Transformer Primaries + CT..

[diddymix]

Hello All!

I was hoping somebody could help me with some advice on something transformer related. I have just bought a new classictone transformer for my marshall amp, and have installed the transformer and added spade lugs to the wires so they can plug onto the amps pcb. Basically after installing it I decided to just do a simple continuity test with my meter.. set to DC volts. When I probe the secondaries and the ground wire also on that side of the transfomer... I get a beep and there is contiuity.. which is what I thought is correct. However when I probe the primaries, and also the CT (HT).. I get nothing and there is no beep. Forgive me if I am missing something here... but am I not correct in saying that there should be... and that the B+ high voltage DC needs to feed my amps output tube plates via the HT centre tap??? Please do get in touch and thanks for your time! I'd rather not try the amp until Ive tested this though maybe it wouldnt hurt it..

[diddymix]

Ah sorry here is the schematic. Thanks!

[Tony Bones]

When testing for continuity, the meter will beep only of the resistance is low enough. The secondaries of OT's are often low enough to get a beep, but not the primaries.

Set the meter to measure resistance and just see what the actual resistance is in ohms. ClassicTone puts the expected DC resistance values on the spec sheet of many of their products.

[sluckey]

The schematic you posted uses a FWB rectifier so the original PT does not even have a CT. I suspect you have a wrong PT for that amp. If you post the data sheet for your classic-tone PT we can tell you for certain.

[xtian]

Not to be a pain...but, there will be no DC continuity between primaries and secondaries.

[diddymix]

The schematic you posted uses a FWB rectifier so the original PT does not even have a CT. I suspect you have a wrong PT for that amp. If you post the data sheet for your classic-tone PT we can tell you for certain.

Hey Sluckey, the new transformer Ive installed and are testing is the output transformer.. Am I correct in saying there should be continuity across the primaries and HT middle connection?? The guy above suggested I may not hear a beep because of the resistance of the primary coils... Im not a techy really but the datasheet says that it is a 4K ohm primary connection.. does this explain no beep as its too much resistance for the meters continuity test? Thanks!

[diddymix]

When testing for continuity, the meter will beep only of the resistance is low enough. The secondaries of OT's are often low enough to get a beep, but not the primaries.

Set the meter to measure resistance and just see what the actual resistance is in ohms. ClassicTone puts the expected DC resistance values on the spec sheet of many of their products.

tanks tony! Hopefully this is the reason and its not broken!!

[pompeiisneaks]

The continuity beep will not work past a specific resistance, it may vary from meter to meter. As was stated, you should measure the resistance. If the resistance from the input to output on the secondaries matches what the datasheet shows you're going to be fine. the primary side should have the center tap and two other leads. There should be roughly the same on either side between center tap and one lead and the sum of the two between the outer sides.

Say you measure 20 ohms from center tap to one side, and then 19 ohms to the other, then from one side to the other it should be pretty close to 39-40 ohms.

I'm making up that resistance measurement from one I had, I don't know what yours is. Due to the fact that it's just copper wound up, it's based upon how long the copper wire is, and how thick it is, to determine how much resistance there should be. The manufacturer knows this and should put it on their transformer data sheets.

~Phil

[xtian]

If the resistance from the input to output on the secondaries matches what the datasheet shows you're going to be fine. the primary side should have the center tap and two other leads. There should be roughly the same on either side between center tap and one lead and the sum of the two between the outer sides.

Say you measure 20 ohms from center tap to one side, and then 19 ohms to the other, then from one side to the other it should be pretty close to 39-40 ohms.

I'm making up that resistance measurement from one I had, I don't know what yours is. Due to the fact that it's just copper wound up, it's based upon how long the copper wire is, and how thick it is, to determine how much resistance there should be. The manufacturer knows this and should put it on their transformer data sheets.

~Phil

"Resistance from input to output" ?!?

This is extremely confused. The PT has primary coils (120vAC input) and secondary coils (6vAC heater, HT, etc). There cannot be any DC continuity between primary and secondary coils.

[pompeiisneaks]

If the resistance from the input to output on the secondaries matches what the datasheet shows you're going to be fine. the primary side should have the center tap and two other leads. There should be roughly the same on either side between center tap and one lead and the sum of the two between the outer sides.

Say you measure 20 ohms from center tap to one side, and then 19 ohms to the other, then from one side to the other it should be pretty close to 39-40 ohms.

I'm making up that resistance measurement from one I had, I don't know what yours is. Due to the fact that it's just copper wound up, it's based upon how long the copper wire is, and how thick it is, to determine how much resistance there should be. The manufacturer knows this and should put it on their transformer data sheets.

~Phil

"Resistance from input to output" ?!?

This is extremely confused. The PT has primary coils (120vAC input) and secondary coils (6vAC heater, HT, etc). There cannot be any DC continuity between primary and secondary coils.

You took only part of my sentence, I did use incorrect nomenclature but that sentence specificially stated 'ON THE SECONDARIES' I meant from a specific tap (was thinking input in my head but it's not the right term) to ground (output)

Yes my terminology was odd, but I explained pretty clearly how to test with resistance. Also OP clearly stated he's talking about the OT not PT. So there are no 120VAC inputs nor 6VAC outputs...

I don't know that I get the implied negativity to my post? Maybe I'm reading too much between the lines there, but everything I said seems apropos, with some odd terminology thrown in to piss people off I guess?

Always ready to be corrected.

~Phil

[xtian]

Sorry, Phil! I reread the first post, and still wouldn’t have guessed it was the OT in question. Carry on!

[pompeiisneaks]

Sorry, Phil! I reread the first post, and still wouldn’t have guessed it was the OT in question. Carry on!

They clarified in the post one or two before mine. I still should be more careful on using proper terminology instead of inventing my own

~Phil

[sluckey]

Sorry, I misread the original post.

[diddymix]

Thanks guys very helpful! I overlooked the resistance thing, and it makes perfect sense to no continuity across the primary side