would amount to about 30V more. Most tubes will still run albeit somewhat hotter and will probably survive for the rest of the gig. Some might even crash if they are biased hot anyway, but the diodes are there to protect the PT which is the most expensive part of your amp. Tubes you can more-easily replace. Besides, there are other steps you can take to 'protect' your tubes against bias-related failure.