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Old 1st July 2011, 02:20 PM   #1
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Default Fully Parametric Tube EQ (with gyrators)

Hoping to build a fully parametric tube EQ and my best bet seems to be 'merging' these two schematics into one so that i can get a tube-based gyrator section (with adjustable Q, Freq and Gain) and use ICs for the input and follower stages (going to use this on a home-built mixer so reducing number of tubes is the reason).

SS EQ: (has Q, Freq and Gain) -

Tube EQ: (has only gain, Q and Freq are static) -

Anyone have any input on how to either 'add' the adjustable controls to the tube one, or to swap out the IC's with Tubes on the SS one?

Also, will i have issues integrating the tubes with the IC stuff?
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Old 2nd July 2011, 01:13 PM   #2
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I did this schematic in hopes of understanding things better. I basically just replaced the transistor with the tube. I tried simulating it in a Spice program but didn't work for me - i've never tried simulation before so probably a PBCAT issue (problem between chair and terminal). I am totally new to 'designing' any circuits, so this schematic may have some laughable elements, but hey, gotta start somewhere...

Here's the schematic i drew up:

If anyone is good at simulation and feels like giving it a shot, here's the netlist:

V4 V4_1 0 DC -150
V3 V3_1 0 DC 150V
V2 V2_1 0 DC 150V
XV1 V2_1 C2_1 R3_2 X12AX7
C2 C2_1 R1_2 270pf
R2A V3_1 V3_1 0.0002
R2B V3_1 R2_3 0.0008
R1A R1_2 R1_2 4K
R1B R1_2 C1_1 6K
C1 C1_1 C1_2 270pf
R6 0 C2_1 100k
R5 V4_1 R3_2 120k
R4 R2_3 C2_1 120k
R3 R1_2 R3_2 10k
.SAVE V3_1 R1_2 C1_2 C2_1 V4_1 R3_2 V2_1 R2_3 C1_1 @v4[p] v4#branch @v3[p]
.SAVE v3#branch @v2[p] v2#branch @c2[p] @c2[i] @c1[p] @c1[i] @r6[p] @r6[i]
.SAVE @r5[p] @r5[i] @r4[p] @r4[i] @r3[p] @r3[i]

* Selected Circuit Analyses :
.TRAN 20n 5u 0 20n

* Models/Subcircuits Used:

*12AX7 Vacuum Tube Triode (Audio freq.) pkg:VT-9 (A:1,2,3)(B:6,7,8)
.SUBCKT X12AX7 1 3 4
B1 2 4 I=((URAMP((V(2,4)/85)+V(3,4)))^1.5)/580
C1 3 4 1.6E-12
C2 3 1 1.7E-12
C3 1 4 0.46E-12
R1 3 5 50E+3
D1 1 2 DX
D2 4 2 DX2
D3 5 4 DX
.MODEL DX D(IS=1.0E-12 RS=1.0)
.MODEL DX2 D(IS=1.0E-9 RS=1.0)

A couple great resources i found, btw:

Gyrator Simulator -
A Waste of Salt gyrator frequency calculator

Rane's in-depth article on the various EQ types -
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Old 2nd July 2011, 03:21 PM   #3
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By the way - i hear that the Electro Harmonix Tube EQ pedal uses a "detuned Twin-T" type format, tho i don't what parts of theirs are tube or IC (nor what they mean by 'detuned').

Anyway, i'm not tied to the gyrator method, so if Twin-T is better for the 'tubification' side, then fine. My main goals are:

1) tube-based gain per frequency (so if i want the sound to break up a little on one frequency and not on another, i can)

2) variable Q, Gain and Frequency

3) hopefully IC's for input and output to reduce number of tubes, but i'm willing to go all tube if it realllly needs to be

Here's Mr. Keen's schematic for a Twin-T with variable Q and Freq:

Here's the full article by him, which is awesome:
Simple, Easy Parametric and Graphic EQ's, Plus Peaks and Notches
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Old 9th July 2011, 10:05 AM   #4
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After some mass-searching, i found this which seems to be perfect:

A Few Interesting DIY Audio Projects c. 1955 | Preservation Sound

no inductors, no transformers.
variable q and freq.
one single tube

my one question is about boost-cut. on the schem there's a lone .5meg pot connected to the grid of V2 that is unmarked (to the upper-left of V2). is this boost/cut for the bass section? and secondly, where does a boost-cut pot go for the treble stage? (i don't see one)
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Old 17th January 2012, 06:02 PM   #5
GCA is offline GCA  United States
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That is indeed an interesting monster.

Try encapsulating that in the feedback loop of a cathodyne phase invertor to give boost/cut.

Since that's a 2-channel, the treble channel need not be made in the first go-round, and the cathodyne could be made from the first triode and the EQ powered from the second, in the same envelope. That gives 1 mini tube socket per channel.

Last edited by GCA; 17th January 2012 at 06:05 PM.
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Old 18th January 2012, 01:26 PM   #6
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Ah, ok - so if i understand you correctly:

Tube 1 - first triode half used for bass signal (as illustrated), 2nd half used as cathodyne inverter (or cathode follower?) for gain side.

Tube 2 - change values of the (bass) r/c network to attenuate treble instead, and use 2nd half of tube for the gain.

Did i get that right?

I have already assembled most of the parts for this thing, its coming up next on the list.

Thanks for the post!
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Old 19th February 2012, 09:48 PM   #7
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Default I know this is an old thread..........!

Hi All
Hope there's still someone out there still reading this. I too am trying to mess aroun with this circuit, with my limited knowledge!!!
When you mean the feedback loop of a cathodyne-do you mean a cathode follower-but I didnt think a cathode follower had a feedback loop? am I being dumb-am I missing something?
COuld someone enlighten me with a brief schemo, incorporating the original 1955 circuit please?
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Old 20th February 2012, 06:22 AM   #8
GCA is offline GCA  United States
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I don't have the ability to frame it into the original circuit, but a cathodyne is a phase inverter that pulls a non-inverted signal from the cathode, like a cathode follower, and an inverted signal from the anode, like a standard gain stage, from the same triode. Read Merlin Blencowe's site (and book for a MUCH bigger explanation!) at Valve Wizard - How to design valve guitar amplifiers. Rk and Rp (Ra using British terminology) are normally the same value of ~100k or so, take a little bit maybe for a cathode bias resistor, so the balance between the two signals is usually really close.

What I meant was to hook up a blend pot from both outputs of the cathodyne to the input stage of the 1955 circuit, then take a signal from the output of the tone control and feed it back into the input of the cathodyne. At dead center of the pot you should get NO signal from the circuit as a whole, so it would be best to run it in parallel with a dry signal. You would have to be very careful with the positive feedback because of the possibility of oscilllation. You would also take the signal to the next stage from the usual output. You would probably not need a recovery stage because there shouldn't really be any insertion loss.

I've never actually tried to vary between an inverted and non-inverted signal into a twin-T filter (which is a notch filter at heart) so I'm not sure how well it would really work. I'm moving out of state right now and most of my electronics stuff is staying where it is so trying it will be impossible for me. It's just a suggested experiment so take it with a grain of salt.

A cathode follower operates under 100% negative feedback, and any time you hook up an output back to its own input, you've created a feedback loop. Even more fun with a cathode follower is hooking its output up to bootstrap the gain stage directly in front of it --> gain of the gods, also covered in the Blencowe book on preamps. Worth a read and re-read for a beginner!
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Old 19th April 2013, 06:43 PM   #9
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hey ValvusMusicus, did you move forward with this at all? I am back in the country with all my project stuff again and am hoping to tackle this thing finally!

GCA - thanks for explaining that. I have to admit, following a schematic is one thing, but I am pretty daunted by not only building this but also implementing something heretofore i had never heard of, due to my newbieness! I basically am wondering if I shouldn't try building it as is, at first - already a wee challenge for me, and then looking for problems in need of fixing, leaving parts accessible for later tweaking.

Would this work as it is, and would it need anything to play nice with other studio gear on the output?

And I do still have the question of how to add boost/cut for the treble section. Would a pot somewhere do the trick, say where that .27M resistor meets the .1 cap on the output leg? I guess that would just be 'cut' and no boost and i'd need another gain stage?

Whoa, so much to learn...
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Old 19th April 2013, 10:39 PM   #10
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Take a look at this. Try sim it first, it may require some modifications, but should get you started.

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