4-400 grid tuning

[W9BHI]

Would a small manual antenna tuner have enough range to directly tune the grids of a pair of 4-400's?
I want to use a flex 3000 as the driver.
Thanks,
Don W9BHI

[KD6VXI]

That would depend on your grid swamping resistor,  I would think.

A 100 ohm grid resistance,  most likely.   A 10 ohm grid resistor,  maybe not.

This is assuming grid driven.

--Shane
KD6VXI

[W9BHI]

Maybe I didn't state clearly what I am trying to do.
I want to use the Flex as an exciter for a class C plate modulated RF deck using a pair of 4-400A's.
Yes, it will be grid driven.
Don

[w4bfs]

Maybe I didn't state clearly what I am trying to do.
I want to use the Flex as an exciter for a class C plate modulated RF deck using a pair of 4-400A's.
Yes, it will be grid driven.
Don

this method needs to be quantified and put in one of the forum reference sections:

***  this is a rough approximation and ignores reactive effects ***

from the 4-400 data sheets plate modulated service: (per tube)

Ep = 290Vp which is 200V average
Ig = .012A
Zg = Ep /Ip = 200 / .012 = 17k ohm
and Pg = Eg x Eg / Zg = 2.35 W

these assume perfect matches and no radiation loss

so 2 tubes will adjust Ig to .024A and drop Zg to 8.5 k

so at this point you need to know how much driving power is available .... If you have an excess then your job is much easier ... if you only have the minimum (5 W) then you will need to use something like a link coupled step up match similar to what K1JJ put in the 813 x 813 transmitter .... you will have to neutralize and that negative feedback will reduce stage gain so will require more drive for proper operation... I would plan to have more rf available

so lets assume you have 25 W available then the matching options ease up .... lets see how low we can take the combined control grid impedance with this much power available

Pg = 5 W and Pswamp  = 20 W so swamping resistor value = Eg x Eg / Pswamping = 40000/20 = 2k

so you are now in the tuning range of antenna tuning units (but not by much !) ... I believe it would be better to use the link coupled arrangement (aka K1JJ 813 rig) with its 10 k swamping resistor as the bugs are worked out

[W9BHI]

I was thinking of using a small 100watt LDG auto tuner so I could  just recall the preset for
a particular band/frequency.
The Flex will supply a 25 watt carrier for drive.
Would a 4:1 unun be of any help?
Thanks,
Don

[w4bfs]

I was thinking of using a small 100watt LDG auto tuner so I could  just recall the preset for
a particular band/frequency.
The Flex will supply a 25 watt carrier for drive.
Would a 4:1 unun be of any help?
Thanks,
Don

you could try loading the tuna then the unun then a 2k 25W noninductive resistor and check all bands you plan to use

if you get a good match and a heated up resistor, you might be in business

[W9BHI]

OK, thanks for the info.
I will do some experimenting.
Don

[AF9O]

Don,
You don't need much power to drive the 4-400s, BUT you need enough grid voltage to well exceed your negative grid bias.  My 4-400 class C amplifier control grid is biased at -225 volts.  The exciter is a Elmac AF-67 with pi-net output of about 30 watts but low RF voltage.  The solution for me was a 6dB T-Pad Attenuator in series with a stepup link-coupled transformer with tuned secondary to get the RF voltage up and drive the 4-400 grid positive so I could get the 12 ma of grid current; all attempts to do this with a common tuner failed.
73
Eugene/AF9O