Can a cascode drive tone controls better than a typical pentode?

[Carlsoti]

Yesterday, I picked up an old Capehart Console, originally advertised as "The Bolero." It's mono, so I don't think I want to try to restore the whole unit. The amp chassis p/n is 4PH55M. It looks like it'll will make a nifty little guitar amp. 2ea 12AX7s up front, 2ea 6V6GT for output, and a 5Y3 rectifier. I've already got another amp with a 6G2 circuit in it. Leaning towards doing a Marshall Studio 15 4001 circuit, or possibly designing a circuit with a cascode input> paraphase PI> PP 6V6.

Does anyone know if a cascode has the same difficulties with driving a tone stack as a pentode?
Any other suggestions as to what circuits use this tube compliment?

T.I.A.

[Merlin]

A cascode behaves pretty much the same as a pentode in that regard

[scstill]

I have a couple of pentode preamp designs with a single tone control which sound good.
for example...  https://stillampd.com/black-and-blues-05
Why does a pentode have difficulty driving tone control? What is the difficulty?

[Carlsoti]

To be honest, I'm not certain, but I seem to recall reading somewhere that a pentode pre-amp tube can't push a tone stack like a triode. Since I've never really played anything with a pentode in the preamp, I didn't retain any of the technical info. Now I've got an old chassis with a pair of 12AX7s, a pair of 6V6s, and a 5y3. I've already got an amp with a 6g2 circuit, so I'm entertaining odd ideas outside of the typical fender copies. I was considering a Cascode input, Bax tone stack (Or is it the james passive?), into a standard triode-driven cathodyne PI. I'll probably end up doing something like a low-powered Orange OR80, though.

[tubeswell]

many examples of amps with EF86 input stages out there. Check out Dr Z

[Carlsoti]

Thanks for the nudge, Tubeswell. Now I've gotta find some sound samples. I pulled my head out of my @55 and in less than 30 seconds, probably less time than it took me to type this post, I was reminded that it was an impedance matching issue where proper circuit design beats out cut and paste nine times out of ten, or better.

[kagliostro]

Don't ask me more (I'm not able to explain it in a better way, Mr Merlin can for sure), the tone controls are a heavy load for the output of a pentode and you loss a high part of the signal, this is due to impedance output level of the pentode

If you do a search you can find examples of pentodes followed by a tone control on old commercial amps but they are very low numbers if compared to the whole guitar amps schematics

---


BTW our friend Tubeswell some years ago posted an interesting proposal with a Pentode joined to a Triode

https://el34world.com/Forum/index.php?topic=17653.msg178019#msg178019

Franco

[tubeswell]

some years ago

At that time, I was in the middle of writing up my masters thesis (I did my Masters later in life) and looking back, I was procrastinating heavily when I should have been focussing on the thesis.

[HotBluePlates]

I have a couple of pentode preamp designs with a single tone control which sound good.
for example...  https://stillampd.com/black-and-blues-05
Why does a pentode have difficulty driving tone control? What is the difficulty?

You been "cheating" by having a Tone control that's not much affected by "source impedance."

Compare:

    Single Tone control 5F2-A Princeton -  signal reduction of -10dB with controls at Half; Volume full-up mostly negates the Tone control's effect.

    Single Tone control 5F2-A Princeton - Pentode driven - "Rin" changed to 680kΩ to mimic pentode; gain is -20dB ---> Not Great, Not Terrible.

    Normal AB763 Tone Stack - gain is -25dB at Midrange (300-1000Hz depending on other controls).  Bass/Treble could be as much as +20dB higher.

    Pentode-driven AB763 Tone Stack - Midrange is now -40dB, which is signal reduction to 1/100^th.  Very noticeably weaker

[AlNewman]

You could get closer to the original values by modifying the load, like this TMB with a pentode source impedance?

https://www.guitarscience.net/tsc/fender.htm#RIN=680k&R1=400k&RT=1000k&RB=1000k&RM=100k&RL=5M&C1=47p&C2=10n&C3=7n&RB_pot=Linear&RM_pot=Linear&RT_pot=Linear

I would imagine it could be noisy, and maybe react differently with the millar capacitance of the next stage.  It does show similar losses compared to the original TMB design.

[DummyLoad]

If you believe John Brokie's TubeCad, cascode Z out is about 1/2 Rp.


--Pete

[pdf64]

... RIN=680k&R1 ...

680k seems strangely high?
A pentode stage's output impedance is pretty much equal to its anode load resistor, eg 220k or whatever.
https://www.valvewizard.co.uk/pentode.html

[HotBluePlates]

... RIN=680k&R1 ...

If you believe John Brokie's TubeCad, cascode Z out is about 1/2 Rp.

680k seems strangely high?
A pentode stage's output impedance is pretty much equal to its anode load resistor, eg 220k or whatever.

I pulled a number out of the air to show the impact a Tone Stack has when Source Impedance is high.  I wasn't much concerned with duplicating a specific amp's circuit, but demonstrating a limiting case to show the impact of change.

Separately, "Gain of a Pentode" is usually given as Gm x R_L, where "high gain" is achieved by making the plate load as large as feasible.  Hence "680kΩ."
    (The challenge is balancing the large anode resistor against supply voltage & anode current, which then alters Gm)

You could get closer to the original values by modifying the load, like this TMB with a pentode source impedance?

https://www.guitarscience.net/tsc/fender.htm#RIN=680k&R1=400k&RT=1000k&RB=1000k&RM=100k&RL=5M&C1=47p&C2=10n&C3=7n&RB_pot=Linear&RM_pot=Linear&RT_pot=Linear

I would imagine it could be noisy, and maybe react differently with the millar capacitance of the next stage.  It does show similar losses compared to the original TMB design.

The changes I see are a 5MΩ load impedance (higher than the stock "1MΩ" meant to represent a Volume pot), and very much smaller capacitor values.

I agree that raising the impedance of the tone stack with these changes reduces the loading effect on the pentode/cascode.  Good job spotting this as a solution!  Most hobbyists will bolt on a circuit-element like a pentode/cascode without recognizing other circuit elements need to be balanced against it to achieve a desirable outcome.

[Carlsoti]

Most hobbyists will bolt on a circuit-element like a pentode/cascode without recognizing other circuit elements need to be balanced against it to achieve a desirable outcome.

   Hey, I resemble that remark! lol FWIW, the idea of trying a cascode came to me in the shower before work and I posted the question before applying any real brain power. After work I googled and found the answer (impedance mismatch) in under a minute.  I didn't receive any formal training on electronics aside from OTJ training at Rockford Fosgate/Hafler on the production line, though I'm not afraid of doing the math when it comes down to it. As a result, I tend to come up with an idea, spit it out into the universe, hash it out on paper, then test it in various modeling programs. Often, I find out that I'm either treading a well worn path many others have used before, or I discover the reasons that the path is NOT used very often. Input from those that have been properly schooled in invaluable to me, so I thank you all.
   In the case of the unit at hand, I think I've decided that a cascode input is NOT the direction I want to go. Instead, I'm going to try something like a 6V6 powered Orange OR80 circuit, with a handful of tweaks. I plan on moving the F.A.C. (essentially a variable coupling capacitor on a rotary switch) between the first two stages, which should give me a bit more distortion from the second triode, compared to a Bax/james ts + volume that's normally there. The tone stack and volume will then come between the second triode and the PI driver, with changes as necessary to drive the 6V6s appropriately.

[kagliostro]

On this old thread there is a schematic of an amp with a cascode at V1 by Jbefumo

https://el34world.com/Forum/index.php?topic=14793.msg217649#msg217649



Franco