Please Peruse This Schematic & Tell Me What You See?

[The Ballzz]

Please take a look at this schematic and tell me what you see. Generally, you can kind of ignore the power amp section, as I have interest in the preamp. The preamp is nearly identical to that of the elusive and rarely even heard of Marshall 200 watt PIG. I know what I think I see, but I want to hear from others.
Thanks,
Gene

 

[2L man]

EQ is done using two parallel triode stages. Treble has high pass filter and bass has low bass filter before valve. Then on output both have potentiometers which are used to adjust EQ and volume.

 

[Quexoz]

I see a diagram of a bunch of mice and insects and their path infiltrating in and out of a house and a shed + the points where they could not get in. The porch is on the left, the shed out back on the right, central A/C unit on the bottom. They couldn't get into it or the back of the shed and turned away, but they punched holes all over the sides of the house and shed, and have a little pathway setup to travel back and forth between them...probably the Queen is in the shed.

It's a pest control estimate on a house in an expensive area. Did I get it right? What do I win?

 

[WingedWords]

I see dark magic arts that I sometimes wish I had the slightest inkling about. Full of admiration for those who understand electrics.

 

[Skyhook]

I know what I think I see, but I want to hear from others

 

[Lynxtrap]

Not sure what we are supposed to look for. I see a "Pig" preamp, but haven't compared it to the original schematic.
Interesting circuit, I've been planning to build that preamp into a micro type amp just to see how it works.
I might have used a post PI master volume if I wanted more overdrive without having to dime the treble and bass controls.

 

[The Ballzz]

Not sure what we are supposed to look for. I see a "Pig" preamp, but haven't compared it to the original schematic.
Interesting circuit, I've been planning to build that preamp into a micro type amp just to see how it works.
I might have used a post PI master volume if I wanted more overdrive without having to dime the treble and bass controls.

Hint: I think I see a crossover network circuit that makes one "channel" V1B pass only treble frequencies above a certain point (high pass filter affected) and the other "channel" pass only bass frequencies below a certain point (low pass filter affected). I'd like to figure out the frequency and roll off slope )dbs per octave) of each of both the HPF and LPF. It looks like they are not actually tone controls or a "tone stack" per se, but rather volume controls for each of both all the bass frequencies and all the treble frequencies. This strikes me as a very unique concept that could lend itself to some very interesting possibilities! below is as close as can be found as a redraw by someone called "vintagekiki" of an original and although the upper and lower channels of the drawing are reverse, darned close to identical :

I have some ideas about advancing and adding to this concept and I'm looking to spitball the idea around with some other inventive types of folks! Where Marshall messed up and abandoned the idea was by trying to make the power section into a very, VERY high voltage (over 700 volts B+)/high wattage monster that was so unstable as to self destruct at the drop of a hat. The problem was not the preamp, but the power amp! I'll elaborate a bit more, once I see a few more replies and comments. Someone who understands crossover networks could be very helpful.
Thanks,
Gene

 

[Tom Kamphuys]

EQ is done using two parallel triode stages. Treble has high pass filter and bass has low bass filter before valve. Then on output both have potentiometers which are used to adjust EQ and volume.

Agree. But I also see a cathode follower (like) stage for the treble and a normal gain stage for bass.

 

[Lynxtrap]

Hint: I think I see a crossover network circuit that makes one "channel" V1B pass only treble frequencies above a certain point (high pass filter affected) and the other "channel" pass only bass frequencies below a certain point (low pass filter affected). I'd like to figure out the frequency and roll off slope )dbs per octave) of each of both the HPF and LPF. It looks like they are not actually tone controls or a "tone stack" per se, but rather volume controls for each of both all the bass frequencies and all the treble frequencies. This strikes me as a very unique concept that could lend itself to some very interesting possibilities! below is as close as can be found as a redraw by someone called "vintagekiki" of an original and although the upper and lower channels of the drawing are reverse, darned close to identical :

Yes, that is how it works and what makes this circuit interesting.
Unfortunately I can't help you with frequncies and roll of slopes.

A couple of things worth noting: The treble "channel" uses a cathode follower. That makes the two channels out of phase. This is designed as an advantage in this case. Theoretically, the frequencies around the crossover point that would otherwise be shared by both channels, are cancelled. That makes the bass and treble controls very independent of each other.

The cathode follower also means that the treble channel would have much less gain than the bass channel. That's why they put a 22k resistor/grid leak as a voltage divider to ground before the bass channel's triode, which reduces that channel's gain significantly.

Because of this the two triodes following V1A can be disregarded as gain stages, being at or close to unity gain. Preamp gain is governed by V1A and the make-up stage where the two channels join. That is why you won't get much overdrive out of the preamp using the master volume.

Without a PPIMV, this thing has to be turned up LOUD for rock'n roll.

 

[andrewRneumann]

I’m scratching my head. Why not just dispense with the CF (V1B) and V2A and just have Input Valve -> Tone Stack -> PI Driver -> PI? V1B doesn’t do anything. It’s not overdriven, should be extremely linear, provides less than unity gain, and is connected to a 220k mixer resistor so has a high output impedance. V2B is unbypassed so a loss of gain and also very linear. What’s the point for using a voltage divider to drop 95% of a signal and then amplifying it 20x back to where it was and slapping a 220K resistance onto its output? (Phase inversion does seem like the only reasonable possibility.) All those resistors in the bottom track followed by a huge attenuator to me also look like a recipe for noise.

IMO that CF should go before the two channels split. The upper channel needs no additional valves before remixing. The lower channel can hit another valve if a phase inversion is needed.

I’m probably sounding harsh. Probably the best sounding amp ever though right? Anything’s possible.

I’d be curious how out of phase the two sides are with the tone shaping going on before the second set of valves. Looks like a job for LTSpice. In general wouldn’t it be a bad idea to mix out of phase signals? You lose the crossover frequencies and don’t have a way to get them back with the controls.

Very interesting. Thanks for posting.

 

[guitar_paul1]

I think the cathode follower (which gives a 180 phase shift) is compensating for the +90 and -90 phase shifts caused by the treble and bass tone circuits.

I'm wracking my brain trying to resist the temptation of going back and re-learning the laplace transform for this situation. All I can recall is that phase angle is related to j's and omegas.

I'm pretty sure the RC network for each could be reduced to a single value of R and C in this case, since there is no L involved.

 

[Telekarster]

I see a diagram of a bunch of mice and insects and their path infiltrating in and out of a house and a shed + the points where they could not get in. The porch is on the left, the shed out back on the right, central A/C unit on the bottom. They couldn't get into it or the back of the shed and turned away, but they punched holes all over the sides of the house and shed, and have a little pathway setup to travel back and forth between them...probably the Queen is in the shed.

It's a pest control estimate on a house in an expensive area. Did I get it right? What do I win?

 

[guitar_paul1]

I see dark magic arts that I sometimes wish I had the slightest inkling about. Full of admiration for those who understand electrics.

Spent my life doing electricity and I feel the same way. It's just like music. Learning something just exposes another layer of complexity.

 

[Lynxtrap]

Marshall supposedly adapted this circuit, called "paraphase tone control", from one of those old Radiotron books (see below). With some interesting differences, as can be seen.

I think @guitar_paul1 is right about the phase shift. In the Radiotron circuit the phase flip happens before the RC network, but Marshall chose to do it differently.

 

[The Ballzz]

I was thinking that by making the cut off frequency of the bass channel and the cut in frequency of the treble channel adjustable, it could be used as a selective mid range boost/cut notch and/or peak allowing movement of the center frequency and width of the midrange? Somewhat like a parametric EQ? I can see how it might already work that way, depending on the cut off and cut in, as well as how much overlap of frequencies there is? It just looks like there are many possibilities that I know only a little about! I'm guessing it is about time to hit the books. I had hoped the possibilities may be more obvious and simple to some folks, but......?
Or Well,
Gene

 

[The Ballzz]

Please note the similarities, even though there are transistors in place of the tubes, although obviously, the two separate channels do not sum together, except at the ears:

What I'd be seeking to correct is the massive gain loss from a standard tone stack that simply bleeds signal to ground for tone shaping. Of course, I realize that using phase cancellation as a means of tone shaping would suck gain also, but maybe in a different way? It makes a bit more sense out of Jimi Hendrix's reputed method of walking up to an amp and starting with everything fully cranked up (tone stack mostly defeated) and selectively removing what he didn't want. As opposed to the concept of starting with everything at zero and adding bits of what one DOES want. A discerning player may end up in the same place, coming from either direction, but I can certainly see the logic of Jimi's reputed approach!

It is also of note that the very few actually functional PIGs in existence are reported to sound amazing. Even the guy who built the lower wattage "PIGLET" from my first post claims it to sound wonderful. I've been considering doing a similar build with a more typical 6V6 power section.
Just Spitballin'
Gene

 

[Ten Over]

I think the cathode follower (which gives a 180 phase shift) is compensating for the +90 and -90 phase shifts caused by the treble and bass tone circuits.

Second order RC filters like this have a theoretical phase shift of 180 degrees, but they only reach this at zero Hertz or infinity Hertz. Single RC filters have 90 degree theoretical phase shifts.

The high pass filter in question has a center frequency of 72Hz at which point the phase shift should be +90 degrees. The low pass filter has a center frequency of 339Hz at which point the phase shift should be -90 degrees. As you increase the frequency, the high pass filter's phase shift decreases towards zero degrees and the low pass filter's phase shift decreases towards -180 degrees. If the two curves were synchronized, then the two paths would always be 180 degrees out of phase. But they are not. Instead they are only 180 degrees out of phase at one audible frequency and who knows what frequency that is.

 

[David Barnett]

Will it sound like Mick Ronson?

 

[andrewRneumann]

Tried to model it in Falstad. Pretty sure I failed.

V1A has Zout = 68.15k
V1B has A = 0.984 and Zout = 615R.
V2A has A = -31.85 and Zout = 68.15k

Can any experts show me how I could use an op amp simulator to model a valve (inverting and non-inverting) in small signal conditions?

 

[W.L.Weller]

You could always have the Doctor build you one:

http://ballseffects.com/balls-piglet/

You could probably spec 6V6s instead of KT66s, even.

But you probably know that already.