|[AMRadio] Eico 730 Modulation Transformer|
ars.w5omr at gmail.com
Tue Nov 12 21:11:44 EST 2013
On 11/12/2013 06:33 AM, Jim Candela wrote:
> Take a look at this thread:
> Several folks took a stab at a question similar to yours. This in my opinion is worth printing out and reading over a few times. Bottom line is you want a turns ratio such that the modulation capability hits a desired target. If that makes a plate to plate load impedance that is low for the tubes in the modulator, then either readjust your RF operating point (load to a deeper dip), or add another pair of modulator tubes. Don't let impedance matching be the main concern; instead concentrate on turns ratio, and then deal with the impedances if need be.
Adding the second set of tubes is what led John/WA5BXO to using (4)813's
in push-pull, parallel.
At the time, he was running either a single or a pair of 304T's (H or L,
he can't remember), but it was all the Collins 32V-2 exciter could do to
provide enough drive.
He was running high I(p) and low B+... something like 1,346V @ 743mA for
as close to 1kW DC input as you could get. This was back before the
1500w PEP power limiting law took effect(*). The Z in the final was
low. The Z of the modulator (pair of 813's) reflected about 2x the Z of
Ronny/WA5GCF (got caught ~what~?) said something along the lines of
'What happens when you parallel resistors?' (to break it down in simple
terms) Add two more tubes, John!
It was an epiphany! (or, to use typical East Texas slangology...) It
was a Pregnant idea! ;-)
Thus, the (4) 813 frankenmodulator was built. Somewhere in time,
WA5BXO/John and K4KYV/Don hand-wound a ~40Hy choke @ beau coup current
to use a modulation reactor for that modulator.
(real names have been used in to incriminate the guilty)
(* = [[For you new guys, we -USED- to be able to run 1kW DC INPUT to the
final. THAT was "Legal Limit". At 100% modulation with a 1kHz tone,
that meant we -were- running ~3kW PEP (assuming 75% efficiency in a
Class C final) Legal limit now is 1500w PEP *output*, reGARDless of
Carrier output power!]] )
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