Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ed22 on December 22, 2020, 10:40:56 am

Title: Power transformer blown on Princeton Reverb reissue : Center tap on heaters
Post by: ed22 on December 22, 2020, 10:40:56 am: Hello,

I'm repairing a Fender 65 PR Reissue. Power transformer is dead.
I've ordered a Hammond 291AEX to replace it.
291AEX has a center tap for heaters :
https://zupimages.net/viewer.php?id=20/52/8c2n.png

Looking at the schematic of the Princeton Reverb reissue :
https://zupimages.net/viewer.php?id=20/52/kfut.png
The original power transformer doesn't use a CT for heaters.

What would be the best way to wire the new transformer?

1) Leave the CT unwired and only connect the two green wires?
2) Use the CT and take away from the PCB the two 100R resistors that creates an artificial CT? In this case, CT sould be wired to ground point GND_420V?

3) what would happen if I leave the 2 resistors and use the CT?

thanks for your help,

rgds,

Ed
Title: Re: Power transformer blown on Princeton Reverb reissue : Center tap on heaters
Post by: sluckey on December 22, 2020, 10:43:43 am: #1 is the best option IMO.
Title: Re: Power transformer blown on Princeton Reverb reissue : Center tap on heaters
Post by: jordan86 on December 22, 2020, 12:40:06 pm: I would never disagree with sluckey but FWIW I used a 290ax on my Princeton and used the provided CT and skipped the 100R artificial CT. Super quiet.

Different scenario for you since it’s a pcb and a replacement vs new build.
Title: Re: Power transformer blown on Princeton Reverb reissue : Center tap on heaters
Post by: thetragichero on December 22, 2020, 12:59:50 pm: i like using an artificial center tap for the heaters even with a transformer that has a heater center tap as a bit of insurance. I'd rather spend pennies to replace a resistor or two than but a new power transformer because the heater winding shorted
Title: Re: Power transformer blown on Princeton Reverb reissue : Center tap on heaters
Post by: pdf64 on December 22, 2020, 02:05:46 pm: Using the CT and ensuring that the HT winding is suitably protected by a dedicated fusing seems the best way forward to me.
If no CT then use beefy heater balancing resistors, eg 5W, that will withstand fault current for long enough for a fuse to blow.
Title: Re: Power transformer blown on Princeton Reverb reissue : Center tap on heaters
Post by: ed22 on December 22, 2020, 05:06:56 pm: Hello all,

thanks for your help. :)
OK so I could use the CT (since the PT has one) AND leave the 2 resistors. Maybe that would add insurance as said thetragichero.
case 2 involves unscrewing the PCB and some desoldering/soldering. Not that I'm too lazy ;) but if case 1 or 3 can do the trick, that would be much easier to do.

rgds,

Ed.
Title: Re: Power transformer blown on Princeton Reverb reissue : Center tap on heaters
Post by: sluckey on December 22, 2020, 05:12:11 pm: If you are in the business of repairing amps then option #1 is the best choice.
Title: Re: Power transformer blown on Princeton Reverb reissue : Center tap on heaters
Post by: drew on December 22, 2020, 05:19:35 pm: Quote from: ed22 on December 22, 2020, 05:06:56 pm
Hello all,

thanks for your help. :)
OK so I could use the CT (since the PT has one) AND leave the 2 resistors. Maybe that would add insurance as said thetragichero.
case 2 involves unscrewing the PCB and some desoldering/soldering. Not that I'm too lazy ;) but if case 1 or 3 can do the trick, that would be much easier to do.

rgds,

Ed.
I think what thetragichero meant was that using the artificial center tap even when the PT has an *available* real center tap is the better approach -- in other words, he's not advocating a belt-and-suspenders deal.
Title: Re: Power transformer blown on Princeton Reverb reissue : Center tap on heaters
Post by: ed22 on December 22, 2020, 05:55:00 pm: I'm not in the business of repairing amps. But I repair tube amps for friends from time to time and for me of course. I also have build some tube amps (and EL34 world is a great source of informations). I'm not an expert but to this day i've never had trouble.
Thinking of case 2, i could just cut the two resistors (no need to take out the PCB).
The idea here was just to know what could be done or not and if one solution is better than the others. From my understanding, the 3 cases are possible.

rgds,
Title: Re: Power transformer blown on Princeton Reverb reissue : Center tap on heaters
Post by: thetragichero on December 22, 2020, 06:06:38 pm: yeah don't do both. keep it simple. heat shrink that center tap, hook up the power transformer like the old one was, and keep it movin
Title: Re: Power transformer blown on Princeton Reverb reissue : Center tap on heaters
Post by: trobbins on December 22, 2020, 07:40:18 pm: Hi Ed, I'd recommend staying with the same fixed resistor humdinger setup. One possible benefit can be when there is a pin 2 to pin 3 arc-over on a 6V6GT, and the fault current is limited or 'fused' by the 100//100 humdinger resistors - compared to no fault current limiting from the heater to ground (due to the CT).

Do you know why the original PT failed, or what part/winding within the PT failed, or if any other parts failed at the same time as the PT?

Ciao, Tim
Title: Re: Power transformer blown on Princeton Reverb reissue : Center tap on heaters
Post by: ed22 on December 23, 2020, 05:15:31 am: Hello Tim,

I don't know why the PT failed. There was absolutely no tension in the amp, except at power switch of course. I've tested the PT and primary was "open-circuit" (no continuity). No burned component, and fuses OK.
All preamp tubes work (i've tested them in my silverface PR).
I'm gonna do a clean power section : new GZ34, new 6V6s (all JJs) and new PT of course. I've already changed the 4 filter caps (it never does harm).
It's not my amp. All I know is that one day my friend turned it on and nothing happened (pilot light off, no sound). No dubious smell as far as I know.

I think i'll keep it simple and just heat schrink the CT as said thetragichero.

thanks,
Ed