EF86 Preamp - ala Route 66: more grind??

[greg]

I have built a 6V6 amp with a preamp similar to a Route 66. It sounds really nice clean, goes up to a nice manageable "grind w/clarity", and holds together well when cranked. I am interested it looking further into what this preamp is capable of in the way of distortion.

Any suggestions on how to distort this EF86 all to hell?

As always, thanks in advance.

Greg

[slajeune]

Hi Greg,

not sure that its the EF86 that is distorting but, to increase the gain a bit you can try this. Increase the value of the resistor from b+ to pin 1 of the EF86. What value do you have now? In my simulations, increasing this resistor value increases the gain of that stage.

I have seen some schematics with 6.8M as a resistor to pin 1!

Cheers,
Stephane.

[greg]

I am using a 1M resistor with a .1uf cap.

Thanks.
Greg

[Matt T.]

You can obviously try different plate and cathode resistors but you could also use a voltage divider for the screen; i.e. one resistor to B+ and another to ground. You'll still need the screen cap to ground but you could try connecting the cap to the cathode instead of ground.

FWIW/IIRC - the last time I messed with my EF86 preamp I think I ended up with a 180K plate resistor, 2K2/22uF cathode resistor/cap, and had a 470K/470K voltage divider on the screen with a .1uF cap connected to the cathode (I don't think it matters here because the cathode resistor is bypassed by a 22uF cap). I liked it better than the usual Vox circuit. I was going to try lower value resistors for the screen voltage divider (like 180K/180K) but never got around to it. I wanted to play the amp like it is for a while.

And FWIW, I wouldn't think you're getting the EF86 to actually distort much but I would expect it to affect the distortion you hear out of the amp a lot.

[Arthur B.]

I have built a 6V6 amp with a preamp similar to a Route 66. It sounds really nice clean, goes up to a nice manageable "grind w/clarity", and holds together well when cranked. I am interested it looking further into what this preamp is capable of in the way of distortion.

Any suggestions on how to distort this EF86 all to hell?

As always, thanks in advance.

Greg

You can use the supressor grid as a control grid as well as the regular one. I believe the gains will mulitply.

You can obviously try different plate and cathode resistors but you could also use a voltage divider for the screen; i.e. one resistor to B+ and another to ground. You'll still need the screen cap to ground but you could try connecting the cap to the cathode instead of ground.

I thought a resistor was unecessary in potentiometer feed?

[markbul]

I have built a 6V6 amp with a preamp similar to a Route 66. It sounds really nice clean, goes up to a nice manageable "grind w/clarity", and holds together well when cranked. I am interested it looking further into what this preamp is capable of in the way of distortion.

Any suggestions on how to distort this EF86 all to hell?

As always, thanks in advance.

Greg

If you want to distort the EF86, put a 12AX7 in front of it.

[Matt T.]

Arthur,
I don't know what you mean (I don't know what 'potentiometer feed' is either). I was just suggesting something to try based on an old post kg made on pentode pres. In fact, I still have it so I'll post the relevant part but keep in mind that this post was made in the context of cascaded pentode preamps. If kg would rather I didn't post stuff like this or if it's uncool in any way, I'll cease and desist. This is from 2002 I think.

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.

[greg]

I think these posts just about answer it for me.

I'll get to work.

Thanks everybody,
Greg

[Arthur B.]

Arthur,
I don't know what you mean (I don't know what 'potentiometer feed' is either).

The suppressor grid is G3. It is pin 8 on an EF86. It's a second control grid placed between the screen grid and the plate. Potentiometer feed biases the screen grid through a resistive voltage divider.