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
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
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
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
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.
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.
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.
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.
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
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
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
http://geek.scorpiorising.ca/Geek-0.html
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?
It took just a few seconds to find the P-TF47609 at www.tubesandmore.com selling for about $20. No doubt more diligent searching could turn up other candidates.
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