So I've got an older PA amp that I think would work great as a rebuild into a nice little guitar amp.
The tube lineup is somewhat inline with this "Octal Madness" amp already, so I got thinking it might be a new and interesting route for me to go with a guitar amp.
Any guitar amp I've built up until now has been some variation on the traditional Fender or Marshall stuff with a 12ax7 cathodyne or LTP phase inverter.
Looking at this thing it looks like it's got some kind of SIPP output stage? Is that right? I'm just not familiar with this type of setup, and was curious what you guys thought about this design? Apparently it can become quite a beast of an amp with as much preamp gain as it has, and a unique sound with the 6SJ7 pentode up front.
The tube lineup is somewhat inline with this "Octal Madness" amp already, so I got thinking it might be a new and interesting route for me to go with a guitar amp.
Any guitar amp I've built up until now has been some variation on the traditional Fender or Marshall stuff with a 12ax7 cathodyne or LTP phase inverter.
Looking at this thing it looks like it's got some kind of SIPP output stage? Is that right? I'm just not familiar with this type of setup, and was curious what you guys thought about this design? Apparently it can become quite a beast of an amp with as much preamp gain as it has, and a unique sound with the 6SJ7 pentode up front.
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Interesting thought. You're just thinking that the 6SJ7 might sound pretty interesting having its grid heavily overdriven by V2a?
I'm more interested in the power stage.. I don't understand how V4 gets an appropriate signal (opposite phase of V3) on its grid #1. It looks like the signal for grid #1 on V4 is taken from the screen grid on V3. Is that out of phase with V3's input on grid #1?
Interesting.. Found this video.. A bit on the slow lengthy side.. I guess the screen grid is 180 degrees out of phase with the grid.
I'm not sure about a lot of the "theory" that this guy talks about though.
https://www.youtube.com/watch?v=vmKil0fb1TQ
I guess that's really the only way this schematic makes sense.
I'm not sure about a lot of the "theory" that this guy talks about though.
https://www.youtube.com/watch?v=vmKil0fb1TQ
I guess that's really the only way this schematic makes sense.
A tube's output is 180 degrees out of phase with the input. The screen grid is in phase with the output making it 180 degrees out of phase with the input as well. So taking the signal off the screen will make it 180 deg. out of phase and give V4 an out of phase signal, so push pull output.
As far as the input being the triode instead of the pentode, it will tend to be a less noisy amp since the input stage's gain will be lower with the triode there. This stage is amplified the most so the noise it generates will be louder as well.
As far as the input being the triode instead of the pentode, it will tend to be a less noisy amp since the input stage's gain will be lower with the triode there. This stage is amplified the most so the noise it generates will be louder as well.
Is this considered an okay way to do the phase inversion in a guitar amp? I haven't seen it done very much as I'm digging around. I realize you wouldn't have the additional gain that certain types of phase inverters would bring, although the preamp/driver stage of this amp looks to be pretty aggressive already.
As for which stage to put first, I can easily play with that I suppose, and see what I like better.
To be honest, this is the first amp I've been involved with that has this type of phase inverter. The reason behind it is that back in the day when things were expensive to build, this saved a tube, no PI tube, so this was one way to do that. I can't speak from experience to the advantages or disadvantages of the circuit. Maybe this thread might help. Also, check out the link in the third post: Antique Radio Forums • View topic - Screen-grid inverter
In the post, they talk about some imbalance in the output stage, so this will tend to add distortion, particularly second order, so it might make the power amp add some nice grind as well, I just don't know for sure. It seems there must be a good reason Fender, Marshall and others never used this circuit. At least, to my knowledge, they didn't. Anyone else know?
In the post, they talk about some imbalance in the output stage, so this will tend to add distortion, particularly second order, so it might make the power amp add some nice grind as well, I just don't know for sure. It seems there must be a good reason Fender, Marshall and others never used this circuit. At least, to my knowledge, they didn't. Anyone else know?
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Following the links in the link above you will find the comment that the screen grid inverter circuit is only good for a Class A output stage.
I'm not sure this is correct but it seems very plausible to me. If true that would be a good reason why it was never used by Fender, Marshall etc.
An option is to add a mosfet as a cathodyne (Split Load or Concertina) phase splitter. This would probably be my preferred option.
You could also add 2 mosfets (or high voltage transistors) as a "standard" differential phase splitter which would add some more gain (again).
That allows you to keep all the existing small signal tubes as gain stages.
Cheers,
Ian
I'm not sure this is correct but it seems very plausible to me. If true that would be a good reason why it was never used by Fender, Marshall etc.
An option is to add a mosfet as a cathodyne (Split Load or Concertina) phase splitter. This would probably be my preferred option.
You could also add 2 mosfets (or high voltage transistors) as a "standard" differential phase splitter which would add some more gain (again).
That allows you to keep all the existing small signal tubes as gain stages.
Cheers,
Ian
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Hm.. That's a good point. Now that I think about it that makes sense about the class A thing.. I could always use an additional triode as well I suppose as a cathodyne PI. I've got the room for it.
Yes to the extra triode.
I was assuming you had no more tube sockets and/or no more heater current capability from the power tranny to accommodate another tube. While there is a small difference between a MOSFET cathodyne Splitter and a triode cathodyne splitter I don't think it will be noticable sonically, particularly if you use significant size grid stops on the output tubes which are required anyway for decent overdrive performance with a cathodyne splitter (Example: for the typical Fender 5E3 circuit with 6V6 outputs I use 39K grid stops on the 6V6s).
If you go with a MOSFET don't forget a gate stop resistor (say 2K2) on the MOSFET and a zener protection diode, say 12V, wired from Gate (Zener Cathode) to the Source (Zener anode).
Cheers,
Ian
I was assuming you had no more tube sockets and/or no more heater current capability from the power tranny to accommodate another tube. While there is a small difference between a MOSFET cathodyne Splitter and a triode cathodyne splitter I don't think it will be noticable sonically, particularly if you use significant size grid stops on the output tubes which are required anyway for decent overdrive performance with a cathodyne splitter (Example: for the typical Fender 5E3 circuit with 6V6 outputs I use 39K grid stops on the 6V6s).
If you go with a MOSFET don't forget a gate stop resistor (say 2K2) on the MOSFET and a zener protection diode, say 12V, wired from Gate (Zener Cathode) to the Source (Zener anode).
Cheers,
Ian
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