Choosing Choke for 5E4-A Tweed Super

[chopstuck]

I have finally .... after 20 years .... gotten my first Tweed circuit sounding dead-nuts to an original Fender. It was all in the choke for the 5E5-A I built in '88. This was pre-internet. I started thinking about power supplies again when looking at a 5E8A low power twin. Turns out I had the wrong choke. I've been using a "universal" 6L6 blackface choke that totally changed the sound. They were not designed to filter plate current.
I put in a big PA salvaged choke that was huge it was as big as the OT and only 85 ohms - BINGO the tone was all there.

Question. What does the tweed 6V6 super need to filter the whole amp.
and or what did Fender use ? Was it the same 5-8H at 200 ma the big 6L6 boys used or could a smaller blackface 6L6 "universal" 125ma choke do the trick ? From the

http://www.dreamtone.org/Calculate_Current_Form.htm

site it looks like my tube config should draw 93 ma.
I don't know if that's accurate.

[Structo]

Depends on the plate voltage but I would guess around a max of 100ma.

[tubeswell]

For an amp with 2 x 6V6s a choke for a whole-of-supply CLC filter I would use ~9H 120mA with a couple of 20uF on either side. Mojo sells one IIRC (and so does Weber).

[chopstuck]

Thank you for the reply. I will try the 9H at 120ma I took out of the tweed pro.

[chopstuck]

Surprise, I tried the 4h at 100-120ma blackface 6L6 choke and the amp sounds constipated and has "fizzy distortion like steel wool on a Corelle plate.
I tried a larger framed unknown 6L6 organ choke that had twice the resistance and the amp sounded so much better. More and higher harmonics no fizz and more solid lows. Bell or glass plate like tones. I switched back just to make sure it wasn't my imagination. The mud and fizz came back. I'm now sold on bigger than spec. chokes.

I'm learning all the time

[surfsup]

I tried a larger framed unknown 6L6 organ choke that had twice the resistance and the amp sounded so much better. More and higher harmonics no fizz and more solid lows. Bell or glass plate like tones. I switched back just to make sure it wasn't my imagination. The mud and fizz came back. I'm now sold on bigger than spec. chokes.


Could you explain "bigger spec"? Resitance was the only difference between the two? In this case it would only affect the voltage (lower). Did the better sounding choke have a larger Henry value or more mA rating?

[Cliff Schecht]

I'm curious as to chopstucks voltages as well when he switches from one choke to the next. I bet that small choke was seriously struggling if it mucked with your tone that bad..

[chopstuck]

THe original small choke was the Fender reissue vibroverb choke -B 036486. The idling voltages were within 2-4 volts with either choke. I couldn't measure the specs of the choke since my meter only goes up to 2H. Is there a way to increase the range of my inductance meter with a bridge or other circuit ?

[Cliff Schecht]

You can measure it with a function generator, a scope and a volt meter. This is about as simple as it gets: http://technologyinterface.nmsu.edu/fal ... nduct.html

[John_P_WI]

My guess is that the big ass salvaged choke is a "swinging" choke and is able to take the current pulses.

The smaller reissue choke is not able to handle the pulsing current as well and is "saturating" losing its effectiveness and is just passing along the pulsing dc to the next cap.

This is especially noticeable on "choke" input or xformer > diodes > choke

BTW, with the wrong / underrated choke on a choke input filter, it is possible to pull too much current from the power xformer - when the choke saturates and its impedance goes down - and melt the power xformer

[bluesfendermanblues]

BTW, with the wrong / underrated choke on a choke input filter, it is possible to pull too much current from the power xformer - when the choke saturates and its impedance goes down - and melt the power xformer

Only if the power xformer is underrated

[tubeswell]

I'd be surprised if the 120mA choke couldn't handle the current demand of a 2 x 6V6 fixed bias amp with three 12A-7 preamp tubes. I'm inclined to wonder whether it doesn't have enough inductance if it is only 3-4H. The recommendation on Merlin Blencowe's page here - http://www.freewebs.com/valvewizard1/smoothing.html - is that "the sum of the two capacitors (in micro-Farads), multiplied by the inductance of the choke (in Henrys) should equal 200 or more for excellent performance". Using the 'stock' 16uF filter caps, this number would only be 128 with a 4H choke. Seeing as how you have a 5U4G you could increase both filter caps to 40uF, in which case it should work better.

[chopstuck]

I'm not sure if it had enough inductance BUT, there was no hum. In that function it met the design requirement. I have seen these chokes listed as 4- 9 Henries. Seems like a large spread.