Ultra Linear Question

[fusionbear]

Hello Gents!

I was thinking of building a little 12-15 watt combo with the super drive preamp into a couple of 6v6's at about 350 on the plates. But, part of the magic of the super drive is the "thump" of the low end. My thoughts are that since the tone is mostly coming from the preamp, using an UL output stage might be the best best so that the amp stays together at a decent volume. SS rectifier will be used. Any ideas? Warnings?

My thought on the B+ string is: B1>Choke>B2>OT winding>Plates, screens.

B2>10K>B3 PI>10K>B4Cathode Follower, Loop>10K>B5 Preamp.

[matt h]

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[fusionbear]

I have the Triode UL OT you mentioned. Most circuit examples for a Hi-Fi 6V6 power section show a resistor or choke with no more than 150 Ohm before the OT B+ feed. I was thinking of using the 90ma standard 4h choke that is 110 Ohm.

[potatofarmer]

I think you'll want more current handling than that for a choke that's going to filter all the B+ for the amp. If you look at some old Sunn (or just Dynaco) schematics, you'll see chokes in parallel to increase the total power handling, which would be all well and good if you could guarantee the chokes will handle power equally. The DCR isn't exactly identical so they won't.

It's not uncommon nowadays to see Sunns or Dynacos that are leaking insulation out of their chokes into the chassis. On paper it's clearly a bad idea to use an underrated choke (or two) but another way to look at it is they've taken 40 years to fail...

I don't like replacing iron so if it were my build I'd probably hunt down a 250 mA choke. Edcor makes some, Classic Tone's 40-18047 would work but would take up a lot of real estate. Actually duhh Triode sells a replacement Dynaco choke that's rated for 200 mA (1.5H)! 200mA is probably pretty close to your overall power draw, but it's cheap. If you could spare the extra $10, the Edcor XC75-1.5H-250mA would be a better choice.

Are you going cathode-biased or fixed? Fender's crude tack of using a bias balance control to nullify the ripple in the output stage might be something to consider, but a pi filter like matt h suggests is definitely the better way to go.

[brewdude]

On a 2x6v6 amp, I would have thought a 150mA choke would be enough. I also assumed you would want more inductance--say 10H, or so.

Disclaimer: I freely admit that I may know only enough to be dangerous--so to speak.

[potatofarmer]

On a 2x6v6 amp, I would have thought a 150mA choke would be enough. I also assumed you would want more inductance--say 10H, or so.

Disclaimer: I freely admit that I may know only enough to be dangerous--so to speak.

Both good points. I'm biased towards overdesigning - look at the max. instantaneous plate + screen current from the load line, double the estimated current draw from the preamp, and round up to the nearest big round number. I'd also want the choke to survive long enough for the B+ fuse to pop, in the event of a tube short. Not necessarily right or wrong, but more expensive, sure.

Choosing an inductance requires math and deciding on an acceptable level of ripple. Definitely not my forte, but looking at a couple off-the-shelf chokes in Duncan's PSU Designer should help.

I guess all I should've said was "a 90mA choke is underrated for this application" but I'm lonely so I talk too much.

[brewdude]

I admit I am not really good at the math but, I am under the impression that at inductance changes with regards to current. So, I suspect that simply over engineering could result in counterintuitive effects on inductance.

[matt h]

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[gingertube]

I use these for PP 6V6 with switchable UL.
The trannies are 10K Raa with 23% UL taps, ideal for a pair of 6V6. Also stunning with ECL86 (6GW8).
http://www.mableaudio.com/en/productvie ... ormer.html
They are medium quality HiFi trannies which also means they are ideal for guitar.
Cheers,
Ian

[fusionbear]

I think I will just not use the choke and take the B+ straight from the diodes. I will play with it and see what happens....

[matt h]

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