Modified BFDR schematic idea with EF86 - comments anyone??

[tubeswell]

I keep on redesigning and rebuilding this amp - Ha!

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, 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 think that don't want much gain out of the EF86 (and the amp is a combo after all), and 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

[Colossal]

That's pretty cool man. Nice work thinking outside the usual box. I've read Merlin's book at least six times now. Always gives me ideas (which I have too little time to act upon, but oh well, still fun to think about this stuff). He made some interesting points about the typical approach to the EF86 which most (commercial) builders that use it just continue to clone the classic noisy/microphonic EF86 stage that we see everywhere. His treatment of the EF86 to lower the gain but to retain the pentode flavor is pretty interesting. I did read a post on AX84 where one guy had tried the values that Merlin listed for his workup of the EF86 and I believe he said he preferred the other (classic) version, but that doesn't make Merlin's approach wrong either, just different. I like the ideas Merlin presented of being able to vary the pentode/triode mode etc. Seems like that would add some very interesting tonal options.

The Dr. Z Prescription ES uses an EF86 in the middle as well and sounds really good and is not noisy. Sounds like a Vox on steroids.

I have a couple of BFDR builds planned and I thought to include the Raw control but wasn't sure it would add value. How do you like it? My plan was to use the amp mostly clean however so going "raw" probably would not help that cause

[M Fowler]

Probably even higher screens to 860 - 1M right?

[tubeswell]

I've read Merlin's book at least six times now. Always gives me ideas (which I have too little time to act upon, but oh well, still fun to think about this stuff). He made some interesting points about the typical approach to the EF86 which most (commercial) builders that use it just continue to clone the classic noisy/microphonic EF86 stage that we see everywhere. His treatment of the EF86 to lower the gain but to retain the pentode flavor is pretty interesting.

That's what got me interested in the concept. That book is compulsory reading - I must read it 20X already

The Dr. Z Prescription ES uses an EF86 in the middle as well and sounds really good and is not noisy. Sounds like a Vox on steroids.

Yeah I saw that too. Mark sent me Dr Z's prescription schematic the other day: Similar concept to the matchless Clubman - but I wasn't sure about the need for a CF stage following the EF86 or not - because of the hi (2M5) output Z of the EF86, and the fact that I have the reverb send/return straight after.

I have a couple of BFDR builds planned and I thought to include the Raw control but wasn't sure it would add value. How do you like it? My plan was to use the amp mostly clean however so going "raw" probably would not help that cause

I just stuck that there because the amp I'm going to do it to has spaces for 11 control knobs (and 6 or 7 pre-amp tubes) and I was trying to think of uses for all them holes in the dashboard. Got no idea how practical the raw control is

[tubeswell]

Probably even higher screens to 860 - 1M right?

I wasn't sure. Merlin's book suggests getting 'improved results and versatile tones' operating the EF86 at a modest gain but preceded by at least one other stage (like the Matchless clubman). So I want the EF86 set up for 'low gain' (i.e. around 100, which I gather is low for an EF86) following another stage, and setting the EF86 up so that the load line goes through the knee to get non-linear compression on low-level signals, but more symmetrical compression on larger signals, and having the variable screen bypass control there to have different variations. The way Merlin describes it in his book, he has an HT voltage of 250 and a 100k plate resistor and has the screen at 70V with a 560k screen resistor, and biased at -1V with a 680R cathode resistor bypassed by 10uF. Out of this he gets the load line passing through the knee of the 0V grid curve. Maybe I should just build that? But I'm not sure about the need for the CF stage. I have spare socket spaces (for dual triodes). The other variation was to have the VC1 stage as a parallel triode (but not having the CF after the EF86, although I'm not sure about the effect of the EF86's output impedance that way - See attached variation

[M Fowler]

Here is an 18w EF86 Mark Huss drew based on Melin's book