Changes to FPIII+ for 6dn7 tubes?

[jrebman]

Just curious, couldd the 6dn7 be used in the FPIII+ and what changes would be needed in order to do this?

Has anybody tried this?  My thinking here is the inherent linearity of the 6dn7 considering it's heritage, and also guessing that maybe it may be possible to bypass the CF stage for short runs as I'm guessing the power half of the tube has a relatiively low output impedance.

I'd be answering most of these questions for myself if I could deal with the curves, but that won't happen anytime soon .

Thinkking something like a 6sn7, 6h8c, or octal 6h30 for the shunt reg tube. -- or is there no tube portion for the shunt reg in this design?

Secondary is that I'd like to go with octals but don't want to use 6sn7s for the main gain tubes.  Also don't want to go with vintage types.

Thanks much,

Jim

 

[jrebman]

Ok, so nobody?  Any other octal tube suggestions for signal tubes?

Decided I will need to go with the CF after all.

Maybe somebody could describe the difference between the 12au7s and the 6sn7s?

Thanks,

Jim

 

[Dr. Toobz]

Based on things I've heard from Paul Joppa and the Doc in previous threads, I'm guessing that the standard 12AU7 CF stage would be preferable to a plate output from the "power" half of the 6DN7 - whose Rp is in a typical application about 2k, IIRC. Snippets of discussions elsewhere on the board (such as in reference to the famed 300B preamp prototype) suggest an output impedance of below 1000 ohms is definitely preferred - see here: http://www.bottlehead.com/smf/index.php/topic,2275.msg17760.html#msg17760.

 

[jrebman]

Ok, yet another idea -- this time back to my old friend the 6j5 but with an MQ B7 output in place of the CF stage.  Can this work with the existing ps configuration, shunt reg, etc.?  And would the 15k/500 B7 be the correct choice?

-- Jim

 

[Paul Joppa]

Just a few notes, since I'm still out of town.

First, replacing the 12AU7 with a 6DN7 is possible, but there's no extra heater current to power a shunt regulator. You would run the power section as the cathode follower, giving it a much larger current so the output signal current is a smaller fraction of the total. It would probably sound better. There are other dual triodes with one larger section; some of them need only 0.6A heater current and could allow a shunt reg.

You could run the power section as a voltage amplifier, with the small section as a shunt reg - assuming the power section has a current source plate load, the shunt reg does not need more than a fraction of he current in the power section. We've done this with a different tube (I forget which one) and it sounds darn good. We've done Magnequest, old-stock Altec, and new Edcor transformers.

We've also tried a plain 12AU7 voltage amp into a 15K:500 output transformer (with C4S and shunt reg), and it also sounds darn good.

But the 300B preamp sounds better.

 

[jrebman]

Hi Paul,

Thanks for the notes and ideas.  Oy, what's a guy to do?  Too many options and with no release date for the 300b pre yet, I'm a bit stuck here.  I also don't have rack space for a TT, two tall amps and a tall preamp (all because of th3e 45/2a3/300b tubes, so something will have to give somewhere.  I also won't be able to use my system until I have a preamp of some sort now.

Guess it's time to ponder this a lot more carefully before I make any decisions.

Thanks again,

Jim

 

[2wo]

There is the 6bn7 at .75, I used it in my 300B amp for a while, or the 6cm7 at .6 but then your back to 9 pin...John

 

[jrebman]

Well, Doc just had to dangle that nice offer in front of my face, so I bought one of the fp III kits on special and added the + upgrade.

I'll build for 12au7s and either a 6sn7 or 6cg7 as a shunt reg tube (still need to figure this out), and I'm sure I'll be more han happy with the results.

-- Jim

 

[intojazz]

Jim,
I'm sure you've already considered this but why not leave the 12AT7 as shunt and the 6SN7's as output tubes. That's how my EFP 3 is currently set up. Actually this seems fairly common. I was using all RFT 12AT7's and switched to the 6sn7 using an adapter. I liked the difference so much that I've wired in the octal sockets. I hope this helps.
John

 

[jrebman]

Hi John,

Well, it really comes down to the fact that the 6sn7 is just not my favorite driver tube -- just a little too thick and rounded for my tastes.  I realize that just about everybody loves them so I'm somewhat of a heretic I suppose, but I'm hoping using one for the shunt reg tube will give just a bit of that roundness to the final sound.

Just my personal taste, not a judgement of anybody else's.

Thanks,

Jim

 

[2wo]

My knee jerk reaction was the 6SN7 might not be up for shunt duty. I looked it up for the first time in years, huh, might be great...John

 

[jrebman]

John,

Yup, could be .  But we shall see.

Really looking forward to building this and the Eros to be my preamplification front end.

-- Jim


The FP was really the last part of the puzzle to complete my system, and while I do have a backup preamp now (that will go in the "ht" system later,

 

[Paul Birkeland]

Hello Jim,

The current run of FP-III plates that we have include the mounting holes for Magnequest B7 output transformers, so you could easily slot in a pair of single triodes on the outboard holes, then a beefier dual triode for the shunt regulator.

The 6J5 octal tube would be well suited to this operation, and looks like it would run OK with the 237 ohm R1 and the LED under the cathode, just like the VA stage in the stock extended FP-III circuit.  For the shunt reg. tube, something like a 12BH7 should work reasonably well.  Not having a cathode follower in the preamp, you could reduce the current through the shunt reg by a fair amount (and change to a different tube to balance out the heater current demand).

-PB

 

[jrebman]

Paul,

Thanks, but it looks like the 6j5 is 300 mA Ih, so I'm not sure what you're saying here:

12BH7 should work reasonably well.  Not having a cathode follower in the preamp,
you could reduce the current through the shunt reg by a fair amount (and change to
a different tube to balance out the heater current demand).

Partly this is because I don't fully understand the shunt reg circuit, but it looks like the 6sn7 or 12bh7 would work fine with the existing available heater current.

Another thought is if the CF stage is eliminated, wouldn't the stock 0d3 still do nicely in this transformer coupled application?  This would also be the 10k - 500 ohm B7, correct?

-- Jim

 

[Paul Birkeland]

Hello Jim,

Yes, you're correct, the heater current would stay the same.

You'll still see a noticeable improvement using the hybrid regulator over the 0D3. 

The 15K B7 is what I would recommend. 

-PB

 

[jrebman]

Thanks, Paul.  My gut tells me I should just build more or less stock first.

I need to better understand what's going on in there first, and who knows what the lead time on the B7s will be like.

I'll also post something over in technical topics on what parameters make for a good shunt reg tube -- hopefully that will get me closer to understanding all this.

-- Jim