HOW do I make a parametric EQ?

[Glennjeff]

Does anyone know how to do a single channel parametric or quasi parametric EQ with a triode or two.

I've managed to get all the Bass and Mid control I ever wanted out of one 12AX7 and it's bias / bypass components. My D'Mars/Glennjeff Hybrid is sounding lovely with just one preamp valve before the PI. I cannot figure out how to do sculpting on the high frequencies. 10pF plate to grid kills it and I don't really want shelving/low pass, I can leave that up to the tone stack, if one is really needed.

Any assistance would be appreciated.

[jjasilli]

It's complicated and involves caps & inductors.  Search the Forum; there are past threads.  Checkout the Ampeg V4 schematic.  Another solution is to patch in a SS circuit like Mesa Boogie or its copies like Acoustic & Carvin.  If you must have it in the amp that's a fine project, but may violate the KISS principal.  Otherwise it also works in an effects pedal or unit, maybe in the effects loop.

[silverfox]

I went through a dreamsigner phase on those for a while. Not sure if it's worth the effort and cost of the extra parts for high gain raw sounding R&R. Would anyone else notice the difference? Perhaps a hash filter would do more but I don't know how to suggest that.

Anyway, here's one of the sites I found and if you search the terms found on the site you'll come across a board designer that has a PC board, schematic and source for all the parts very reasonably priced.

The big cost and work related object is the inductor which is a multi-tapped coil, (inductor for those purists'.)

http://www.gyraf.dk/gy_pd/pultec/pultec.htm

For some cantankerously flavored opinions and suggestions you could email patrick over at turner audio.

Fox.

[Glennjeff]

Thanks folks,

I can take the mid section out of that gyraf pultec, simplify / modify it for guitar amp and build / implement it fairly easily.

All the best.

[PRR]

The Pultec is scaled for 600 ohms in and out.

You will NOT drive 600 ohms well with "a triode or two". (At least not without a Power Triode near as big as your main power amp.)

If you scale the inductor value for higher impedance (6K or 60K) you get into really unlikely values, unlikely because implementation leads to fairly large core with many-many turns of fine-fine wire.

Send/Return jack and some Para-EQ box.

However "sculpting treble" shouldn't be that hard. There's tons of treble tone plans.

10pFd plate-cathode is like 500pFd plate to ground, that will cream ALL the highs off. Try plate to ground, not plate to grid. 100p-330p are likely values. Throw a 100K reostat in series so the top-top treble doesn't go away completely.

[Glennjeff]

PRR,

Thanks, got a few spots in the treble that are eardrum splitters. Will try anode/cathode caps.

Rheostat in series with bypass cap making it a variable shelf rather than low pass?

Can series LCR be used A/K, was experimenting with it today but with no success. Also was trying to get some negative feedback happening by going LCR series between V1b cathode and V1a cathode + V1b cathode to V1a grid. Must have my ideas all wrong coz there wasn't even a feedback squeal.

All the best.

[eleventeen]

"You will NOT drive 600 ohms well with "a triode or two". (At least not without a Power Triode near as big as your main power amp.) "

PRR, I meant to question you about this. Can't a 12AX7 cathode swing a dozen volts sitting on top of a 2.2K resistor? (Not that it does, normally)  So why can't it kick out 2-3 volts in a cathode-follower config on a 600 ohm output impedance? Can you elaborate some on your statement?

[PRR]

Dozen-V/2.2K is 5.45mA. Ever see a 12AX7 flow 5mA?

Think parallel 12AU7, maybe.

[jazbo8]

Or you can get a step down transformer similar to this one, so the poor 12AX7 is not unduly stressed. Or just a FET source follower.

[Glennjeff]

Cap in series with variable resistor did a fair job.

Took the edge off a new greenback clone very nicely.

What you can do with an amp is fundamentally restricted by how much real estate you've got on the front panel!

That Jensen transformer reads like a nice piece of hardware, will find justification for getting one methinks.

Thanks,

All the best.

[PRR]

$73

[silverfox]

At the risk of flaring a sunspot, I've got a question:

Why not redesign the circuit to omit the impedance matching transformers?

Another site I found while researching tube based tone controls, go to the end for a three band tone control: http://www.angelfire.com/electronic/funwithtubes/Amp-Tone-A.html

This is another approach I found; An inductor-capacitor-resistor based filter network as an EQ.

www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/3BandPultec/P3bandDoc.pdf

There are other designs available and Tubecad is also a site I looked at for a while.

Fox.

[Glennjeff]

Good stuff silverfox, thanks.

The +/- 5dB in the mids on that angelfire.com circuit is probably not enough for guitar amp applications. Definitely worth experimenting with.

Being a "sound guy, I find guitar amp tone stacks to be very weird little beasties. They say Treble, Middle and Bass but that is not what they do, and the controls interact with each other quite significantly. Can't get my head around it.

[PRR]

> They say Treble, Middle and Bass but that is not what they do

Fender Stack:

"Mid" really controls "everything".

"Bass" and "Treb" are boost-only (unlike hi-fi/PA thinking).

With both Bass and Treb advanced, there is a phase cancellation and a mid-dip. In this mode he Mid knob mostly trims the depth of the mid-dip.

For "flat" EQ, set Mid to 10, Bass and Treb to zero.

The large bass mid-dip large treble is not very unlike what we used to do with cheap hi-fis. They weren't flat anywhere, but mondo bass and treb, they were exciting. (See also: Disco.) As the natural sound of a naked steel string is not very exciting, can be narrow nasal twangy, the Fender Stack opened a more "full range" or "orchestal range" sound. Its popularity attests to a desire for such a sound. It is not a sound you can easily get on sound-board EQ systems: the bass/mid slope can be very steep, much steeper than handy EQs.

If the Fender Stack, or other popular sounds like bass-cut screeming-highs, are not what you want, Happy Hunting. ROSS did sell a lot of little 8-band graphic EQ pedals.

[silverfox]

GlennJeff, Does it make sense to combine a 5 band OpAmp based EQ circuit with the tube portion of one of those other circuits previously posted, to obtain a hybrid circuit. Tube and OpAmp based EQ. Why? Trying to get the compression from the tubes and the control and filtering from the OpAmps.

Fox.

[Glennjeff]

silverfox,

Personally, I don't like the idea too much, but whatever gets your wheels spinning I suppose. Op amps sound awful IMHO. Seems like EQ is pretty doable with tubes, you just have to use more tubes. It's all a bit "theoretical" now coz I got the amp pretty much where I want it, always nice to have options though, especially for future builds.

Planning on using a dbx Pro Vocal processor in the FX loop, which is digital AD/DA stuff, horrid really, it's got a parametric EQ but not planning on using that, just the delay, reverb and maybe compression. Go figure !

All the best.