Want to make small inductor for tone circuit

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
My Fender Super Twin has normal tone controls, then has a rotary graphic section. One band is out due to an intermittent inductor.

The first section is labelled 'presence' and only has bost, not cut.
.033 mfd 200 v cap in series with an unknown inductor.

2300 HZ band: .047 mfd 200 v cap in series with an unknown inductor.

1250 HZ band: .082 mfd 200 v cap in series with an unknown inductor.

485 HZ band: .22 mfd 200 v cap in series with an unknown inductor.

235 HZ band: .47 mfd 200 v cap in series with an unknown inductor.

100 HZ band: 1.0 mfd 200 v cap in series with an unknown inductor.

Enzo posted on another forum with the same situation but on the 100 HZ band:
EQ inductor for old Fender

He determined:
"The inductor is 2.5Hy. It is 1" long cylinder, half inch diameter. ROughly the size of a 5 watt cement resistor. ANd heavy - feels like ferrite."

So mine was he next band over; the capacitor about doubles for each octave band; I would imagine the inductor should also. So he has 1.0 mfd and 2.5 henry for 100 HZ; I'm going to guess mine (with .47 mfd) is 1.25 Henry.

Those are still pretty large values, so for such a physically small inductor it's probably quite a bit of thin wire.

I suspect physical or thermal-cycle physical damage, not electrical burnout.

I've got some questions:

1) What's the easiest way for me to make some kind of bridge circuit to measure my intermittent inductor when it's working? I have a VTVM, a signal generator.

2) Any good ideas where to purchase a nice core and wire to wind my own, or an approproate commercial product.

I guess I could certainly find some core with insulated leads, wind thin wire on, and test with pink noise and the running circuit, watch the knob's effect on a RTA and get it close enough for a guitar amps.
 
Can you post pics, color code, or part number? I think have most of a set from a Super Twin Reverb just kicking around in my parts stash, maybe even all of them.

Oh man, if you actually have the real thing that would be incredible! It looks like a capacitor but is heavier and the body is black ferrite and the ends are sealed with epoxy where the leads come out. The only markings are the Fender part numbers. The one I need is marked 011945 in white print.
 
It's about an inch long, bit less than 1/2 inch diameter tube.

It seems the wire is probably very fine, to achieve that value in such a small package. The leads come loose, and break the wire. So I might put an extra dab of cleaner and epoxy on each, just to help prolong their life.
 
The color codes are on the schematic, but some of them are pretty hard to read. The Studio Bass schematic that I have is much clearer and we should be able to glean the actual values from the color codes. Looks like I have five of the six, including the one you need. I'm guessing I may have left the Presence one in circuit, but it's been a really long time since I worked on that amp, probably at least twenty years.

PS: If you want to change this to the Studio Bass EQ circuit you'll still need another one of the 011945 inductors, or something equivalent, as the SB uses two of those.
 
PS: If you want to change this to the Studio Bass EQ circuit you'll still need another one of the 011945 inductors, or something equivalent, as the SB uses two of those.

I don't think it really does, I'd always assumed that was a typo on the schematic, unless you know that to be true from looking at one. Perhaps they wanted the lowest bass contols to have more gradual overlapping low-pass???

I would be more interested in how to vary the =/- range.
 
I don't think it really does, I'd always assumed that was a typo on the schematic, unless you know that to be true from looking at one. Perhaps they wanted the lowest bass contols to have more gradual overlapping low-pass???

I would be more interested in how to vary the =/- range.

I'll do a quickie model in LTspice and see what I can figure out. I really don't want to mess with my friend's Studio Bass anymore if I can help it.
 
I don't think it really does, I'd always assumed that was a typo on the schematic, unless you know that to be true from looking at one.

Don't think it's a typo after running some quick online calcs to solve for simple LC resonance frequencies. Here are the actual inductor values, as best as I can figure:

011941: 50mH
011942: 100mH
011943: 200mH
011944: 500mH
011945: 1H
011946: 2.5H

A few of the frequencies are as much as 20% or so off the schematic/front plate labeled values -- close enough for rock 'n roll. :cool:
 
You're right, it does show the color codes on the schematic; I guess I'd assumed that was a wire color with stripe color or something long ago, and has been "invisible" to my selective brain since!

Also, as I investigate possible power supply upgrades I discover the Studio Bass has a lower 395 V B++ compared to the Super Twin 400 V B++ (B+ is shte same 500 V on both).
 
I want to make sure I thanked you publicly for really coming thru on this one! If I get the right test setup I'll try to measure these, so they are reproducible. If nothing else, it is an interesting tube graphic EQ.

I added epoxy to the ends of each, where the leads exit the cylinder body. Then I added some GOOP glue to anchor them to the turret-board. Several leads were loose, making it just a matter of time before fatigue failure from the vibration of setting a head atop a cabinet.
 
Cyclecamper,
The REAL values for your edification

Frequncy Band (centre) is set by the LC resonant frequency which you can calculate from
f = 1/ 2pi (LC)^-2

or 1 over 2 pi root L C (in words.)

OR cheat:
Resonant Frequency Calculator

2300 Hz and 0.047 uF => 101.88mH, call it 0.1 H
1250 Hz and 0.082 uF => 197.7 mH, call it 0.2 H
485 Hz and 0.22 uF => 489.48 mH, call it 0.5 H
235 Hz and 0.47 uF => 975.9 mH, call it 1 H
100 Hz and 1 uF => 2.5 H

You can match those values up with the Fender Part Numbers above.

Next go to Mouser or similar and search for those values. Should be cheaper than a Fender spare part.

Edit: responded from page 1, just noted Passinwind's response above - well this is his design review, he passes (if you'll forgive the pun).

Cheers,
Ian
 
Last edited:
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.