Question? Filament Circuit on PR

[TTOCS2]

HELP!(lol) hello all I've been rebuilding/restoring a PR,pretty much have gone completely through it with new caps and all,I've been going off the build PeavyBandit did on his new PR Deluxe OT 12" spkr,5AR4:My question is on the Bias control and Filament Circuit where the 100k resistor is upgraded to a 100k 2watt.The note below says if you use a pair of 100ohm resistors on the Filament Circuit you must lift this GY center tap or cause some smoke.Is whats being said there if your using 2-100k 1watt?I'm new at working on amps with just a little no-how.It would be real easy IF mine had a GY center tap,what I have is a RY and a R.This is the only thing left on the rebuild the new cabinet came last week and I'd love to get it all back together and completed any help would be great.I really dont want to see the smoke part...

[Enzo]

What notes, and where? Bias and filament circuits are pretty much separate items. 100k resistors are usually plate load resistors in preamp tube circuits, or maybe grid resistors. I don't recall 100k resistors connected to the filament wiring anywhere.

But 100k resistors are not 100 ohm resistors. The "k" means 1000, as in 1000 ohms A 100k resistor is 100,000 ohms, while a 100 ohm resistor is well, 100 ohms.

We like to connect the filament - also called heater - circuit to ground to reduce hum. One way is to conect the center tap of the heater winding on the power transformer to ground. Since the heater winding is often green wires, the center tap wire for that is green with a yellow stripe. Not all transformers have that center tap. In those cases, what we often do is connect a 100 ohm resistor from each of the two wires to ground.

What I believe you are being warned about is not to do BOTH. If you HAVE a center tap, AND already have the two resistors, then DO NOT connect the center tap to ground. IN that case, you could either remove the two resistors and ground the center tap, or leave the resistors and tape off the center tap where it won't touch anything. Just don't have resistors and the grounded center tap.

A most common place to find those two resistors is soldered to the terminals of the pilot light socket. If you have them.

[TTOCS2]

Ttocs2

this one,Thank You
http://music-electronics-forum.com/s...8461#post48461

[TTOCS2]

Ok,I got it

I get it,Thanx I just hated the thought that it might have gone up in smoke,coolness so I'll tube it up and give it a go.Mine does not have the resistors on those wires.Thanx again....TTOCS2

[Rick Erickson]

There's no danger of smoke from doing both the resistors and having the center tap connected, it's just redundant. The idea of using the resistors is to isolate the heater wires from the chassis AND provide the hum cancelling that the center tap was used for. Just because you don't have a green/yellow wire doesn't mean you don't have a center tap ground. Early Fender transformers had the heater center tap internaly connected to the transformer core. You would have to dismantle the transformer to disconnect this tap. Check for continuity from the heater wires to chassis to confirm whether you have this internal tap connection or not.

RE

[Steve A.]

... Just because you don't have a green/yellow wire doesn't mean you don't have a center tap ground. Early Fender transformers had the heater center tap internaly connected to the transformer core. You would have to dismantle the transformer to disconnect this tap. Check for continuity from the heater wires to chassis to confirm whether you have this internal tap connection or not.

Interesting! Do you have any idea which models used such a transformer?

A lot of the 50's models (like the 5E3) had one end of the filament tap connected to ground.

The reason I ask is that if I didn't know better I might just assume that the filament windings were internally shorted (as in "bad").

Thanks for the heads up!

Steve Ahola

[Rick Erickson]

Steve - I don't know which exact models, or even years - but I know one when I see one...
You can usually find the bare wire poking out between the cover and the core. The best way to tell though is by measuring continuity.

RE

[Steve A.]

Steve - I don't know which exact models, or even years - but I know one when I see one...
You can usually find the bare wire poking out between the cover and the core. The best way to tell though is by measuring continuity.

RE

Thanks for the advice!

[CitizenCain]

Is there any reason the filament circuit shouldn't be grounded in the middle of its run instead of at one or the other ends? I'm using a balance pot in my filment circuit, and the most convient place to mount it is the space between the power tubes and the PI. This puts the balance ground between the power amp and preamp. Am I inviting trouble?

[Rick Erickson]

[Is there any reason the filament circuit shouldn't be grounded in the middle of its run instead of at one or the other ends?]

Souldn't make any difference at all.

RE

[CitizenCain]

Thanks for the reply. I forgot I posted this question!