Why usually there aren't Tone Staks after a pentode ?

[kagliostro]

I noted that is very difficult, if not impossible, to see a Tone Stack that follow a pentode

only simple tone circuit can be find in that place

so I've think there is a particular reason for that.

Pentode tubes have gain higher than triodes, so what ?

May be something about pentodes output impedance ?

Thanks

K

[John]

I have a TS following a 5879, no issues. But maybe it's because many times the pentode is used in V1? 

[HotBluePlates]

My general guess: Overall economy of parts vs. the amount of gain needed.

Many amps that use pentodes have that tube in the V1 position. Further, there's more gain available than with a single triode stage. That allows the output of the pentode to feed the driver/phase inverter, and reduces the total number of tube stages needed.

But a single pentode only very rarely can match the gain of two cascaded triode stages. If you add a tone stack bigger than a simple tone control after the pentode, the loss in the stack would then require another tube stage to provide enough make-up gain to deliver a big enough signal to the driver/phase inverter.

You could choose to add a triode or pentode after the bigger tone stack. But if you'll add another tube stage, you could have more total gain with a dual triode, and save yourself a socket. And pentode -> tonestack -> driver may give you too much gain, which you'd then need to throw away.

So pentode alone won't get the job done, but pentode & triode is more than you need and costs more in parts. Dual-triode has the right balance of gain and economy. The same argument could apply to why we don't see cascode gain stages in guitar amps; this circuit yields pentode-like gain from a dual-triode, but has similar cost and less gain than two cascaded triodes.

However, you can choose to do things amp companies wouldn't because of cost.

[Merlin]

I noted that is very difficult, if not impossible, to see a Tone Stack that follow a pentode

The output impedance of a pentode is high, but the input impedance of most tone stacks is low, and highly variable. You would therefore get a heavy loss of signal, especially treble, if you attach a tone stack to a pentode. It can be done, but it is making life difficult for yourself.

[John]

And I realized that my "tone stack" is indeed just a one-knob control - doesn't really count as a "stack" . My tubes are paralleled triode>pentode>tone control>PI>power tubes.

Sorry I wasn't more clear to start with in my first post.

[printer2]

Fender B-T tone stack for a pentode. Haven't tried it but tried it with Tone Stack Calculator.

R1 - 200k, C1 - 50p, C2 - 10n, C3 - 10n, R2 - 1M, R3 - 1M, R4 50k

[The_Gaz]

Wasn't it discovered in the 65 amps Soho thread that they used a cathode follower after the Pentode to drive the stack? Seems like a nice and tonefu solution to me.

[tubeswell]

I did a tilt control following an EF86 in one of my amps a year or so ago using 1M resistors and pots, and it worked reasonably well.



Soundbytes (with my strat and a Celestion G10 30Wer)

Me manglin' Jimi Page
http://www.nzguitars.com/forum/download/file.php?id=8347

M' manglin' blues
http://www.nzguitars.com/forum/download/file.php?id=8346

[kagliostro]

Thanks to all

However, you can choose to do things amp companies wouldn't because of cost.

It can be done, but it is making life difficult for yourself.

based upon what I have read I understand that there are more reason because we don't see Tone stack following pentodes and the principal reason is cost, may be not directly in components, more easily the cost regards the expenditure of time to arrive at a result practicable (in my case very high, missing the relative know how  )

K

P.s.: I forgot ..... Nice samples Tubeswell 

[HotBluePlates]

Fender B-T tone stack for a pentode. Haven't tried it but tried it with Tone Stack Calculator.

R1 - 200k, C1 - 50p, C2 - 10n, C3 - 10n, R2 - 1M, R3 - 1M, R4 50k

Remember that you'll need to change Zsrc (source impedance) of the driving stage to get an accurate model.

Zsrc for a preamp pentode is very nearly equal to the plate load resistor value. So if you use a 100kΩ or 220kΩ plate resistor, change Zsrc from 38k (the default) to 100k or 220k. You'll see the stack load down the signal even more.

The problem is to get more gain from the pentode, you need to make the plate load bigger. But this also raises the source impedance, and causes the tone stack to cause more loss, just as you try to raise gain.

Wasn't it discovered in the 65 amps Soho thread that they used a cathode follower after the Pentode to drive the stack?

Good catch, cause I was forgetting this. Adding the cathode follower restores a low source impedance so the pentode can work into the tone stack with less loss.

Tubenit and Geezer's recent amp design make use of this fact as well.

But now the problem is the cathode follower helps to maximize pentode gain (because of its high input impedance), but adds no gain of its own. you still need 2 sockets at a minimum, and you could have gotten more total gain using dual triodes in those sockets.

But if nothing but a pentode will do, this is a reasonable approach.

[Geezer]

But now the problem is the cathode follower helps to maximize pentode gain (because of its high input impedance), but adds no gain of its own. you still need 2 sockets at a minimum

Not if you use a MOSFET for the CF

G

[jjasilli]

I noted that is very difficult, if not impossible, to see a Tone Stack that follow a pentode

The output impedance of a pentode is high, but the input impedance of most tone stacks is low, and highly variable. You would therefore get a heavy loss of signal, especially treble, if you attach a tone stack to a pentode. It can be done, but it is making life difficult for yourself.

Agreed, and thanks for the reality check!  I have a bunch of old pentode/triode small bottles, and ran into this issue of the hi output impedance of the pentode.  Might as well use the pentode to drive a triode - the triode could be a CF or another gain stage. 

[DTM]

Hi everyone. First post. I remember someone on the AX84 Forum in the “Homebrew” area stating very strongly that a tone stack after a pentode killed the “pentode chime”. I don’t recall whether they meant a tone stack directly after the pentode (CF driven or otherwise) or if they meant a tone stack at any point after a pentode gain stage but somehow I was left with the impression that they meant the latter.

I’ve not tested this effect for myself but I can see how the impedance mis-match would kill the top end if the tone stack was just after the pentode. But l also wondered if any phase changes caused by the TS could also be responsible for this lack of “pentode chime”.

Basically does the combination of pentode and tone stack sound “bad” enough that nobody uses it?

[kagliostro]

I think the problem is when a TS is immediately after a pentode

Tubenit & Geezer used TS after pentode+CF and had good results in more amps

here the last one

http://www.el34world.com/Forum/index.php?topic=14305.0

K

[HotBluePlates]

Hi everyone. First post. I remember someone on the AX84 Forum in the “Homebrew” area stating very strongly that a tone stack after a pentode killed the “pentode chime”. ...

Basically does the combination of pentode and tone stack sound “bad” enough that nobody uses it?

I think the problem is when a TS is immediately after a pentode

I think the effect they heard was actually a reduction of the pentode's gain, leading to a reduction of its distortion and a lack of "chime".

Triode gain stages typically have a low enough output impedance you normally don't think about all the things that impact the stage's operation.

To get good gain with a triode, you need to make the plate load several times bigger than the internal plate impedance of the tube. Because you cap-couple the stage to the following gain stage, you need to make the grid reference (the resistor from grid to ground) of the following several times bigger than the previous stage's plate load.

The plate load resistor forms a voltage divider with the internal plate impedance, so gain increases as the plate load gets bigger. At a.c., the following stage's grid reference is in parallel with the earlier stage's plate load. So, you gotta keep that grid reference big to keep it from reducing the effective size of the previous stage's plate load. Hence, a tube with ~50k internal impedance (12AX7) often has a 100k or bigger plate load, and keeping the following grid reference around 1M means little effective reduction of the plate load at a.c.

Pentodes don't work exactly the same, due to their very high internal plate impedance. But making the plate load bigger leads to more gain. You have to balance this with the need to keep the screen resistor a certain multiple of the plate load; making the plate load too big means the screen resistor needs to be even bigger, which reduces screen voltage, which cuts pentode gain.

However, assume the plate and screen resistors are optimum size. If the pentode drives a low impedance load, that appears like a "resistor" in parallel with the pentode's plate load, and cuts the pentode's gain.

[printer2]

Fender B-T tone stack for a pentode. Haven't tried it but tried it with Tone Stack Calculator.

R1 - 200k, C1 - 50p, C2 - 10n, C3 - 10n, R2 - 1M, R3 - 1M, R4 50k

Remember that you'll need to change Zsrc (source impedance) of the driving stage to get an accurate model.

Zsrc for a preamp pentode is very nearly equal to the plate load resistor value. So if you use a 100kΩ or 220kΩ plate resistor, change Zsrc from 38k (the default) to 100k or 220k. You'll see the stack load down the signal even more.

The problem is to get more gain from the pentode, you need to make the plate load bigger. But this also raises the source impedance, and causes the tone stack to cause more loss, just as you try to raise gain.

I used 200k as the source resistance and ended up with the same loss as a normal Fender circuit.

[DTM]

“I think the problem is when a TS is immediately after a pentode”

K, you are correct - my memory is failing me.

“I think the effect they heard was actually a reduction of the pentode's gain, leading to a reduction of its distortion and a lack of "chime".”

Now that makes perfect sense as does the rest of the explanation. Thanks.

[HotBluePlates]

I used 200k as the source resistance and ended up with the same loss as a normal Fender circuit.

That's good to know; I'm glad to see you accounted for the pentode's characteristics.

However, you also have to be smarter than the sim, or know how it might lie to you.

The TSC sim assumes a perfect voltage source in series with a resistor the value of Zsrc. The losses shown in the plot correspond to a voltage-divider effect based on the value of Zsrc and the effective impedance of the tone stack at the control settings indicated, and swept across the whole frequency range.

What the sim doesn't account for is how the load presented by the tone stack changes the operation and characteristics of the stage ahead of it. So there will be more loss than indicated by the sim, for the reasons I noted in my previous post.

[printer2]

I used 200k as the source resistance and ended up with the same loss as a normal Fender circuit.

That's good to know; I'm glad to see you accounted for the pentode's characteristics.

However, you also have to be smarter than the sim, or know how it might lie to you.

The TSC sim assumes a perfect voltage source in series with a resistor the value of Zsrc. The losses shown in the plot correspond to a voltage-divider effect based on the value of Zsrc and the effective impedance of the tone stack at the control settings indicated, and swept across the whole frequency range.

What the sim doesn't account for is how the load presented by the tone stack changes the operation and characteristics of the stage ahead of it. So there will be more loss than indicated by the sim, for the reasons I noted in my previous post.

And as I noted in my previous post, I never tried it. No warranties implied.

[topbrent]

Dr.Z amps have pentode front ends with a tonestack.  

Take a peek at the Dr.Z Route 66 and Z28

[printer2]

How is the Dr Z tone control? The circuit seems a little odd with the bass having to go through the treble pot. How interactive are the knobs?

[HotBluePlates]

Dr.Z amps have pentode front ends with a tonestack.  

Take a peek at the Dr.Z Route 66 and Z28

Notice the 1M pots, and the fact the caps feed the pots. The overall impedance is higher (and load lighter) than the typical Fender/Marshall tone stack.

The Tilt control referenced earlier has a similar arrangement, though only one knob.

[printer2]

Dr.Z amps have pentode front ends with a tonestack.  

Take a peek at the Dr.Z Route 66 and Z28

Notice the 1M pots, and the fact the caps feed the pots. The overall impedance is higher (and load lighter) than the typical Fender/Marshall tone stack.

The Tilt control referenced earlier has a similar arrangement, though only one knob.

But if you adjust the Fender values for operating with a pentode? The Route 666 shows a 1000pF cap in the treble circuit with a 1M pot, the modified Fender stack a 50pF with 1M. The 666 shows the bass going through a 10nF and 150k resistor with the Fender a 10nF and 200k. The mid dip in the Fender the result of the capacitor slopes and mid pot, but that is how we get the desired mid scoop.

[jojokeo]

I must really have a tube sickness because I'm reading threads while sipping a cocktail in my condo on Maui while looking out at Molokai & Lanaii islands. 
I've not had too many issues at all regarding this topic. I've built six or seven pentode variations at least and the differences aren't severe. You'll get something that leans more towards blues over rock or vice versa. In a current amp I can bypass a cf stage w/ a BF stack after pentode and it sounds terrific. In fact I normally run pentodes at a lower gain which helps reduce microphonics too. But I still get all the pentode mojo.

[Willabe]

I must really have a tube sickness because I'm reading threads while sipping a cocktail in my condo on Maui while looking out at Molokai & Lanaii islands. 

Must be a nice sickness.   


               Brad     

[jojokeo]

  yes but it's a good sickness Brad!
I really like pentodes and what they sound like. Plus there's more ways they can be manipulated offering greater variety. Some days I like lower gain or more chime & others it's great to crank it all the way. There's a feel & sound you just can't get with a triode.

[tubenit]

Anyone try a 5879 with a mosfet CF in the V1 position like this going into a "typical"  TMB tone stack? 

I've done this further down in the signal chain but not as a V1 tube. 

IF you tried it,  did you still get the touch sensitivity of the 5879 tube in the V1 with the mosfet CF?

With respect, Tubenit

[MakerDP]

I've been wondering the same thing Tubenit. I'm tossing around the idea of a dual-5879 build driving a SE KT66 output with a James tonestack between the 5879s.

Wasn't sure if I needed a MOSFET CF before the tonestack or not.

[Tone Junkie]

I like the dual 5879 idea I was going to do an SE kt88 build. Might change to this one with a tube driven effects loop.
Bill

[kagliostro]

I was wondering about a dual paralleled 6ak5 (5654) as V1, followed by a TS without a CF in the middle

(I've a bit stock of those tubes)

Franco