Plate modulating a PL-172A/8295A

[VE3AJM]

Has anyone ever tried this. I know its a rare tube to obtain. Looking at the Eimac transmitting tube manual there are specs listed there to run the tube in Class C CW but not for phone operation. Looks like a very easy tube to grid drive.

Any thoughts?

Al VE3AJM

[Opcom]

Not me, but at 3KV it makes >2KW according to the NJ7P site, so if it will run well at 1500V it should be straightforward. That is a rare and costly tube.

I wonder why the Eimac sheet shows only 1770W output though. (besides less current, etc., so what's the deal? Eimac usually gives specs that push the device to maximum ratings and output.


Eimac - 8295A
Class C Amplifier
Plate Voltage ................................. 3000 V
Grid No. 3 Voltage ............................ 35 V
Grid No. 2 Voltage ............................ 500 V
Grid No. 1 Voltage ............................ -200 V
Grid No. 1 Current ............................ 10 mA
Plate Current ................................. 820 mA
Grid No. 2 Current ............................ 42 mA
Driving Power ................................. 2.1 W
Power Input Power ......................... 2460W
Useful Power Output  ......................... 1770 W


NJ7P - PL172
Class C Amplifier
Plate Voltage ................................. 3000 V
Grid No. 3 Voltage ............................ 75 V
Grid No. 2 Voltage ............................ 500 V
Grid No. 1 Voltage ............................ -175 V
Grid No. 1 Current ............................ 24 mA
Plate Current ................................. 900 mA
Grid No. 2 Current ............................ 56 mA
Driving Power ................................. 4.8 W
Power Input (my edit) ......................... 2700W
Power Output (approx) ......................... 2170 W

[VE3AJM]

Interesting on the Eimac ratings. I do have a HB amplifier that uses the tube and some spares. It runs 3kv on the plate of the tube. The amp was built by Fred Hammond and is very well built. A huge Johnson roller inductor and a 15kv rated vacuum tuning capacitor. The power supply appears to be able to handle it being used to supply B+ to a modulator. It uses a passive grid input circuit using noninductive resistors. Perhaps bias the finals off into greater Class C operation for AM plate modulated service?   Al

[KL7OF]

I am looking to purchase a pl172....anyone have one for sale or trade?  TNX

[Opcom]

I don't know the tube from experience, only read data sheets on that one and try to understand what they really mean for an application.

The big difference looks like the drive and bias and therefore plate current, so maybe Eimac didn't publish the full-blown capabilities for the tube due to "ham reasons", such as FCC pressure. The professionals wanting maximum power would use the tube performance calculator or call Eimac, I guess.

It was rumored the published 3-500Z grounded grid ratings were toned down because of FCC pressure but I have never seen the "high power" data sheet for it. That is to say the max 3-500Z plate volts was 4000, yet it is interesting that Eimac showed exactly 750W PEP for one tube at 3000V. So, where's the "4000V" PEP rating? Did they dare not say it might do 1KW PEP? on ssb, not claiming a CCS rating.

- this same phenomenon may have happened on the Eimac PL-172 data sheet except with plate current, but wherever N7JP got his info didn't respect the limited power scenario so he shows the 900mA specs. The PL-172 is rated 1A plate current, per Eimac.

820mA makes 1770W - 72% efficient - Eimac

900mA makes 2170W - 80% eff. -N7JP
if it were 72% efficient- then P=1940W

1000mA makes  - 2400W - 80% eff. -Patrick's wild assumptions
if it were 72% efficient- then P=2157W

There is a discrepancy -  the efficiency. Could the Eimac ratings be lower due to the reason of operation for lower harmonics, or that they take into account the output circuit and the other specs do not?

The higher the bias, the narrower the angle of conduction.

The higher the peak current (till saturation), the better the efficiency (I think - comments?)


This is all pretty trivial, It sure looks like enough tube for the job. One question, does the suppressor need any special modulation applied to it?

You have an existing amp, can you temporarily modify to class C (to realize maximum power) and use it for a test?

[VE3AJM]

When I get this amp going, I'll let you know. As is, it will apparently run in class AB1 or class C CW. The amp was taken apart to get it moved here. I may have more time for this this coming autumn.
My intent is to get it going as is and eventually have it loaf along on phone at 1kw DC input. We're working on it. I have a modulator built up already using a pair of 5868 triodes.

Al