[AMRadio] Matching a 100 watt PA am to drive 833'S

Geoff w5omr at w5omr.shacknet.nu
Tue Jan 4 11:19:34 EST 2005

Don Moore R Moore wrote:

>I have a older BC 1F series gates transmitter.  I would like to do away
>with the 845 driver tubes and drive the 833's with a 100 watt PA amp. 
>Does anybody have any Ideas what I would need to match the amp to the
>driver transformer of the 833's.
>Don Moore

I use it here, with an old Bogen PA amp.

I've run two different versions of this, with the help and aid of WA5BXO.

Originally, the audio was taken from the plates of the output tubes in the PA, through some DC 

Blocking caps (0.6uF @ 450v  as I recall) to the hi-Z side of a small'ish (6AQ5 type) output 

transformer to act as a choke, straight to the bases of a pair of ECG164 vertical output 

transistors, configured in an emitter-follower fashion.  The emitter output, with bias voltage 

applied through some high-wattage resistors, is then fed directly to the grids of the modulators.

In my case, I'm using a pair of 250TH's in the modulator, which, in normal/typical operation, 

require around 450vpk grid to grid drive.  With 300vcc on the collectors, a maximum of 

around 600vpk is available.  The regulated bias supply swings from 0 ~ -150VDC.  250TH's

require around  -75v w/3kV on the plates, to idle the tubes at around 100mA.

The end effect is, no inter-stage driving transformer is required, which would introduce a 

loss in frequency response due to a non-linear hysteresis curve through the driving transformer.

Whatever audio component is produced on the plates of the speech-amp, is what is applied

to the grids of the Class B modulators.

The 2nd incarnation of this circuit has the speech-amp audio coming straight from the plates

of the output section, to a Hi-Z (6kOhm) 1:1 transformer, with the secondary actiing as the

Hi-Z choke.  

What a FINE Job it does, too.

Unfortunatly, I'm infected with the ham operators most dreaded disease;  neighboritis.

As a result, I normally only run ~100w after 6am... and on the Early-Risers AM (ERAM)

session, the rig is up around 200w.  Since I'm using seperate power supplies, wired in

series, when running lower power, I turn off the modulator supply, to use common voltage

for the final and modulator.

Positive peaks suffer, but when you're talking only a couple of hundred miles, band conditions

don't really factor a lot.  Some QSB, but conditions to 200miles are stable all through the day

here in South Texas.  Long Haul stuff gets the higher B+ on the modulator, to produce positive

peaks that approach 4:1.  (see 


(BTW, John - I copied the files to my server, because of how slow QSL.Net has been of late)

Pictures and schematics of modulator are available at


73 = Best Regards,


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