[sorenj07]

Got some lower-powered PP OPT's, very similar to the ones I used before in my triode-mode EL84 push pull amp. They're probably from a Hammond organ, are drop-through mount, and look like they can handle about 10W and as little current as possible.

During my brainstorming, I figured that pentode mode with these things would necessitate feedback, which I guess the cathode feedback winding could provide. However I'm more interested in a zero-feedback PP triode setup. A lot less hassle to figure out, and no need for regulated screens etc.

I'll post pictures of them ASAP. Right now the laminations are drying off from having the grimy silver paint wire-wheeled off and painted with low-gloss metal enamel.  I also have some matte finish spray-on enamel for the endbells, once I clean them off a bit.

Here's my proposed operating point using a 6F6 tube. The datasheets all list push-pull triode mode as an option, so I figured I might as well. I haven't quite decided between cathode-bias or fixed-bias.. It depends on the parts I have at hand.



What do you think so far? I was thinking of driving them with a single 12AT7 in LTP, fed by a CCS.

[sorenj07]

Upon checking the datasheets, it looks like I need to step up the power supply.  At 350V B+, fixed-bias at -38V 24mA/tube into a 6K load gets 13W at 2%.

I had been considering using a 5V4 with a 6K7VG transformer in choke-input but if I use SS rectification, I have 330V to play with.

[Niteshade]

What about 300v b+, 6K-10k primaries & 200-275ohm cathode resistors? You can always change the cathode resistors to your liking. At least you won't have to make a bias supply.

[sorenj07]

Hm, any reason for that particular range for cathode resistance? The nice thing about a bias supply is that you can use a low-wattage trimpot to fine-tune whatever bias you want..

[JoshK]

I'd go fixed bias. 

[sorenj07]

Yup, that's the plan so far. I have an amp to go before I start that one, so there's some time in any case. -38V is a bit steep though, I might have to go 3-stage