Is the 4-125A just lame, Or is the 4-65A seriously underrated?

[KI4YAN]

So, working on my 6 meter reciever/transmitter projects has rekindled interest in the Big'un. I finished the exciter and started on the power supply and the cabinet years ago and so I puttered around on gathering up the parts that were spread all over the shop and house, just getting them in one spot. Kinda needed to do that so I had room to put the engine in my pick-up truck soon anyway.

Went to look at the data again on the 4-65A and 4-125A tubes, double-check the recommended op points, and such-making sure I'm not building myself into a corner-and I find the Class-C Telephony recommended operating points for 2000V plate supply.

4-125A says at 2000V plate voltage, 150mA plate current, and 350V screen voltage, it'll run 300 watts input for 75W plate dissipation. In telephony, the plate dissipation limit is 85 watts, here it's set up at 70 watts. But then it claims only 225W RF output power.

Now, looking at the 4-65A datasheet for Class-C Telephony, I find the following:

2000 plate volts, 120mA plate current, 250V screen volts, and it'll run 240 watts input power, dissipate 40 watts on the plate, and put out 200 watts RF output power.

Both tubes are limited to 2500 plate volts when used for telephony, and the 4-65A will do 225W output at 2500V and the 4-125A still only manages 300W output.

Is the 4-125A just a dog, or are 4-65A's just real heros?

[Opcom]

That's interesting. Is one spec giving the figures for carrier conditions and the other for 'single tone' output?

[KI4YAN]

Both claim to be "Class C Telephony, values given are for a single tube, high-level plate modulated"

https://frank.pocnet.net/sheets/140/4/4-125A.pdf

https://frank.pocnet.net/sheets/140/4/4-65A.pdf

That's the data I am using. (Or was-I think that's a slightly different 4-65A sheet, it only calls out 195 watts in this one.)

[kb3ouk]

Never seen two of them side by side to tell but I always thought the 4-65 and 4-125 plate looked very similar, and wondered if the extra dissipation on the 4-125 wasn't mainly just from a better design for airflow around the tube to allow for higher power.

[w4bfs]

a 4-65 is a good deal smaller than a 4-125 with the 4-65 being just a bit bigger than a 6146 et al .... the 4-65 has a 24W fil and a 4-125 has a 35W fil so in that sense the 4-125 is more emission limited than a 4-65 ....a 813 has a 50W fil for its 125W dissipation plate

I did some emission testing on a 4-125 a few years ago and observed a Collins am tx using a 4-125 at 300W out with a red plate and all seemed in order with the owners statement that he got good life out of the 4-125 ... that tx had 3kV b+

if both tubes have the hr6 plate radiator and unimpeded air flow the 4-65 does not require forced air and the 4-125 only needs 2 cfm

always remember that the tube specs were for a 1000 hour minimum operating life with more life available at lower power and better cooling

[WBear2GCR]

They get bigger incrementally.

The next two steps are 4-250 and then the 4-400.
You can generally speaking put a 4-250 in the same hole as a 4-400... etc.

The 4-65 has a not terribly big plate. Smaller envelope too.

No dogs.

                _-_-

[Opcom]

It is my opinion, which I am not prepared to substantiate at this time, that both of the tubes in question as well as others built in the same way do not reach their maximum potential until the highest voltages are used.

AM carrier at 2KV: 4-65 = 195W (780W pep)
AM carrier at 2KV: 4-125 = 225W (900W pep) = 115% x 4-65 output

AM carrier at 2.5KV: 4-65 = 230W (780W pep)
AM carrier at 2.5KV: 4-125 = 300W (1200W pep) = 130% x 4-65 output

If the 4-125 could operate at almost twice the voltage of the 4-65, the situation might be moot.

I agree with the opinion that the filament current (and emission which is linked to it) is one reason for the apparent discrepancy.

[W3RSW]

There's a beautiful 4-65 driven by a 5763 all band transmitter in the 1957 ARRL handbook, w/ clamp tube and several vfo, doublers, tripled and buffer circuits along with Electronic keyer.
"75 to 300 WAtts vfo Controlled". By w8etu in those days.

Power supply Xfor is 4500 vac at 200ma, 866a's, single choke /following cap filtered w 200k 50 watt bleeder.

Final pi net coil cap constants are chart given for 600, 1k, 1.5k and 2 Kv.

Interestingly the handbook's next step up is a very similar 813 driven by a 6146 and labeled "A 500 Watt Multiband Transmitter."  A 4-125  version is skipped. Of course all these are plate input ratings.

So if you already have a bunch of 4-65's , a pair in the final looks promising.

[KI4YAN]

I currently have (4) 4-65a and (2) 4-125a, and the plate iron will do 2300vac at 440ma, CCS. It is a non-centertapped winding, so will have to be bridge rectifier. I have (5) 115uf, 2300v DC film-in-oil capacitors, but I don't have a filter choke or swinging choke yet.

I did get a Simpson model 29 meter face re-labled from 0-20kv down to 0-2.5kv today, and got a little more work done on the 6meter reciever-it is about ready to button up!

[W3RSW]

Helpful to look at the last of the potentially no fan or blower series, the 4-250.
https://frank.pocnet.net/sheets/140/4/4-250A.pdf

Note that in the cooling remarks, if the tube is mounted correctly and at frequencies below 30 MHz and at lower duty cycle, natural convection is fine.

Note also the ratings at 2500vdc compared to the 4-65 and 4-125 although the 4-250 is rated at 3kv.

Now plot the plate power input or power output for the three tubes at 2500 VDC.

These are rugged tubes designed for CCS and as such progress in a conservative order.
Slight blower air up through lower seals from chassis of course will insure longer life, better duty cycle and all that.

With today's armada of fans and blowers, scarcity of tubes, etc., as several have mentioned, it never hurts to put forced air on all our creations, be it a fan on even an 813, a rugged a tube as was ever built.