Oops.. 5f6a.. zap

[Lindz]

Long post, sorry

After many successful builds the past year I got a bit cocky and impatient with my new 5f6a build. Variac was at work and I never did build a light bulb limiter, so being impatient and such a great builder now (ha ha) I fired it up anyway.. what could go wrong.. its a simple build, right?

- Powered up to standby no problem. Voltages look correct
- popped in rectifier tube, comes off standby fine, voltages look OK, bias looks like it will adjust +/- appropriately.
- popped in 5881's (reissue Tungsol), powered up ok, comes off standby OK w/435v on the grids (no preamp tubes or biasing done yet so I figure that is OK albeit a little high) and figure I am probably in business now
- pop in preamp tubes and it powers up ok, but when I come off standby, bzzzzzt.....arcing in the power tubes. I had it off within a couple seconds but I really wished I had been patient and waited until I had a limiter or variac on it instead of just firing it up..

Anyway. during my post zap inspection I carefully traced all my and connections and I cannot find an obvious problem - I measured continuity for pretty much every connection I could, checked grounding resistance, and all my under board connections seem correct measuring continuity from the top of the board. All my caps are new cork sniffer type stuff (Red Jupiters, BC Electrolytics), resistors all measured OK before the install etc.

But.. 2 things come to mind

1. I dropped the rectifier tube on the floor before installing it. It appeared fine and since I did not have another tube on hand I thought I would try it. Seemed to be OK since power on and coming off standby voltages were in range. Any chance that a rectifier tube would seemingly be OK until there is a bit of a load on it from the preamp tubes, and then go south?

2. I noticed that the 100k resistor on v2 between pins 1/6/7 on the tube must have been damaged when I was bending the leads as the carbon comp part of the body was cracked. Even with that issue I doubt that could have caused arcing in the power tubes/rectifier..

That was the only glaring flaw I could find so I replaced that resistor and went over everything again a few times and then thought I would try a second power up without power tubes or preamp tubes (probably a bad idea in hindsight especially since I had issues already and could have already fried tubes, a tranny or something and made it all worse with the 2nd try)

came off standby fine, voltages look OK

popped in power tubes, bzzzzzt... arcing again in power tubes, rectifier and a blown b+ fuse. Had it off in 1 second

OK so after all that I finally have the sense to stop, but need to assess the damage and decide how to proceed from here..

I am thinking my PT is OK since it powers up and comes off standby OK with no tubes and or just the rectifier tube in place. correct?

Rectifier tube.. I am going to replace it in any event assuming that any arcing likely compromised it, but am just curious what knowledgeable folks think about the fact that it will power up and come off standby, and puts out the correct voltage after rectification even after this event?

Power Tubes were band new matched pair.. but risky to use I think after arcing. Wish I had a tube tester but will likely replace.

OT.. not so sure. Suppose I could have fried it. Power was only ever on for a few seconds but if there was a bad short then I suppose it could be done. Will do whatever low tech winding tests I can, but would appreciate any comments on this..?

Tube sockets - they are beldens and seem fine but after arcing what would you experienced builders do? just inspect and reflow? rewire? replace the socket(s)?

Any other tips on how you would proceed from here? I am still somewhat of a noob at trouble shooting something like this. I have not had a failure like this before fortunately. Guess I had to learn the hard way, be patient and use a limiter or Variac !!! duh! ha ha

thanks!

signed,

should have been patient

[martin manning]

- popped in 5881's (reissue Tungsol), powered up ok, comes off standby OK w/435v on the grids (no preamp tubes or biasing done yet so I figure that is OK albeit a little high) and figure I am probably in business now

Grids? You must mean plates, but 435 is not out of line. Do you have any idea of where the power tube current was? That might not be too important here but that and voltages are the first things to check and set when the power tubes are installed. I don't think installing the preamp tubes did it since you have the problem either way. I'm thinking the rectifier is the root cause. Notice the arcing occurs when you come off standby, when there is a current surge. Is your circuit wired per the original in that the standby is before the reservoir? If so I would change that around so that the reservoir is charged slowly as the rectifier warms up.

[tubeswell]

Tubes don't take kindly to being dropped. With the standby off, the full load goes onto the rectifier

[Lindz]

Ummmm.. when taking a fresh look at it today I noticed that my cable from the amp speaker jack was not plugged into the cabinet = no load when I was powering up. How brilliant was that? I have 2 attenuators on top of my cab an cables dangling from each and did not pay close enough attention to what was plugged into what and ended up with the amp plugged into one and the other plugged into the cab. On bypass of course

I know that no load is a no- no because of potential flyback voltage, but I am curious if no load might cause some visible arcing in the tubes, and might be have been the issue during the first fire up as opposed to the rectifier or some other short?

Martin, layout is old school, ala Fender but with no ground lift switch, and with Merlin-ish grounding (main chassis ground at input jack)

Grids at 435? ha ha.. typing while frazzled. Good thing I am airing my competence in all these different ways to the whole Ampgarage audience.. ha ha. Nothing like embarrasing yourself in public

On the other hand, to try and show something I did that was not completely stupid, my scratch build cab turned out great...

At least I have a great cabinet to put it in when I fix it..

[Reeltarded]

Yes that would cause a bit of pent up electricity.

Nice rig.

[martin manning]

Absolutely. Flipping the standby with the rectifier hot would charge the reservoir toute suite and produce a lot of ringing and high voltages between the OT and the choke. Do you have a shorting jack on the OT secondary or the 4x RCA jacks? Its not likely that four RCA's would come out, but I'd feel better with a Switchcraft shorting jack. The (possible) good news is that the arcing in the tubes may have saved your OT. I would replace the rectifier and power tubes and hope for the best. The caps may have taken an over-voltage too.

[Lindz]

It has 1/4" jacks - I did not have a shorting jack, but even if I did, I had the cable plugged into the amp, but not the cabinet so it would have still run open

Another dumb question..

I assume that if I fried the OT and it has a short now, that I would not be able to come off standy and idle (with just a rectifier and no other tubes installed) right?

I think if it has a short it would/should likely pop the b+ fuse with the big current draw, right? I am trying to decide if I need to disconnect the OT leads to check for shorts or the fact that it can idle at 450v (with no other tubes installed) indicates it could still be OK. I assume running it under load may still be another story

I don't want to put new tubes in and zap them if I fried the OT but my ability to truly test the OT is limited to what I can do with a meter.

thoughts?

From Martin's comments I should probably check the choke too? I assume I want to see somewhat close to specified resistance across the leads?

Of course I have already grabbed my Variac from the shop so at least I can do a reasonably controlled power-up this time

[JazzGuitarGimp]

When the output tubes are removed, the OT primary is effectively disconnected from its return path to ground. So the fact that you can come off of standby with the output tubes removed doesn't really mean anything - the transformer could be defective. Depending on the nature of the failure, it may be hard to discern whether or not it has a problem by measuring its DC resistance. But you can do this without unsoldering the primary wires. Just make sure the caps are discharged, remove the output tubes, then measure each half of the transformers primary resistance; measure from pin 3 (plate) of one output tubes to the transformer's center tap, then from pin 3 of the other output tube to the CT.

[martin manning]

You could test the OT for resistance and shorted turns, but the winding damage that you get from this type of incident is pinholes in the insulation, which won't show up until you get up to working voltages. I'd re-tube it and see what it sounds like. Your OT is 2-ohm only or a multi-tap?

[Lindz]

it's a classic tone 2, 4 and 8 ohm

[martin manning]

I came up with this for shorting the secondary with two impedances. https://tubeamparchive.com/viewtopic.ph ... 223#267223 If you want all three available I think you'd need to use a selector switch.

[Firestorm]

Wow, you did all kinds of terrible things! GZ34s have easy weeny tolerances. Dropping them is bad. If you've pinholed the OT (and maybe you haven't) it won't always test bad because it may need normal voltage to jump the gap.

[Lindz]

Wow, you did all kinds of terrible things!

Yes and my penance is to shame myself in public, and never do it again....

As I said, having a lot of my recent builds just fire up with no issues I started to get cocky and lazy.. should know better

[Lindz]

OK so my new OT came today and I installed it and hooked up my NEW LIGHT BULB LIMITER (ha ha), PLUGGED IN THE SPEAKER CAB and fired it up.

seems OK with just rectifier - bulb flashes a bit then dims almost off, stays the same dim light coming off standby

add new power tubes - same as above though the bulb stays light for several seconds gradually dimming and then glows dimly

add preamp tubes - same as power tubes, initial bright flash, then dims (maybe 50%) though the bulb stays a little brighter vs just power tubes, going from standby to on it flashes once more and dims back to similar level

Does this seem normal for a 5f6a (2amp b+ fuse) with 38 watt bulb in limiter? Never used one before. At least there were no fireworks this time

Now for the odd issue. Measuring voltages I see voltage (a little more than 1/3 of what is expected without limiter) when following the HV points in the amp until I get to v2 - on the one side of the 10k 2w resistor I see 120v dc but on the other side I see zero volts and none going to the front of the amp past that point. see my picture - I cut and pasted on the Ceriatione 5f6a layout as it was handy online.

Checked for continuity to ground after that point, resistor measures OK, not sure about the 10uf cap but easy to replace. However before tearing things apart I wanted to see if the limiter may be causing this or do I have some other issue? I am guessing I have some issue but I am not sure if the limiter may be dropping the current so much that the resistor literally blocks it at that point ? I would think some voltage should trickle through even with less current but am puzzled... Maybe I have something going on at v2?

I do not have any bigger bulbs here right now, nor do I want to simply hook it straight to mains after my last fiasco so I thought I would post another dumb question...

[martin manning]

Seeing zero voltage at that PS node could mean there is a short to ground. You should be measuring some voltage on the downstream side of the 10k resistor unless it's open. There is no supply voltage threshold below which you won't. I'd start looking for a wiring error. What resistance do you measure to ground from that point? I'd expect something on the order of 10k ohms with the preamp tubes in or out.