2e26 tube PA

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Dale Hunt:
Quote from: 9A5BDP


...I am very interested to find some data regarding pi (collins) anode network for antenna tuning purpose...





This is fairly easily calculated by the formulas in the standard handbooks.

For a tetrode class AB linear amplifier the plate load resistance (RL) is given by
2 * ( Va - Va_min) / I_peak

Va is the anode DC voltage, and Va_min is the minimum voltage on peaks.  Usually
this would be a bit less than the screen voltage for good performance.

I_peak is the peak anode current, which is 1.4 times the average (not to be
confused with the reading on voice peaks, of course.)

So for the recommended conditions given above (500V anode, 200V screen,
maximum current 45mA) we might use 185V as the minimum anode voltage,
so the plate load impedance is (500 - 185) / (0.045 * 1.4) = 5K ohms.

Then the reactances of the pi-network components are calculated using
some standard formulas that are too awkward to try to type here.  But
the same copy of the RSGB Radio Communications Handbook has
a list of reactances for a Q of 12 for various load impedances, so it looks
like:

XC1 = 417 ohms
XL = 437 ohms
XC2 = 75 ohms

should get you pretty close.

[Disclaimer:  it wouldn't hurt to have someone who is more familiar with
tubes check my calculations here.]

With lower plate voltage you may need to use two tubes in parallel to get
sufficient output power.

Dubravko Pozar:
Thanx for math that you provided. I finaly got one free day from work so I will make some calculations and graph exercise on tube datasheets.
How stabile must be screen voltage for PA stage? I saw somewhere that in past is some VR tubes used for that purpose (also for suplying stabile voltage to VFO).

Tom Allinson:
Quote

Lets go from the end..life of 2e26 tubes are short? Please tell me more..did tube with short life powered from the PA HV pover supply? Maybe is problem with overdisipated anode...

Wow!  That's taxing my memory just a bit; its over 50 years ago.  As I recall, the drivers were biased Class C as it is a CW type (FM) transmitter.  The plate supply of the 2E26 was generally fed from the mid voltage supply and was, typically, in the range of 250 to 300 volts (don't ask the current, I'd only be guessing!) .  Normally the high voltage supply fed only the plate(s) and screen(s) of the final PA amplifier(s).  The single 2E26 would feed one or two 6146 tubes or, in some cases, a single 5894 (or variant) for the higher power version.
That configuration seemed to be, at the time, almost an industry standard as General Electric, Motorola and Link all used it in their VHF Lo and High band radios at one time or another.  In the older Link HiBand radio, that single 2E26 even fed an 829B for a (supposed) 50 watt output.  As expected, that rarely happened, though.
Unfortunately, that's about all that I can recall.  Sure wish I'd kept some of that old equipment.  In my experience, that was the fun stuff to work on unlike the current breed of "throw-away" gear.
One final comment.  Over the years, various tubes would gain a reputation as to their life expectancy.  The 6146 and, especially, the 5894 were quite highly regarded but the 2E26, 829B and 2C39 were some of the ones regarded as "losers".  Obviously, the design of the circuits that those tubes operated into were equally or even more significant in that appraisal, but the tube seemed to be the item that garnered all the praise or condemnation as the case may be.
Tom

KQ6Q:
The Gonset Communicators of the 50's used a 2E26 final on 6m and 2m. They weren't pushed very hard, the units had a power supply that worked on AC or 12V DC with a Vibrator.

Scott Schrader:
2E26s weren't a bad tube, but they ran quite hot in that envelope, and if you hit a pretty good bump, as driver in things like Motorola business band sets locked in the trunk, they would short.  yes, short life.  I always though the interns designed that one while the paid tube engineers were doing the 6146 family.

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