New Mojotone 6G16 build - shorted OT (I was wrong, just a failed rectifier)

[Medley]

Hello all,

Update to my post below: turns out that the primary on the OT is not dead shorted. Between center and brown wire, resistance is 68.4ohms and center to blue is 60.9ohms. The OT is a MOJO770, https://www.mojotone.com/transformers/Fender/Output/MT-FENDER-MOJO770.pdf. I don't know if those resistances are healthy or not; can someone advise? Is it possible what I experienced was simply a failure of the GZ34?



I'm a lurker here generally, but I'm reaching out for some help. I've been working on building the new Mojotone 63 Vibroverb kit. This is the second amp I've built, the first being their 5F4 Super. That build went trouble free, apart from a JJ 6L6 that was DOA.
This kit has been a challenge, mostly for missing parts and a few other things that are probably a sign of the fact that this is a new model/kit for them. Long story, won't go into those details.
I'm a pretty careful builder, checking my stuff multiple times as I go. Yesterday I fired it up for the first time, and everything seemed to be working. Some hiss on the vib channel, but nothing unbearable. Plugged in my Gretsch and everything seemed to work fine. It's still on the bench, chassis not mounted in the cabinet yet. I went to bed, planning on biasing the tubes today. Couldn't get enough current flow with the default configuration so I replaced the 15k grounding resistor from the bias adjust pot with a 10k and got the idle plate current to about 31ma with 454v on the plates. Still too tame and there was now a lot of distortion at modest volume. But I was doing this over lunch and had to get back to work so shut everything down with the plan to come back to it after work today.
Well, this afternoon, I flicked on the power with standby off (ie, no HT to the circuit yet). When I flicked on the standby switch, I immediately saw a flash in the GZ34 and things went dead. The fuse is blown, of course. But more distressing, when I check the resistance across the OT windings, there appears to be a dead short on the primary.
I reached out to Mojotone this afternoon, and am waiting to hear back from them. So completion of my project is now delayed by another couple of weeks while I wait to hear from their tech support and (hopefully) get a replacement OT and rectifier shipped up here to Toronto.
Any idea what could cause this? Could a failed rectifier take out the OT or vice versa? Fortunately the PT is still OK.
Suggestions?

[pdf64]

the resistance across the OT windings, there appears to be a dead short on the primary

What exactly do you mean, please provide detail?

[Medley]

the resistance across the OT windings, there appears to be a dead short on the primary

What exactly do you mean, please provide detail?

Well, I've updated my post to clarify that the primary windings on the OT are in fact not completed shorted, but what I meant was when I measure resistance between the center tap and the wire that connects to the plates of each of the power tubes, there appeared to be no resistance at all, however after measuring again a little while ago I found this not to be the case. I provided the resistance on each winding in my update. If I had another GZ34 and 2 amp fuse, I'd try powering up again and measuring voltage drop across those windings, but unfortunately I can't at the moment because I have neither of those on hand.

I am not terribly familiar with the specs for transformers; I know how they work but not really what normal readings are at various points. I suppose the more likely failure mode of an internal short would be an open circuit rather than zero resistance. At any rate, I'm hoping my OT is OK and it was just a failure of one of the cursed JJ tubes. Last kit I bought from Mojotone came with a 6L6 that was dead on arrival.

[PRR]

The DC (Ohmmeter) resistance tends to be ABOUT 1/20th to 1/10th of the nominal audio impedance.

So a "4000 CT ohms" winding, we might expect 100-200 ohms each side. So 60 ohms looks low.

But a winding is a complex problem in economics and marketing. A wire gauge which is comfortable for the winder may be bigger than the theoretical optimum.

And it is super unlikely that two halves of a winding will have the SAME fault (within a few %) (an unbalance is expected cuz the two windings same number of turns but are one over the other so one is longer per turn).

Also a short IN the primary (end to end, end to center) will not be blowing rectifiers or line fuse.

[pdf64]

The 5F4 has a hit switching standby. That will put excessive stress on a valve rectifier when standby is flipped to operate mode.
A blown rectifier is typical.
Don’t use standby.

[Willabe]

The 5F4 has a hit switching standby. That will put excessive stress on a valve rectifier when standby is flipped to operate mode. A blown rectifier is typical. Don’t use standby.

Depends where you hook up the 1st B+ filter caps. Put the 1st filter caps/caps before the standby. Like on the BF Fenders. (You know that.)

I played and hung out for years in Chicago, mainly blues clubs. I was in those clubs 3, 4, 5 nights a week. I saw hundreds of shows. At the 2:00am bars, 3 to 4 sets, at the 4:00am bars, 4 to 5 sets a night. So that standby switch was getting used several times a night.

Everyone that had a standby switch on their amp used it. Never saw a single amp go down live, never. Never heard about anybody having that problem. Never had anybody tell me their amp went down on the gig, never heard anybody tell me so and so's amp went down on the gig, never. All the guitarists and really all the musicians talked shop every night, that's what you did. If that was a problem everybody would have known.

There were literal hundreds of guitar players gigging ever night in Chicago since the 50's and it would have been passed on by word of mouth from the older players to the younger players not to use your standby switch. In fact they told you to use the standby switch. 

I played through a BF Dual Showman for a couple years, then a BF Super Reverb for many years live, never had either amp go down at a gig.

Only 2 things went bad with those amps; 1. A single 6L6GC (out of 4) died in the Showman, but the amp still played. 2. The Super blew a filament faux center R. That's it.

And I played a lot of gigs and practiced at high volume, volume set at at least 6 to 7, many, many, many hours with those amps. Took the BFSR on the road, short trips, several times too.

In fact out of the hundreds of bars shows and a couple hundred or so big concerts, with several to many bands in the line up that I have seen, I can only remember seeing 1 amp go down live. It was definitely the guitarist in Focus because I remember he had a hissy fit and threw his guitar to the ground.

And I think it was a Marshall.   

[acheld]

Great story and review of experience.

I have nowhere near that kind of first hand knowledge, but all of my gigging friends use their standby switches every day, w/o problems.   

Personally, after trying all of Merlin's "improvements" to the standby portion of the circuit, all of which had minor negative side effects, I've gone back to standard BF circuitry.

But of COURSE it was a Marshall that failed -- Jim was just trying to "improve" Fender circuits!   

[sluckey]

The 5F4 has a hit switching standby.

And so? The amp in question is a 6G16 and the reservoir caps are hard wired to the 5AR4, before the STBY switch.

[Willabe]

But of COURSE it was a Marshall that failed -- Jim was just trying to "improve" Fender circuits!   

To be fair, it might have been the guitar or cord that was the problem, I don't know. I seem to remember a roadie coming out a couple times and plugging into a different stack? 

[Medley]

The 5F4 has a hit switching standby. That will put excessive stress on a valve rectifier when standby is flipped to operate mode.
A blown rectifier is typical.
Don’t use standby.

Appreciate your input, but my query is not about the 5F4. It is about a problem with my Vibroverb build and its GZ34 rectifier, which doesn’t produce a “hit”. Your comment did spin off some great stories though.

[pdf64]

The 5F4 has a hit switching standby.

And so? The amp in question is a 6G16 and the reservoir caps are hard wired to the 5AR4, before the STBY switch.

Apologies for the typo, I meant ‘hot switching’. 
In post #1 Medley mentions previously building a 5F4.

Everyone that had a standby switch on their amp used it. Never saw a single amp go down live, never. Never heard about anybody having that problem

Back in the 70s and 80s when I was starting out, I recall it being not unusual for an amp fizzle out / pop a fuse when standby was flipped. Of course, we still used standby because everyone knows standby is good for valves, right? And Jim Marshall was obviously an electronics genius who could do no wrong 
Whereas the JMP50s we were using had one of the stupidest standby arrangements ever 

Bear in mind that standby is a variety of things, not every standby arrangement hot switches a valve rectifier, eg BF don’t, tweed era usually do. And a golden age valve might last for numerous hot switching incidents before failure. The point is its useful life will tend to be shorter than otherwise. So it seems a bad idea to build an amp with such a feature.

[acheld]

OK, back to your amp,  and back to basics.

Suggest plugging into a light bulb limiter, removing your tubes (except the rectifier) and report back voltages at each node -- these are point XA, XB, XC, and XD on your schematic. Following this, you will know if your transformer and your rectifier are working.  Since you are limiting current with the lightbulb, your voltages will be lower than expected.      If your light bulb is glowing more than "a little", then you have a short somewhere.

[Medley]

OK, back to your amp,  and back to basics.

Suggest plugging into a light bulb limiter, removing your tubes (except the rectifier) and report back voltages at each node -- these are point XA, XB, XC, and XD on your schematic. Following this, you will know if your transformer and your rectifier are working.  Since you are limiting current with the lightbulb, your voltages will be lower than expected.      If your light bulb is glowing more than "a little", then you have a short somewhere.

Thank you for your helpful suggestions. I will do so as soon as I procure the stuff for the light bulb limiter (have been wanting to build one for a while anyway). I do use a variac for ramping up power slowly when I power my builds on for the first time. Also have to wait for my replacement rectifier as I only had the one GZ34.

Cheers.

[Medley]

After communicating back and forth with the great tech guys at Mojotone, they suggested that it was likely a failed rectifier. They sent me a replacement, and after installing it, and biasing the 6L6s, things are all right in the world.
Of course, with a new build like this, it always takes me a period of time using it to give me peace of mind.
Thanks for all your assistance everyone.

[Medley]

The 5F4 has a hit switching standby.

And so? The amp in question is a 6G16 and the reservoir caps are hard wired to the 5AR4, before the STBY switch.

So here's something I've revisited in the last few days, Sluckey, and may explain why my 5AR4/GZ34 rectifier tube failed (which happened the second I turned off the Standby switch, by the way). Turns out the 6G16 does have the standby switch before any of the filter caps. And Mojotone's layout diagram for the Vibroverb kit shows the wiring this way. However, the schematic they also include has the standby switch after those first two caps. Puzzled, I checked the original schematic for the 6G16, and it was designed with the standby switch directly after the rectifier, before any filter caps. I believe this might explain why the rectifier arc'd and the fuse blew when I flicked the standby off.
Unfortunately it's not trivial for me to rewire the B+, because I'd have to remove the circuit board to get at a couple wire runs on the back side of the board.I'm inclined to perhaps tack a 47k, 2watt resistor across the standby switch as Vox ended up doing with their AC30CC2 when they made the same mistake.

[pdf64]

...
Unfortunately it's not trivial for me to rewire the B+, because I'd have to remove the circuit board to get at a couple wire runs on the back side of the board.I'm inclined to perhaps tack a 47k, 2watt resistor across the standby switch
...

That will help, I’m sure, but excessive surge rectifier currents are still likely. To remedy that, it hardly seems that moving a couple if wires around would be that complex?

[tubeswell]

… was likely a failed rectifier.

Google ‘R G Keen Immortal Amp Mods’


(for a cheap method of protecting your PT from a tube rectifier going dead short)