[Milsurplus] Powering the TBW-4

[Ray Fantini]

L303 and 302 are important for the function of the oscillator, keep them in there. Also remember that the path to ground for the osculator is thru R213
The cathode of V-301 is the return path for the osculator and you will need all that junk for it to work. I just used a regular 24-volt center taped transformer with the original circuit.
I was able to build both transformers onto the bottom deck but only used the filament rheostat R201 for the 10 volt transformer for the 803, at first was worried about preserving the 803 but later learned that the 803 is just about indestructible. In tune mode they keep the 500 volts applied to the screen of the 803 but remove the plate voltage, that would kill most power tubes but the 803 just sits there like its nothing! Brought this up years ago but have to say that I have never seen a failed 803 or know anyone who has.
The second deck was used for the 500 volt supply all standard construction but I kept the original idea of keying the incoming AC to the HV and Plate supply and discovered that when they did this with 800 cycles the size of the filters were small but at 60 cycles the larger filters provided enough DC after you released the PTT or Key that you would hear the MO drift down the band. For that reason, I installed external bias at K301B that kills the oscillator and IA stage when the key is up. M301 is a good point to interduce bias, just remember that when K301B closes it will short the output of the bias supply to ground so you will need a current limiter on the bias line.
R205 is in the negative side of the 500 volt supply to ground to develop a small negative DC voltage. In CW mode the suppressor is grounded and in AM its routed thru T204 and dumps a bunch of audio on the suppressor grid to produce modulation. Don’t think I got more the fifty watts or so in AM
I did an external plate supply being it would be just about impossible to do the plate supply in that limited space.

Thought Exercise:

Is the TBW a repackaged TDE? Much of the TDE is similar to the TBW, was the TBW and attempt to build a light weight TDE for shore operations? What about the GO series that were in things like the PBY? What came first? TDE, TBW  or GO?



Ray F/KA3EKH

From: Mark K3MSB <mark.k3msb at gmail.com>
Sent: Thursday, June 13, 2024 5:49 PM
To: Ray Fantini <RAFANTINI at salisbury.edu>
Cc: milsurplus at mailman.qth.net
Subject: Powering the TBW-4


Hi Ray

My RMU had two of the three 800 Hz transformers missing.    Since I will be running my TBW off of 120 VAC I decided to get rid of all remaining 800 Hz related parts and wiring.    This resulted in the bottom deck being completely devoid of parts and the middle deck almost free of parts.   The middle deck still has R-219 which is still needed but can easily be relocated.   I've attached two photos.

I plan to mount all new transformers (except the 2 KV supply) onto the bottom two decks of the RMU.    This will be the 500V supply as well as the 2.5V, 10V, and 12.5V Filament transformers.

At present, I plan to have all primaries go through the Filament Rheostat control.

The two 837's require 12.6V for the filament but they are fed by an 18.6 VCT secondary off of  the non-existant T-202.  I think this increased voltage was needed due to the voltage drop in L-302 and L-303 in the Oscillator filament circuit.    I think this then required dropping resistor R-213 in the filament circuit of the Intermediate Amplifier.   I plan on bypassing (shorting) L-302, L-303, and R-213 so that I can use a standard 12.6V transformer for the two 837's.

The 2 KV supply will be external to the RMU and will be fed in via a new plug on the bottom of the RMU, then to S-208C.     The new 500V supply will feed S-208B.

What I still haven't figured out yet is what voltage the 803 suppressor needs on Phone.   This voltage is developed from the non-existant T-203 HV secondary CT via the bias adjustment pot R-205.  There is nothing in the manual about adjusting R-205 (or maybe I'm not seeing it....).  Looking at the 803 data sheet,  for 2 KV plate voltage in Suppressor Modulated Class C phone operation, I'd need about -110V on the suppressor.     Still not sure of this area yet,  but I should have room for a bias supply in the RMU.   The bias supply would feed into the ungrounded end of R-205 then to the 803 via S-209A in Phone operation.

The front panel AC ON/OFF and DC ON/OFF switches will be used as intended.    Both required a good bath in DeOxit for them to work  (Well, they are 85 years old.....)

I plan to repurpose some of the 4 front panel AC Compensation SPST switches.

One will be for a  “TX/RX” switch for lack of a better term.   In TX, relay K-301 in the HF transmitter will close as it's supposed to do.    Except.....   I plan to ground the Oscillator with a new modern relay instead of K-301B.    That new relay will be controlled by the key or mic switch from the RMU.

When receiving, the 2KV and 500V power supplies will still be providing voltages to the 803, but the oscillator will not be enabled.   I was discussing this with Howie and we feel this is OK as long as plate dissipation is not exceeded, and that concern may require me to run less than 2KV on the plates.

I ran a TBW-5 for a few years with 1200V on the plate and that gave me more than 100W output on CW which is adequate for my needs.     Running phone with the TBW will be a new experience for me.

Any and all comments welcomed.
73 Mark K3MSB

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