I think the OT in my buddies JCM800 might be a goner. I'm getting very little volume out of it. I've tried multiple sets of tubes that have all tested well. I've poked aroung with my chopstick so much that I can't think of anything else to poke. i've probably looked over every connection about 3 times...
He's run it on an improperly mismatched cab for a loong time, doesn't worry about the whole "standby switch" (why should he be bothered with TWO switches when one turns it on? He also doesn't seem to care about the fact that it still sounds pretty good, he just wants it earbleedingly loud so that he'll never be able to use it.......sorry bout the rant....
the preamp gain seems to be fine. It just has really low output. I've also replaced half of the pots (including the MV) wich helped alot. Now it doesn't crackle and cut out completely....
Are there any more tests I should consider? If I do test the OT how would I go about that?
FWIW.. I have a Fluke 867B meter (I'm very luck to have been gifted this amazing piece) that I use and love, yet know reletively little of it's true capabilities. It is supposed to be able to do some scope-like functions, but I know essentially nothing other than scopes are handy things to have if you know how to use them....which I don't.
The last question I have (for now...I promise) is what OT would you replace it with? Tubes and more had one for about $75... and he doesn't want to spend a dime...of course.... I'm on 10 piece pricing with Mercury, and I love them for what I've built before, but he won't go for that high of a price....so what do you reccommend? Whould the tubes and more be ok?
Thank you for your help, and your patience with the ranting....
Testing an Output Transformer...argh....
Moderators: pompeiisneaks, Colossal
Re: Testing an Output Transformer...argh....
A simple continuity test on the OT is a good place to start. Pull the power tubes. Lift the center tap connection. Check for ohms across the primary -- that would be pin 3 on one tube to pin 3 on the other tube. If 2 pairs, check across the two outer tubes, then the two inner tubes and result should be the same between inners and outers.
Do the same on the secondary. Probe across the sleeve and tip. You should have some resistance.
If all is well, check the turns ratio. With tubes out and CT lifted, jumper the 6.3v filament connection to pin 3 -- one leg to each side (use 2 outer or 2 inner sockets). Use your volt meter to see what comes out at the output jack. Simple division tells you the turns ratio. Chances are you'll get less than one volt out of the secondary. If you don't get a reading you can work with, run the 6.3v on the secondary and measure what comes out the primary. Pay attention if you do that, as it can fry an OT. At 16:1, 6.3v into the secondary is 100v out. But you've got a Fluke, so you have a meter that should be able to tell you what you are getting out on the secondary.
Sorry, I don't know this particular amp, and don't want to tell you what to expect. Also, I'm awful at this tube math and you shouldn't rely on me. The formula is the primary load divided by the square of the turns ratio = the correct speaker impedance. Look up your tube load and go from there.
Do the same on the secondary. Probe across the sleeve and tip. You should have some resistance.
If all is well, check the turns ratio. With tubes out and CT lifted, jumper the 6.3v filament connection to pin 3 -- one leg to each side (use 2 outer or 2 inner sockets). Use your volt meter to see what comes out at the output jack. Simple division tells you the turns ratio. Chances are you'll get less than one volt out of the secondary. If you don't get a reading you can work with, run the 6.3v on the secondary and measure what comes out the primary. Pay attention if you do that, as it can fry an OT. At 16:1, 6.3v into the secondary is 100v out. But you've got a Fluke, so you have a meter that should be able to tell you what you are getting out on the secondary.
Sorry, I don't know this particular amp, and don't want to tell you what to expect. Also, I'm awful at this tube math and you shouldn't rely on me. The formula is the primary load divided by the square of the turns ratio = the correct speaker impedance. Look up your tube load and go from there.
Re: Testing an Output Transformer...argh....
An easy test to do on an output transformer is to measure the turns ratio. You need a source of low voltage AC. A wall wart will work or you could use the heater voltage in the amp. Be extremely carefull if you are using the heater voltage because any short could damage the power transformer. If the heater voltage uses a grounded center tap, use one side only. If you are using the heater voltage, remove the tubes and the HV fuse. Connect the AC voltage to the 16 ohms tap on the OT. Make sure the speaker jack is not shorting out any of the other taps. Carefully measure the AC voltage on the 16 ohm tap, then measure the AC voltage between the two ends of the primary at the power tube sockets. A 50W amp has about a 3.5K primary and a 100W about half that. Divide the voltage on the primary by the voltage on the secondary, square it and multipy by the impedance of the secondary tap you used. If the number you get is reasonable, the transformer is good.
Example: 3.1V on the 8 ohm tap 62V on the primary. Turns ratio is 20:1.
20 squared is 400. 400*8=3200 ohms.
On a blown transformer where I expected 3800 ohms, I got 850.
Example: 3.1V on the 8 ohm tap 62V on the primary. Turns ratio is 20:1.
20 squared is 400. 400*8=3200 ohms.
On a blown transformer where I expected 3800 ohms, I got 850.
Re: Testing an Output Transformer...argh....
I have seen OT that ohm ok but sound like poo. It seems like you would have a very distorted sound if it was the OT. They sound like a blown speaker if they are not opened up or shorted. You need to do some voltage checks.
I've got blisters on my fingers!
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novosibir
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Re: Testing an Output Transformer...argh....
An easy way to check for a toasted (and conducting) insulation between the primary & secondary is to play the amp, struck the strings hard and measure the voltage drop at the B+
A voltage drop of 40...100V at the B+ with the amp dimed and strigs hard strucking is normal. When the insulation is shot, you'd get a drop of 200...400V in the moment, when the sound becomes nasty - in the moment, when the primary is arcing over to the secondary (which de facto is nearly ground)
Larry
A voltage drop of 40...100V at the B+ with the amp dimed and strigs hard strucking is normal. When the insulation is shot, you'd get a drop of 200...400V in the moment, when the sound becomes nasty - in the moment, when the primary is arcing over to the secondary (which de facto is nearly ground)
Larry
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