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Mosfet or other semiconductor substitute for cathodyne?
Mosfet or other semiconductor substitute for cathodyne?
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Old 25th April 2016, 12:16 AM   #1
exeric is offline exeric  United States
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Default Mosfet or other semiconductor substitute for cathodyne?

One of the primary ways to advance is to use the organizing idea of applying known principles to new applications. Tubelab and others have found that semiconductors, specifically some mosfets, can be used in special applications and will not affect the sound in a tube circuit. The best example is in source followers to drive tubes. There a numerous advantages there. I will let others more experienced to advance the argument.

I've recently been interested in phase inversion for push pull right before the finals. The cases where mosfets have been used successfully in tube circuits,( besides CCS) is where there is no gain in the stage. I'm thinking now that a mosfet may be a very good candidate for use in a cathodyne stage. That kind of stage also has no gain. Is there a case to be made for using them there in a tube circuit and not influencing the sound in a negative way? Inquiring minds want to know.
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Old 25th April 2016, 01:32 AM   #2
Eli Duttman is offline Eli Duttman  United States
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A power MOSFET is just fine as a split load, AKA "concertina" phase splitter. Somehow, "cathodyne" doesn't fit, where there is no cathode.

You hit the nail, right on the head. To date, successful integration of FETs into tubed circuitry requires them to amplify current, not voltage. A voltage follower is such a situation and, being a "tricked out" voltage follower, a "concertina" phase splitter is too.

As a FET, like a pentode, can swing closer to the B+ rail than a triode, it can prove to be superior in the split load phase splitter role. Check this AA thread and all associated links out. A triode would not have worked.
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Last edited by Eli Duttman; 25th April 2016 at 01:33 AM. Reason: spelling
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Old 25th April 2016, 02:32 AM   #3
exeric is offline exeric  United States
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Jeez Eli, I wish I'd seen that thread before. I spent a lot of energy to engineer a separate high voltage section for the concertina tube stage and already bought the parts for that section. Just preliminarily it looks like I may be able to use the B+ used for the rest of the amp for the concertina.

Actually I'm not put out that someone got to this idea before me. It is the nature of the world when there's billions of individuals inhabiting the planet that someone, probably many people, will get to an idea before you. At least it's nice to know that others are on the same wavelength as me.
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Old 25th April 2016, 04:20 AM   #4
exeric is offline exeric  United States
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Eli,
Finished reading that thread. It was very informative. Actually using a mosfet wouldn't have changed the B+ requirement for my needs. There really isn't a need, at least in my case, for a higher B+ for the concertina. All you have to do is forego direct coupling from the drive and its resolved.

Something that I found out when I was simulating it on LTspice - When you bias the cathode or source of the concertina at exactly 1/4 of the total voltage applied to that stage then the upper harmonic distortion just disappears! It's pretty remarkable. It seems the 1/4,1/2, and 1/4 rules isn't just a good way to load it for maximum signal output. At low to medium signal levels in there were not even any harmonics above the third. At higher signal levels, for example 2 volts rms in, there were no harmonics above the fifth for my simulation. You should try it yourself Eli, if you have access to LTspice.

I'm inclined to believe it's a real thing and would show up in a real circuit. I'm thinking that any nonlinearity in the transfer curves shows up as an additional interference between the split loads if those loads don't have equal and proportional "space" in which to operate. I'm speculating here.

The bottom line is that if you don't have the voltage necessary for the concertina to be biased at the 1/4 voltage level of the it total voltage, then it may be more important to give up direct coupling than to deviate from that biasing level. That's the scheme I'm aiming for. I going to capacitively couple it and do away with the complication of a high voltage supply for concertina. I don't need that high voltage for driving el84s or 6v6s. YMMV.
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Old 25th April 2016, 02:35 PM   #5
316a is offline 316a  England
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Quote:
Originally Posted by Eli Duttman View Post
A power MOSFET is just fine as a split load, AKA "concertina" phase splitter. Somehow, "cathodyne" doesn't fit, where there is no cathode.
'source-o-dyne' is probably am much better description

316a
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Old 25th April 2016, 02:48 PM   #6
Tubelab_com is offline Tubelab_com  United States
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There is a sticky thread in the instruments and Amps forum....

http://www.diyaudio.com/forums/instr...challenge.html

It was a challenge to see who could build the best guitar amp for less than $100. The thread got ugly several times, especially when a few vocal people protested the use of "sand" in a tube amp. After the formal challenge ended, several of us continued tweaking our amps and making new ones under new rules, basically.....rules, we don't need no stinkin rules, lets just make good cheap amps.

The mosfet PI, both split - load, and LTP were discussed during the challenge, but not used. Since the elimination of rules, mosfet PI's have appeared. They can be seen in a few designs starting with post # 1605.

The schematic of that amp as it exists today is included. The PI is a mosfet "cathodyne" using an LND150.

Yes, this is a low $$$$ guitar amp. Distortion in the PI and output tubes under high drive conditions is welcome, and encouraged. The same PI has been used in HiFi applications.

A split load PI, tube, mosfet, or Darlington BJT, works perfectly as long as the both loads are IDENTICAL for AC and DC currents, due to Kirchoff's law. Any imbalance in the loads from output tube grid current WILL cause distortion. For this reason a buffer stage between the PI and the output tubes should be included if this is a possibility.....Yes, mosfet followers are the hot ticket here too. An ordinary BJT will have some imbalance due to base current, but it can be managed if you need to use a BJT for some reason.
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Old 25th April 2016, 09:50 PM   #7
exeric is offline exeric  United States
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Quote:
Originally Posted by tubelab.com View Post

A split load PI, tube, mosfet, or Darlington BJT, works perfectly as long as the both loads are IDENTICAL for AC and DC currents, due to Kirchoff's law. Any imbalance in the loads from output tube grid current WILL cause distortion. For this reason a buffer stage between the PI and the output tubes should be included if this is a possibility.....Yes, mosfet followers are the hot ticket here too. An ordinary BJT will have some imbalance due to base current, but it can be managed if you need to use a BJT for some reason.
One nice aspect of a split load PI is that if you overload it and have grid current on the output it's the plate impedance that really changes, much more than the cathode impedance. If you have to have overload distortion then that's the preferable way. It will provide 2nd harmonic distortion rather that odd harmonics. So it's kind of benign. So if one goes cheap here it's not necessarily a bad thing, especially in an instrument amp. Even a hifi amp probably would have sound most would not find objectionable at low levels of overload.

I haven't yet read that thread, but will. I'm always up for seeing a good cat fight. Of course, that only goes so far, gets tiresome after a while, and isn't at all enjoyable if you're in it.
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Old 26th April 2016, 03:53 AM   #8
dsavitsk is offline dsavitsk  United States
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Quote:
Originally Posted by tubelab.com View Post
The schematic of that amp as it exists today is included. The PI is a mosfet "cathodyne" using an LND150.
I made a guitar amp a while back with an emitter follower after a gain stage driving a cheapo 10K:10K Edcor transformer as a phase inverter (sort of a tubelab meets Gibson topology). Might push the $100 amp to a $110 amp, but it works great.



Quote:
Originally Posted by Eli Duttman View Post
You hit the nail, right on the head. To date, successful integration of FETs into tubed circuitry requires them to amplify current, not voltage.
Nonsense.

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Old 26th April 2016, 04:35 AM   #9
Eli Duttman is offline Eli Duttman  United States
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Yeah, I forgot about AW's hybrid cascode, until after the post was made. My bad, sorry.
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Old 27th April 2016, 07:17 AM   #10
exeric is offline exeric  United States
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Quote:
Originally Posted by tubelab.com View Post
There is a sticky thread in the instruments and Amps forum....

http://www.diyaudio.com/forums/instr...challenge.html
I finally got through the entire thread. I have to say it did get ugly there for a short bit with some attempted bullying by an individual who will go unnamed. (Not you, George.)

It is almost embarrassing how coincidentally close in time my own understanding of the ability to substitute mosfets came to the same understanding communicated by others. It feels a little bit like saying, "You know, I'm theorizing that there might be this universal force that holds us to the ground. I tentatively calling it gravity." I actually hadn't read any of those threads saying the same thing before I came to the same conclusion about cathodynes.

I really do believe that there might be an aspect of consciousness that not only exists within each individual but also in the space between all individuals and that is shared by everyone. Enough of the psychobabble, I guess.
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