New 6F6 Amplifier Project

[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.

[revintage]

Hi,
What is the PP primary impedance, 7,6k?

[sorenj07]

Yes, the loadline is for SE, but since I'm staying mostly in class A, it should be equivalent with half the primary impedance. The anode-to-anode impedance of the OPT is 7.6K to one single 8 ohm winding. I'm assuming it's the case based on my previous experience, but I'll measure just to make sure. Looking for a ~31:1 ratio, or about 185V after putting 6V on the secondary.

Anyway, I just had a couple 12AT7's on hand. Yes they have distortion but isn't it mostly second-harmonic, which gets canceled out in a topology like the LTP? Any other good double triodes that can put out +-26V from line input?

[revintage]

Your loadline is good. You will get 3W Class A in PP.

About distortion-cancelling I am no great friend of it, think it is a way of fooling yourself. I think all stages should be linear in themselves. But I have seen the 12AT7 is hyped at the moment.

The 6DJ8 is a good cheap linear candidate but you should need a see-saw PI to get the same gain as 12AT/ wit LTP. Didn´t you use the 6DJ8 in the EL84?

[kuka_v]

I would like to suggest a little bit other loadline and some results of matching in the table...
Ugrid = -30v Pout=1.45 Harm = 3.3% but Ra = 7.7 kom - ? it could be critical if you will not wire transformer for this project...

[sorenj07]

2 things: What's a "see-saw" PI? And why the new loadline? Is there any way I could get closer to ~6W Po from PP triode mode?

Upon checking the datasheets, it looks like I really 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.

[sorenj07]

I've changed my mind; I think I'm going to use the cathode feedback windings and run some 6005's in pentode mode. That way, the demands on the driver are lessened, output power is increased, and some NFB is already being applied..

Thing is, I don't know the polarity of the CFB windings. Anyone got a wiring diagram/schematic? I think a Hammond PR40 would be a good place to start.

[Nihilist]

What ever happened with this project ? Have you used 6f6's , and what is your opinion of them ?



........................Blake

[sorenj07]

Nothing yet I'm finishing a 6V6 amp, then I've got a 6005/6AQ5 amp, then this one. Don't worry though, I'll be building it before the end of the summer.

[moonbird]

Any progress on the 6005/6aq5 amp?? thx