Solid state CF harmonic trem

[Esquirefreak]

If one used a solid state cathode follower, one could build a single channel revision-A brownface amp with only 4 tubes.

The CF part was posted by Tubenit in this thread: https://el34world.com/Forum/index.php?topic=23811.0

I'm trying to fit all the pieces together, does this look about right to those with knowledge?

[tubeswell]

Yes, you can use an N-channel MOSFET (like an IRF820, which, in the TO-220 package, is rated for 500V and 1A without a heatsink) as a Source Follower in place of where you might otherwise use a CF as an impedance buffer/current driver - and you don’t need to worry about heaters (or H-K insulation). And an IRF820 will just sub straight in place of a 12AX7 triode with no other circuit modifications needed, bar one -Good practice to use a 12v Zener between the Gate and the Source.

[sluckey]

INT pot is wrong. Phase splitter has grid connected to cathode. Modulator lower tube grid has no ground reference resistor.

[Esquirefreak]

Ah! I missed the 1M resistor from the original schema. Would please correct my wiring of the INT pot? Thanks a bunch for helping out.

/Max

[sluckey]

.

[Esquirefreak]

A thousand thanks, Sir 

/Max

[Esquirefreak]

Are the gate and source internally connected or do I need to bridge them? (I have VERY little experience from messing with transistors, let alone Mosfets.

/Max

[dwinstonwood]

...I have VERY little experience from messing with transistors, let alone Mosfets.

/Max

First, thanks Esquirefreak for posting about this. It's a cool idea that eliminates an extra tube and socket from the chassis.
I also have practically no experience with solid state parts, and I can't really read their schematic symbols. The thing I've circled confuses me. Does that mean no connection is made at that point?
Thanks!

[sluckey]

The thing I've circled confuses me. Does that mean no connection is made at that point?

That is just an arrow that indicates N-channel or P-channel. It is not a connection. There are only three leads on an IRF820.

[sluckey]

Are the gate and source internally connected or do I need to bridge them?

DO NOT CONNECT THE GATE TO THE SOURCE! Simply connect the three leads of the device exactly as shown.

[Esquirefreak]

Now I see the middle/gate connection. I didn't get it at first. Zener between the gate/source as stated above.

Thanks!

/Max

[dwinstonwood]

Thanks sluckey. Another reading list to compile...

[Esquirefreak]

It'll be a while until I get to work on this build, due to economics and so on. I have made and populated the boards. Will keep you updated when there's progress made.

However, I bought some IRF820 transistors and 12V Zeners, and figured that, while playing the waiting game, I might try the CF network on my center volume based (6G4) build.

Does this look right?

[sluckey]

no

[Esquirefreak]

That bad, huh? Hmm, back to research, I guess 

/Max

[sluckey]

Didn't like my simple drawing?

[Esquirefreak]

Yeah, of course I did! But I tried to adapt your drawing to a 6G4 rather than the -A revision, since the phase inverter stage and filter is different. As that amp is already built I'm trying to change as few parts as possible. I just don't understand how and where the CF is incorporated 

I'm using your drawing for my next build. Since I'm using that circuit from scratch I made the board accordingly.

[Esquirefreak]

Aha, the Zener is in the wrong place, right?

[sluckey]

The whole point of this thread was to replace the CF in a 6G4-A trem circuit so you could use only 4 triodes. We did that.

Now you're trying to put a CF in the totally different 6G4 circuit. Guess what. Ain't no CF in the 6G4 circuit!

[Esquirefreak]

Point taken, I won't go off topic.

/Max