Seymour Duncan eating rectifier tubes

[davoux]

I just got a Seymour Duncan convertible 100. Supposedly was working before shipping. When I got it I plugged it in and had some arcing in the 5U4GB then nothing but the fan, which led me to believe the fuse was good. Fuse read 3 ohm I thought, but it must have been 3 meg. When replaced, the heater circuit came up fine so I thought I had fixed it and put tubes back in including a new rectifier (5U4GB). When I turned on the power switch there was severe arcing in the rectifier and some snap, crackle, and pop.

With the 5U4GB out, I read .5 ohm from pin 2 to 8, 34 ohm pin 2 to 4, 36 ohm pin 2 to 6, and 36.3 ohm from pin 6 to 8. Does this sound like the trans. secondary is intact? Once I get more fuses tomarrow, what volt readings should I get pin to pin or pin to ground?

Any best guesses where I could look for the problem? I'm so looking forward to pulling that PCB what a pain in the ass it looks to be. Last time I work on anything other that point to point.

TNX for your help

[Enzo]

The recto tube arcs because WAAAAAY too much current is flowing through it. That is to say the B+ is shorted to ground somewhere. Could be the power tranny or elsewhere. Can the amp power up WITHOUT the rectifier tube and not take a fuse out? I mean with BOTH power and standby in operate position.

With power removed You can measure the HV secondary which is wired from pin 4 to pin 6 of the recto tube. I assume there is a center tap that is grounded. HV are red, and CT is red/white. SO there should also be a reading from chassis to either of those pins.

BUT. You get .8 ohm across 2 and 8. That is OK, that is a reasonable reading for the 5VAC winding. But you also get 36 ohms from that winding to the other. THAT is not good. Verify that, is there low resistance from pins 2,8 to chassis? That sounds like the 5v winding is grounded. This can happen a couple ways.

Make sure there are no shorts on the tube socket or arc marks.

If the 5v winding shorts to the transofmer frame, it won't matter with the recto tube out. ut would be no different from the grounded 6v winding on many amos. But when the recto is in there, it is trying to put 500 volts on that winding - as well as on the B+ rail. So if it shorts to frame, it is grounding off the B+. In this case the standby switch makes no difference.

If the main filter caps are shorted, that too will ground off the B+. The ohm meter might tell, but of course the ohm meter can't differentiate between winding shorted to frame and shorted filter caps. Although the trany short likely will be very low resistance, and the filters being two in series will likely have a higher fault resistance.

If the standby switch matters, then first thing to look at is the power tubes. Got one with a broke off center pin, and it is in the socket wrong? Or it could just be a bad one. Does pulling the power tubes out stop the recto light show?

The output tranny could be bad. A winding short to frame there would be as bad as one in the power tranny. Also some versions of the drawing have the pentode/triode swith mis drawn. One position of hte swithc leaves the screens unterminated and the OT primary winding partially shorted across. DOn't know if that would arc the recto, but it won't work right. If someone wired it like the schematic, change it.

[Enzo]

Oh, and check the output tube socket for arcs and shorts too.

[Alf]

I'm so looking forward to pulling that PCB what a pain in the ass it looks to be. Last time I work on anything other that point to point.

You're right there and take care when soldering , the pcb copper traces will come off
if you don't take the greatest care ! Happened to me in a convertable 60.
I will never work these amps again.

Good luck, Alf

[davoux]

Further Info

I got some more fuses and tried turning both the power and standby switches on. I had all tubes and modules out. It did NOT take out the fuse. I tried it first with both jack1 and jack 3 disconnected to isolate the problem away from the PCB. Then I plugged in the jacks. It did not seem to make a difference. One thing of note, though I don't understand it, is that about 1/3 of the time when switching the power switch EITHER on or off, the house GFI trips.

Here are the various readings from tests from the recto pins. No power on I get .5 ohm from pin 2 to grd. ; .3ohm from pin 8 to grd. ; 33.5 ohm from pin 4 to grd. ; 35 ohm pin 6 to grd. ; and 68.5 ohm from pins 4 to 6.

Here are the reading with power and standby switches on. From pin 2 to grd 5.35 vac ; pin 8 to grd. .01 v ; pin 2 to 8 5.35v ; pin 6 to grd. 489v ; pin 4 to grd 489v ; from pins 4 to 6 900+ v (didn't leave it on long meter beaping).

I looked carefully at the sockets and I don't see any obvious arcing or burned traces. None of the power tubes have a broken center pin so they can't be in wrong. With the rectos so arced out, is there any point in putting in only the recto without the power tubes, would it still arc?

I understand how a bridge rectifier works, but I don't understand how the tube or that circuit works so I'm just swimming in the deep water without a clue. Does any of what I found give good clues where to go other than selling it on ebay as is ?

TNX for any help

[Enzo]

Yes, as surmised, your B+ is shorted to ground. Pins 2 and 8 of the recto are its heater, and the heater winding of the power tranny. They will have a very low resistance to each other because you are measuring the heavy wire of the transformer winding. But they DO NOT want to be grounded. That 5VAC sits floating 500VDC above ground. If it shows shorted to ground - as your measurements show - then either the winding is shorted to the frame, or the filter caps are shorted or the wiring is shorted to ground somewhere.

Rectifier tubes are simple. If you understand diodes, all the recto is is a pair of diodes. Imagine two diodes, one each connected to pins 4 and 6. Anode end to the pin. Then the cathode ends are connected together and go to pin 8 or 2, whichever they chose. That is what the tube is. The heater IS the cathode on the recto tube. In fact you could wire exactly that in place of the tube and the amp would work - assuming it worked with a tube there as well already. That is what is inside those solid state plug-in recto tube replacement things.

There should be a pair of wires to pins 2 and 8 from the power tranny. There is also a wire from either 2 or 8 going off to the filter caps and elsewhere. Disconnect this lonesome wire. NOW measure pins 2 and 8 to ground. If they still show low resistance to ground, the transformer is shorted to the frame. Or perhaps there is an unused center tap that someone wired to ground. They shouldn't do that.

[davoux]

I pulled the wire going from pin 8 to the standby switch which would take the filters out of the circuit. When I measure 2 and 8 to grd. they still read about .5 ohms. I assume this means the trans is shorted. Any fix other than new trans? From what I understand, no one makes these so I don't know where I would find one.

Could this kind of trouble come from shipping? Could a banged up recto cause this problem when plugged in? The shipping was insured, but the box didn't show signs of anything other than a flat drop.

Right now I'm a little discouraged. Even if I found a trans, some of the solders were be near impossible to get at and I don't imagine its a good idea to cut and splice the new leads.

Enzo, thanks much for your help!!!

[Rick Erickson]

Do you have any recourse with the seller? At this point I would recomend having the amp looked at by a professional service shop before assuming that the PT is bad. As for cut-and-splice: IF the job is done correctly there shouldn't be any danger. That would be a last resort approach for me, but if the splices are done well and insulated with heat-shrink tubing it should work fine.

RE

[Enzo]

You want to be sure, so disconnect those wires from the recto socket. We want the wires from the tranny going nowhere but the sky. Stick them in mid air. Now see if they are still shorted to frame. From your resistance readings it sounded like the one to pin 8 was grounded. SO look closely to make sure there was no little bead of solder or something shorting the socket pin to chassis. If they still have continuity to frame while in the air, then the thing is shot.

What is the part number stamped into that transformer?

[Trout]

Hi, I just signed up when I saw This Thread, I couldnt resist.

I have a Seymour Duncan Convertable 100W I purchased new back in the 80's.
Every once and a while I pull the ole girl out and rip it up for a few hours. I can atest to the 5U4 arc issues. Mine has done that ever since it was new, The only rect tube I have even been able to use in the amp was limited to New or very close to new Sylvania Tall Bottle black plates. Everything else would arc to varying degrees. Why? I have no idea, But Looking at the PS schematic, It reads worse than VCR instructions haha

It has always done this, From day 1.

My amp also pops the ground faults at equipped outlets also.
In case you have not seen this site, There is an excellent information source compiled by
Paul Marossy at http://www.diyguitarist.com/ .
I have built several modules for mine based on info from that site.

Anyway, I felt it was a design issue in the PS, Though I would like to degug it also.
Trout

[Amp Kat]

I think one of the problems is the high capacitance of the first filters in that 200uf 350 volt cap arrangement. The first cap has about 650 volts on it's positive side and although the voltage divides down to 350 volts it's that first side that may not be happy. A coinsidence is the tube in the one I'm working on is a Sylvania black plate I brought from the house because the other one was arching like a frankenstein movie. Another problem I found with this amp is the connections on those 200uf 350 volters are proned to touching the chassis especially if replaced because the caps dont' go high enough up into the chassis and it pushes the connector closer to the chassis which has the HV supply there. I put some electrick tape under those connectors and on the connector itself to isolate it more from the chassis. The PCB connections can break off very easy so care needs to be taken if you pull that board. Lots of connections in this amp that can wreak havoc and the modules are not very stable in there by themselves and the mods on the website provided above are a plus for stable operation. Amp sounds great when working properly but age can make it a TLC type of amp tstl.

[Trout]

Ya Know?

They had some great ideas back when they built to Convertible 100. The Triode/Pentode switching, The Module concept, The Variable output, Fairly nice effects loop and switching. Its a shame some of the finer details were not executed better.

I have had persistent random noise issues in mine for over 6 years, weird scratching noises and random popping noises. I figure one of these weeks when I have nothing else going on, I will pull the chassis, Service the caps, the pots and all those jacks that I suspect are contributing to the issues.

I converted a couple of normal modules into classics, Built from scratch a presence and a hi-gain. I dreamed of having a full set at one time, but I started building amps for fun and the Duncan ended up spending the better part of the last 5 years in the closet.
I thought about selling it, But I got it new($999.00) and no way could I get near that for it.
Trout

[slajeune]

Hi,

I think that Amp Kat is on to the best lead. The 2 x 200uF arragement in such a fashion gives 100uF from the rectifier's tube perspective.

Looking at this: http://www.mif.pg.gda.pl/homepages/f.../001/5/5U4.gif

The maximum capacitance AFTER the rectifier tube is 32uF.... what the rectifier is seeing is WAY too much capacitance.

I would replace the 2 x 200uF for 2 x 47uF. This will give about 22uF for the rectifier. Way safer than the 100uF you see now.

I am wrong in thinking this? That would explain why the rectifer arced since day one for Trout.

cheers,
Stephane.

[Trout]

Hi,

I think that Amp Kat is on to the best lead. The 2 x 200uF arragement in such a fashion gives 100uF from the rectifier's tube perspective.

Looking at this: http://www.mif.pg.gda.pl/homepages/f.../001/5/5U4.gif

The maximum capacitance AFTER the rectifier tube is 32uF.... what the rectifier is seeing is WAY too much capacitance.

I would replace the 2 x 200uF for 2 x 47uF. This will give about 22uF for the rectifier. Way safer than the 100uF you see now.

I am wrong in thinking this? That would explain why the rectifer arced since day one for Trout.

cheers,
Stephane.

You may indeed be very correct,
I had found that when using a brand new Sylvania 5U4 the arc issue would go away, For A While, Then after a good number of hours, It would happen more and more often until I replaced the tube.

BTW, that arcing is brutal on power and output transformers, I ruined a brand new bandmaster power trans like that.

I have seen that exact symptom in a couple of amps I built that had way to much capacitance combined with a low decoupling resistor it was a huge issue. I fried several 5Y3's trying to run 47uf,r100,47uf,r100 47uf. That is not even close to how high the Duncan is.

In all honesty, that much capacitance is not required for fairly good filtering and it makes no sense that they had it that high.
This might be just what mine needs.
Trout

[davoux]

Sorry guys, I did the unthinkable yesterday and quit on the Duncan. After pulling the wire from pin 8 to the power switch and still reading only .5 ohms to ground, I concluded that the Post office had abused it enough to take out the PT. So I brought it to them for an insurance claim.

We will see if they accept it or not. There was some signs of it being abused. They had broken off a welded nut on the chassis to connect the chassis to the case. Was that fun to get the chassis loose. Fortunate for the hole cut in the top for the modules. They also beat it enough that the modules tore up the foam on the module cover, came loose and then bounced around in the chassis.

Reading all your tales, I not sure this is the amp for me. It would go to my son and their band does not understand TLC. I'm not much of a tech but would probably spend much of my life under its hood. It bothers me that it has such expensive tastes for 5U4s. I know it spit out the Mesa Boogie in about 5 seconds. That was all I could find locally. Also, I think getting the PCB out would be a breeze, BUT, I don't know how you would ever get the connectors back on those fragile pins.

If the Post Office rejects the claim, I'll perhaps be back for more advice like it seems Enzo must have an idea on the PT as he was asking about the stampings. Also, I would follow up on Amp Cats idea of backing down the filter caps. To be honest though, I don't know if I any longer care that the post office uses it for landfill. I think I would look around for a Traynor YGL 03. I sort of understand them, they are tough, and easy to work on.

Thank you all for your posts, help and concern - especially you Enzo.