Originally posted by eschertron
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advice on OT
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Originally posted by d. spree View PostChuck
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!!!
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Originally posted by Bruce / Mission Amps View PostDon't feel bad... I get punished and humiliated all the time and I've done hundreds and hundreds!!!
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Originally posted by Chuck H View PostYes. 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 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!!!
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Don't feel bad... I get punished and humiliated all the time and I've done hundreds and hundreds!!!
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Originally posted by g-one View PostThe 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.
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 . . .
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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.
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Originally posted by g-one View PostThe 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.
Why would the readings have been so divergent on the 12AX7 (pin 6 well below proper voltage and pin 1 well above)?
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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.
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Originally posted by Chuck H View PostYes. 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.
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!!!
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Originally posted by d. spree View PostIts 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?
Originally posted by d. spree View PostAlso 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.
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.
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advice on OT
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 pleaseTags: None
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