1-channel BFDR with EF86 Schematic comments please

[tubeswell]

As it is well over a couple of months since I redesigned my last amp, I thought about re-designing this one. Now it has JJ6V6s in the output stage and a 12AT7 LTP (i.e. its looking like a BFDR back end, but with bias vary trem) - everything about that sounds groovy and I may even go to 4 x 6V6s there since there is plenty of juice in the Pt and OT and I've got speakers to burn etc, however...

The real reason I am posting this today is I am thinking about the pre-amp again, and I have been toying with the idea of an EF86 stage for experimenting with the tonal variation.

So employing Merlin's excellent book, I knocked up this schematic today, which I've modelled after a BFDR front end (but with the EF86 and CF in place of where the second half of the '12AX7' would otherwise have been).

I don't want super hi gain out of the EF86 (and the amp is a combo after all), so I was thinking of a lowish screen voltage (around 50-60?) but given the HT is most likely going to sit around 320-330 there (altho' I could easily lower it), I wasn't sure about the screen resistor value, so I guestimated 680k.

So thoughts about the EF86 anyone? (too noisy? wrong screen resistor value? etc?)

TIA

[octal]

Aaah, I was just thinking along similar lines after looking at a Selmer Zodiac schematic with the EF86 as the tone stack recovery stage!

I think you might be able to lose the ac coupled cathode follower after the pentode stage, since it's driving 1meg in parallel with 3.3meg- doesn't seem like too challenging a load even for a pentode. Dunno if you need an extra triode section for anything though?


Nathan

[tubeswell]

I think you might be able to lose the ac coupled cathode follower after the pentode stage, since it's driving 1meg in parallel with 3.3meg- doesn't seem like too challenging a load even for a pentode. Dunno if you need an extra triode section for anything though?


Nathan

Thanks for the feedback Nathan

Funnily enough I'd thought about that variant earlier too, but I was worried that with a ra (which is the same as the effective output impedance in a pentode right?) of 2M5 from the EF86, even a 3M3-odd 'grid load' would result in significant attenuation. (Wouldn't you need quite a bit more impedance bridging? - like 10M or something? - or would you perhaps up the gain factor in the EF86 to compensate? - at the risk of increased microphonics?)

I'd be curious to hear other's thoughts on which would be better, the one I posted before, or the (variant) attachment on this reply. (FWIW I put the 'extra stage' in parallel with V1 and made it a 12AY7 - which oughta drive the tone stack really well with a low noise floor)

Cheers

[octal]

Well, that EF 86 is going to have more gain than the normal 12ax7 stage you'd have there... so it's a question of how much extra gain do you want. I knew the ra of a pentode was high, but I didn't think it'd be THAT high. I guess you could do the math and figure out what your total gain would be in each scenereo....

[octal]

I was looking at the articles on Merlin's site and he says that a pretty good approximation for a pentode is Zout=Ra. So in this case, the output impedance of your pentode stage is 100K. There's definitely no need for a cathode follower IMHO.

Let us know how this sounds when/if you build it!


Nathan

[tubeswell]

Hi Nathan - yes you are correct, I was getting ra confused with Ra. 100k it is. That simplifies things quite a bit. Cheers