Hello Bob and Steve:


Thanks for getting me in this loop. 


Would you like to have your email exchange posted in the "written contribution" section of www.PhilbrickArchive.org ?


I too have thought of putting some of my solid state Philbrick modules to use as stereo amplifiers, but have yet to do it. 


But I have a power amp tube design in mind that I would like to try out first. The idea is to run a tube like the 5894 twin power beam pentode in it's grid forward bias region. I came upon this idea when I saw how much more linear the forward bias gm is than the normal negative bias gm. You can check the 5894 curves at the end of the data sheet found at http://www.mif.pg.gda.pl/homepages/frank/sheets/049/5/5894.pdf


Normally, forward grid bias curves would not be available for a tube designed for audio use, but  the 5894, was designed for class C operation in power RF amplifiers. In class C, the grid gets driven hard positive to saturate the heck out of the tube and reduce the internal voltage drop.


The general approach that I have in mind is to use one 5894 twin driving a center tapped transformer load to a relatively low B+, somewhere between 150V and 300V. I still expect to get plenty of power with the modest B+ because of the added plate current with the grid forward biased.


Driving the grids into forward bias would not be easy with tubes, unless you have to have some K2-W's, possibly combined with external cathode followers inside the K2-W inverting loop. This way, I can easily generate a 100Vp-p swing with little distortion and low output resistance to drive the non-linear input current of the forward biased 5894 grids. I hope the K2-W has enough slew rate to keep up at the high frequencies. I will check www.PhilbrickArchive.org for this.


Another appealing aspect of the high plate current-low plate voltage approach is the possibility of using AC power transformers instead of audio power transformers for the audio output. Normally, power transformers don't have enough inductance (about 1H) to appear as a transformer load on the plate at low frequencies. Output transformers are usually designed with primary inductances of at least 5H. The reduced operating output impedance of the plate circuit may make it possible to use cheap surplus power transformers as audio output transformers. The reduced inductance may benefit the high end because of the implicit lower leakage inductance of the primary.


Having the K2-w around, makes this design easy to contemplate, if not easy to implement.






At 11:50 AM 10/15/05, Robert A Pease wrote:

 *** Hello  Steve,  see at  ***



Steven Weber wrote:


Hi Bob,


Back in the early 70's, I was a EE major at Lowell Tech in Lowell, MA. With

in easy walking distance of the dorms, there was an electronics surplus

store in one of the old textile mill buildings. One day I found some

Philbrick P65AU op amp modules there. I bought several of them, for maybe

$5.00 total.  *** A bargain...


I decided to build a stereo hi-fi amp around them. Took the better part of

the semester to build it, and nearly flunked out because of it, but I ended

up with a pretty good stereo amp at the end. I learned a lot building the

amp, a lot more then they were teaching me in class at the time! *** Check.


Limited by the 30 volt supply the op amp would run at, it didn't put out a

lot of power, but with some efficient bass reflex speakers, it was loud

enough.  *** Check.  The P66's were  useful? If you had  enough of them, you could parallel several of them.  Or add other  BOOST  transistors. 

A few years later I took the amp to a Tech HIFI audio clinic, where they

got a good chuckle when they saw my home made amp, but when they ran the

frequency responce and distortion, they were quite impressed with the



*** The P65 was  not  very fast  for  slew rate. It had  about  0.6 volts per microsecond, so at 20 kHz it  could barely  swing 8 volts p-p.  But  most program material  does not  have a large amount of content at  20 kHz.   The P65  was also a good low-noise preamp for phono  cartridges, etc.


I used that stereo amp for many years and eventually gave it to a friend of

mine, who might still be using it. I still have a *spare* P65AU module and

a P66A booster follower, which I never did find a use for.  

*** Take good  care of them!   We may need them someday!   I've  

recently been running some  P65's in a 1-millisecond  delay line, using  

10 henrys of inductance.



Steve's Electronics

Berlin, NH 


*** Thanks  for the comments.  / rap