Ceriatone JTM45 blowing mains fuse

[Heliotricity]

Is yours old? They might replace it or maybe give you a discount on a replacement.

This one is older, bought it used so that hadn't occurred to me. Good idea though. I've already removed the transformer and ordered a new one. Once I get it I'll test it with no tubes and just the primaries connected first, if the fuse holds then I'll wire the secondaries, if that goes good then I'll add tubes, leaving the power tubes for last. I'll keep you all posted how it goes.

I would do one final test just to be certain, and that is to disconnect everything and use the 'neon test' to discover whether you have a shorted turn anywhere.

What is the 'neon test'? I googled the term but only found a bunch of other unrelated information.

[52 Bill]

What is the 'neon test'? I googled the term but only found a bunch of other unrelated information.

I'm pretty sure he is referring to R.G. Keen's transformer tester. It's over on the Geofx site.

It uses a neon bulb and a battery to ring the transformer windings, which will show if there is a shorted turn.

[DRH1958]

Here's the neon short test. I don't think RG will mind since it's on his site, public domain and all that stuff. LOTS of great de-bug and other knowledge on his site! Very helpful and regular contributor to this forum. Thanks, RG!

[Enzo]

Well gee, put up a link, anyway...

New Page 1

[Mick Bailey]

That's the one. Works better on the HT or primary winding rather than the heater winding and gives a pretty reliable indication of a shorted turn (which can't be measured with a DMM).

[Heliotricity]

New Classictone 40-18033 PT arrived today.



Installed it and hooked up just the primaries (blue COM wire and the black 120v wire) and powered it on with no tubes installed and the mains fuse held. So then I connected all the secondaries and the fuse held, I checked AC voltages on pins 4, 5 and 9 of the preamp tubes, they all read 2.7 volts. I checked pins 2 and 8 on the rectifier tube and there was a reading of only .1 volts. There should have been more than 5 volts there.

I then plugged in the rectifier tube (no other tubes plugged in) and powered on and pin 2 read 8.6 volts and pin 8 read 7.5. They should have about half that on them if I'm correct. I also checked the DC voltage on the standby and only got 005 volts DC, when there should be 325 volts. Something is not right.

My question is this: are the yellow 5v @2A secondary wires on the PT polarized? Did I put the wire that should have been on pin 2 on pin 8 and the pin 8 wire on pin 2?

I am going to check the rectifier tube socket pins and make sure they are clean, maybe it is an issue with the tube socket.

[Heliotricity]

I just noticed on the ceriatone layout that on the rectifier tube socket they have resistors between pins 4-5 and 6-7 and also a .01 uF 3kV cap between pins 4 and 6.



My amp doesn't have those. I rewired this exactly the way it was before, with the red secondary 350v wires going to pins 5 and 7, is this why I'm getting the strange readings? Here is a photo of how mine is currently wired.



I noticed in other photos of jtm45 builds that it appears the secondary 350v wires go to pins 4 and 6 and don't have any resistors or the .01 uF cap, should this be how mine is wired as well to work properly?

[Justin Thomas]

If you have your HT wires going to pins 5 & 7, then there's no connection inside the tube to the plates. So yes, that might explain some funky readings. And those are probably diodes, not resistors.

Justin

[Heliotricity]

If you have your HT wires going to pins 5 & 7, then there's no connection inside the tube to the plates. So yes, that might explain some funky readings. And those are probably diodes, not resistors.

Your right, I just realized I wired them wrong. Got confused by the ceriatone layout with the wires going to 5 and 7. I'll fix it and take new readings.

[52 Bill]

I just noticed on the ceriatone layout that on the rectifier tube socket they have resistors between pins 4-5 and 6-7 and also a .01 uF 3kV cap between pins 4 and 6.

Those components are safety diodes and a cap to filter any noise caused by the diodes. If as Justin suggests, you have the high tension wires connected to the wrong terminals, you will not get any high voltage dc readings.

How are you reading the ac voltages, from pin to pin or from pin to ground? You should read the 5 volt rectifier filament directly from yellow wire to yellow wire and not to ground.

[Heliotricity]

I fixed the wiring and it appears to be all good. Voltages throughout are correct with the new PT. Played through it for an hour and it sounds great.

When I first powered it up I replaced the Gold Lion with a different KT66 which I know works and there was quite a bit of hum from the speaker. Nothing was plugged in and both volumes were at zero. I checked the bias and the replaced tube was fine but on the original tube which I was using before with the Gold Lion there was no bias reading. So I shut it down and put in a spare KT66 set I have and the hum was gone and bias read perfectly for both. So I know that tube which caused the hum and had no bias reading is bad. It is possible that the Gold Lion tube is also bad, not really sure how to check it but I'm going to trash them both.

BTW 52Bill you nailed it, I was reading those AC voltages on pin 2 and 8 on the rectifier socket to ground.

Thanks all for your help, now I can get back to making the music!