Smoking OT

[foxlemieux]

I'm in the process of putting together my first build, a Mojo 5F1 kit, and when I turn it on the OT smokes quite a bit with an electric sounding pop and static from the speaker after the tubes warm up. Adjusting the volume does not alter the loudness of the static.

This is the second OT I've had. The first was burnt out in a very similar manner. When it burnt out I went back and triple checked all my connections and solder points. I found a few small problems (i.e. points I resoldered a bit cleaner and the most serious was a filament wire on the 12AX7 that need to be clipped so it didn't short with another pin).

The other issue was that I had neglected to see the note that came with the OT that said the red and blue wires are backwards from the schematic. I had the blue wired to the 6V6 pin 3 and the red to B+. On this OT that is backwards. When I wired in the new OT I wired the red to pin3 and the blue to B+. Same results. Smoke.

I quickly turned the amp off (not before the smell and quite a bit of smoke came pouring out) and went to work checking everything again. I can't find any shorts measuring resistance to all the pins and to the Caps.

I don't have the measured voltages with me right now but I can post them later if need be. Where should I be looking? What would make the OT smoke like that? A bad 6V6? Improper grounding on the speaker jack? Too high B+ signaling a bad 5Y3? Maybe even a bad PT?

This is all assuming I have wired everything properly. As far as I can see everything is wired to schematic and is in line with a lot of pictures I've seen online.

Thanks for any help you can provide

Jason

[GibsonLover]

Yikes! You may want to fuse the B+ to save money on OT's for now :-)
Try and disconnect the B+ from the OT, then fire it up. The output stage
won't be running, but at least you'll be able to tell if the power supply is working with reasonable voltages in the preamp. That, and very close visual inspection.

[tubeswell]

Okay since you had a speaker connected, you must have wired it wrong.

The PT secondaries need to be wired like this:

* HT winding wires to pins 4 and 6 of rectifier tube, with the HT Center tap going to the ground return path chassis grounding point

* Rectifier 5V winding wires to pins 2 and 8 of rectifier tube.

* 6.3V winding going to the lamp and filament pins (pins 2 and 7 on the 6V6, and pins 9 and 4-5 on the 12AX7) (how have you got this wired? Is there a heater winding CT? if so that should either be going to the the ground return path chassis grounding point, or pin 8 of the 6V6)

B+ from pin 8 of rectifier tube to first filter cap/OT primary (I assuming you have the filter caps the right way around)

OT other primary to pin 3 of 6V6

second filter cap node (after filter dropping resistor) to pin 4 of 6V6

And of course have the speaker connected to the T secondaries

[foxlemieux]

Everything is wired as you described it.

There is no center tap from the 6.3V winding. I have a 100Ohm resister going from each leg of the pilot light to ground.

All the filter caps are wired correctly with respect to polarity.

I disconnected the B+ from the OT (good advice) and measured B+ and voltages on the 12AX7. I have an analog meter and the higher the voltage being measured the less specifically accurate the measurement is. All of these measurements were taken on the meters 500V setting.

B+: 4/5th full scale. Roughly 400V.

12AX7 pin 1: Slightly higher than 2/5th full scale. Roughly 210V.

12AX7 pin 6: 2/5th full scale. Roughly 200V

All those voltages appear to be in line with what I've read elsewhere.

Here are some pictures of the build. I fully expect a "messy wiring" lecture from someone. Once I get everything working I intend to go back and clean up my wire runs to reduce hum etc. I'd just like to get the thing working and not smoking for the time being. Although, is it possible the messy wiring is somehow contributing to the problem? From everything I understand I can't see how that would work, but I readily admit my understanding is still in its infancy.

[lmolter]

If nothing else burned up then your wiring might be ok. But for the OT to smoke and pop, there is a major miswiring somewhere. If I may, can I please ask a stoopid question or two? Is perhaps the secondary wired up as the primary and vice versa? If not, are you sure the B+ is going to the center tap on the primary? There's definitely a lot of current passing through those windings (hence the smoke), so there's got to be a B+ wiring gaffe somewhere.

Please post what the outcome is when you find it. I promise I won't give you any lectures about anything. I don't think anyone else will either. The folks here are pretty good. If it wasn't for them, I would have never finished my 5F6A.

Larry

[foxlemieux]

As far as I know the OT is wired correctly. The black wire (ground) is soldered directly to the OT casing by the factory. The yellow wire (secondary) is wired to the positive RCA jack.

On the 5F1 schematic that came with the kit it shows the red OT primary wired to B+ and the blue to the 6V6. There was a note packaged with the OT stating that that particular MOJO OT is wired backwards and that blue should be wired to B+.

Every day I take a few minutes to check my wiring trying to find a mistake. I've *yet* to find one and I'm really at a loss. There is obviuosly a problem somewhere.

[Enzo]

With power removed, measure resistance from pin 3 of the 6V6 socket to chassis. Is it shorted to ground? Or is there a low resistance? It should measure like it was a filter cap. A rising resistance that goes off scale eventually.

Inspect the 6V6 socket very closely. And little burn marks around pin 3 on top or bottom side of the socket?

The only thing that is going to burn up a transformer here is if it shorts to ground and shorting out the B+ through itself.

Unless you really did coincidently get two bad parts in a row. I'd be betting on the wiring.


You want the messy build lecture? It is far harder to go back into a wired amp and fix the layout than while building. And I know this is a world of instant gratification, but take the time to install parts neatly and as you will want them in the first place.

We always have time to do it over, but we never have time to do it right.


The amp is a 5F1, which is a Champ. That is a single ended amp, there is no center tap on the transformer. The red and blue wires being reversed ONLY affects the NFB. The amp would work wired "backwards" or not other than that. It wouldn;t burn uop the transformer.

[Sir Cuitous]

foxlemieux, can I suggest also to double check the diagram for your OT. I believe that your secondary ground might be the yellow and the black is your positive to speaker. It was that way on a Champ I built last year. It also might explain a lot of what's going on.

[foxlemieux]

The black wire of the OT comes soldered to the casing of the OT from the factory. Even if I de-soldered it there wouldn't be enough wire left to make it to the RCA jack. I'm not looking at the diagram right now, but I'm pretty sure I remember it showing that the black wire is the ground wire.

That does bring up a question I have though. Could a grounding issue on the secondary be causing the short that's burning out the OT?

Testing resistance at pin 3 of the 6V6 doesn't even give a blip on my meter. I talked to the guys at Mojo about this previously when I thought maybe they had given me a bad part. They said an ohm meter was the wrong way to measure resistance and look for an open coil on an OT. As far as there being a short to ground on pin 3, infinite resistance rules that out.

As an aside, I didn't mean getting a lecture was a bad thing. I think at this point I'm a good lecture or two shy of getting this amp working.

[lmolter]

That does bring up a question I have though. Could a grounding issue on the secondary be causing the short that's burning out the OT?

If it did, you would have seen it with the meter, no? On another point, I misspoke about the B+ going to the center tap - I hadn't looked at the schematic for the Champ. I was _assuming_ push-pull output. Anyway, with the circuit as simple as it is, this is a mystery. How much current is the 6V6 pulling? Of course, as it stands you can't measure it without smoking another OT. To the forum: could a large plate resistor be added in series with the OT for current-measurement purposes? There's not much left to blame for the problem.

Larry

[martin manning]

Could the 6V6 have an internal short? Like Enzo said check pin 3 to ground with it plugged in to the socket to see if the plate is shorted to ground inside the envelope or out, but maybe check the other pins with it removed. The only pair that should have a measurable resistance are 2 and 7, the filament. Just a thought, but I'm wondering if the screen (pin 4) could be shorted to the suppressor, and taking the B+ to ground through the internal connection to the cathode. That wouldn't run high current through the OT, though.

MPM

[Enzo]

ANy chance the center post of teh 6V6 is snapped off and the tube is in the socket facing the wrong way?

[foxlemieux]

I can't find any shorts on any of the 6V6 socket pins with the tube in or out. As you mentioned, Martin, only the filament pins show resistence. The 6V6 still has its center post and is inserted correctly.

Thanks guys for taking the time to think about this with me. It's fairly frustrating on this end. My next step is to take it to someone locally who can take a look at it.

[lmolter]

Any luck yet? We're all dying (at least I am) to know what you've found.

[foxlemieux]

lmolter, thanks for staying interested. The short answer is 'no' I haven't found anything out yet. For a lengthier explanation read on.

The second OT I bought burnt out the first time I switched it on back when I thought I had figured out the issue. I was hoping it had just smoked a little and that I had turned the amp off before it burnt. No such luck. I don't have the money right now to keep buying OTs, so I don't have one at the moment.

I asked around town at the various guitar stores and none of them (yes, none) have an amp tech on staff. I can't even find a store locally that stocks 6V6s. They all send amps out to either Atlanta or Charleston for work. I'd like to sit with the tech who looks at it, and learn what the issue is.

There are a few guys in town I know that have some tube electronics knowledge. Between my crazy work schedule and their's I've yet to hook up with any of them. Without an OT it wouldn't matter much at this point anyway.

I'm about to go into a wall to wall schedule at work, so I don't expect to be able to do anything more with the amp until the end of the month. I will post back if and when I learn something new.

Thanks again guys for all your advice and info. It's great that there is a resource like this with such a nice community contributing.