My EF86 circuit

[Shea]

I use an EF86 to drive a passive Baxandall (i.e., James) tone stack. I like the rawness of having only one gain stage before the pi, so I didn't want to use two cascaded gain stages to make up for the gain lost by the tone stack. Instead I used the higher gain of the EF86. I also did some tweaks to the pi and used a 1-meg volume pot to keep the p-p voltage swing up. It worked out pretty well. It's not as gainy as my one-knob and two-knob 18's, but there still a good amount. I'll bet that some people will like the clean headroom of my 3-knob amp better, because the one-knobbers and two-knobbers have very little headroom to speak of.

I use Svetlana EF86s and have not had any problem with microphonics.

Shea

[whiteboyblues2001]

Shea,

I am headed in that same direction. I just got my Svet EF86 in the mail, but I went to my local electronics store and it went out of business. Bummer. I guess that I will have to order the other parts online. Oh well.

I am going to start by replacing the trem channel with the EF86 and keeping the same single knob tone stack. Then I might replace the tone stack with a TMB tone stack. Does the Baxandall reduce the signal as much as a standard Fender, Marshall, Vox style tone stack? I think I remember people saying it wasn't quite as reducing.

How did you modify the PI to get more gain back? I would be interested in your experiences since I may follow suit.

Thanks for your help.
Joe

[Shea]

In my opinion - and this is purely subjective - a properly-designed Bax can do the same tones as a Fender / Marshall tone stack plus some others. I downloaded the Duncan Tone Stack simulator and tweaked its standard James tone stack circuit until the midrange hump / cut occurred in about the same place as the Marshall tone stack.


I had also though the insertion loss was supposed to be lower, but it doesn't seem any lower. The main advantage is that the tone controls are not as interactive as in the Fender / Marshall stack. Sometimes it seems like Fenders and the higher-powered Marshalls do one basic tone, and the tone stack only fine tunes it. With the Bax (or James, to be precise), I feel like I have more control over the tone. I also feel that a midrange knob is unnecessary with it . Turning both knobs down gives a mid boost, but it doesn't get blurry-sounding like the Fender / Marshall stack does when you turn down the treble & bass knobs. Turning up both knobs on the Bax sounds like a mid cut.

Shea

[zaphod_phil]

Actually, according to common wisdom, the Bax/James stack has a much higher insertion loss, although as you've discovered it can be tweaked. You're dead right about the limitations of the Fender and Marshall TMB stacks! Possibly the common Marshall variant is slightly worse....

[Shea]

I found that it's pretty simple to make a bypass switch using a DPDT switch. I used a 500k push-pull pot for the volume knob. Pulling the pot out disconnects ground from two of the components in the tone stack (a cap and a resistor), and connects the treble cap (330pf) directly to the hot lug of the volume pot. It gives a big gain boost.

I suppose I could make it connect the treble cap to the wiper of the volume pot, rather than the hot lug, if I wanted to make it sound bright.

Also, just using a SPST switch to disconnect ground from the tonestack might work.

Shea

[MarkB]

Shea

Help me out here - I'm trying to understand something. I'm confused about you reference to headroom.

If headroom is maximum undistorted volume, and
If headroom is controlled by the tube stage that distorts first, and
If distortion occurs when a given voltage swing is exceeded in the tube stage that distorts first, then
How does changing the gain in a previous stage affect headroom?
Isn't changing gain in the first stage the same as changing the setting of the volume control? And how does that change headroom?

I don't doubt your observation, it just seems to my non-electronics mind that the only way to increase headroom would be to change the maximum voltage swing in the tube stage that's distorting first.

MarkB

[Shea]

MarkB -

Well, you're probably right in a literal or technical sense. What I really meant was that a bigger portion of the volume knob's sweep is clean - about a third, as opposed to a tenth. The highest clean setting is probably not any louder than it is in my other amps, so I guess it's not exactly headroom. I couldn't think of a better word to explain what I was trying to describe.

Shea