The Harvey Wells TBS-50D Bandmaster
The vintage looks of the TBS-50D have been tempting me each time I saw one at a swap meet. Recently I had the opportunity to acquire one and discover that it is also an interesting and capable transmitter.
When I bought it, I had no idea what sort of circuitry might be inside. The vintage looks suggested there might be tubes with 2-digit numbers and probably low power output as well. But after downloading a schematic from BAMA, I found that I’d really acquired a nice transmitter that was of the same general capability as an AF-67/68 or Ranger. The 807 final modulated by p-p 6L6s should be capable of nice performance, though I doubted that the 6 and 2-meter performance would be worth bothering with.
My first thought was to build a power supply for it, but the more I looked at the schematic I began to realize that it wouldn’t be all that difficult to use the Multi-Elmac M1070A power supply for my AF-67/68 to power it. It might be as simple as making up an adapter cable so that the right voltages would get to the right place in the TBS-50D. It was almost that simple, but I kept finding “improvements” to incorporate.
The nice thing about the TBS-50D is that the octal plug and two terminal strips on the back of the unit bring out just about all the circuit points that one would need in order to provide power to it. By mounting a 15-pin male Cinch-Jones plug on metal standoffs between the two terminal strips on the rear of the unit I would be able to attach the power output cable of the M1070A directly into the Bandmaster just the same as I do with my Multi-Elmac transmitter. All I had to do was run wires to the appropriate pins on either the octal plug or the terminal strip contacts.
Originally the Bandmaster operated from a single 400 VDC supply for the high voltage, and dropped that through various resistors to provide the low B+ and screen supply voltages. The M1070A would provide a 600 VDC high B+ and a 300 VDC low B+. It looked like the Bandmaster could handle that and would put out much more power than Harvey Wells had originally intended. The end result proves this to be true.
The diagram below shows the basic hookup between the M1070A supply and the Bandmaster. The leftmost column identifies the various inputs and outputs from the power supply. The next column shows the pin numbers at the Cinch-Jones socket at the end of the M1070A power supply cable. The next column shows the tie-strip terminals utilized (including some jumpers and a diode), and finally the rightmost column shows the connections to the octal plug on the back of the transmitter.
Note that I have wired it so that the original Bandmaster On-Off switch turns on the AC to the 1070A power supply, and I have wired the original “Standby-Transmit” switch to function as a Spotting switch to allow convenient zeroing of the VFO or finding your location on the band. Basically the Spotting switch just applies the unkeyed (always on) low B+ to the oscillator/driver stages. However, if you prefer not to retain the original function of the Standby-Transmit switch, just wire it to ground the PTT line instead. The diode between Terminal Strip pins 14 and 9 is required for the modulator changes identified in the next section. Use a 400-600 volt rated diode.
Changes Inside the Bandmaster
A bit of study of the Bandmaster schematic turned up a couple of oddities. For one thing, the cold end of the RF final’s plate choke is not bypassed to ground. This is a sure way to get unwanted RF on the Hi B+ line. Similarly, the grid current shunt resistor does not have an RF bypass capacitor, and this allows RF to reach the front panel meter. Both of these probably contributed to the Bandmaster’s reputation as a TVI producer. A friend once suggested it should have been called the “TVI-50” instead. I installed a .001 2kv disc ceramic capacitor at the cold end of the RF choke and a .005 1kv disc ceramic across R12, the 1k grid shunt resistor.
While the original modulator 6L6s would probably have run fine on the elevated Hi B+ provided by the M1070A, I didn’t really like the idea of RC cathode bias and an unregulated screen supply. I decided to run the screens directly from the +300 vdc low B+, which is nice and stable, and to use a Zener diode instead of the original RC components at the cathodes of the 6L6s. A 24-volt Zener provided me with an idle current of about 70 ma through the modulators. You should adjust the Zener value if necessary so as to not exceed this current.
However I couldn’t just tie the screens directly to the keyed +300 volt low B+, because in the CW mode the modulator plate voltage is switched off and the screens would over-dissipate if voltage were applied. I needed some way to kill the screen voltage when the plate voltage is removed. Fortunately, there is a section of the AM/CW switch (SW6) that is only used to ground the key line in AM mode. I replaced the original key jack with a shorting-type jack that would automatically ground the key line as needed, allowing me to reuse that section of the switch to open up the 6L6 screens in CW mode. The figure below shows the overall modulator changes.
Pin 6 on one of the 6L6s was originally used as a tie point for several wires. That needs to be changed so that the only wire terminated there is the one from Terminal 14 on the back panel Terminal Strip. Route the +600 vdc feed from the AM/CW switch so it now goes directly to the center tap of the modulation transformer (on mine it originally went to pin 6). The remaining wire originally tied in at pin 6 was the high B+ feed to the 15K 5W dropping resistor for the speech amplifier section. You can remove it and the resistor. I then used the keyed +300 vdc and a 220 ohm 2 watt dropping resistor to supply the speech amplifier section instead. The 220 ohm resistor mainly provides current limiting for the filter capacitors in the speech amplifier chassis.
There was one remaining annoyance with the Bandmaster and that involved the overly small loading capacitor in the final tank. Because it is so small, the manual tells you that you will need to attach some additional parallel capacitance in order to properly load into a 50 ohm antenna on 80 and 40 meters. I decided to install a small three-position rotary switch on the back panel so that I could switch in the needed extra capacitance easily when going from band to band. I used a 512 pf capacitor for 40 meters and a 750 pf for 80 meters. The three-position switch lets me select the 750, the 512 or open (for all higher bands).
The way I like to power up a project like this, is to do things sequentially so that you can have confidence that various things are working right before you try the “whole enchilada”. For this project, I began by wiring up only the needed jumpers for the AC power on-off switch and the filament supply. This allowed me to turn on the power via the Bandmasters Power switch and to verify that all the Bandmaster’s tube filaments were lighting properly.
I then removed the 6L6s (for their own protection), and wired up the +300 vdc keyed and unkeyed supply jumpers. With the CW-Phone switch in the CW position, the meter switch in the Grid position and an appropriate crystal installed, I could now throw the Spotting switch (Standby-Transmit switch) and see grid current being delivered to the finals. At this point I had to do some alignment of the oscillator buffer stages to get enough current, so I checked out the other bands as well. Each of them needed a bit of touch up.
Next I installed the PTT jumper and verified that the M1070A’s T/R relay was activating properly when the microphone transmit switch was pressed. Finally, it was time to get serious and wire in the +600 vdc supply jumper. At this point I loaded up the final for the first time and verified I was getting good RF power out. This went well so I was ready to move on to the modulator.
After making the wiring changes shown above and installing the 6L6s, it was time to verify that the modulator idling current was in the right range. I temporarily installed a current meter in the cathode circuit of the 6L6s and keyed the transmitter (AM-CW switch now in the AM position). You should not exceed 70 ma of idling current for the modulators when the PTT switch is closed. If it is higher you need either a higher value Zener or you can just put some regular diodes in series with the Zener to boost the voltage up slightly.
When the modulator idling current is set properly, you are ready for some on the air checkout.
I’ve been very pleasantly surprised by the performance of my Bandmaster Deluxe. It puts out 40 to 45 watts out on most of the low bands and 30+ watts out on 6m. I don’t expect to try 2m, however. Signal reports have been good and no TVI complaints have been heard. The only problem I’ve had with using the higher B+ on the Bandmaster is a some plate tuning capacitor arcing on occasional voice peaks. I’ve eliminated this by substituting a slightly wider-spaced tuning capacitor.