The 50 dollar Amplifier
by KC5RT
I purchased a boat anchor classic amplifier RF deck a few months ago for $50.00. It was in pretty bad shape. The front panel meter had been poorly repaired, the cabinet and the chassis was rusting. There were several bad components under the chassis. In general it was a mess. Then again it was all there and it looked like it would be an easy and fun project. The Loundenboomer Mark II was manufactured sometime around 1962 by a local Kansas City company, Radio Industries Inc.
The RF deck is nearly complete now. I changed the cabinet paint color to a darker gray. I restored all of the switch contacts and replaced all of the bias resistors and metering resistors under the chassis. All but one was out of tolerance. I was able to purchase a chimney from ebay at a cost of 70 dollars and now I have about 150 dollars invested in the RF deck and I still don't know if I have a good tube. The power supply is another issue.
A little Loudenboomer Information.
The Amplifier covers 80-10 meters, output 1000 watts P.E.P. SSB, 600 watts CW, features full metering, band switching and built in cooling fan. The Amplifier uses a seperate high voltage power supply. The only tube used in the amplifer is a single 3-400Z. The "Loudenboomer" Mk 2A is nearly identical to the Hallicrafters HT-45 except for the style of knobs, type of meter and front panel trim. It was produced by Radio Industries of Kansas City, this company was bought by the Hallicrafters Co in 1963.
As you can see in the picture to the left the construction is very simple and clean. I replaced all of the resistors and added the bias resistor and terminal jack that is used to control idle current when the amplifier is not in use. (see the schematic) When in transmit the antenna relay must short out this 150 ohm resistor to remove the self bias created by the resistors IR drop..
The cabinet lid is hinged and provided access to the tube and chimney. Note the filament transformer in the left rear and the interlock switch on the bracket that is bolted to the transformer. This switch turns cuts power to the high voltage power supply when the cabinet lid is opened.
Here is R2, a 150 ohm 7 to 10 watt resistor and the two wire shorting plug that is used to connect the T/R relay shorting contacts. The original Mark II did not have this resistor or terminal strip. These were later adapted into the HT45. I used a two wire mike jack instead of a a cheap terminal strip. I also used an adjustable resistor so that the idle current could be adjusted if needed. I initially set the the resistor to 150 ohms as in the HT45 schematic.
Here is a snap shot of the Amplifier on the bench. Note the bundle of wires coming out the back. These will terminate in a homebrew high voltage power supply to provide the 2500 volts needed to "fire up" the 3-400Z.
The original Loudenboomer amplifier lacks an antenna transfer switch. I have seen one modification where a transfer relay was added to the amplifier but it was ugly. I decided to build a seperate antenna T/R relay and input attenuator box and mount it on the rear of the cabinet.
If you look closely at the schematic you will see that this amplifer does not have any input matching circuitry. The HT 45 manual suggests placing a 3dB attenuator between the exciter and the amplifier in order to provide a 6 dB minimum return loss. This would represent a 3.0 to 1 VSWR or better to the exciter. However it cuts a 100 watt drive signal down to 50 watts.
The 3-400Z tube has not been in production since 1968 and good tubes are getting hard to find and expensive. Ebay sold two NIB last month for 300 dollars. Since the 3-400 and 3-500 are nearly the same electrically it maybe possible to sub the still available 3-500Z into the Loudenboomer. The critical dimension is the height of the tube. The 3-400Z is only 5.5 inches tall while the 3-500Z can be as tall as 6.1 inches.
The power supply and antenna transfer relay will be shown in next months project page.
Scanned HT45 schematic.