Suggest 2 band tube EQ for amp! 😊

[daxliniere]

Hey all,
I just picked up a fantastic little tube reel-to-reel which sounds amazing as a guitar amp. EL34 output, tube rectifier, a bunch of EF86 tubes, and a built-in speaker.

The unit has a pair of high and low pass filters, but I'd like to replace them with boost/cut bell bands. Can anyone suggest some nice, versatile EQ circuits for this? It's all point-to-point under the hood, but I'm happy to make a PCB.

Thanks in advance!
Dax.

[Bergheim]

Does it have to be active?
A well known circuit is the Fender tonestack with a resistor in place of the mid pot.
I myself like the passive James tonestack with dual bass capacitors. With some component tweaking, it can produce both a fairly deep mid scoop as well as a mid hump, while keeping the frequency of the scoop/hump the same.

[TUBEDUDE]

Are you thinking of adding a gain stage for the "boost and cut" circuit, or just a different passive EQ?
If the latter, there are many possibilities. Most will likely sacrifice some gain, so you may need to coax more from one of those EF86's. You might consider something along the lines of a tilt control with a presence, or a tilt with a mid, like the Bone Ray control from Merlin Blencowe.

[lonote]

There are lots out there. I have experience with only a few & often have the same question you have.

Maybe try fooling with these calculators? Lots of circuits to try & to tweak.

https://www.guitarscience.net/tsc/info.htm


https://www.duncanamps.com/tsc/


https://tonestack.yuriturov.com/

[daxliniere]

Thank you everyone for your responses. @PDF64, I saw your reply on the EL34 forum just before I was permanently auto-banned for using a VPN. (I always use a VPN, it's 2024. :shrug: )

As the unit has only 2 tone control knobs and I'd like to keep it looking original, I think I'm going to go with the Baxandall pair. The Ferrograph has a LOT of gain with the 2 normal preamp tubes (V2+V3) and even more if I connect to V1 which amplifies the signal from the playback head.
This means I don't think I'll have to worry about the static gain loss through a passive EQ, plus it's nice to keep it all tube.

My plan so far is to add a 3-position rotary switch to connect the input jack to 1, 2 or 3 gain stages (green, red or blue) for varying degrees of distortion (none to raging). I figure that I'll need to add a (bypass??) capacitor between V1 and V2 (orange lines) after I remove the existing high- and low-cut filter section.
The pink box is where I was thinking to add the new EQ section. Does that seem about right? Should I remove R32? If not, should that go before or after the new EQ section?


I'll let you know how it all turns out. One thing I started already was to turn the control panel upside down. (it's originally used flat on a desk, but when I use it as a guitar amp, it will be upright.) I figured it would be a pretty easy thing (cross-wire the controls and sockets), but I ended up needing to rework a lot of the metalwork. Would I do it again? Ehh.. probably, because I'm me and I want to do things 'properly'. Did it take a buttload of time? hell yeah.

All the best,
Dax.

[dorrisant]

If you flip the panel upside down, the pots will not magically change their sweep... Full CCw for zero and full CW for 100%. The only thing that would change is their left to right order. So no rewiring required... Unless I'm totally not getting what you are saying.

Post a pic of your work, maybe?

[daxliniere]

Hey Dorrisant, thanks for the reply. I will take some photos tomorrow when I'm in front of it again.

As for changing their order, no, that would just put the gain knob in the treble hole, etc..
Also, flipping the entire panel upside down doesn't require the sweep to be inverted, it's literally just turning the whole panel upside down.