RCA BTA-1M on 75 Meters?

[w8khk]

Has anyone ever converted an RCA BTA-1M or 1MX to 75 Meters?  If so, I am curious how they accomplished neutralization.  I do not expect the stock transformer neutralization method to scale to 4Mhz. 

I have considered altering the final to a push-pull design with balanced bridge neutralization, but I thought I would see if someone had a simple solution.  Parallel 833s work ok on BC and 160, but not sure if that is a good idea at higher freqs.  Thanks for any info or tips!

[w8rpw]

Just saw your question. I just finished a Gates BC1G, also two 833s in parallel. I did not trust the grid neutralization scheme and installed a double ended tank ckt with plug in coils so I could do the " normal criss cross neut" left them in parallel to try. surprise, surprise,   the grid neut worked fine. changed the plate tank to a pi l with two vac variables and it even goes to 40 meters!  unbelievable with those 18 inch long leads.  I used the old B&W B series grid coil and the original gates tuning cap.  Bernie W8RPW

[w8khk]

Hello Bernie,

Thanks for the reply and info.  I was not able to find a schematic or manual for the BC1G, but I did find an online copy of the BC1T schematic.  I have read notes that the 1T is very similar to the 1G, except for cosmetics and control circuits.  I do not know if that is accurate.

I looked at the BC1T and see it has a center-tapped grid coil, with the neut cap from the plate of the 833s to the end of this coil opposite the grid connection.  If this is the way your BC1G is configured, I will try this method for 75M on the RCA BTA-1MX.  Certainly easier than going with the balanced push-pull plate circuit.  I have already ripped out the transformer neutralization used on BC and 160, so it should be relatively straightforward to implement the capacitor neutralization with a balanced grid circuit.

Does your BC1G use parallel 807s for drivers?  If so, do you know how much grid current you run on the pair of 833s?  That info would help me to validate the driver configuration before trying to load it up.   Thanks!

73, Rick

[k4kyv]

Where did you find the online BC1-T schematic?  I think Grant, W4BVT was looking for one.  Mine is mounted on a large poster board and would be a PITA to scan or photocopy.

The stock neutralisation scheme holds perfectly on mine from one end of 160 to another, but I have never tried it out on any other bands.  If I wanted to work multiple bands, I would probably convert the transmitter to push-pull, using plug-in coils, variable link coupling and cross-neutralisation.

I replaced the stock bakelite PA grid/driver plate coil with a piece of air core inductor stock of approximately the same dimensions, and that alone achieved about 35% more grid drive on 160.  I was able to get the tap in a correct spot so that neutralisation occurs with the variable cap about mid-scale, on the 2nd or 3rd try.

I have built homebrew triode finals with grid neutralisation, and never could get one to neutralise well, even after taking every precaution with symmetry, shielding, grounding, etc.  I think Gates did it very sloppily with the 1T, but somehow they managed to make it work damn well.  Even with perfect neutralisation, mine was a little unstable at first, but I improved it by replacing the 18" long #14 wire with a piece of 1/2" wide copper strap, and relocating all the rf ground connections associated with the driver plate/PA grid to ONE spot on the chassis.

[w8khk]

Hi Don,

I found the BC1T manual, with schematic, on the BAMA site.  I tried to get it from the BAMA mirror, but the download produced a zero length file.  The download from BAMA took almost half an hour for the 5.5 meg file, bc1tgates.pdf.  The schematic is spread across several pages, but is very clear and usable.  If you or Grant would like a copy, and have difficulty downloading, I could email it.  I was surprised to see how similar the Gates audio drivers are to the RCA BTA-1M.

I am still not clear whether the BC1T is similar to the BC1G.  The schematic looks very similar to the BC1H that I helped Bill Boyd install at WABV in Abbeville, SC last May.  Unfortunately, that station went dark in July due to lack of advertizing funds, and I believe the BC1H is now up for sale for $1,500.  I was impressed at how clean the unit was, looked almost new!

I recall reading Bill Orr's writing on neutralization.  He recommended that a single-ended triode be neutralized by using a split stator balanced tank in the plate circuit.  He said that split stator in the grid circuit, with a pi or pi - L in the plate circuit is typically unstable and very hard to make work reliably.  That is why I considered going push-pull instead of balanced grid only.

I am looking at making this rig work only on 75M, so I may still try the balanced grid approach.  I am using a split stator breadslicer in the plate circuit, because it is what I have with sufficient capacity and spacing.  If I build the balanced grid circuit, I could always add the balanced plate circuit if the former does not work correctly.  I think I can implement the hardware physically to allow the original plate tuning knob to turn the breadslicer, using one right-angle drive and a gear reduction at the cap.  I plan to put the loading knob just above the existing power control variable resistor that adjusts the amount of grid leak bias to the final.  If I change to push pull with link coupling, the loading cap will be the capacitive reactance in series with the output link.  This BTA is so modified already there is no interest in keeping it "stock".  But I do want to mod it in an asthetically pleasing manner.

I am curious how much grid current you run on your Gates to handle modulation peaks, if you measured it.  RCA used a funky "percentage" meter for all functions except plate voltage and current, filament voltage, and RF out.

I will definitely replace the grid coil, as the one in the unit is way to large in turns to be practical for 75.  It is interesting to hear that the bakelite was so inefficient in your rig.  Any other pointers you have to offer are much appreciated.

73,
Rick

[W8IXY]

Having worked on several Gates BC1G's over the years, the PA grid current (all I worked on were above 1.3 mHz) was specified at 100ma.  However, I used to run it up to 150 ma and the thing modulated better, with less IM and THD distortion.  Also tuning the PA output circuit about 10 to 20 ma off the dip, in the direction of higher output, also resulted in a better THD and IM distortion spec.  Another thing, getting the neutralization right also greatly affected the linearity of modulation as well.  The driver 807's are modulated about 20% to allow for the greater grid current peaks at higher mod percentages.  You can "play" with the grid leak resistor across the 833's grids to ground to also affect the PA grid current as well as the modulation linearity.  (Make sure you adjust the balance pot on the audio driver board for minimum THD).   When you "get it all right", you may be able to get the THD down to the 1% level, and the IM distortion to less than 3%.  Those 833's sure sound good when properly "treated".

73.
Ted W8IXY

[w8khk]

Thanks for the details, Ted.  Those numbers for the Gates should also work for the RCA BTA-1M.  There is no balance control in the audio section of the RCA.  It does have push-pull inverse feedback from the plate of each 833 back to the cold end of each winding of the input transformer. I will be testing the audio separately with no RF fired up.  The RCA does have an "Output Power" control on the front panel, which is a variable grid leak in the final.  Should be easy to adjust with the help of a scope.