I just went through this with my 4 x Drake L4B's.?
I used 3 x new Gigavac GH1's.? ?You can buy em with either a 12 vdc coil or a 26.5 vdc coil.? ?Jennings version is called an RJ1A. The kilovac version is called a HC-1.....and the coils in all 3 x brands are field swappable to either a 12 / 26.5 / 115 vdc coil.? ?The Gigavac uses metric threads on the big lockdown nut.
I used em for input / output / bias.? ?In my case, I used the 12 vdc coil version.....and wired all 3 x coils in series.? The oem drake fil xfmr also has a 22 vac winding on it to run the oem 3PDT clunker TR relay.? Drake used a 1/2 wave rectifier, so the output was 31 vdc no load.? ?In my case, I changed that to a FWD ( full wave doubler) to obtain 63 vdc no load.? ?A 175 ohm @ 25 watt metal finned dropping resistor is used in series with the 3 x series coils. 36 vdc used to operate the 3 x series 12 vdc coils.? 27 vdc is dropped across the 175 ohm series drop resistor. Drop resistor dissipates 3.9 watts on TX....and 0 watts on RX.?
IE: when T=0, the relay coils get hit with an initial over voltage. (63/3 or 21 vdc per coil)....which will? speed em up.? To speed em up even more, a 50uf @ 150 vdc lytic is wired in parallel with the 175 ohm drop resistor.? This is called a ...'RC hotshot' circuit. Jennings, Kilovac, Gigavac all mention it's use.? ( Alpha used it on their older amps, that used a vac relay.? Alpha used 36 vdc applied to the 26.5 vdc coil, and also a small lytic across the series drop resistor).? With the small cap in parallel with the drop resistor, it sustains the over voltage a hair longer.? The concept works superb.? ?All 3 x vac relays in each drake amp operate in slightly under 1 msec.?
80% of the total operate time is spent just building up the magnetic field in the coil. (The XL of the coil impedes DC current flow).? The over voltage? makes the current flow faster.
The ONLY thing that needs to be sequenced is the RF output from the xcvr itself.? You want to key the amp NOW,? but delay RF from the xcvr by 1-2 msecs.? This gives the sped up vac relays in the amp a head start.?
Modern xcvr's have an adjustable TX RF output delay...which works on both CW + SSB.? ?My yaseu MK-V has an adjustable 0-30 msec delay, ( in 1 msec increments) to prevent hot switching? the slow 3PDT mech relay in the older tube amps. The drake 3PDT operates in 17 msecs.? ?The 3PDT in the ameritron amps operates in 15 msecs.? ? The delay in the MK-V was set at 20 msecs to ensure the oem drake relay got a head start.? BUT the adjustable delay in the MK-V only works on CW.? ?For SSB use, plan B.... which is to re-use the spst tiny relay in the MK-V? (was used to key an amp)? to instead GRND the TX inhibit lead in the MK-V.? ?UNTIL u grnd the TX inhibit lead, you can key the amp, but zero RF coming out of the xcvr.? ?To actually key the relays in the amp, I use an opto isolator. The input of the opto is keyed? via the 'electronic TX grnd'? from the MK-V.? Input of opto only draws a few ma.? ?You can get em with a 5-15-24 vdc input ( and each of those has a super wide operating range)..and the output side is rated for either? 60 vdc @ 3 amps CCS...or 250 vdc @ 1 amp CCS.? 5 kv? isolation between input side and output side.?
You can also use a 2nd opto to do the bias in the amp, which works good.?
Ok, back to the henry 2K-3.? ?The small relay? xfmr in the 2K-3 is 8.5 vac on either side of the CT.? ?It's a FWCT config.? Output is 8.5 vac x 1.414 = 12 vdc.? ?I would reconfigure the small relay xfmr to a FWB..and use the entire sec winding ( CT not used).? ?Then it becomes 17 vac x 1.414 = 24 vdc no load.? ?Ok, then? wire the 3 x 12 vdc vac relay coils in parallel.? ?Each 12 vdc coil is 80 ohms.? ?12 vdc / 26.67 ohms, ( 80/3 = 26.67 ohms)? ?= .450 amps.
The series drop resistor will be 26.67 ohms.? Diss in the resistor will be (.45 x .45) x 26.67 ohms = 5.4 watts.? ?Each relay coil will diss? 1.8 watts.? Each coil has 150 ma flowing through it.? Wire a 50 uf @? 100-150 vdc (axial lead) lytic across the drop resistor.?
Depending on xcvr used, it has to handle? 24 vdc OCV (open circuit voltage)? @ 450 ma under load.? You do NOT want to use a slow mech relay in the xcvr to key the sped up vac relays!? ?Hence the use of either an opto isolator or a fast switch transistor.?
Ok, the 3 x relays will operate / rls in just a hair < 1 msec.? No hot switching.
I found a sidewall on the L4B, that would take all 3 x vac relays.? ? A 9/16" hole is required.? ( A 5/8" will also work).? ?The greenlee 1/2" -? 9/16"? - 5/8"? punches all use a 1/4" draw stud....and hence require a 1/4" pilot hole.? I used my roper whitney hand punch to punch the 3 x 1/4"? pilot holes....bam, done.? Then the greenlee 9/16" punch (die and cup, just tighten the wrench).?
You could use just 2 x vac relays...for input / output.....and an opto for the cut off bias.? That would save one vac relay.? ?You could maybe, use a super fast SPDT tiny mech relay for the input side.? I wanted vac relay on both the input and output, so the amp will handle 2 kw on bypass mode.? ?I do that, since I string several drake amps in series, one per band, or one at each end of 80m etc, or any other combo.? A simple rotary switch in the key line of the xcvr ensures only one amp can ever be keyed at one time.? ? ?On the drake L4B, the only way to calibrate the internal wattmeter is to take it apart to get to the underside of chassis....and apply 1 kw cxr from a 2nd amplifier ( with a bird 43)? to the drake amp...which of course is powered down, and on bypass.? IE: it has to handle a kw on bypass mode anyway.?
You could mount one vac relay near the input coax connector, and a 2nd vac relay near the output side....or near the load cap.? Then a short length of RG-400 between the NC contacts of the input + output relay. ( aka..the 'bypass leg' ).? ?
All 3 of my vac relays are dead quiet.? ?The gigavac GH1 hi -pot tests to 7.5 kv.?
The idea of using a 12 vdc coil came from the eng dept at Kilovac, during a phone call I had with em years ago.? "IF u are voltage limited, use the 12 vdc coil".?
There is also another method to obtain 28 vdc in ur henry amp.? It uses a 10 vac fil xfmr ( with fils in series).? ?Tap off the 10 vac sec...and use a FWD ( doubler)? to obtain 28.28 vdc.?
Without the speedup circuit, the vac relays typ operate in 2-3 msecs.? ?In the past, folks would use a semi complicated sequencing mess...so the output relay would operate 1st.....like say 3 msec.? ?Then the input relay next, say 5 msecs...then the bias relay last....say 7 msecs.? When going back to RX, the rls sequence was reversed, with bias relay releasing 1st, then input relay, the output relay last.?
Simplify it....just operate all 3 of em at once ( even if u don't speed em up....and all 3 operate in 3 msecs) .? The only thing that has to be sequenced is to delay grnding the handy dandy? TX inhibit lead in the xcvr.?
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Sri? for the diatribe.?
Jim? VE7RF
