advice on OT

[d. spree]

I have a champ build going on and I wonder how to test whether the OT is good. Had problems with arcing on the power tube between pin 2 and pin 3. The 470 ohm cap was damaged, dont know if that caused the problem - this cap





Want to be sure the OT is good. Its a mojotone OT - has one blue wire wiring diagram shows to B+ and one red wire for power tube and only one yellow wire on the output side. The yellow wire shows continuity to ground - is this normal? Also the mojo drawing seems to show both the yellow wire from OT and white wire from the board (as in the attached drawing) both wired to the tipe of the speaker jack. Its a lousy drawing but that seems to be what it shows.

Help please

[Chuck H]

Its a mojotone OT - has one blue wire wiring diagram shows to B+ and one red wire for power tube and only one yellow wire on the output side. The yellow wire shows continuity to ground - is this normal?

Yes. The yellow lead is one end of the secondary. The other end of the secondary is connected to the transformer casing so there SHOULD be continuity there.

Also the mojo drawing seems to show both the yellow wire from OT and white wire from the board (as in the attached drawing) both wired to the tipe of the speaker jack.

I'll guess that is the NFB loop. So that seems correct.

If the blue and red leads don't show continuity to the case then the easiest way to test the OT would be to wire it up and see if it works. You COULD use a small AC voltage at the secondary and measure the voltage on the primary to see that it steps properly. Remember the transformer case is one end of the secondary.

[d. spree]

Yes. The yellow lead is one end of the secondary. The other end of the secondary is connected to the transformer casing so there SHOULD be continuity there.



I'll guess that is the NFB loop. So that seems correct.

If the blue and red leads don't show continuity to the case then the easiest way to test the OT would be to wire it up and see if it works. You COULD use a small AC voltage at the secondary and measure the voltage on the primary to see that it steps properly. Remember the transformer case is one end of the secondary.

Thanks Chuck. Gotta start reading about how transformers work.

This is a friend's build. I checked his wiring front and back of board and wiring to sockets etc. It all looked correct against the drawing Mojo supplied. Started it up without tubes in a dim bulb tester and increased wattage of bulbs etc. It all seemed good. Same with rectifier tube in. Voltage at first filter cap seemed high by about 35v. Tested with 12AX7 in, seemed good. Tested with power tube and got some flickering back and forth on dim bulb tester - that settled. Ran amp without the dim bulb tester. Voltages were within specs except at pin 6 and 1 of 12AX7 - and when I measured volatge from filament leads go a 12V read?? drawing shows voltages should be 165v. They were 125 at pin 6 and 200 at pin 1. suddenly got arcing at power tube - from pin 6 to 3. Switched off immediately - checked the board and saw that the 470 Ohm resistor from pin 1-8 of the power tube (coupled with the 25 uF electrolyic capacitor) was split. Don't know when in the chain of events this happened. Have taken out the board and rechecked the wiring and it is all correct. Ordered replacement resistors and want to figure out what might be the problem before I go ahead. Any advice would be helpful. This build should be simple!!!

[g1]

The 470ohm resistor from pin 8 to ground is the cathode resistor for the power tube. Usually it burns up if the power tube is bad. Yours has blown twice. Are you still using the same power tube as in post #1? The power tube seems the most likely suspect.

[d. spree]

The 470ohm resistor from pin 8 to ground is the cathode resistor for the power tube. Usually it burns up if the power tube is bad. Yours has blown twice. Are you still using the same power tube as in post #1? The power tube seems the most likely suspect.

Thanks - I thought that to be the case, it only happened the one time - but I have lots of spare tubes and will discard that one. It is interesting but the Mojo supplied rectifier was bad and I had to replace it with a NOS one I have. Both were JJ tubes and I wonder if they got shaken around in the shipping process. I am waiting for a replacement resistor before I can power it up again. (Actually ordered a few of these.

Why would the readings have been so divergent on the 12AX7 (pin 6 well below proper voltage and pin 1 well above)?

[g1]

The 12AX7 voltages could also be tube related. Double check the plate and cathode resistor values for each half of the 12AX7, should be 100K's and 1.5K's. Then try another 12AX7 in there and see what happens to those voltages.

[d. spree]

The 12AX7 voltages could also be tube related. Double check the plate and cathode resistor values for each half of the 12AX7, should be 100K's and 1.5K's. Then try another 12AX7 in there and see what happens to those voltages.

Could it be that Mojo shipped 3 different tubes that were all bad!!!??? Thing is the wiring was correct - it was checked and triple checked against the drawings which are pretty simple circuits I will check the resistor values - they are old style carbon comp resistors (very nice aesthetically but maybe a bit dodgy from a consistency basis. I will disconnect them and test. The other weird thing is that when I emailed Mojo to ask if my assumptions about wiring the OT were right I got a very fast response (3 minutes would you believe) - but weird in that the tech wrote that sometimes the red and blue wires to the board and OT have to be reversed because the manufacturer sometimes reverses things. (They are evidently made by Heyboer so you'd think that Mojo would have some consistency in the wiring scheme). Think they might warn you before you build. How hard is it to give clear instructions - the 5e3 builds I did from Mission were accompanied by simple and clear instructions.

Feel bad for the friend I am helping - I suggested this amp for him and his son because it is so simple!!!! It's already taken twice the time and a lot more hassle than my Deluxe builds. I guess that's the mysterious and wondrous world of amp building. Just when you think you know a little . . .

[Bruce / Mission Amps]

Don't feel bad... I get punished and humiliated all the time and I've done hundreds and hundreds!!!

[d. spree]

Yes. The yellow lead is one end of the secondary. The other end of the secondary is connected to the transformer casing so there SHOULD be continuity there.



I'll guess that is the NFB loop. So that seems correct.

If the blue and red leads don't show continuity to the case then the easiest way to test the OT would be to wire it up and see if it works. You COULD use a small AC voltage at the secondary and measure the voltage on the primary to see that it steps properly. Remember the transformer case is one end of the secondary.

Chuck

Thanks for the advice. Had a chance to check last night. There is continuity between the red and the case and the blue wire and the case and the yellow to ground. So I guess the OT is in good shape? As soon as I get a replacement 470 Ohm resistor I will wire her up and use a different output tube and fire her up through the dim bulb tester! Makes me wonder what tester is there to see if the amp builder is indeed the dim bulb!!!

[d. spree]

Don't feel bad... I get punished and humiliated all the time and I've done hundreds and hundreds!!!

Yeh you've written that before - and I appreciate it. Amp building is humbling! I'm sitting there at 1:30 in the morning on a saturday night having pissed off the wife as I am preoccupied with why the simple little champ ain't right, and I get an arc and even manage to give myself a shock! That's when I knew I should have stopped hours before when I wasn't dead tired and pissed off. I guess never work on an amp - even a simple little one - when wired and tired!!!

[eschertron]

Chuck

Thanks for the advice. Had a chance to check last night. There is continuity between the red and the case and the blue wire and the case and the yellow to ground. So I guess the OT is in good shape? As soon as I get a replacement 470 Ohm resistor I will wire her up and use a different output tube and fire her up through the dim bulb tester! Makes me wonder what tester is there to see if the amp builder is indeed the dim bulb!!!

I hope you mean there ISN'T continuity between the red (or blue) and the case. That would be bad. That's why Chuck says to check that. Continuity is a big FAIL on the primary side insulation.

[d. spree]

I hope you mean there ISN'T continuity between the red (or blue) and the case. That would be bad. That's why Chuck says to check that. Continuity is a big FAIL on the primary side insulation.

Then I guess I'll be sending the OT back to Mojo as it has continuity between the lue lead and the case and the red lead and the case.

[J M Fahey]

The other end of the secondary is connected to the transformer casing

Well, this is a HORRIBLE way to do things.
What if you don't want speaker ground just where the transformer is bolted to the chassis?
What if you want to invert secondary phase for any reason?
What if you are testing your amp or at least a transformer, unmounted, just sitting on a wooden bench?
WHAT WERE THEY THINKING !!!!

[d. spree]

Well, this is a HORRIBLE way to do things.
What if you don't want speaker ground just where the transformer is bolted to the chassis?
What if you want to invert secondary phase for any reason?
What if you are testing your amp or at least a transformer, unmounted, just sitting on a wooden bench?
WHAT WERE THEY THINKING !!!!

John thanks for the response - I guess you are talking about how the mojo OT is wired to the speaker jack? I also presume given the continuity issues (as I posted above) the OT is faulty. Do you suggest I order a different OT which has a ground wire along with the yellow wire to speaker for separate grounding? that's how I built my 5e3s.

Sorry if the question seems overly simple - I'm just learning this stuff.

[eschertron]

Then I guess I'll be sending the OT back to Mojo as it has continuity between the lue lead and the case and the red lead and the case.

Just to verify: with the blue and red leads disconnected from the circuit, you get continuity from either lead to the case (which is grounded)? Think about it. One of those leads would be connected to B+, providing a short to ground from the PSU. Sounds to me like whatever took out the cathode resistor also punched through the winding insulation.
If you didn't have the leads off the board when measuring, a short *somewhere else* could have given you that continuity reading. Just thinking out loud.

I agree with Juan that the secondary common on the case is a silly and unwieldy way to implement the OT. For what? to save 9 inches of green wire?


edit: what is the Ohmage between the blue and red leads? That tranny should have a few dozen to a hundred ohms or so DC resistance on the primary?