Paraphase Tone Control

[RicharD]

Looks interesting.  I wanna bread board it.  I think there's a typo.  V2 -  Ra seems like it should be a 1k and not a .1M

 

[VMS]

Isn't the second triode just a regular voltage amp with 1k cathode resistor and 100k plate resistor.

What interests me is the mixing of out-of-phase signals. 

[kagliostro]

Two 22,000ohm plate resistors or must be 220,000ohm plate resistors 

Kagliostro

[VMS]

Two 22,000ohm plate resistors or must be 220,000ohm plate resistors 

Kagliostro

Maybe I'm wrong but because of the 20uF capacitors the 22,000ohm resistors are not plate resistors.

[RicharD]

This is copied directly from the Radio Designer's Handbook 4 so I would suspect the likelihood of a typo to be quite slim.  The reason I ask is because: a grounded cathode amp with Ra = 23k and Rk = 1k looks like a perfect bias at 250V & 4.45mA.    100k over 1k looks all kinds of nasty, plate starved. 

>What interests me is the mixing of out-of-phase signals.
Totally what caught my eye.


Here's my quandary for the week.  I am experimenting with a really clean amplifier circuit (see Janitor thread).  This circuit could quite possibly become a low power (80ish Watt) bass guitar practice amps.  So far I've tried 3 different V1 stages and I have settled on using the 12SJ7 circuit over the cascode 12SN7 and another miserable failure not worth talking about.  The last experiment involved: a 6SN7 in the V2A position wired as a cathode follower > a James tone stack > V2B, 6SN7 wired as a plate follower to match the loss of the James tone stack.  This amplifier sounds better w/o any tone stack whatsoever.  I'm guessing this is because it's a bass amp.  Typically I shave a little bottom with V1's Ck and cut some highs in front of the volume control.  With this circuit, I have the cathode bypassed big with a 150uF.  Shaving highs isn't necessary because the amp isn't super high gain.  In fact I think if anything, I want to boost lows and highs.  This brings be to the point of experimenting with different tone stacks, and here I am.

This amp may very well end up w/o a tone stack or even a volume control.  My bass guitar has both of those anyway.  Life sure was simpler when I just copied Fender stuffs.

[HotBluePlates]

V2 -  Ra seems like it should be a 1k and not a .1M

No. Forget prettiest load-line, and copy what they provide. For what it's worth, I have a copy of RDH4 in hand, and the same .1M is present. So it's not a fly-speck from copy of a copy of a copy.

What troubles me is that there is no value given for the Treble pot. I did an internet search based on the reference listed in RDH4, and found this. Scroll to page 10, and you'll see the same circuit citing the same author and source as RDH4.

Turns out the Treble pot should be a 100k pot, and the 2nd stage plate load is 100k as well.

[RicharD]

>there is no value given for the Treble pot

Last night this is where I got tripped up.  It just doesn't seem right for a 6SN7.  I saw it with a 100k treble pot but the RDH4 I copied from that Headfonz guy says 1,000 (right next to the 82,000.  Maybe I'm misreading this.  I was hoping to better understand this circuit before bread-boarding it.  1 reason being I am working at 150V higher B+ which of course I can easily reduce for the sake of experimentation.  I'm sure you're probably right, RDH is not know to be full of misinformation,  but it just seems odd for the load line to be almost horizontal.  My thought was to use both pots as 1k, eliminate both the 20uF caps, and make that 100k a 1k or eliminate it all together.

[HotBluePlates]

You added the 1,000 ohm marking that wasn't previously present in the drawing. Take it back out.

Forget loadlines for a moment. Use a 12AU7 if you want instead of the 6SN7. Click the link I provided, as you'll see another diagram from the same original source, and it shows a 100k treble pot. Use a 100k treble pot, and use a 100k plate load resistor for the second stage.

[RicharD]

>You added the 1,000 ohm marking that wasn't previously present in the drawing. Take it back out.

I did not!
http://headfonz.rutgers.edu/RDH4/
Chapter 15.

I'm kinda done banging my head for the moment.  V1 looks like a Concertina that was left to the user to set the grid bias.  V2 looks like a plain ol grounded cathode amp that's somewhat irrelevant to the tone circuit itself.  Coming off the plate are 2 RC hi pass filters and the signal inverts.  Coming off the cathode are 1 hi pass and 2 low pass filters & the signal does not invert.  Whatever is left should cancel out.  This leaves me with 1 great big ol, "I don't get it.... AT ALL."     

[octal]

If I had to guess, I'd say that at some point that tone control is going to have a huge notch due to the out of phase outputs of the driver colliding with the phase shift of the high pass and low pass networks. That is a guess though.

I don't see what's "paraphase" about this circuit at all. It's a concertina phase inverter followed by a gain stage.

[HotBluePlates]

>You added the 1,000 ohm marking that wasn't previously present in the drawing. Take it back out.

I did not!
http://headfonz.rutgers.edu/RDH4/
Chapter 15.

I apologize profusely!

When you made your first post, I flipped to an original copy of RDH4 that PRR had mailed to me in Iraq. My 1952 original doesn't have 1,000 ohms written below the word "Treble" the way the digital copy you linked has written. I'm sorry for falsely accusing you of alteration.  

However, I think our second source linked gives the right method, which seems to be the same as your original inclination. The 20uF caps and 22k resistors are only about creating decoupling networks; they might be necessary due to the fact that at least 1 output from the first stage will be of the same phase as the plate signal of the second stage. That said, I think you can do without the decoupling networks, at least given the good power supply present on your breadboard.

The 100k load may look very horizontal, but the 1k load will be pretty vertical (and therefore high-current) by comparison. Anyway, 100k and 1k are not unreasonable values, even for a 12AU7, as we know from plugging them at random in guitar amp circuits. I too have a leaning to see V1 as a split-load, except 1k seems like an awfully low load resistance for the plate. I'm starting to see both stages as simply being common-cathode with 2 different-impedance outputs.

Try-n-see is the best way to sort this out. It is possible that my copy has an omission.

[RicharD]

No apologies necessary.  I thought you were making a joke.   

It's a funky circuit.  I need to bread-board it stand alone and finger it out before I try to deploy it in an existing circuit.

[PRR]

> mixing of out-of-phase signals.

No. It's out-of-out-of-phase.

Recall your R-C filter theory. Each R-C stage has 45 degree phase shift at the -3db corner, 90 deg at infinity. Low-pass lagging, hi-pass leading.

Two R-C stages have 45+45= 90 degree shift at the corner.

One 2-pole hi-pass and one 2-pole low-pass gives a _180_ degree phase difference at the corner or crossover.

Simple 2-pole crossovers tend to null-out at the crossover frequency.

In loudspeakers, you can swap the wires to one of the drivers, so that (in concept) there is no null and the crossover sums correctly in the room.

In an electronic crossover you need to flip polarity of one output.

In this case they flip first, filter, then mix with a simple summer.

Both pots are "1,000" in my PDF copy of the book.

> an original copy of RDH4 ... My 1952 original doesn't have 1,000 ohms written

How odd. The PDF version that floats around does have it. It may be that you have (I sent) an older printing that was corrected later, in time for the copy that the PDF was scanned from.

> 4.45mA

Which is better: gasoline at $4.45 or at $2? Why would you dump 4+mA down a mere tone-control buffer? And get less than 4V peak output across your 1K resistor? (The 22K+20uFd is a ripple filter and a solid AC ground.)

Starved?

2V peak in gives 27V peak (20Vrms) output at 2% THD. Yes, it is near-bottomed and clips at -29V; if you go right to a power-tube grid you may want to refine the operating point. For ANY other use 20Vrms is way more than you need.

> It just doesn't seem right for a 6SN7.

Yup. 2K-3K would be a more mainstream cathode resistor for a DC operating point. But Rk is unbypassed. We need some gain, we don't need a lot of gain. With Rk=3K the gain would be much less. We could fix that with a C and another R. Or we can fudge so the DC values also give a useful audio gain and ample output.

> Concertina that was left to the user to set the grid bias.

Ha, didn't notice that. 1Meg to ground will cover it.

> off the cathode are 1 hi pass and 2 low pass filters

That "hi-pass" is shown as 1uF and 1Meg||(3*0.82Meg) or 766K, 0.2Hz. It's just DC block.

Ignore V2. Well below 400Hz it is flat and in-phase. Well above 400Hz it is flat and flip-polarity, a minor oddity which the ear may overlook. The only deep question is what happens AT 400Hz? If you run these 2-pole networks same-polarity they will null. (This is used in some for-guitar tone controls.) The phase-splitter "fixes this problem", 400Hz is not nulled.

[kagliostro]

From VMS

Two 22,000ohm plate resistors or must be 220,000ohm plate resistors  

Kagliostro

Maybe I'm wrong but because of the 20uF capacitors the 22,000ohm resistors are not plate resistors.

From PRR

(The 22K+20uFd is a ripple filter and a solid AC ground.)

With this schematic and PRR explanation all has sense also for me - Thanks

100kohm plate resistor (22kohm+20uF) ripple filter

Kagliostro

[VMS]

> mixing of out-of-phase signals.

No. It's out-of-out-of-phase.

Recall your R-C filter theory. Each R-C stage has 45 degree phase shift at the -3db corner, 90 deg at infinity. Low-pass lagging, hi-pass leading.

Two R-C stages have 45+45= 90 degree shift at the corner.

One 2-pole hi-pass and one 2-pole low-pass gives a _180_ degree phase difference at the corner or crossover.

Thanks PRR for explaining this!

I had an inkling that something is happening with those RC-filters.

[PRR]

> Paraphase Independent Bass-Treble Tone Control.jpg

Where did you find this?

It looks like the same plan, but shows the treble pot as 0.1Meg (100K).

Seems to me that both outputs should be the same, so that when mixed equally we get "flat" response.

I asked the computer to calculate it. With 100K I get a huge treble boost. With 1K it is nearly flat.

First stage current is very high with these values. Wasteful. I wonder if it should be 1K bass pot, 1K treble pot, and 100K-121K dropping/filter resistor.

Cathodyne/network loss is about 13db (pots centered). With booster shown, about 6db gain.

[RicharD]

The term generator seems to mean potentiometer therefore I assume the RDH is correct and both should be 1k.  I still haven't had a chance to scratch the circuit together but as I see it, about 1/3 B+ is dropped across the 22k resistor above the treble generator.  Seems like it want to suck crazy current.  I just need to build it and see what happens.  I'm gonna use a 12AU7 since I have plenty of them to burn and not so many SN7's.

[DummyLoad]

The term generator seems to mean potentiometer therefore I assume the RDH is correct and both should be 1k.  I still haven't had a chance to scratch the circuit together but as I see it, about 1/3 B+ is dropped across the 22k resistor above the treble generator.  Seems like it want to suck crazy current.  I just need to build it and see what happens.  I'm gonna use a 12AU7 since I have plenty of them to burn and not so many SN7's.

use 6C5 or 6J5 - won't care either... 6J5 is exactly 1/2 6SN7GT. sacrifice tubes we'll likely never use?

[kagliostro]

From PRR

> Paraphase Independent Bass-Treble Tone Control.jpg

Where did you find this?

I find it in the file HotBluePlates linked in his previous tread (red link here)

V2 -  Ra seems like it should be a 1k and not a .1M

No. Forget prettiest load-line, and copy what they provide. For what it's worth, I have a copy of RDH4 in hand, and the same .1M is present. So it's not a fly-speck from copy of a copy of a copy.

What troubles me is that there is no value given for the Treble pot. I did an internet search based on the reference listed in RDH4, and found this. Scroll to page 10, and you'll see the same circuit citing the same author and source as RDH4.

Turns out the Treble pot should be a 100k pot, and the 2nd stage plate load is 100k as well.

Kagliostro

[HotBluePlates]

> Paraphase Independent Bass-Treble Tone Control.jpg

Where did you find this?

It looks like the same plan, but shows the treble pot as 0.1Meg (100K).

Click the link I posted.

I got "smart" when we forst talked this topic. I looked in RDH4 for the reference cited for this circuit; it mentioned a guy named "Jaffe" who wrote up this circuit in a 1934 issue of Radio Magazine. Google turned up 1 file that someone scanned that also referenced the exact same original article. That snippet is what Kagliostro posted in jpeg form.

[KeithR]

Has anybody breadboarded this yet? If so did you try different center freq?

[silverfox]

How does a circuit like this compare to a traditional Fender Vox Marshall or other type tone control I frequently see here? I've often wondered if the specialized tube driven tone controls are superlative to traditional guitar amp designs.


Silverfox.

[KeithR]

Really couldn't say. There's a reason most guitar amps follow similar topologies,  and it's not due to lack of imagination.  The  responce shown for this circuit is unusual looking as far as guitar amps go. This is why I wondered if anyone tried it yet. For just two knobs it covers a lot of sonic territory.