Converting a BC500G for 80M & 160M by adding a separate 833RFA for 80M

[K2DUV]

I recently became the proud custodian of a BG500G that once was WRAR in Tappahannock, VA.  I plan to convert it from 1000KC (its assigned BC frequency) by first getting it to operate correctly at 1000KC and then moving it up slowly to 160M.   Once I have it running correctly on 160M, I plan to add 80M.  I would appreciate readers  comments on the following plan for adding 80M:

The BC500G only has a single 833 RFA but provisions are there for adding a second one if it is upgraded it to a  BC1G. I am planning to add a second 833 which will be the 80M RFA.  This 80M 833RFA will have its own RFC and Pi Network which I will fit on a shelf in the spacious cabinet.  Adding 80M in this manner will avoid having to switch antennas and high current RF. I will have to switch the 2000VDC from one 833RFA to the other using a vacuum relay.  I will also have to use a high current relay to transfer the 833 RFA filament voltage from one 833RFA to the other.

The 807 RF driver circuit will have to be modified to operate on both 160M and 80M. I would appreciate any available experience on how to change the 807 coil taps and tuning capacitor.    I believe other BC1G owners have done this successfully.

 It appears to me that when I switch from 160M to 80M, I can leave the grid of the unused 833  connected to the 807 drivers since I will have disconnected the filament voltage and the plate voltage from the unused 833RFA.  As soon as I have the transmitter back on 1000KC, I can try this out since I have an 833 with burned out filament.

If I proceed as suggested above, I should be able to leave the original 833RFA 160M tank circuit in place (with changed capacitors per the Gates manual.)  I might be able to convert the original tank circuit for 160M tank from Pi-Tee to simple Pi thus freeing up a rotary inductor or tapped coil for use on 80M.

I would  really appreciate thoughts on this approach.  Having the second 833RFA in place but with floating filament and no plate voltage may change the neutralization and tuning slightly, but I don't think any other effects will result (will they???)

I look forward to an enlightening response!!

Floyd K2DUV

[MikeKE0ZUinkcmo]

Hello Floyd,

I also got possession of a BC500 a few months ago, and plan to eventually put it on both 160 and 80, but using existing components and hopefully just changing taps and pulling fixed caps, as necessary.   It needs work of course, and it'll take a bit of time for it to come up in the queue.

I'll be very interested in your progress.

Mike

[K2DUV]

Thank you Mike.  I will make postings as I progress. 

By the way, my BC500G used a smaller plate transformer, modulation and filament  transformer than the BC1G.  I am certain of this with the plate and modulation transformer but not the filament transformer as I cannot find any numbers on it.  I understand that Gates made an upgrade kit for the BC500G but this must have also required that these transformers be upgraded as well.  Do you know anything about this?

Please tell me more about your plans to add 80M to your BC500G.

73, Floyd, K2DUV

[KL7OF]

Floyd, I don't know how enlightening this will be.......Will the filament transformer for your BC500 light up (4) 833's ? I thought they used the same filament tranny for the 500 and 1000 watt rigs.. The oscillator and driver tuning will have to be changed when swapping bands....The driver coil tuning taps could be changed with a relay but the oscillator is a different story...You could switch in an entirely separate oscillator for another band.  I have Mod transformers and chokes for both the 500 and 1000 and they have the same part numbers stenciled on them but the iron for the 500 is physically smaller ...All the iron I have is made by the same outfit..(electro something)..I have some other info on how I modified my BC1G....I will check it out later today when I am in my shack and forward it on to you...Good Luck...Steve

[W7TFO]

All the Gates G, H, T, and J had the same iron regardless of .5 or 1 kW that I've seen.

The 250's were different.

73DG

[K2DUV]

Thanks for the replies.  I think Dennis is correct that all Gates BG500G transmitters shipped with the same iron. Mine however has a different HV transformer and audio input transformer as shown in the attachment.  They may have been replaced during the active life of WRWR.

As for the multi-filament transformer that powers the three 833s, mine looks original but bears no part numbers.  In the attachment  I show the size of the transformer core laminations.  Perhaps someone with a BC1G can measure theirs and see if they are the same.

Thanks & 73, Floyd

[flintstone mop]

The older tube TX's usually cooperate well to get to 160M. 80M will be a challenge from all of the extra lead lengths within.
If you have the manual they are clear on how to move the TX to the top of the BC band and that is usually enough to get to top band.
Try to avoid to complicate the mission. BC500G is a nice TX,,,plenty of power with one 833 in the RF for 160.

[KL7OF]

I sent some scans of conversion data to Floyd and Mike.....Hope you boys can sort out the pages...I was having some trouble with Microsoft outlook and google chrome...They are NOT easy to operate...Good Luck  ......Steve

[W9ZSL]

We had a BC-1T at the station where I worked.  The owners were contemplating an application for 1KW day and 250 night.  For some reason they started with a basic 250 watt version and when the CP came through, it was converted.  As far as I know, the only iron that had to be replaced was the plate xfmr.

[W8IXY]

For the factory stock transmitters, the plate voltage for a BC500G was about 2000, and for the BC1G about 2700. Both of the transmitters had a high/low power position to switch to half voltage (and along with it, half current). That let the 1000 watt day and 250 watt night stations power down at sunset with the push of a button.  The 500 watt version with the single 833 PA tube had the power reduction that went to 125 watts (half voltage).  Other contactors also switched the bias on the modulators to allow for the lower plate voltage. The earlier BC1T version of the transmitter had a modification kit from Gates that used a couple of huge resistors to lower the PA plate voltage to about 70% of the high power so the station could run 500 watts, mostly for pre-sunrise power allocations that started back in the 1980's. The iron (mod and HV) in the 500 watt version was somewhat smaller than in the 1000 watt version. A 500 watt transmitter could be upgraded to 1000 watts with the install of the heavier iron and the addition of a 2nd 833 in parallel with the original. The RF and modulator driver components were the same for the 500 and 1000 watt models.

A couple of decades ago, I also modified a BC500G to allow the station to run it at a power level of about 50 watts (for "micropower" night use), by installing a variac (switched via an additional contactor) to allow adjustment of the HV to about 750 volts, as well as adding a switched in series resistor in the modulator 833's bias supply, otherwise at the lower voltage, the 833's would be biased past cutoff. It worked fine at 50 watts but was a waste of power because just heating 3 - 833's with their 100 watt filaments, plus the heater power of the 807's and the 12BY7's in the oscillator/buffer, pulled 10 times as much AC power as the transmitter did for 50 watts RF output. The station eventually installed a new solid state transmitter that would run efficiently at any power level.

73
Ted  W8IXY

[Opcom]

With only 2KV on the plate, is it reasonable to leave the HV on both tube plates, bias the unused tube close to its max negative grid voltage, and just remove the filament power from it? It would avoid an expensive HV relay.

The so quoted "ne plus ultra", left the HV of 2000V on all the unused amplifiers' plates and switched the tubes with the low voltage controls.

http://amfone.net/Amforum/index.php?action=printpage;topic=21388.0
"In the 1962 handbook, the article on page 194, "One Band Kilowatt Amplifiers" showed a stack of parallel 813 amps with a common power supply. This was called the "ne plus ultra" by the decently educated author.
The 10M one had a tap in the pi coil for the tuning cap, so that it was plate-PI-coil-C-tuning-PI-coil-C-loading. This was said to help with the large plate capacitance of the 813 pair. These were not GG amps but control-grid-driven amps with bias and screen voltage. Maybe this will help with the plate circuit design."

A similar setup with HV on the unused tubes is found in QST Nov 1961.

[W9ZSL]

Our BC-1T had a tap on the primary that was switched by a contactor.  Secondary produced either 6200 or 4670 VCT.

[K2DUV]

Thank you W7TFO, KC4MOP, KL7oF, W9ZSL, W8IXY and KD5OIE for the responses.  I have figured out what transformers I have (except for the 833 filament transformer) and thanks to w7TFO I have the socket for the 2nd 833RFA and a replacement bias transformer which failed when first lighted up.

My next step will be to get the rig operating on 1000KHz into the dummy load and then to start the move up to 160M.  When that is done I will add the second 833RFA and recheck 160M with both tubes in parallel.

Could someone who has a BC1G measure the size of the 833 filament transformer? I think measurement of the width, length and thickness of the laminations and a description of the wires coming from the transformer will do.

Many thanks & 73, Floyd K2DUV