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EF86 - Reloaded

1/4/2006 9:11 AM
Matt EF86 - Reloaded
Hopefully the 'sequel' to my first EF86 trials will be better than the Reloaded sequel was to the original Matrix.;)  
I tried the 'typical' EF86 circuit (220k plate, 1M/.1uF screen, 2k2/22uF cathode, 1M grid load, etc.) and it never sounded very good to me. Very little headroom and way too sqishy/chewy for me. The 'best' circuit I came up with (kg provided most of the details for this, I still have the post if anyone is interested) was this: 100k plate, 2k2/22uF cathode, the screen was connected to a voltage divider of 270k to B+ and 820K to ground and the .1uF screen cap was connected to the cathode (not ground). The grid load was 470k and the output was a .0047uF cap and 500k-A volume pot.  
In any case, anybody have any other tricks/circuits to try with an EF86? My B+ at this stage is only ~350vdc, this will be the second channel of a 2-channel amp and will be primarily for cleaner tones. The EF86 will be the only gain stage before the PI and I was going to try the Route 66 tone stack. Any input appreciated. Thanks.  
1/4/2006 9:15 AM
Just because it's good info, here's the post by ken:  
[QUOTE]i've messed with cascaded pentodes a bit.  
one thing to keep in mind is that the character of the distortion will be more organic, a bit less compressed, with a bit less sizzle  
to it, when using say one or two cascaded pentode stages vs. two or three cascaded triodes.  
the reason boils down to the number of stages... every time you go through another stage you get a little sharper clipping of the waveform. one stage, massively overdriven, usually has a LOT of even harmonic distortion--looking at the waveform you'll see one half fairly accurately reproduced (usually the +ve half, when vg1 is going -ve), whereas the other half has been clipped hard (usually at the grid when vg1 is goive +ve and passing Vk, leading to grid clamping). now, futzing around with the quiescent point can alter these relationships--for example, using a low value of Rp and not dropping much voltage across it will cause the +ve plate half to be hard clipped at the b+ rail earlier--but generally you'll find the stage giving you one half of the waveform a lot cleaner than the other.  
as a result, when you cascade two of these such stages, you end up with something resembling a square wave, since due to the polarity inversion each stage clips a different half. as you cascade your way through more of these stages, the wave becomes more and more squared off. as you can guess, using fewer pentode stages vs. more triode stages will lessen this effect.  
the other thing about pentodes is that their operation at the extremes (like in guitar preamps) produces a little softer clipping  
characteristic. this boils down to the screen grid drawing current during the peaks of cathode current. the Vp(min) will have a  
round bottom instead of a flatline like in a triode. to some extent it is a form of compression, which can be very useful. cascade  
two stages like this and you get more of it.  
honestly, my tastes for the lead type of distortion come from a good hard clip, and GENERALLY i achieve this the good old marshall way--with a cathode follower. of course you can put a CF at the end of two cascaded pentode stages and see what it  
will do--i think you might be pleased with the result.  
a few hints:  
a) use a fixed voltage divider from b+ to gnd to generate Vg2. i never liked the datasheet method of a simple series resistor from b+ and a cap to ground... any variation in tube characteristics and the Vg2 is all over the place, plus it tends to wander around as you drive it hard. a lower impedance divider will give you a bit less sag, and a bit less compression as the stages are  
driven hard. a higher impedance will give a slightly spongier sound. bypass the divider with a cap. a smaller cap will give you a quicker recovery from the compression/sponginess, a larger will give a slower recovery. if you use cathode bias then tie the  
bypass cap to the CATHODE, not to ground. this will help with preserving the transients.  
b) the vg2 is what sets your vg1 range for the quiescent bias of the tube.. higher vg2 gives more -ve vg1 for the same Ik.  
personally i like a large -ve bias on g1 so that clean headroom is preserved. this helps with the touch sensitivity.  
c) keep Vp around the same voltage as Vg2. too much lower and Ig2 goes through the roof. higher doesn't seem to be a problem up to the voltage limit of the plate. Vp is of course set through the available b+ voltage, the Ik, and the value of Rp.  
d) the value of Rp can be tweaked to adjust the gain through the stage. in pentodes it's handy to think of the gain as gm*Rp,  
instead of mu*Vin, since the plate impedance is so high. clearly as Rp is made higher, the gain goes up. keep an eye to (c)  
however since you can't arbitrarily adjust Rp without affecting other parameters.  
e) as a corrolary to (d) the value of Rp also affects the value of output impedance.  
f) as a corrolary to (e), with pentodes you can get away with much smaller coupling caps to net the same LF -3dB point, since  
the output impedance is higher than in triodes.  
that's all i can think of now. i say give it a shot... it's a different sound than triodes, and i kind of like the squishiness and pliability of the clipping characteristic.  
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1/4/2006 9:18 AM
I forgot to mention that kg's post was in the context of cascading pentodes and getting distortion. In my case, the EF86 will be the only preamp tube and from there it will go straight into the long-tail PI. I would like a great clean tone but if I crank it I'd like it to overdrive the PI nicely.
1/4/2006 10:40 AM

Well, I use a 6186 which can be wired pin-compatible with a 6AU6.  
It is very tasty as a solitary gain stage before a LTP or a SE power tube, but there will not be much overdrive except from a high-output guitar.  
And if you put in a tone stack that will cause enough insertion loss to virtually eliminate distortion, I think a pentode input can be real good this way for a clean preamp topology.  
To preserve the gain available from the pentode stage you really have to use high impedance components downstream from the plate output, for instance do not put a tone stack there but instead use a single carefully-selected coupling cap to go to the grid of the PI, and have that grid with no less than a 1Megohm grid load.  
That way you have a baseline performance level to compare with after you put in a tone stack so you will know firsthand how much gain it costs you.  
Also consider higher-mu power tubes like EL84 or EL34 since in operation they seem like they overdrive twice as easily as 6V6 or 6L6 types with equal levels of signal coming from the same preamp.  
1/4/2006 12:14 PM
Thanks Mike. I can't get Duncan's site to work right now, are those pentodes? If so, have you tried running them with a voltage divider on the screen and the screen cap connected to the cathode?  
I was concerned about having enough gain if I used a tone stack but that's what these little trials are for.:)  
This amp is cathode-biased EL34s. The other channel will be similar to a Soldano Atomic 16 (or tweaked JCM800, same thing). There will be an FX loop but I want to get the base tones first. The channels will be in-phase with each other so who knows what I'll be able to get out of it (haha, that's what these little trials are for).
1/5/2006 11:29 AM
Ian Anderson

has anyone here actually built a high-gain cascading EF86 circuit? - not necessarily multiple EF86s either, maybe an EF86 frontend with another two ECC83 gainstages after it.  
... Ian
1/5/2006 11:55 AM
Rick Erickson

"maybe an EF86 frontend with another two ECC83 gainstages after it."
Does anyone have a source for EF86's that could work in a high gain situation? I have a hard enough time trying to find one that will work in a Matchless without being overly microphonic.  
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