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]

Originally Posted by foxlemieux

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.

[lmolter]

If you live in the States, you could send it to me. I'll look at it no charge and send it back. Just a thought.

Larry

[foxlemieux]

I appreciate the offer, and it might come to that. I'll see what other local options I can exhaust first.

[loudthud]

In the first picture it looks like one of the red wires going to the 5Y3 socket has a yellow stripe. That can't be right, that wire should go to ground, usually one of the transformer bolts.

[foxlemieux]

Thanks for taking a look at it, loudthud. The is the sort of mistake I'm sure I made somewhere but have yet to find. That wire is going to ground along with earth ground on a PT lug. The angle at which I took the picture makes it look like it's connected to the 5Y3.

[foxlemieux]

A quick update: I'm back working my second job which is far less time consuming than my other job. I can finally put a bit of time into trying to sort this problem out.

In the mail tomorrow I'm expecting a new OT (the third for those playing along at home) and new a 5y3 and 6V6. I don't believe the tubes are the problem, but it's safe to say I'm not an authority on the matter. I figured as long as I'm already paying shipping I might as well since I can't buy those tubes locally.

We'll see how it goes.

[erndawg]

Before you install the new OT, why don't you temporarily substitute a power resistor in place of the OT. A suitable value would be 5k at 10 watts.

Disconnect the OT primary, and install the resistor in its place. Also, disconnect the 22k resistor used in the negative feedback loop. Just lift the leg that gets connected to the 12ax7's cathode.

This way, you can measure voltages without worry about burning up the new OT. Report back with your measured voltages.

[theunrulychef]

Maybe you could put a fuse on the OT before blowing out another one? Someone smarter & more experienced than I would have to chime in & say whether this should work & give you a fuse rating suggestion.

[foxlemieux]

As per the good advice I ordered a 10W 5k from Mouser. Thanks erndawg for the resistor value. Thanks to theunrulychef and actually Gibsonlover as well who suggested something similar a long time ago. I feel less likely that I'll burn out another OT with a power resistor in circuit rather than a fuse on the OT. Color me paranoid.

Just so I'm 100% clear I just directly connect the 10W 5k from pin 8 on the 5Y3 to pin 3 on the 6V6? Or should I jumper from the filter cap eyelet? Does it matter?

For my further education if the 22k NFB resistor is still grounded by the output jack does it still need to be disconnected from the circuit because the OT isn't wired in? I'm still studying up on negative feedback theory and function, so the signal path is still a bit nebulous to me.

[erndawg]

What you need to do is disconnect the OT primary leads from the 16 uF/10k/pin 8 node, and pin 3 of the 6V6.

Then connect the 5k/10W power resistor between the eyelet board 16 uF/10k/pin 8 node and pin 3 on the 6V6 socket.

[foxlemieux]

Gotcha, erndawg. That was plan A. A bit more soldering but seemed the best way to go about it. The part comes in tomorrow. A long weekend of work ahead of me, but I'll try to get some voltages posted by mid week.

[robotman]

I didn't see anyone suggesting that each lead from the OT be checked for resistance to the case of the OT. On the output side or speaker side of the OT one of those leads could be to ground normally. On the power tube side neither the blue or red should have any ground ref. at all if measured out of the circuit, ie un-hooked.

[foxlemieux]

Current config as suggested: NFB resistor leg lifted. 10W 5k resistor from 16 uF/10k/pin 8 node to pin 3 on the 6V6. Voltages in the configuration:

5Y3 pin 8: 412V

6V6 pin 3: 205V (low from the 10W 5k resistor?)
6V6 pin 4: 353V
6V6 pin 8: 21V

12AX7 pin 1: 190V
12AX7 pin 3: 1.62V
12AX7 pin 6: 190V
12AX7 pin 8: 1.60V

Those numbers seem in line with what I've read elsewhere except for that 6V6 pin 3 voltage. Any thoughts?

[erndawg]

The voltages look good; a 200 volt drop across the 5k/10W is to be expected. The 6V6 is drawing roughly 40 mA, so the numbers are right.

A Champ OT can have a DC resistance, depending on the OT, of somewhere between 200 - 700 ohms. Its impedance, depending on the OT, can range between 5k and 8k.

The 5k/10W resistor limits the current sufficiently to protect burning out your PT, in case you had an ultrasonic parasitic oscillation or an unexpected circuit path.

For the record, give us the resistance readings of the failed OTs. Measure from red to blue, red to black, blue to black, and yellow to black. I assume your black lead is still connected to the transformer's mounting bracket.

I looked back at your photos, and was wondering about photo 2. The resistor below the screw head, to the left of the 22k (red, red, orange). That is supposed to be 10k (brown, black, orange) but it looks like brown, black, brown (100 ohm). Not that it would burn out the OT...

Report back on the OT readings and the resistor value.

[erndawg]

Also for the record, are you still using the original 6V6 that you have used all along?

[foxlemieux]

Erndawg, when I ordered the 10W 5k resistor I also ordered a new 2W 10k resistor for that position. I didn't like how I seated the original resistor and how it sat over that first position eyelet, especially with all the soldering and desoldering I've been doing at that point. I offset the new resistor by making an 'L' with the leads so that it sits slightly lower on the board leaving room to work around it.

Long story short, the old resistor was brown, black, orange and a quick resistance check verified the rating. The exposure in that 2nd shot shot is what makes the band look brown instead of orange. Kudos for the sharp eye though.

As very much an aside - when I bought the new resistor I got a metal oxide resistor as the original in the kit was also a metal oxide resistor. Would a metal film resistor have done the same job in the same way or would there be a substantial difference in the quality of the power through the different type of resistor?

To answer your second question, no, when I bought the new OT I figured as long as I would be paying shipping I'd go ahead and get a new 5Y3 and 6V6. Those voltage measurements are with the new tubes installed; however, the numbers are very similar to the readings I was getting with the old tubes. I'm still a bit wary of trying the new OT in the circuit just from changing the tubes out.

Wait...actually your second question was about the OT resistance. Unfortunately (and perhaps foolishly) the old OT's found their way to the garbage. The first I actually took apart to get a look at the insides. The second very quickly found the circular file with a bit of force. I'll chalk that up to frustration. Where the red lead tucked into the winding was charred black and chrispy, so I assumed it was well beyond repair or future usefulness.

[erndawg]

Would a metal film resistor have done the same job in the same way or would there be a substantial difference in the quality of the power through the different type of resistor?

No electrical difference.

Before installing the new OT, take some resistance readings. Measure from red to blue, red to black, blue to black, and yellow to black. I assume your black lead is still connected to the transformer's mounting bracket. Let's rule out any goofy manufacturing defect before installing.

You should have nearly 0 ohms between black and yellow, somewhere in the range of 200 - 700 ohms between red and blue, and open (no continuity) between black and red or black and blue. That is, no short between primary and secondary or primary to bracket (assuming black is still connected to transformer bracket). If something looks odd, or you are not certain of what you are seeing, report back before doing anything further.

Verify that you are reading ~ 3.2 ohms (the DC resistance of the speaker) through your speaker jack. So, in your case, you should be measuring from the chassis to the center pin of your RCA speaker jack. Again, if anything looks odd, report back.

If all looks good, install the new OT in series with the 5k/10W resistor.

Disconnect the 5k/10W power resistor lead at the 6V6 socket pin 3. Connect one of the transformer primary leads (red or blue, depending on your OT) to the 6v6 socket at pin 3. Now connect the other transformer primary lead (red or blue, depending on your OT) to the 5k/10W resistor lead that you just disconnected.

Keep the Negative Feedback resistor lead disconnected for now.

Before you apply power, look at your soldering on the 6V6 socket. Make certain there are no solder bridges, wire whiskers shorting anything, wires/pins inadvertently touching each other, or cracks in the socket.

Verify that you have wired to the correct pins. Pin 1 - looking from the wiring/soldering side of the tube socket - is to the left of the socket key. Pin 8 is to the right of the socket key.

With this setup, you will have a very good amount of current limiting, hence OT protection. You should have sound, albeit at a substantially reduced volume. Measure the voltage from chassis ground to the first filter cap, then the 5k/10W resistor/OT primary lead, then 6V6 pin 3. Then measure the 6V6 cathode voltage. Report back.

Good luck Fox.

[foxlemieux]

Success. Or at least good progress.

The resistance of the OT was inline with what was expected so I went ahead and wired it in.

The voltages with the OT wired in series with the 10W 5k:

6V6 Cathode at Pin 8: 21.23V
10W 5k/OT Primary Lead: 215V
6V6 Pin 3: 200V

In regards to measuring the filter cap: to measure that would I attach the common lead to chassis ground and the hot lead to the positive side of the cap? I didn't want to do that without double checking as it sounds extremely close to the way I drain that filter cap, and I've heard bad things about leaving a draining resistor in that configuration and then turning the amp on. Just trying to keep my heart beating by erring on the side of caution.

With the amp in this configuration it works with very little hum. The volume doesn't seem substantially reduced or this is one hell of a loud amp once that 10W 5k comes out.

Thanks erndawg and everyone else for getting the amp and I along this far.

[erndawg]

Good.

For now, continue to keep the Negative Feedback resistor lead disconnected.

In regards to measuring the filter cap: to measure that would I attach the common lead to chassis ground and the hot lead to the positive side of the cap?

Yes, that is exactly how you would connect the probes. If you wanted to, you could keep the Digital Multimeter leads connected there permanently - not that you'd want to. The point is, the DMM draws very little current since its effective resistance is > 10 M. Note that this is the same as measuring pin 8 of the 5Y3 s they are the same node.

When you are discharging a filter cap, the discharge resistor is a much lower value compared to the effective resistance of the DMM. That's the point of a DMM; they don't upset/load circuits. Back in prehistoric times (when I was a kid), a non-amplified meter (the good ones were tube amplified) could load down a circuit.

If your measured B+ looks about right, go ahead and remove the 5k/10W resistor. The volume should increase, and the voltage on pin 3 of the 6V6 should also increase. Make sure your 6V6 cathode voltage (pin 8) also looks good.

Finally, if all looks good, connect the Negative Feedback. If the amp starts squealing, roaring, or making a horrible noise, turn it off immediately. To correct that, swap the OT primary leads - the red and blue wires - and nothing else.

Good luck.

[foxlemieux]

Allright. Whew. Well that's the end of it. 100% working.

The Mojo OT ships with a little piece of paper that says the OT is wired with blue to B+ and red to the 6V6. I wired the amp in this configuration and got crazy high pitched squelch. Positive feedback instead of negative feedback. I disconnected the NFB and actually really liked the sound of the amp.

I wanted to have everything working 100% so I swapped the blue and red OT primaries and low and behold the NFB 'works.' I sort of like the sound of the amp better without the NFB. I'm contemplating taking it out of the circuit. The jury is still out. But a word of caution for anyone who gets the NOS 771 Mojo OT: the piece of paper that comes with it about the reversal of the primaries is incorrect. I read that in another thread on this forum as well. I might contact Mojo and let them know about it.

A huge thanks to all the people who have very generously given their advice and time. I can't thank you guys enough. This amp working is as much your work as it is mine.

I wish I could sit back and see the mistake I made that was burning out the OT's, but honestly reading back through all this I can't find a single thing that I changed from the last burnout to the current working circuit other than the tubes. I did change the 6V6 and the 5Y3, so I guess I can chalk it up to a bad tube. I'm not certain about that by any means.

As far as the sound of the amp (which is what this is all about, right?) I really dig it. It's way louder than I had anticipated which is sort of mix of good and bad. I was hoping for a slightly quieter practice amp, but even at low volumes it sings pretty well. If I had a lower output guitar (like a tele) I think the amp would sit in a better volume range. My PRS has always had a really high output even with the bridge humbucker split. I do love the way the amp really showcases the tone of my guitar. Without any tone shaping in the amp (beyond the color of the circuit) the tone of my guitar really shines through for better or worse. It's definatly the type of amp I'd take with me and play through to preview a potential guitar purchase.

[tubeswell]

Glad you have it sorted. Anyway I built a 5F2A last year with a NFB lift ("Brite") switch that really makes the amp do a screamin' rendition of the stairway solo with everything dimed. So I understand why you might like the NFB lifted sometimes.