triode / pentode

[Colas LeGrippa]

Hi,

The EL34 data sheet shows that a pentode connection with lower plate voltage than a triode connection produces more power ( 3w more if I remember ). Most amps I have built have their power tubes screen grid at the same potential as the plate, and then are being considered as triodes. Is it because it is hard to lower the B+ without using a valve VOLTAGE regulator  ( 0C3, by example, used in a LESLIE 147 AMPLIFIER to supply the screen grids of the 2 X 6550 by 105V less  ).  Since I prefer an early breakup from the power tubes, which way is the best? Triode or pentode ? Could somebody post a schematics showing a switch that toggles between the triode and the pentode connection ?

Happy new year 

Colas Colada, Colas LaGrappa .............

[navdave]

Google it 

http://www.el34world.com/Forum/index.php?topic=12525.0

[Colas LeGrippa]

my google is frozen, sorry, but I have connected my booster cables and it's warming up now.thanks for the link

[PRR]

> power tubes screen grid at the same potential as the plate, and then are being considered as triodes.

Same _DC_ potential. But in "triode" the screen goes up-and-down with _signal_, in pentode it does not.

The triode screen goes down just when we need more curent. That's why a triode makes less power than a pentode, and often needs more supply voltage to do well.

The triode/pentode switching is well-covered elsewhere.

[kagliostro]

Kagliostro

[HotBluePlates]

Anyone have a diagram for Triode-Ultralinear-Pentode switching?

I'm likely looking to have an individual switch for each tube in a push-pull pair. Probably won't matter, but it makes me slightly nervous to bring both tubes' plate and screen-tap leads all to one point.

[tubeswell]

Another (untried but possible?) variant idea for triode-pentode (morph) at output stage (utilising Merlin Blencowe's pre-amp morph circuit). You could have a double-ganged pot or 2 separate pots (for experimentation)

[kagliostro]

it makes me slightly nervous to bring both tubes' plate and screen-tap leads all to one point

that I can understabd

but

Anyone have a diagram for Triode-Ultralinear-Pentode switching?

???? ????

---------

some possible different "solution" for a PA

note the "Superpendode" is claimed 80W with a pair of EL34 (without stress on the tubes)



http://www.next-tube.com/articles/Veen2/Veen2EN.pdf

Kagliostro

[Willabe]

Doesn't Menno own/run a toroidal transformer company?

I think #5,6,7,8 are torids and are for dealing with the need of toroidal OT to have balanced dc currents?


        Brad     

 

[kagliostro]

Doesn't Menno own/run a toroidal transformer company?

May be, all is around money (to get), remember it 

Kagliostro

[Willabe]

I'm not trying to knock Menno. I have nothing against him. I've read a little about him and he's been very involved with the development and use of torids for audio and MI amp's.

I'm just trying to say that I think those circuits (#5,6,7,8) are/were developed for toroidal OT's, not standard EI lam. OT's.

I could be wrong.     


             Brad        

[kagliostro]

Sometime ago I had a response from Merlin Blencowe

about the use of toroidal transformers as OT

the verdict was negative

but I admit that I know near nothing about the use as Menno do and my knowledge is so limited .........

Kagliostro

[John]

Speaking of Merlin, he has a schematic using a pot or 2 for pentode/UL/triode morph. It's in the preamp book though-addressing the small signal pentode, so not sure how useful it is for power section. If HPB wants, I'll certainly attach it, but did not want to muddy the waters.

[sluckey]

Anyone have a diagram for Triode-Ultralinear-Pentode switching?

Here's the basic drawing. I've never tried this...

[kagliostro]

Here is the preamp version of the triode/pentode switch

Kagliostro

[PRR]

> I think #5,6,7,8 are torids and are for dealing with the need of toroidal OT to have balanced dc currents?

No. They assume a custom transformer and work the tube in more elaborate ways than a simple/stock transformer could.

"Super Pentode" applies positive feedback to the screens, increasing gain and maybe power output. CFB is a low-ratio McIntosh. "Super Triode" is just UltraLinear plus low-ratio McIntosh connection.

Some of these will work better with tightly-coupled windings. Mac used C-cores. Today toroid is the obvious choice.

None of these have any real business in a guitar amp.

[HotBluePlates]

"Super Pentode" applies positive feedback to the screens, increasing gain and maybe power output. CFB is a low-ratio McIntosh. "Super Triode" is just UltraLinear plus low-ratio McIntosh connection.

And I think you meant to mention "Unity Coupled" is the original McIntosh wiring.

I answered my own question... While I thank everyone for showing Triode-Pentode, UL-Pentode and/or Triode-UL wiring, I had asked for was a Triode-UL-Pentode switch. That has 3 positions, and selects any of the 3 operating modes. See the front panel pic below from one of London Power's amps, right above the power scale control.

The needed switch is a DPDT On-On-On switch, which enables progressive switching (you could do an Off-Standby-On switch with this part). When you have the switch to one side, both circuits are in "state 1". Flip to the middle, and one circuit stays in "state 1" and the other switches to "state 2". Flip to the 3rd position, and both switches move to "state 2". Sorry, I know a picture is worth a thousand words, but I just figured it out on paper, and don't want to spend 20 more minutes drawing it in Visio! 

The first circuit switches the screen between the screen supply node and a different terminal. That "different terminal" is hard-wired to a second circuit, which can select between the UL tap or the plate connection.

You could also use a single switch (4PDT On-On-On) to switch both sides of the output stage at one switch.

Question for PRR in that circumstance: Do you think there would be a problem bringing the plate, screen node, screen and UL wires for both tubes together in close proximity like that? I suppose there isn't, since KOC seems to do it in his amps, but then again, I wonder if he's using the panel switch to control a relay somewhere. Anyway, I ask because I'm thinking about adding such a switch to a build-in-progress (Standel-inspired). The convenience factor is probably higher with a single switch for both push and pull side.

[kagliostro]

About to put all this wires in one place I think must considered that often there is a Triode pentode swich implemented

so no much difference if you add also the UL configuration

if I have  to do that I'll use a 1 way 3 position (double) rotary switch (is a 2 way 3 position) and I'll prefer a ceramic one (like in many RF apparates with high voltages)

like this one



probably also this kind of non ceramic switch can afford the needs





EDIT: The only way I can see to have it without a rotary switch in only one command is with a rotary switch acted as a normal switch like this



but don't know if it will be a safe realization

Kagliostro

[HotBluePlates]

Good thinking, but same in a toggle switch. See below.

EDIT: The needed part is (for one example) the C&K 7211 DPDT On-On-On switch. See the second half of page 2 of the linked datasheet.

The key thing I didn't know about was this type of switch controls both poles, but does not open/close both poles in the same switch position. When you move from "position 1" to "position 2", only one of the poles is switched to the other available circuit. Moving to "position 3" switches the remaining pole to the other circuit. Once you look at the data sheet, you'll see it's different than what you are familiar with in a normal 2-position DPDT, or in a DPDT center-off switch.

The diagram showing the DPDT On-On-On switch is for 1 half of the push-pull output. A 4PDT On-O-n-On switch (ex. C&K 7411) is needed for switching both halves on 1 switch.

EDIT: Corrected schematic is shown below

[kagliostro]

I've post an edit to my previous post at the same time you posted your scheme

Is correct if I understand you are using two different switch acted alternatively ?

Kagliostro

p.s.: Looking to your circuit may be I've understand how it can be done with a 4DPT on-on-on switch (if can be found)

EDIT: No, don't see a way to achive it with only a 4DPT toggle switch

[HotBluePlates]

Check my edited post above (now with a correct drawing!!).

[kagliostro]

Thanks HotBluePlates

Now I think I've understand
(a bit of misunderstand also was because I was thinking that the On-On-On Switch will act differently)

I've found this 4DPT Toggle Switch (don't know if is adequate to this voltages)



http://www.guitarelectronics.com/product/SWT36-17/3-Way-4-Pole-Guitar-Toggle-Switch-Chrome.html

is this schematic what that you mean ?

Kagliostro

[printer2]

Good thinking, but same in a toggle switch. See below.

EDIT: The needed part is (for one example) the C&K 7211 DPDT On-On-On switch. See the second half of page 2 of the linked datasheet.

The key thing I didn't know about was this type of switch controls both poles, but does not open/close both poles in the same switch position. When you move from "position 1" to "position 2", only one of the poles is switched to the other available circuit. Moving to "position 3" switches the remaining pole to the other circuit. Once you look at the data sheet, you'll see it's different than what you are familiar with in a normal 2-position DPDT, or in a DPDT center-off switch.

The diagram showing the DPDT On-On-On switch is for 1 half of the push-pull output. A 4PDT On-O-n-On switch (ex. C&K 7411) is needed for switching both halves on 1 switch.

EDIT: I drew the diagram wrong. I'll upload a corrected version later.

They have a nifty mechanical arrangement in order to stage the switching of the two poles. You can think of it as in one direction it needs a little more push to get the slider in the switch moving. I picked up a bunch of these and they came in yesterday. You can feel the difference from a regular DPDT switch when you change positions. I am using this switch to change the bias and capacitor on a duel input amp such as a 5E3.

[HotBluePlates]

Awesome idea Printer!

Below is my drawing of the actual switch wiring for the Triode/UL/Pentode switch, shamelessly stealing your physical switch diagram.

[printer2]

Awesome idea Printer!

Below is my drawing of the actual switch wiring for the Triode/UL/Pentode switch, shamelessly stealing your physical switch diagram.

Oh yeah? Well I will just have to steal yours right back and use it somewhere. I'll wait to see how you like it first though. 

[HotBluePlates]

Steal 'em, and save 'em for later.

Once I finally get this amp done, I'll give a run-down of what I think. However, I'm fairly certain all settings will be useable, because the amp's overall character is clean, clean.

[kagliostro]

A friend, member of an italian forum, who has experienced with such configurations,

told me he didn't like the UL configuration, so the amp was wired only for pentode / triode operation

BUT, if you like the UL, sure you can add the switch and benefit of the more options

Kagliostro

[Colas LeGrippa]

UL may be good for Hi-Fi, not for guitar.( already tried, hate that ).  The Marshall silver jubilee 2555 shows a simple dpdt identified ''HI-LO''  that disconnects the screens from the B+ and connects them to the plates, asat.

[sluckey]

UL may be good for Hi-Fi, not for guitar

I don't know. There was a time that all the big boys were playing UL amps.

[Willabe]

Marshall Major 200w, UL. Ritchie Blackmore, SRV.   

Dr. Z makes a few UL amps that guys love. But  --No FB loop -- in them.


       Brad     

[HotBluePlates]

The amp build in question is a Standel, with some miinor changes/additions. Since I'll be using Hammond iron, and the Williamson circuit (supposedly an inspiration for the output stage) was triode-strapped 807's (and re-thought later in UL form), it seems worthwhile to make use of the available UL taps.

And if you're familiar with Chet Atkins' playing, or the JBL D-130, clean is where it's at. The amp may be something of a 1-trick pony, but I want it to do the trick better than anything else. Besides, I already have other amps/pedals for dirt or "dingy clean".

Oh, and surpisingly, there's no feedback loop around the output stage of the Standel (though there defintiely is in the Williamson).

[Willabe]

I've read a number of times that those Standel amps were used on many classic C&W recordings. Quite a who's who list of early getar players had them.

Look forward to hearing what you think when you get you'rs built HBP.

Huh, UL and no FB loop?

KOC and Dontay (?) like this set up on some amps quite a bit. IIRC they said that the UL taps add a little feed back to the OP stage tubes, just enough to damp the speaker, yet not kill the amps tone.   


        Brad               




 

[HotBluePlates]

The original Standel, though, was a pentode output stage. It just seems like a good option to have all three modes, at only the extra cost of a switch.