Smoking OT

[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.