20W cathode follower amplifier

[Fuling]

My 12B4A CF amp continues to impress me, so I´ve decided to build a bigger SE version before I start with the PL519 PP project.
Setup will be like this:
PL36 (25E5) triode wired output tube.
(http://www.tubes.mynetcologne.de/roe..._triode_v2.pdf)

James 6112HS OPTs (http://euphoniaaudio.netfirms.com/ea...pdf/6112HS.pdf)
I bought a pair a while ago to build a 6S45PE spud but I changed my mind as I don´t need another 1W amp.

A pair of choke (40H 50mA) loaded 12B4A will take care of the driver stage duties, with >300V on the plates and fixed bias these tubes can deliver enormous voltage swings.

Haven´t decided what input tubes to use yet, but anything that can swing +-50Vp will do.

A Hammond 370HX power transformer together with a 5AR4 rectifier and a 10H 200mA choke will deliver B+ in the range of 320-350V.
As I´ve mentioned before the output tubes will operate with cathode bias (to drop some voltage) while the driver stage will have fixed bias to allow maximum output swing.

[Fuling]

Addendum:

As I´ve mentioned before I have already run some bench tests on the PL36-12B4A combo. That time I used a 125DSE OPT wired to 2,5k/8 and got 7,5W out with 260V 60mA across the output tube.
This time I´m going to use a 3,5k OPT and run the tubes at a slightly higher voltage and lower current, the calculated output power is 6,9W and the Zout and distortion should be slightly lower.

The James transformers are definitely higher quality unit that the cheap Hammonds which should be reflected in the overall sound quality.

There are two things "wrong" with the PL36 tube in this application though: With four cats in the house I would prefer a tube without top cap, and a little higher gm wouldn´t hurt either.

[tubelab.com]

I have recently built an Augmented Cathode Follower Power Amplifier. It doesn't work right yet, but I will report on it in the Augmented Cathode Follower thread.

[Fuling]

Augmented cathode follower = cascoded cathode follower?

[tubelab.com]

Sort of. Send me an email and I will reply with the technical paper from 1957. I see no way to enclose it in forum mail.

[MRupp]

Would that be the document ? http://www.pat2pdf.org/ then patent number 2965853

[tubelab.com]

That patent describes the circuit. The design that I am working with is shown in figure 4 in that patent. There is an IRE (now IEEE) publication that is not in the public domain that covers the same circuits as well.

[Fuling]

My latest contribution to the world of CF power amps:

Even though I got my PL36 amp up and running I never liked the sound of it, the 12B4A amp sounded much better even though it has much cheaper iron. What to do? Well, I obviously tore the thing apart (shame on a fairly nice chassi) and rebuilt the thing with a whole new tube lineup.
Instead of 1/2 6SN7 - 12B4A - PL36 per channel I went for only the 6SN7 and the 12B4´s, this time each channel employs both sections of a 6SN7 in a cascade to drive two paralell 12B4A´s.

The first 6SN7 section works as a resistor loaded gain stage with a non-decoupled cathode resistor (to decrease the overall gain) and the second acts as a driver stage with Hammond 156C (150H 8mA) chokes as plate loads (two in series).

The output stage consists of two 12B4A+s in //, each with a 1k2/100uF combo on the cathode for self bias. This stage runs on a 280V rail delivered by a 5AR4 rectifier and a CLC filter while the input and driver section operates at 360Vdc to get maximum swing and linearity. Both voltage rails are taken from the same PT winding but the higher voltage is rectified separately with silicon diodes and filtered by an IRF 840-based capacitance multiplier.

Due to the high gain in the input-driver sections I had some problems with oscillations and noise pickup that took me several days to fix. PSU decoupling, copper screens between the 6SN7 sections and rearranged ground buses took care of most of the problems but the amp is still a bit touchy when it comes to picking up noise.

[tubelab.com]

It looks cool!

I have done some experimenting with 6S4's and 12B4's. I went looking for a 9 pin miniature tube that had a little more grunt. The 7233 is easily equal to two 12B4's but they are not too easy to find. I have done some experiments, and the results are promising, but I only had one tube. I got 10 more of them at a hamfest yesterday, so there will be an amp built in the next few months. The Bendix 6877 looks like a contender too, but they are even harder to find and not cheap. I got some at the hamfest. I also found one Bendix 6080 with graphite plates for $4. It is lonely and needs a friend!

[Fuling]

Thank you!

I sort of fell in love with the sweet little 12B4A years ago and I´ve used them in a bunch of different applications ever since and they´ve never let me down.

Aside from chasing RCA 808´s for my future big A2 SET project I´m trying to stay away from expensive and/or hard-to-get tubes.
What I´m looking fo next is a cheap and easily available lowRp, high Gm tube capable fo delivering >8W into a 750R load at <120mA plate current.
The thing is that I have a pair of Lundahl LL1664 OPT´s that I´m planning to use in a quite unorthodox way. These are 3k/8R transformers with two identical primary sections that are supposed to be wired in series, my plan is to wire them in // to get a 750R/8R impedance ratio with twice the current handling capability.
Not sure it will work as intended, but the datasheets for some of Lunahl´s interstage transformers shows application examples where the windings can be wired in series or in parallel to achieve different winding ratios.

E130L would probably be the perfect tube for this application, but I´ll try to find something less rare and expensive.