Your preference on center taps?

[jordan86]

Half way through my first build. A Mojotone Princeton Reverb Kit with "upgraded" transformers.

What do you guys do with center taps?

I'm in a bit of a pickle with the center taps on my Hammond 290AX. It has dual HV taps, and center taps for both the HV and filament taps. I've not found a single Princeton example

Could I get some help on these few questions...

1) I'm 99% sure I just trim my unused 275v HV lines to preferred length, heat shrink ends, and tuck them away. I would like to try them out down the road.
2) The HV CT gets grounded to the main ground tab, per my schematic.
3) Most AA1164 circuits seem to use a artificial center tap for filament. The two 100ohm resistors coming off the pilot light. That's what my Monotone schematic shows as well.
4) My mojotone schematic uses a transformer without a filament CT. The original schematics I am looking at shows a CT for both HV and filament sent to ground.

Would it be best to use the artificial CT with the resistors AND ground the filament CT (grn/yel) to ground as well? Or simply choose one or the other? Would be less work to skip the 100 resistors and just ground the filament CT to the main ground tab with HV CT? 5) FWIW, my kit calls for a gz34 not a 5u4. I wouldn't mind trying a 5u4 down the road though if that matters.

[jojokeo]

Half way through my first build. A Mojotone Princeton Reverb Kit with "upgraded" transformers.

What do you guys do with center taps?

I'm in a bit of a pickle with the center taps on my Hammond 290AX. It has dual HV taps, and center taps for both the HV and filament taps. I've not found a single Princeton example

Would it be best to use the artificial CT with the resistors AND ground the filament CT (grn/yel) to ground as well? Or simply choose one or the other? Would be less work to skip the 100 resistors and just ground the filament CT to the main ground tab with HV CT? 5) FWIW, my kit calls for a gz34 not a 5u4. I wouldn't mind trying a 5u4 down the road though if that matters.

Lots of confusion for you unfortunately that shouldn't really happen on a kit build. But it's not surprising either. Look at it as a good thing in that it is giving you much more learning opportunity ;)


1) B+ center tap goes to main star lug via a short wire run if possible and the first main filter cap's ground is soldered here also. Most will solder the power tube's screen here along with the output jack's ground.
2) Be sure that there's not actually TWO grounds for high V/B+ because that is not likely? Unless it's a toroid coil or some other weirdness kind of tranny?? Likely it's a low voltage 50v bias tap or similar? Double and triple check this first to be absolutely sure.
3) Heater/filaments: if there's a CT then use it. Do not use 100ohm resistors as these are only used when the filament wiring does not have a CT. Never do both at the same time.
4) The filament CT should be soldered to the cathodes of the power tube's socket pins or that side of the cathode resistor and cap (if it will be cathode biased?). You will get "elevated heaters" without extra parts and helps a bit on reducing floor noise by doing this. If not (depending on the Princeton model) then to a normal ground.
5) Unless the transformer has 5V windings rated for 3A do not use the 5U4 rectifier since it runs at 3A. The GZ34/5AR4 uses 1.9A and a 5Y3 uses 2A.

[kagliostro]

About heaters, use only a CT, the real CT or a virtual one

To me the right procedure will be to use the real CT and, if  you have a humm problem, give a try to the virtual CT

About virtual CT I consider the use of a pot (humdinger) an improvement respect the use of a pair  of resistors

if you have enough space, good practice will be to add an elevated earth for heaters CT connection (real CT or virtual CT will both benefit)

Franco

[mresistor]

I just built a Princeton Reverb clone using a Hammond 290AX and I used the filament center tap and grounded it to the same bolt as the HV center tap.  I used the 325V HV taps and heat shrinked the 275 taps and tucked them away as neatly as possible. I don't think I will ever be using them.
I think what Kagliostro is saying can be found here http://www.valvewizard.co.uk/heater.html   

You can't elevate the heaters in this amp using the power tubes as the cathodes are grounded.  Merlin discusses how to do it with a voltage divider which I didn't need to do.
He also talks about using small resistor on the filament center tap to ground and I think I'm going to add that to my PR clone.
 I do also  suggest you add 1 ohm 1W 1% resistors on the cathodes which will make adjusting the power tube bias very easy to do.
 

[jordan86]

Thanks, all. I will go ahead and use the provided center taps for ground, and skip the 100ohm artificial CT the Mojotone suggests.

The 5v filament winding does have 3A, so I should be good there.

I was surprised how many questions the Mojotone kit leaves unanswered. It’s not the greatest directions. Fortunately, I am a bit of a tinkerer and have figured most of it out. In this case though, it’s a bit self inflicted since I am not using the provided kit trannys.

Surprisingly, StewMac has incredible instructions for their PR kit, which just so happens to be the Mojotone kit repackaged. I downloaded their instruction manual from their site along with a few other A1164 schematics for reference.

[mresistor]

Actually you are better off not using the supplied power transformer as it might have only been rated at 70ma whereas the Hammond 290AX is rated at 100ma.  The original Fender PR PTs were only rated at 70ma and I have read quite a few failed over time. Also the OTs are kind of puny but some say that is where the mojo comes from.    I personally have replaced one PT and one OT (originals) that were bad. I also used a Hammond PR OT in my build, what are you using?

[jordan86]

Yes, that is exactly why I grabbed it.

As for the OT, I picked up a TO20B from David Allen. I wanted his Heyboer made TO20, but his stock is depleted at the moment and suppliers are either way behind, not doing small custom runs, or flat out closing due to covid-19.

Unfortunately, I had to drill new mounting holes for the TO20B. It fit the mounting holes fine, but it is probably a half inch wider than the mojo OT. The mounting screw hole was so close to the OT, I would not have been able to get the screw in at all with the TO20B. Mojo apparently likes them real close. So I had to move the holes about 3/4” further away from the PT to clear the screw hole.

[yorgle]

Forgive me if this is slightly sideways from the subject of this thread, but I'm wondering- would you create a virtual center tap even in a DC heater circuit?

[kagliostro]

It may be applied

I'm not sure that it is beneficial, however you can do it

a friend of mine, after a try to different options, decided for this



Franco

[shooter]

would you create a virtual center tap even in a DC heater circuit

I wouldn't, things are complex enough without trying to figure out a split rail PS.