Homebrew linear using 715C tetrodes ?

[F4GGA]

Hi ,
I've almost achievd my first RF linear amp using a pair of 715C. Why using this tube? I have more than 24 left NOS or in a good shape ... I wanted the exercise to be cheap , using lots of unusued parts from shack (microwave oven xformers and so on...) . Anyway, it's working !
My question is : what's the best operating point ? (300v on G2, 1,6kV on plates) , what max power ? in and out ? Any experience whith these bottles ?!
cheers, 73'
Pierre

[NY3V]

I have no experience with that tube.

Here are the RCA specifications for the 715C Tetrode >>>

http://www.mif.pg.gda.pl/homepages/f...049/7/715C.pdf

It has only 60 watts maximum Plate Dissipation, so the maximum Class AB2 maximum Power would be limited to less than 170 watts Output per tube.

[KM1H]

Sell them on FleaPay where they often bring a good price. Then find a tube that will be linear which the 715 family certainly will not be at any power level worth using.

There was a short flurry of use with them especially in rags such as 73 Magazine that quickly died out.

Maybe sell them in groups of 10 for CB amps

If you want to be different take a look at the 803/VT-106; ignore its published ratings and treat it as an 813 on steroids, they thrive on 3000-3500V but work well at 2000. The only caveat is they run out of steam above 20M. They fit a generic 3-500Z/4-400A 5 pin socket.

The 814 is the 813's little brother and totally ignored. The military VT-154 version is especially a sleeper, Ive picked up several NIB at $2-5 each even on FleaPay. A quad will make decent power in GG or swamped grid driven.

Carl

[F4GGA]

Thanks for your inputs I have some 803's ... for a future AMxmitter , the VT154 i 've had went away fleabey , one another linear is on the way with a QB5/1750 ...
and i 'm almost QRV with the twin 715B/C ... So i precise my question : How hard shall i push them (before smoke ) ?! 73'
Pierre

[KM1H]

Use as high a voltage as you have and keep the drive and plate current down. Load to full output and watch the anode color; a bright red key down should be OK when on SSB. Select components for a loaded Q of 10-12 at what ever plate load you end up with.
You might sound like an Italian but others will let you know soon enough


GM3SEK has an excellent Excel spread sheet on his site.
http://www.ifwtech.co.uk/g3sek/

Carl

[F4GGA]

Thanks Carl, i made it yesterday : almost 400watts , red dull plate on CW ...not as bright as your 304TLs I'll modify the input circuitry ... Stay tuned ! Are these 304TL usable in linear RF ? (and all the other sisters , aka 100TH, VT127A..) ...have some of these left too
Pierre

[KM1H]

Pierre, I use all those tubes but as AM modulators or Class C finals. The first KW amp I saw as a Novice in 1955 was PP VT-127A's as a 20M monobander. The anodes were white and the plate transformer was a "pole pig" so I suspect the 1000W INPUT rule was a bit extended

Those old triodes require lots of drive (and bias) and are rather inefficient in GG AB2 linear service. Look around for a couple of 6C21 pulse tubes, they can be pushed quite hard.

Im converting a 2 x 250TH Class C RF deck to 6C21's to take better advantage of the full audio capability of the PP 304TL modulators.

Carl

[F4GGA]

Hi HAMs !
I made lots of tests, mods and so on...result is : Not worth the effort For 100watt input, i "only" get 250watts .... I'll swap the 715C for a pair of 803 , after a rebuilding of the PSU'rack. GG or passive loaded g1 Carl ? Thanks
Pierre

[KM1H]

The 803 works well in GG Pierre and is very stable. In G1 driven it might need neutralization, depending upon frequency and construction.

Carl