EF86 Variable Screen Bypass Capacitor

[PharmRock]

It seems I'm in "design paralysis" as I inch ever so closely to finally drilling the board for this EF86/18watt build.  Every time I read something interesting, I figure "hey, I've got a clean slate...why not build that into the layout as an option for later?"

I have Merlin's book on preamp design, and he talks about some cool things to do with an EF86, at least as far as the screen bypass capacitor goes. Essentially what he proposes is an "Active Bass Cut" control, where a 1M pot is placed between the EF86 screen bypass capacitor (e.g. 0.1uF) and ground.  (a smaller parallel 4.7nF screen bypass cap also goes from screen to ground).  This seems like a pretty neat control to have in conjunction with a 6-position rotary tone control which would be placed after the EF86, and would open up a lot of tonal shaping options.

In searching the forum I came upon this thread by tubeswell where he has a very similar version of this in his schematic:
https://el34world.com/Forum/index.php?topic=9640.0

Tubeswell, did you implement this in your final design?  How did it work out?

Anyone else try a similar approach for an EF86-based amp?

[tubeswell]

Yes, I had a lot of fun with EF86s. The one I liked the most from Merlin’s books was the triode-pentode morph control - with the Morph control pot using a log taper pot wired backwards (which gives a smoother morph, but is counter-intuitive w.r.t. gain). If you use a DC-coupled CF stage after the EF86, you can (usefully) hook Cg2 to the CF cathode for a low impedance load for the screen/morph control, while doubling as a low impedance tone stack driver. This too I have done. I liked the EF86 in V1 best, where you can get a nice chime with the pentode mode (control) at max, with the pups dialled back. Some people prefer another stage in front of the EF86 tho. YMMV

[PharmRock]

Thanks Tubeswell...I read about the triode/pentode morph control too and was considering it as an option.

The EF86 is replacing the normal channel in an 18-watt (1974), so no CF in this amp.  But worth keeping in mind for future builds.

In looking at Merlin's schematics, it seems I could build a layout where I can try each one separately (morph, variable capacitor) with changing only a couple of components and see which one I like most.

Thanks again.

[PharmRock]

Hi Tubeswell,
Trying to source the parts for this Pentode/Triode Morph control that you implemented in your EF86 BFDR variant.  I reattached your schematic to this thread for ease of reference.

I suspect I need a fairly high voltage rating for the 470nF cap coming off the EF86 plate which goes to the 1M edit: "Raw" "Tr/Pt Morph" pot (basically the "anti-scratchiness" cap).  Likewise, I am anticipating around 100V on the EF86 screen, so I probably need a voltage rating at least twice that value for the 100nF cap which goes to the wiper of the 1M pot. 

I have film (Dijon) caps in these values, but they're freaking huge!!  And they are rated for 630V.  I was wondering what you ended up using for these caps in your build.  Electrolytic caps are hard to come by in capacitances <1uF (although I did find a source but I've got to buy 10 of each: https://www.tedss.com/Capacitors/Browse/aluminum-electrolytic-axial

Thanks again for your help on this.  Seems like a really cool option to build into the design. 

[HotBluePlates]

... EF86, at least as far as the screen bypass capacitor goes. Essentially what he proposes is an "Active Bass Cut" control ...

But "active bass cut" could be done at the cathode with a lower-voltage cap & a pot.

Passive Bass Cut could be a cap in-series with an existing coupling cap, and a pot (wired as rheostat) across the added cap.

I sound like a broken record, but the most useful control I've encountered for an EF86 is variable screen voltage.  The AC10 gives this via the Amplitude control, which then varies the resistance in the voltage divider supplying the screen.

The AC10's setup allows going from a very-clean EF86 to an easily-distorted EF86 (and noticeable coloration of the sound).

[PharmRock]

After putting this on the back burner for a while, I am finally getting around to wiring up the amp.  I have 4 versions of how I might run the EF86 screens:
1) Normal: typical set up with 220K plate, 1M screen, 0.1uF screen bypass. 
2) Morph: screen AC voltage fed into plate via morph control.  Requires the addition of 470uF cap from plate to morph control. Found in Merlin's book and used by tubeswell.
3) "Squish" - variable screen resistance: adapted from Vox AC10 amplitude control as suggested by HBP.  Wouldn't this have DC voltage on the pot and be scratchy?
4) "Squish" - bass cut: 1M pot after screen bypass cap.  Found in Merlin's book.

I've drawn the schematics out and attached them.  I was hoping y'all could take a look and tell me if you see any issues.  The "base" values use the typical 220K plate resistor and 1M screen resistor as I've seen on many EF86 circuits.  Also, all versions have a cathode resistor of 2.2K, cathode bypass cap of 1-10uF (will tweak this), 33K input grid stopper, and 1M input resistor.  The "Squish" name came from the 18-watt forum. 

Thanks, and I appreciate any feedback.

[PRR]

> Wouldn't this have DC voltage on the pot and be scratchy?

Cap bypass absorbs the scratch.

What you really want to do is try all four. It's only a few extra lugs and a few bucks of parts.

[PharmRock]

> Wouldn't this have DC voltage on the pot and be scratchy?

Cap bypass absorbs the scratch.

What you really want to do is try all four. It's only a few extra lugs and a few bucks of parts.

Thanks PRR...the board layout is currently set up for the Morph control, which would be easy to switch to a "normal" layout by tying the screen cap (C2) to ground rather than to the Morph control (R6), and lifting one leg of the 0.47uF plate cap (C4) vs. going to the Morph control. 

I'll try to come up with the layouts on DIYLC later on today.