Regulating G2 in Pentode Operation

[alexmoose]

Hows everyone doing?

My guitar played friend convinced me to try a pentode operation (switchable to UL or Triode, of course) when I upgrade my output iron. I've heared a good way to approach pentode mode is using a high voltage Mosfet to regulate the screen voltage, I wanted to know if anybody could seggest a circuit that they have used personally, or found online.

If no one has found any design could someone please explain to me how to design my own circuit? I figure its a very simple design (5 parts or less), I just have no clue where to begin!

Thanks a bunch
-Alex

[Tubes4e4]

Hi Alex,

for a start, some data would be useful to know:

- What tubes are used in the output stage, in which "standard" configuration (SE, PP), you are starting from?

- What is the "standard" operation point (voltage across tube and Vg1 (bias) voltage) those finals are working at?

- since you also mentioned switchable triode and UL mode besides pentode mode with regulated screens, what is what you are heading for? I mean, f.e sound flexibility, while Po could be sacrifized, and so on.

Knowing all that , suggestions for a suitable Vg2 and an according regulator could be made.

Since it seems you want all that stuff being switchable, a simple pass regulator for Vg2 using two or three VR tubes, a resistor and a small cap, seems to be out of question, which sad, because those VRs certainly would add Mojo

Tom

[ray_moth]

Pentodes are used in guitar amps to maximize the OP power and to deliver the kind of sound you just can't get any other way. However, the screens are not regulated in a guitar amp because to do so would impair the sound rather than improving it, from a guitar musician's point of view.

For a hi-fi amp with OP pentodes in class AB, the sound is definitely improved if you have a stable voltage for the screens. This can be achieved either by having an independent supply for the screens that has very low impedance or by using a regulator circuit for the screen supply. Although a regulator is a less expensive solution, it does mean that you must also regulate the bias (if fixed bias is used), otherwise the operating points of the OP tubes could drift with mains supply fluctuations.

a simple pass regulator for Vg2 using two or three VR tubes, a resistor and a small cap

That arrangement would not provide a stable voltage for the screens, it would only guarantee that the screen voltage is always a certain number of volts below the plate voltage, determined by the VRs. As the plate voltage sags, the screen would follow it, which is exactly what you want to avoid. A shunt regulator using VR tubes could be a solution.

However, a much more difficult problem in this case is that Alexmoose wants to be able to switch between modes and the required load impedance will be different according to the mode used. The same applies to the required amount of NFB necessary to attain a reasonable damping factor. Switching between triode and UL is OK but switching betwee pentode and either UL or triode is usually not practical.

[alexmoose]

wow, extreemly great responses. thank you

Right now the amp runs el34s in SE, but in a few weeks it will most likely be KT88s. So it will always be a Class A operation

The purpose of the amp is to be a Hi-fi amp one minute, then with the flick of a switch be able to be a Pentode Guitar amplifier (I have a bunch of gain/ distortion stages that could be plugged in when operated in this mode)

The '88s will be operating at around 400v, with probably 100ma (give or take) on each plate.


-Moose

[Miles Prower]

[QUOTE]Originally posted by alexmoose
Hows everyone doing?

So far, so good.

My guitar played friend convinced me to try a pentode operation (switchable to UL or Triode, of course) when I upgrade my output iron. I've heared a good way to approach pentode mode is using a high voltage Mosfet to regulate the screen voltage, I wanted to know if anybody could seggest a circuit that they have used personally, or found online.

Here's a screen regulator I designed for my latest project. This is basically nothing more than the same SS design reimplemented in hollow state. As for performance, I can't complain. Dropping the DC rail by some 40V caused a loss of 0.8V at the output (any lower and the VRs lose regulation). After running for some six months, the output hasn't wandered a bit.

If no one has found any design could someone please explain to me how to design my own circuit? I figure its a very simple design (5 parts or less), I just have no clue where to begin!

Thanks a bunch
-Alex

It isn't difficult at all. Can supply design details. However, I don't think it'll be too useful for a guitar amp, since those have characteristics very different from HiFi, which is what I do.

[Tubes4e4]

Hi ray_moth,

regarding VRs I meant shunt regulator, of course

Tom

[Geek]

I use this on the screens of a stereo PP 6V6 (4 tubes) and it's simple and solid

http://geek.scorpiorising.ca/Geek-0.html

[SY]

And, finally, I showed a simplified Maida-style regulator for the Red Light District screen supply. Adjustable for 250-300V. It's here on the forum somewhere...

[alexmoose]

SY, I took a good long look at your regulator circuit and I liked it, in fact, I hadn't looked at that amplifier design since I just got started, when I first started, The Red Light District looked scary to me, now it looks like a good project that I could make without too much hassle. is there a power transformer you seggest?

[SY]

I used one salvaged out of an old Dyna SCA-35. These are selling new now over at Triode Electronics, branded "Dynaclone." For the driver stage, just about any 280-300V unit will work- it doesn't take much current.