The AM Forum

Well I have spent the last couple days mesing around with the RCA 1 Kw in preparation of the freq move from 720 KHz to 80m.One thing I want to do is drive the TX with an external carrier feq generator and I was messing around with an RF signal generator and was actually able to put the thing on the air using the sig gen to drive the grid of the 6146B driver.This was just trying to prove an idea and I would in no way use a sig gen on the air LOL. I found the drive requirements to be MUCH lower than I had expected and was wondering about driving the 4-400's directly.The exciter will be a Kenwood TS-820S in the TUNE mode.In the tune mode the rig will produce from 1-20 watts output depending on the position of the carrier control.I was just figuring that if all I needed was a couple watts to drive the 4-400's then I would dispense with the trouble of retuning the driver stage etc. I would imagine a single 6146 in CCS service would not be producing that much power.I could run around 5 watts and pad it down into the driver stage but driving the finals directly would be much better I would think.So has anyone built up a HB plate modulated 4-400 rig or driven a pair directly and if so how much drive was required? The TX runs 3100 volts and about 350 mA for the full Canadian legal limit of 750 watts carrier.

I don't know how far off base this is but I remember a pair of 4-400's in a rig I had and 20Ma of drive current
Fred

Actually that is probably close. The spec calls for 17-20 mA grid drive for 1000 watts O/P. I suppose I could calculate impedances and figure how many watts that would require.

I rebuilt an RCA BTA-1R1.  Did it in a couple stages.

First - keeping the RCA fairly stock - I replaced the original tuned circuit of the 6146 driver with an LC tuned circuit for 75M.

Second stage (a complete repackage of the BTA-1) I added a link coupled bandswitched circuit to the PA grids and drove it with a Ranger.  I recall the Ranger would run 30 to 40 Watts output to drive the 4-400s to 18mA grid current.  Worked great.

Thinking about other similarly tubed rigs, all have pretty much the same topography.  T-368 with single 4-400 uses a 6000 driver - appx a 6146 in horsepower.  Viking 500 with single 4-400 uses 5763 - abt 1/2 a 6146.

The other things these rigs have in common - mainly because they will operate CW - is that they use a clamp tube on the PA screen to clamp the PA off when there is no grid drive.  I had some T/R sequencing issues when using the outboard Ranger in that the RCA would be 'ready' before the Ranger supplied drive & the RCA's PA would occasionally trip the overcurrent relay. I it by adjusting the sequencing rather than cramming in a clamper.

Have fun!

73, Bill   N2BC

So Bill, if I understand you correctly, you scrapped the 6146B driver stage altogether and drove the 4-400's directly via a tuned input? That is sort of what I was thinking about doing. The TX BTW is an RCA BTA-1S.My plan was to either drive miliwatts into the driver stage or bypass it and drive several watts into the PA directly.I was surprised at the low drive requirements into the 6146B. I could maintain the TX on the air with an RF sig gen as the carrier generator. The big problem is how to mount the vacuum variable tune/load caps I plan to use. The RF cabinet is not friendly to components being mounted so that they can be adjusted from the front.Oh well if I sit and stare at it long enough something will come to me.Unfortunitly it will probably be my wife telling me to go the heck to bed! LOL.Getting the freq conversion done is only half the project.The other half is the TR sequencing,antenna changeover,RX muting  etc. and remote operation as the TX is in the basement and I plan to have PTT control from the shack directly above it.I can go to the basement to turn it on and tune it up on freq but I want PTT from the operating desk. I am also redesigning a cable access point in the wall behind the operating position and need to accomodate the extra plugs and wiring required for this project.The panel is getting pretty crowded already.

Quite similar to what I did initially except the shack is in the basement and the RCA was in the garage.  I drove the buffer stage with a PC card oscillator.  The PC at the RCA also had a bunch of relays controlled via the printer port and I wrote some software to control the oscillator and the relays.  The garage PC was linked to the basement PC with 100' of null modem cable, quite the kludge.  Could do everything except tune it.  I had planned to add motors to the vac variables but ended up rebuilding the entire thing into a standard rack - in the basement!

I'm not sure if the S uses the same cabinet as the R... but I was able to mount the vac variables on the inner front plate and turns counters on the outside front cover. They were connected with fiberglass shafts and universal joints to handle the offsets.

I did away with the PI-L configuration, scrapped the huge PI coil and used the original L coil only.  I may have some pics of the layout if you want them.

The R and I am sure the S are setup to allow remote operation of almost everything but I did all the basic control stuff with the PC controlled relay setup and used a multi-conductor cable to bring the metering down to the shack where I rigged a small remote panel with Grid I, PA I, Mod I, and HV monitoring. 

One drawback to the remote operation was getting interrupted one day by one of the kids screaming that it was on fire! Turned out it was a coil on one of the relays billowing smoke.  At this point my motivation to bring it into the house increased a bunch.

I you would like some pix, send me your email addr:  n2bc@stny.rr.com


73, Bill

I pull about 40ma grid drive to a pair of 7527A Amperex (similar to 4-400) in a 1953 homebrew rig.
E_p is around 2200V, and running a B&W turret-style bandswitched grid input tank.
I'm using a separate screen supply with about 30hy series inductance to improve modulation. 
Modulated by a pair of 810s, and the final tank is the HDVL swinging-link setup.

The Eimac spec for grid drive for the 4-400A in class C plate modulated service is 12 ma.

In my homebrew pair of 4-400As plate modulated by a pair of class B 833As, I run 28 ma of grid drive. This is developed across a 9000 ohm grid leak resistor with -120 VDC of fixed protective bias through a steering diode, and this arrangement provides -252 volts of grid bias. Eimac further recommends -225 volts of grid bias for class C plate modulated service, but I obviously drive the tubes just a little harder than they suggest. I run 2600 VDC on the plates of the HPA and modulators.

I think you will find that about 15 watts of drive will be adequate to drive a pair of 4-400As in class C plate modulated service, and this leaves some reserve for circuit losses, etc. In my transmitter, I have an Amperex 8505A beam power pentode that I use as an IPA to drive the pair of 4-400As in the HPA; this stage takes about 750 mw input across 50 ohms to drive the tubes to 25 ma of grid drive. If I did not have this IPA as part of the amplifier configuration, I would measure the power input across 50 ohms for you that develops this drive level.

Good luck with your project!

73,

Bruce

I recently finished converting an RCA BTR 1R1 to 75 and 160.  I use a
solid state transceiver cut back to about 20 watts output as an exciter.  This gives me a digital readout for frequency and solid state stability on the vfo. The rf from the transceiver is fed through a small antenna tuner into the grids of the 4-400's.  Very basic, but it works quite well. 
   As far as output from the RCA, I use a simple pi-network with air variables (no vacuum caps).  The loading and tune variables are mounted to a metal plate that is then bolted inside the top of the transmitter cabinet.  The control shafts for the pi-network variables are extended out through the front of the cabinet above the door.  This requires drilling a couple of holes in the cabinet for the shafts to pass through, but doesn't detract from the overall appearance of the RCA.
     Band switching is accomplished by a heavy duty rotary switch that is mounted with the shaft extending into the tube compartment.  Changing bands is only a matter of opening the front door, rotating the band switch and closing the door.

The more I look at it the more I think I will bypass the driver stage altogether and drive the 4-400's directly. The driver will be a Kenwood TS-820S and it will provide from 1-20 watts in the tune mode depending on the position of the carrier control.I figure a small Pi network on the input of the grids is just as easy as messing around with the driver and the weird way it is tuned by adjusting turns on the plate choke. I'll install an input Pi network as I may drive it with my FT-857 if the Kenwood takes a dive.I did some studying last night and I have a plan that will see a pair of Jennings vacuum variables installed as tune and load caps as well as using an existing coil as the tank coil but moving it slightly to make room for the caps. I can mount the caps inside the RF cabinet on the inner front panel and extend the shafts through to the outer front panel where the turns counters will be mounted.The shafts will pass between the tall fiberglass tube chimnies.Well time to go play with it some more since I am off work today. I think I'll go and see why the power cutback does not work. It is installed but is not working and I would like to cut the power back to 250 watts while messing around with it.I could just hardwire everything over but that is a PITA.I think it is just a problem with the control voltage.I probably left yet another jumper out somewhere.  ???