This weekend I have been thinking if it would be possible to design a semi parametric EQ using Tubes.
Variable Frequency and if possible some sort of variable Q.
Also a way to make it work like a lo or hishelf.
Most of the time A gyrator is used but in tubes this get's a little bit difficult.
After some reading I've found a way using a wien bridge in a feedback path.
Problem with tubes is that the output impedance of the amplifier stages is rather high and affects the working of the circuit + gain is most of the times limited.
After some thinking and a good night sleep I came up with the following.
2 pentodes in a differential stage with a current source in the kathodes.
the output used for the feedback circuit is buffered by a DC coupled Kathode follower.
Output impedance of an ecc88 kathode follower is low enough.
Using pentodes gives a little bit more gain.
I simulated this in ltspcice and the results looks promising.
The dual 100k pot has a resistance in series of 10k which defines the minimum frequency.
The 5k1 and 5k6 resistances should theoretically be the same but using these values the gain in the middle position is almost 0db (1X)
Actualy the 5k1 could be trimmed to around 5k2 to be exactly correct.
with a 6k8 resistor to gnd this gives a range of +/-18db.
If a resistance of 10k is inserted in series this wil go to around +/-12db and the Q will be lower.
So inserting a pot of 10k in series makes this like a sort of adjustable Q
Higher Q is possible but make this to high and it will become an oscillator. (around 5k1 the system will oscillate => wien bridge oscillator)
Then you could also leave the cap or the resistor out of the wien bridge circuit to gnd and make it shelve.
It would be possible to make the feedback resistors higher in value but I think noise could become a problem.
For a channel strip you could use 4 circuits of this.
2 ecc88's and 8 ef86's :d but if I remember there are some dual pentodes that exist. Dual triodes like the ecc83 could probably also work.
It's just a rough idea. Don't know if I will ever make something like this but when I searched the internet for a parametric tube EQ there was not much I could find.
Variable Frequency and if possible some sort of variable Q.
Also a way to make it work like a lo or hishelf.
Most of the time A gyrator is used but in tubes this get's a little bit difficult.
After some reading I've found a way using a wien bridge in a feedback path.
Problem with tubes is that the output impedance of the amplifier stages is rather high and affects the working of the circuit + gain is most of the times limited.
After some thinking and a good night sleep I came up with the following.
2 pentodes in a differential stage with a current source in the kathodes.
the output used for the feedback circuit is buffered by a DC coupled Kathode follower.
Output impedance of an ecc88 kathode follower is low enough.
Using pentodes gives a little bit more gain.
I simulated this in ltspcice and the results looks promising.
The dual 100k pot has a resistance in series of 10k which defines the minimum frequency.
The 5k1 and 5k6 resistances should theoretically be the same but using these values the gain in the middle position is almost 0db (1X)
Actualy the 5k1 could be trimmed to around 5k2 to be exactly correct.
with a 6k8 resistor to gnd this gives a range of +/-18db.
If a resistance of 10k is inserted in series this wil go to around +/-12db and the Q will be lower.
So inserting a pot of 10k in series makes this like a sort of adjustable Q
Higher Q is possible but make this to high and it will become an oscillator. (around 5k1 the system will oscillate => wien bridge oscillator)
Then you could also leave the cap or the resistor out of the wien bridge circuit to gnd and make it shelve.
It would be possible to make the feedback resistors higher in value but I think noise could become a problem.
For a channel strip you could use 4 circuits of this.
2 ecc88's and 8 ef86's :d but if I remember there are some dual pentodes that exist. Dual triodes like the ecc83 could probably also work.
It's just a rough idea. Don't know if I will ever make something like this but when I searched the internet for a parametric tube EQ there was not much I could find.
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Back in the 1960's I remember some one fitting B139's above a celling with enormous brick/breeze block horns in to the room with Kelly ribbon tweeters all working because he was using a John Linsley Hood valve parametric equaliser.
I remember the set up was described as Quad ESL's (only 57's at that time)with bass and volume. Is there a diagram in JLH's books.