Dual Impedance and Filament Q, 5F2A First time builder

[Bangy]

Hello

I have an RCA and a 1/4 inch switcraft 12A for jacks. I want to run the RCA with 4 Ohm tap, and the 8 Ohm Tap to the 1/4" Jack. I WILL NEVER use them simultaneously.

I read a discussion from a previous post on the dual impedance issue and sorry didn't understand if using a 12A 1/4" with an RCA is doable or not. The RCA is basic like the old tweed amp had and the 12A has a ground, Switch, and Tip lug, as Im sure you all know.

My plan is to share the (-) from the OT to both Jacks by running to the RCA (4 ohm) then to the 1/4" (8ohm). Any issues with this idea?

Question is the negative feedback connection. Would I simply connect both Speaker jacks to the 22K Resisstor and call it a day?

Filament question--

I have an actual Lupe wired 5F2A, I am staring at as I build this and her filament wiring is super simple compared to What I believe I am supposed to do when reading Doug's literature. If I am seeing things correctly in my Fender amp Lupe didn't use a pair of filaments to V1 and V2, only one. She takes one 6.3V (green) form PT to ground, the other to the Light (which has only one lug) - V2 pin 7 - V1 Pin 4/5.

Question should I follow Lupe or Dougs illustration that has both 6.3V green wires to the lamp (which has 2 terminals)- then twisted pairs to V2 pin 2 and 7 - V1 pins 4/5 and 9?

I hope this makes sense. . . Thanks for your help. . . Bangy

Subsequent to this post, I talked with my tech. He explained how the filament treatment of this circuit was varied and that mine has a center tap that lupe cutoff, thus the ground of 1 wire and the run of one other green filament wire. This necessitates the grounding of pin 1 jumpered to 2 on V2- which is exactly how Lupe did it.  He suggested since I have it set up with a center tap to ground to stay the course. So thats the plan.

The output jacks he has me sending only one of the speaker outs to the negative feedback. He doubts when I compare them that I will notice much if any of a change. He also said to take the black secondary OT lead to a solid ground which I hadn't thought of.

Any thought Id share this with you guys.

[sluckey]

Would I simply connect both Speaker jacks to the 22K Resisstor and call it a day?

No! Connect the NFB wire to the 4Ω jack only.

Question should I follow Lupe or Dougs illustration

Follow Doug's instructions.

[Bangy]

Thanks Sluckey, my tech agreed with you on both fronts.

Why the 4 ohm tap and not the 8?  I will be using the 8 ohm tap regularly? For the NFB.

[eleventeen]

Which tap you use for the NFB has nothing to do with which output impedance tap you use to drive your speaker(s). ALL output taps supply an output signa at all times, of differing voltage levels, assuming the tranny is working.


Think of the laminations of the OT as a wall. Although the NFB is drawn from the "outside" side of the wall, once it is selected, it's selected, based upon how many volts it puts out. As a matter of circuit design. That amount of volts, fed back into the amp, creates the neg feedback action. It's done.


The chosen speaker impedance tap is only there to match the outside of the tranny to your speaker load most efficiently. Not saying this isn't fairly important, it is. Yes, if there is a mismatch here, it will indeed reflect backwards into the amp. This we do not want. One day, the amp drives an 8 ohm load, some other day, it drives a 4 ohm load. The way to have the amp act uniformly is to change (only) the speaker impedance tap and that's that.


It has to be this way. The selection of the NFB tap from the output side of the OT is a soldered connection. The load impedance selection is done via binding pot or switch. You only change one, the load select tap.

[sluckey]

Because that's the tap Fender used (I think). The NFB circuit doesn't care which tap you use for the speaker. Anyhow, it ain't critical. If you want more NFB then connect to the 8Ω tap. JUST DON'T CONNECT THE 4Ω AND 8Ω TAPS TOGETHER!

[Bangy]

Sluckey and 11teen very much appreciated.

Am I understanding that when I choose the 4 ohm Speaker line out for the NFB, I am giving the circuit what Leo designed AND regardless of which speaker Tap I am using the NFB will be incorporated in the output?

And I hear you only 1 and never together.

Bangy

[eleventeen]

Exactly right.


As Sluckey said "The NFB circuit doesn't care which tap you use for the speaker."

[Bangy]

Awesome. . . Thanks guys. Board is wired components are fit.  I am going to check values one more time today and then solder the board. Sure has been fun-NOw the hope it works vibe cometh!

Bangy