Test we can perform to determine damage to output tranny.

[cole]

Hi All,

I wanted to get some info on how to test an output tranny to see if it has been damaged by a no-load condition.

Generally, I am aware that we'd want to test for resistance and shorts, etc., but wanted some info specific to the test as it relates to a JTM 45 output transformer.

Cole

[novosibir]

When the 'no load condition' has created a creeping burn path from the primary through the insulation to the secondary, then you can't measure it with usual test equipment. You'd have to test this with a test voltage of at least 1,500 volts, usually 2,500 volts, like the transformer manufacturers do after a OT is finished.

The only what you could do is, to measure the B+ on the open amp, while you're playing it dimed into a (correct) load and watch for considerable voltage drops. A B+ of i.e. 450V at idle may drop down to 380V temporarily by hard strucking the strings - but when you notice 'strange notes' and additional see, that the voltage is dropping below 350V or even 300V in this moment, then you know, that primary voltage will be at least partial shorted through the secondary due to a bad insulation.

Good luck!

Larry

[cole]

Thanks Larry...you are always there to help...appreciate it very much.

I can certainly monitor the B+ and see if this occurs. The amp in question only seen a no-load condition for a second...that's it. No smell, no smoke, nothing strange occured as a result...

However, now I get maybe 20% of the bass frequencies I used to get. This happened before (not due to any incorrect loading) where I was playing one evening and all was fine, then the next evening (nothing changed) I plug in and get the exact same bass frequency loss - I suspected preamp tubes and on a whim reversed V1 and V3 and surprise it corrected the loss. I felt this kinda strange but alas this was the case.

I did the preamp tube switcheroo last evening again (after the amp was mistakenly plugged in to a dead cab for a second) but noticed no change. I suspect I either f**ked up the OT or the power tubes. I have a set of tubes on order from George and if they do not correct the problem then I can suspect the worst.

Funny thing is the amp sounds fine but not on the low end where it should be farting out when I turn the bass control past 2-3. Now I can dime it and there is barely a difference between it being set at 2 and dimed.

I blame this all on myself, but I just bought this Marshall 4X12 from a guy who did not inform me of the faulty 16 ohm input on the speaker jack out. I knew he replaced the speakers but I should have checked for proper resistance on the speaker jack before I plugged in my Metro JTM 45.

Cole

[Marvin_45]

Hello Cole

try this: The transformer has a turn ratio of 22,36:1. Take the 6,3 Volt heater winding of a transformer, connect it with 16 Ohms of the OT and measure each half of the primary. Your DVM should now read about 70,5 Volts for each half or around 141 Volts over the whole primary

Best regards
Marvin

[lectricman]

you could also use a megohm meter. it is used to test insulation at pretty high voltages some are 1000v or more.

[darrell]

When the 'no load condition' has created a creeping burn path from the primary through the insulation to the secondary, then you can't measure it with usual test equipment. You'd have to test this with a test voltage of at least 1,500 volts, usually 2,500 volts, like the transformer manufacturers do after a OT is finished.

The only what you could do is, to measure the B+ on the open amp, while you're playing it dimed into a (correct) load and watch for considerable voltage drops. A B+ of i.e. 450V at idle may drop down to 380V temporarily by hard strucking the strings - but when you notice 'strange notes' and additional see, that the voltage is dropping below 350V or even 300V in this moment, then you know, that primary voltage will be at least partial shorted through the secondary due to a bad insulation.

Good luck!

Larry

Wich pin should i do thi on Im having the same thing..When I first buit the amp all was fine I baised the tubes at 15 watts But one went tube went bad and blew a fuse..It has never sounded the same since and like his amp the bass pot just doesnt do anthing.

[novosibir]

A blowed fuse due to a bad tube and a not working bass pot are different animals and doesn't influence each other - so you have to look for 2 different and independent defects in the amp!

Larry