The AM Forum

I'm trying to help a friend with his project. At 3KV, what is the true peak or single-tone wattage needed to drive one 3-500Z on HF?

The two Eimac data sheets here seem to disagree about the wattage required to drive the single 3-500Z in the cathode-driven circuit.

03-15-1968:
"R-F LINEAR AMPLIFIER GROUNDED-GRID CLASS B"
Single Tone Conditions:
Eb = 3000V
(Cathode Voltage not stated, assumed to be zero)
Zero-Sig DC Plate Current = 160mA
Po = 750W
"Max-Sig Driving Power" = 30W


08-05-1980:
"RADIO FREQUENCY LINEAR AMPLIFIER CATHODE DRIVEN"
Class AB2. Peak Envelope or modulation Crest Conditions:
Eb = 3000V
Cathode Voltage = +10V
Zero Signal Plate Current = 62mA
Po = 740W
"Maximum Signal Driving Power" = 46W

What gives?
More drive needed in 1980 data sheet because of the 10V bias on the cathode?

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I also submit that a pair of 3-500Z as a modulator takes only 25W grid drive to make 1420W, so one tube would make 710W with 12.5W drive. That is drive to the grid though. The same for the grid driven specs in the 1980 datasheet - 14W.

I think you're pretty spot on about it having to overcome the bias.

I say this as the two values are about correct for idling current.

Also, 30 watts into a 115 ohm load is going to be 58 volts.  46 watts will be 72 volts.  That's a 14 volt increase.

I believe the additional 14 volts of drive will be necessary to overcome the bias (10V) and drive the tube harder for the gain drop as you bias the tube more towards C.

Maybe I'm off in left field....  And yes, I know it's the power that drives the tube, not voltage.  But they are related.

--Shane

One data sheet is for Class AB2, the other for Class B.

Having built several amplifiers with a pair of 3-500Z's used as modulators in Class B, grid driven and a pair in GG cathode driven, drive requirements are different.

In grid driven audio service, 30 W is about what is needed to get about 800 to 1 KW of audio with a decent mod transformer with a pair driven in push - pull.  A little negative bias is required to keep the resting current down, perhaps -8 V or so, +/- 2 V depending on the tubes.

In GG cathode drive for RF, 60 to 80 W is needed for a pair to get the same power out due to the inherent negative feedback in the GG configuration.  Also, a small amount of bias is needed, usually with a few forward conducting diodes in the cathode return from the CT on the filament transformer.  Again, 6 to 8 V or so.

This is with 3 KV on the plates. For a single tube in GG RF service I would expect the drive to be about half at 30 to 40 W.  This is using a tuned RF input of course.

Hope this helps...  Chuck

That's what I was hoping for. 30-40W and no more. A pi network is what I had in mind. The project is limited by budget, and his only radio is one of those little backpack ones, 6-10W PEP I believe. BUT he wants the amp to be able to work with a desktop 100W rig when he gets one (attenuator on amp input is assumed for that).

I want to think about the grid driven method, but I've had bad luck with neutralizing a 4-1000 in class C, and not confident that I could make a 3-500 stable as a grid driven linear amp. once bitten..