• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Cathode follower rigorous definition.

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hey folks,

Anyone have any suggestions where one might find a mathematically rigorous definition of cathode follower output stages, preferably with OTL in mind? I understand loadlines can be used similarly to a common cathode/plate follower but I'm interested in a more in depth description of the CF workings, AC conditions, etc. I have the MJ book and several others but there is not a lot said on the matter.
 
I've been meaning to pick up a copy, I guess I should bite the bullet. Looks like it can be had for ~ $40 on Amazon.

That makes sense. I think I just want to see all the gory details. I'm a first year EE student but unfortunately valve electronics is not in modern day curriculum.

Thanks!
 
Last edited:
DD,

Dig up a copy of the 1st or 2nd edition of "Basic Electronics for Scientists", by James Brophy. Mathematically rigorous discussions of both tube and SS are in there.

BTW, the closest thing in "sand" to a tube is a FET. The drain curves of FETs are very similar to the plate curves of pentodes.
 
Hey folks,

Anyone have any suggestions where one might find a mathematically rigorous definition of cathode follower output stages, preferably with OTL in mind? I understand loadlines can be used similarly to a common cathode/plate follower but I'm interested in a more in depth description of the CF workings, AC conditions, etc. I have the MJ book and several others but there is not a lot said on the matter.

Cathode Follower

There is not a lot said because there isn't a lot to say. A loadline is a loadline is a loadline. The VT, itself, does not and can not care how that load is connected: between the DC rail and plate, or between the cathode and DC ground, or returned to a negative rail, or split between the plate and cathode: it makes no difference. None.

"cathode follower output stages, preferably with OTL in mind"

This is where a lot of people go very wrong. The cathode follower can't work magic and make the VT into something it is not, and can never be: a low voltage, high current device. If you are attempting to use the CF as a power final, you have two choices: connect the CF to an OPT, just as you would if using it as a grounded cathode stage.

The other alternative is to parallel up enough VT's so's the parallel combination can source the required current while operating into a very steep, and very low voltage swing, loadline. That's the halibut: there are very few types that can operate like this: the 6AS7 power dual triode (originally designed for series pass regulators) that big Russian triple-nipple power triode (same deal: series pass regulator for the MiG fighter) or TV horizontal deflection pentodes operated as pentode CFs. There just aren't that many VTs that can source the bigamps at lowish Vpk's.

As for OTL topologies, these become problematic since there are no "P-Channel" VTs, and so you'll have to "fake it" with various totem pole topologies like the SEPP, SRPP, Futterman, Inverse Futterman, or Circlotron.
 
Miles, thanks for the response. Lot's of good info. Funny you mention 6AS7's as that's exactly what I'm experimenting with. I'm paralleling two 6AS7 plates (one tube per channel) in a headphone design and driving the CF stage with a single direct coupled 6SN7. If I'm calculating correctly I should have a Zout of about 50 Ohms. I'm running the 6AS7's pretty conservatively, 55mA /plate (110mA per channel) which I think should be adequate for headphones. B+ is 300v for both stages. I welcome your thoughts on this. Obviously I could push the 6AS7's much harder current wise but the main goal here was to lower the Zout.

I'm now hung up on figuring out output de-coupling. Part of the reason for the low Zout was to utilize my lower impedance cans. I have 600 ohms cans that are easy to design for but for lower impedance I'm looking at about 250uF per channel for a 20Hz corner into 32 ohms. So I'm going back and forth between using ASC's or some sort of poly (Solen etc. bypassed with an auricap or something like that. Any thoughts are welcome here as well.

I actually had a few pair of 6C33C's for a while and even bought two giant chassis to build a pair of mono-blocks. I still have a handful of 5U8C rectifiers that would have gone nicely with them. As I got further into the process and started to realize the immensity of the design I decided to save it for another day. I just remembered I still have a pair of 600ohm transformers that I bought for that design laying around. Maybe I should sell them for output cap money. Ha!

The thing I'm finding in regards to OTL is that people either seem to love the idea or hate it. Most of my (limited) experience has been with this topology. I've built several variations of WCF using 6SN7's and 6N1P's and had good luck with them. Admittedly though I've never had a good transformer design to listen to make an honest comparison. When I get out of school and have more time, room, and most importantly money I'm going to start looking at building some larger scale power amplifiers and at this point I'll probably start looking at transformered designs.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.