1-ch BFDR with EF86 Schematic comments welcome

[tubeswell]

I posted this on a couple of other forums but I thought this place would be good to get some feedback from as well so...

This is an existing amp that i keep rebuilding and redesigning.  It started out life as a 5F6A /JTM45 variant  (has a 330-0-330 200mA PT with a 5A heater, and a 50W OT and a 5F6A 90mA choke) - used to have 2 x 6L6s, but 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 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.

Taking up Merlin's excellent ideas about EF86s, I knocked up this schematic 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), like a one ch BFDR (kinda)

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

[tubenit]

That looks like it would be a very cool amp and a really innovative idea.  I like it & if you build it, I hope you'll share the results.

With respect, Tubenit

[Fresh_Start]

I don't know anything about applications for AC cathode followers.  Even after re-reading Merlin's essay on the subject, I don't understand what the benefit of the AC cathode follower would be in this circuit.  Is it to maximize the gain from the pentode?

Please edumacate me!

Chip

P.S.  Just read a reply on AMPAGE... how about parallel 12AX7 triodes for the input stage?  Alternatively, how about a DC cathode follower driving the tone stack to boost the signal going into the pentode?  Just throwing things out to see if anything sticks...

[tubeswell]

I don't know anything about applications for AC cathode followers.  Even after re-reading Merlin's essay on the subject, I don't understand what the benefit of the AC cathode follower would be in this circuit.  Is it to maximize the gain from the pentode?

Please edumacate me!

Chip

P.S.  Just read a reply on AMPAGE... how about parallel 12AX7 triodes for the input stage?  Alternatively, how about a DC cathode follower driving the tone stack to boost the signal going into the pentode?  Just throwing things out to see if anything sticks...

Yeah thanks Chip (and tubenit  ). I replied to that ampage feedback too. I had also thought about a parallel 12AY7 driving the tonestack instead (and losing the CF after the EF86), but pentodes have a high output impedance (its effectively equal to the plate resistance) AFAICT - so the EF86's output impedance would be 2M5, and with only 3M3-odd following the stage without a CF, I don't know how strong the signal would be - hence the CF. But I'd be curious to hear people's views on that subject.

TIA

[PRR]

> an EF86 stage for experimenting with the tonal variation.

"Tone" in this context means pushing near OverLoad.

Plot the signal levels through the amp. You have up to 40V at output tube grids, gain near 15 in logtail PI, etc to the front.

See pic below.

I estimate 1V out of the pentode and its CF. It can make maybe 60V; however anything over 1V at pentode plate means heavy 6V6 clipping so pentode "flavor" is quite clean until long past the point the 6V6es are flat-topped. I feel it will not have "tone", it will be a "clean" boost.

I also see very small input sensitivity: 2mV. This is a LOT of gain. Maybe it won't be that extreme. And of course you can turn-down the Volume pot. See the (alternate levels) for turn-down use. However this leads to the signal at pentode grid being lower than at the input: hiss is dominated by the second stage (under our control) rather than by the source (arm and pickup; nominally not under the amp-designer's control).

For pentode flavor, put the pentode where it will work HARD. Unless you play very gently, actually the first stage is hard-worked. Strong strum may be 200mV, pentode gain ~~100, plate level is 20V. This is higher than any other point in the amp until 6V6 grids. The 6V6 driver would be another good place, but push-pull may negate some of that "tone" you know from SE pentodes.

> I don't want super hi gain.., so I was thinking of a lowish screen voltage

Low G2 voltage gives high gain. For low gain, nail G2 to +300V, increase Rk to get plate voltage in a zone.

> what the benefit of the AC cathode follower

Pentode gives higher gain "because" it has very high plate impedance. If you use a 470K plate resistor to drive a 220K tone-stack, gain falls badly.

BTW, distortion ("tone") is reduced by buffering.

However here we have 100K plate resistor and 750K load. Loaded gain is 88% of un-loaded gain, a 1dB difference, not audible. If it were mis-biased to distort... but it is "well"-biased, and it runs at 1/60th of overload, it's clean as a cat's nose.

[tubeswell]

> an EF86 stage for experimenting with the tonal variation.

"Tone" in this context means pushing near OverLoad.

Plot the signal levels through the amp. You have up to 40V at output tube grids, gain near 15 in logtail PI, etc to the front.

See pic below.

Thanks for the feedback (and the pic) PRR

I estimate 1V out of the pentode and its CF. It can make maybe 60V; however anything over 1V at pentode plate means heavy 6V6 clipping so pentode "flavor" is quite clean until long past the point the 6V6es are flat-topped. I feel it will not have "tone", it will be a "clean" boost.

So there's really no difference from a conventional triode stage (in place of the EF86 and CF stage) there?

I also see very small input sensitivity: 2mV. This is a LOT of gain. Maybe it won't be that extreme. And of course you can turn-down the Volume pot. See the (alternate levels) for turn-down use. However this leads to the signal at pentode grid being lower than at the input: hiss is dominated by the second stage (under our control) rather than by the source (arm and pickup; nominally not under the amp-designer's control).

For pentode flavor, put the pentode where it will work HARD. Unless you play very gently, actually the first stage is hard-worked. Strong strum may be 200mV, pentode gain ~~100, plate level is 20V. This is higher than any other point in the amp until 6V6 grids. The 6V6 driver would be another good place, but push-pull may negate some of that "tone" you know from SE pentodes.

So would that be the EF86 and the CF stage at the start? (or just the EF86 stage?) The tone stack load is high but if the EF86 is set up for more gain does that counteract that?, or does it just make the EF86 more prone to microphonics?

> I don't want super hi gain.., so I was thinking of a lowish screen voltage

Low G2 voltage gives high gain. For low gain, nail G2 to +300V, increase Rk to get plate voltage in a zone.

Huh? I thought lowering screen voltage increased plate resistance and reduced gain (and increased input sensitivity)??  :confused2:

So would you put a 560k screen resistor? Or 470k? (or lower?)

> what the benefit of the AC cathode follower

Pentode gives higher gain "because" it has very high plate impedance. If you use a 470K plate resistor to drive a 220K tone-stack, gain falls badly.

BTW, distortion ("tone") is reduced by buffering.

So buffering will remove harmonic distortion from the pentode? (I thought it would just replicate anything that came out of the pentode, albeit with better impedance bridging)

However here we have 100K plate resistor and 750K load. Loaded gain is 88% of un-loaded gain, a 1dB difference, not audible. If it were mis-biased to distort... but it is "well"-biased, and it runs at 1/60th of overload, it's clean as a cat's nose.

What I'm looking for is some pentode colour (not necessarily clipping - but more harmonic distortion). How do I get that here? (By lowering the HT? and increasing the screen voltage?, or vice versa?)

If I use the 'spare' triode as a parallel triode in V1 to drive the tone stack, and just use the EF86 by itself as a 'tone stack recovery', (albeit with more gain) what would the gain structure be? (i.e.: see attached variation of schematic - Note that I haven't changed the EF86 g2 voltage or bias in this amendment because I'm still confused about what you said about lowering the screen voltage to get more gain). Many Thanks

[bigdaddy]

I would say eliminate the first triode and forget about the cathode follower completely. Just put the TS between the EF86 and the 12AX7. The second plate of the EF86 has a great ability to cut some gain by adding a pot after the 100n cap. But I would just add a rotary stitch with a few resistors. There's not that much difference but enough to warrant having some flexibility. Some people call it a squish control, whatever, it does work on the EF86 but not on some other pentodes as effectively .

The way you have it set up looks like you will have a lot of gain and a lot of possible issues, it's a bit overkill IMO. Just taking a simple Princeton reverb, adding a long tail PI and the first stage being a pentode instead of a triode seems like the best way to me. Also using that unused triode for a second channel seems like a better idea. That way you have typical fender and a higher gain more British voiced channel, both very usable. You can use the same tonestack just use some type of switching jack when you plug into the other channel or a dpdt. As it is with the extra triode for the reverb you'll see a lot of extra gain with the long tail PI.

Also why a 4K primary? As far as I know a 6V6 likes an 8K much better. A 50 watt OT is overkill. I think it looks like the design is kind of more is better, when in fact with 6V6 power tubes less is better from my experiences. the overall design would probably work better with a El-34 or 6L6GC.

[tubeswell]

Also why a 4K primary? As far as I know a 6V6 likes an 8K much better. A 50 watt OT is overkill. I think it looks like the design is kind of more is better, when in fact with 6V6 power tubes less is better from my experiences. the overall design would probably work better with a El-34 or 6L6GC.

Thanks bigdaddy

That's a relic OT from a Bassman build. I've actually got it reflecting 8k (but I forgot to put that on the schematic). I'm thinking I'll probably make it 4 x 6V6s just because I like the sound of them. Thanks for your other suggestions as well.

The amp has very much been a 'morish' indulgence - started out from a thread over at ampage where someone else was asking how you add reverb to a tweed bassman, which gave me an itch to try. But what I should done is just built a bassman and used the existing standalaone reverb unit I've already got. Instead I got carried away, but that's a whole nother story. Now I'm simply using it as a platform for wild experiments. the chassis already has space for 11 knobs and 6 or 7 pre-amp tubes - and the Pt has plenty of juice. It is a spare amp so it is not critical for me to have it lying around for a while. I yanked the 6L6s out the other day and set up the BFDR back end, which works superbly. Now I'm tinkering with the front of it again.

I was reading Merlin's book where he suggested getting 'improved results and versatile tones' operating the EF86 at a modest gain but preceded by at least one other stage (kind of like a Matchless clubman).  My thought was therefore to have 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?

Seeing as how the matchless clubman has the FX send and return after the EF86, I thought of putting in into a 1 channel BFDR/princeton in front of the reverb send-return. I wasn't sure about the necessity of the CF (and I still am not). I haven't built with an EF86 before so its all a bit crazy for me.

What did you mean by 'the second plate of the EF86'?

[PRR]

> buffering will remove harmonic distortion

distortion ("tone") is reduced by buffering. Not "removed".

> I thought lowering screen voltage increased plate resistance and reduced gain (and increased input sensitivity)??

http://www.pentodepress.com/home/amp-technology/pentode-gain/

In audio: Pentode plate resistance is always FAR higher than other circuit values and does not influence design. (It is cited for tuned-circuit IF amplifier design.)

> So would you....

I would not be looking for strong flavor in the BFDR; up until the 6L6es overload it is very clean, nothing works hard.

[bigdaddy]

It's actually not a plate sorry, I get a little confused sometimes.....lazy mind on morphine......it's called screen grid(G2), pin 1. Messing around with the voltage there changes how the tube will sound, it's a balance between the plate and that grid. Experimenting with different resistors and seeing the effects on sound along with the cathode resistor is a three legged dance so to speak, change one and all the other will too. No need to use a triode to drive it, it sounds best in a push pull amp if you go directly into it. But if you try to get maximum gain it gets microphonic, that's why I guess a lot of people use the 5879. But IMO nothing has the highend gain the EF86 has. I think surrounding it with 12AX7s kills the tone only that tube can give you. Adding a 500KL after the 100n cap on G2 will give you some ability to change the gain, I would rather use a switch and fixed resistors.

The 50 watt OT is overkill, unless you do decide on 4 6V6s.

6V6s are easier to drive and no need for so much gain. That can only cause problems with trying to control it which means adding things that actually is what I call a tone suck, like snubber caps. Look at the simplicity of the Vox preamps for instance. One of the best sounding amps I ever hear online was a simple 18 watter with one EF86 right into the PI, no NFB, no TS only a tone control. What a great natural organic sound, and I am not a fan of EL84s. This amp was amazing, just adding a TS and a triode probably would have made that amp into an amazing sonic device with a pair of 6v6s instead, similar to your amp.

I always wonder if people really use/need the tremolo? if you really don't need it, leave it out. Same for things like effects send/return. All those things just add to making an amp complex and harder to build. make it first without those things and then add them on later. Get the basic amp working and dial in the tone/sound, then add on the bells and whistles.

[tubeswell]

I always wonder if people really use/need the tremolo? if you really don't need it, leave it out. Same for things like effects send/return. All those things just add to making an amp complex and harder to build. make it first without those things and then add them on later. Get the basic amp working and dial in the tone/sound, then add on the bells and whistles.

Thanks again bigdaddy. The amp is already built and the trem etc works fine - I am just pondering a rebuild/modification to the front end with the EF86 in mind. Like you I really love simple amps and I have built some like that already (e.g.; 5E3, 5G9, 5F2A, 5F6A, 6G15, AA1164 - of those I love the 5G9 the best - which is why I bother with trem).

[tubeswell]

I don't know anything about applications for AC cathode followers.  Even after re-reading Merlin's essay on the subject, I don't understand what the benefit of the AC cathode follower would be in this circuit.  Is it to maximize the gain from the pentode?

Please edumacate me!

Chip

P.S.  Just read a reply on AMPAGE... how about parallel 12AX7 triodes for the input stage?  Alternatively, how about a DC cathode follower driving the tone stack to boost the signal going into the pentode?  Just throwing things out to see if anything sticks...

Yeah thanks Chip (and tubenit  ). I replied to that ampage feedback too. I had also thought about a parallel 12AY7 driving the tonestack instead (and losing the CF after the EF86), but pentodes have a high output impedance (its effectively equal to the plate resistance) AFAICT - so the EF86's output impedance would be 2M5, and with only 3M3-odd following the stage without a CF, I don't know how strong the signal would be - hence the CF. But I'd be curious to hear people's views on that subject.

TIA

I've just realised (after having it pointed out to me on another forum, that the output impedance in a pentode is effectively equal to Ra (and not ra as I was thinking)). So with a 100k output impedance, that makes it quite a bit simpler.