Output transformer from old radio

[TurboGuitarMelton]

Hi guys,


Ive ditched my single ended parrallel princeton idea and am going simpler for my first scratch build. Its going to be a 5f1 champ with the 1 tube reverb.


So with that said, I have a cool old OT from an RCA radio. I measured the winding ratio with a small AC voltage and it seems to be 50:1 .That means the impedance is 2.5k righr? So with a 4 ohm speaker we are looking at 10k for a single 6v6gt. That checks out okay. I know a 6v6 is around 7500 ohms. Close enough for me.


Im attaching a schematic of what the OT came out of. There are 3 primary taps on the transformer. Why are there 3 for this OT? Would it work okay in my application? Which leads shohld i use?

[sluckey]

Connect red to B+ and blue to plate. Tape off the brown and don't use.

[sith]

If that is correct schematics on your attachment, then you have all you need to know on schematics…  connect two taps that says 500ohm, blue wire to anode, red to B+

[sith]

Sluckey beat me to it 

Btw. if you are contemplating 5f1 champ, may I suggest same era 5f2a princeton,  not much complicated, but much more refined amp 

[jjasilli]

It looks to me like the BRN wire is feeding the screen (and then roaming off in ways I don't get).  Que passa?

[sluckey]

That portion of the OT is being used as a poor man's choke. Pretty common back then with small consumer electronics. If I was using that OT to build a champ I'd use the choke tap, but I was reluctant to say so in my original post. Just trying to keep it simple.

[jjasilli]

 

[TurboGuitarMelton]

Thank you guys for the replies. I was hesistant to jump in and assume i could ignore the brown wire. It helps to know why its there.




Thats awesome it can double as a choke! Ill probably do that!




Sith, I already added more filtering to the power supply. And I dont want the tone knob from the 5f2a. I like the simplicity of the 5f1 with the one volume.   Plus, I think the tone knob on the 5f2a is really just a variable bright cap. I've heard it makes little difference when you have the amp turned up louder.

[sith]

well yes and no.... but to my mind not 'classic' bright cap…. tone pot works with two caps on it and to cut story short, if you turn it all the way you got some sort of bright cap, turn it other way you got highs to ground and therefore much warmer (softer?) sound… I like that option of dialing specific tone instead of champ that only barks loud and louder 

[shooter]

I like the simplicity of the 5f1

Yup!  the guitar and player have all the knobs needed 
when you get it up, plug it into a 4X12, let the neighbors hear your smiles

[PRR]

> There are 3 primary taps on the transformer.

It's not a choke-y choke.

The raw 265V has ripple. Some of this ripple flows toward the 6V6. A small inverted copy of this ripple flows to R33 and then to screen and driver. If the tap position is *critically* selected, for the circuit action, ripple is cancelled. Nearly.

RCA had a patent on this scheme, and used it routinely, and made a few bucks licensing it to other radio makers.

I have never seen it done on a guitar amp. In today's economics another RC filter on the B+ will drop ripple way down without critical adjustment.

What he said. Snip and tape the brown lead, feed screen filter direct.

> the winding ratio ...seems to be 50:1 .That means the impedance is 2.5k righr?

The impedance ratio, righr. And yes, 3.2/4 Ohm speakers were common, and that makes 8K. 6V6 can take many load impedances, if you adjust V and I. But save your brain.

"Can I use this OT?" Yes, obviously. Any radio is a Tuner and a Power Amp. Omit the tuner, you have a Power Amp. RCA's engineers were no fools, the power amp is fine. With just one small tube in front of the big tube, gain is low for guitar, you will want another stage. If you are hacking a radio, this can be one of the IF tubes. If you cut the whole thing up, then picture a Champ and fade-in the RCA output section. RCA aimed for 3 Watts, whereas Fender ran the 6V6 hotter for 5W-6W. Otherwise there's about no difference.

This re-creation will "work" but the high resistor values will shave 5KHz. Good for AM radio, less-bright for guitar. I would lean more to Fender driver values.