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EF86 input tube (wow!)

4/6/1999 2:59 PM
MJ Harnish
EF86 input tube (wow!)
I finally put together the 2nd preamp for my DIY 5F6A. It uses just a single EF86 and is based on the AC-15 preamp. Essentially I just have the option of either using an EF86 or a 12AY7 as the input tube (no footswitching yet though) since I mated the two preamps just prior to the tone stack and switch between them using the unused bright switch. The results....  
YOW!!!! The EF86 has mega-gain. The amp is screaming with the volume at 3. It's the most dynamic thing I've ever played; you can control the amp's clean/distortion tone entirely with the guitar's volume control and pick attack, and subtle changes make a BIG difference (and they sound good).  
I still have to tweak the circuit a bit (it hums more than it should and I think I want to back off the gain a tad), but I'm really surprised at the results so far. Tonight I'm going to really give it a trial by fire tonight at practice but so far it gets two thumbs up. Now I know why so many people really seem to dig those Dr. Z amps that use the EF86. Maybe I need one... :O)  
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4/7/1999 8:58 PM
Bob S

What kind of tube are you using? I have several Vox amps with EF-86's and they have been a problem with being microphonic. Matchless had the same problem and had sent a box of those things to my friend(who was a dealer) and was having ringing problems because of them being microphonic. I'm thrilled to see you so excited about a self-creation. It's sure is rewarding when you get it right, Isn't it?  
Bob S.
4/9/1999 10:14 AM
MJ Harnish

I'm using a Svetlana EF-86 that I got from Triode electronics. Ned assured me that he pretests them for microphonics and so far so good.  
Yes, it is quite satisfying. It would have to be considering all the headaches I've gone through to get the amp working like I want it to. My next step is to put in a different volume pot; I must have screwed up and put in a liner taper since it goes from practically nothing to full out between 2 and 4 on the volume control. I also have to get rid of the extra hum I've picked up now...
4/8/1999 9:04 AM
Ken Gilbert

Hey MJ...  
You've got my interest piqued! I was DL-ing and printing out the EF86 data sheets from Svetlana's website, and I took a look at some of the suggested applications. Here's something I found interesting:  
"Figure 2 is a suggestion for a very simple voltage-controlled amplifier for use in electronic-music equipment. Although distortion is greater than with conventional VCAs, this circuit gives a distinctive sound which has musical value. . . . An extra feature of the EF86 is that the suppressor grid is separately accessible (pin 8), which allows its use as an additional CV input if desired, in a manner similar to the screen grid. . . .The voltage gain of this VCA varies exponentially with the CV, making it suitable for most voltage-controlled music synthesizers with their exponential CV response."
In the typical connection, as pentode, you can expect a voltage gain of about 150-200. That's a LOT... most 12ax7 stages poop out at about 70 or so.  
The thing that's got me thinking is that the voltage gain can be controlled by the screen voltage, and that it's an exponential relationship. This might be a great place to attempt to impliment something like Dave Stork suggested to more closely mimic the dynamic response of a power section. It's a very trick gain control, in any event--a gain control that DOES NOT inherently create a different frequency response according to its position.  
If you (positively) fed back a portion of the output signal, and used this as the CV (or control voltage) then you could actually create an EXPANDER to INCREASE the dynamic range. Likewise, with NFB, then it becomes a handy compressor.  
With a simple op amp based LFO, you could easily create a tremolo circuit that does NOT depend on those damned LDR's. In fact, due to the fact that the Svetlana EF86 has a seperate pin out for the supressor grid, you could use THAT for a CV input as well! Talk about getting your money's worth...  
Now I've got to get my hands on some EF86's. I've had very good luck with Svetlana tubes, so I'll try to get some of those--anyone out there have a source handy?  
4/8/1999 9:34 AM

>>Now I've got to get my hands on some EF86's. I've had  
very good luck with Svetlana tubes, so I'll try to get some  
of those--anyone out there have a source handy?<<<<<  
You might drop a line to Eric Barbour at Svetlana for a free sample - especially if you are posting a design idea using his tube. If he drops by here I'm sure he'll read this thread.  
4/8/1999 9:46 AM

If you can get your hands on a copy of the RCA Receiving Tube Manual printed before the late 1950s you will find a compressor/expander schematic either in the back or in the circuit design explanations in the front which uses a 6L7 pentagrid tube which has two actual control grids and was specifically designed for audio applications (and some RF). The basics of the circuit could be used with the EF86 but would also apply to any of the miniature pentagrid mixer tubes that were later designed. These would probably be more adaptable than the EF86 and can be pickedup for a song at any hamfest or other place where "non-general-guitar-use" NOS tubes are available (or Antique Electric, etc.). Some types are 6BA7, 6BE6, etc.  
4/9/1999 10:17 AM
MJ Harnish

Well you lost me once you hit the gain controls... :O)  
I got my EF-86 from Triode Electronics. Ned told me that he pretests them for microphonics which is a big + since they seem to be very susceptible. Time will tell with this tube, but it's mounted in a head that doesn't sit on top of the cab so it's exposed to very little vibration. It's a good thing b/c as I said this tube is like a jet engine compared to the 12ay7 in the other channel. :O)

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