Extend the life of 75S-x Receivers

How to Extend the Active Life of a Collins 75S- Receiver

By Don Kang 3-2-2005


Heat is the major contributor for components degradation in the radio. By replacing three oscillator tubes, two audio tubes and two lamps with solid-state equivalents, the total power consumption of the radio is reduced from 70 watts to 40 watts. Those tubes handling RF/IF signal are not disturbed. Thus the vacuum tube radio performance is maintained.

1.0 Introduction

We all know that the vacuum tubes have limited life. Other components also get aged as well. Heat is the primary enemy. An obvious remedy is to install a cooling fan and many users have actually installed a fan on the S-line cabinet cover. Even today, original designers will resort to the use of a fan if no other simple solution is available.

Here is my approach to this problem, and two self-imposed restrictions.

No circuit modifications
(b) Incoming signal stays in the original vacuum tube circuit until it becomes an audio signal.

With the above restrictions, six items will be replaced with solid states devices.

They are: (1) 100khz Xtal marker oscillator, 6DC6

(2) Main tuning PTO, 6AU6/7543

(3) CW BFO, 6DC6 (NA for 75S-1)

(4) Detector and audio voltage amplifier, 6AT6

(5) Audio power amplifier, 6BF5

(6) Two lamps for S-meter and main dial, GE 47

Heater current: 6DC6 0.3A x 2 — 0.6A

6AU6/7543 —- 0.3A

6AT6 ———–0.3A

6BF5 ———– 1.2A

Lamps (0.15 x 2) —0.3A

The total current saving is 2.7A and the total power reduction is 2.7A x 6.3V = 17.01watts.

2.0 Finding Solid-State Devices for The Vacuum tubes

Many thousands of discrete solid-state devices have been produced in the years following the vacuum tube era. Today, most of those discrete devices have become a thing of the past. Some of them are more difficult to find than vacuum tubes. And unlike the vacuum tube, the appearance or the part number does not reveal what kind of device it is.

2.1 An insight of solid states (semiconductor) devices

There are two types of charge carriers, electrons and holes in the semiconductor. A hole is simply a missing electron. If the electrons are dominating, they become the majority carriers, and the semiconductor is called N-type. When holes are dominating it is a P-type material. Many solid state devices have an amplifying function. They are called transistors. The transistors are classified into bipolar and unipolar transistors based on the operating principle. For the bipolar transistor, the minority carriers are injected into the base region by a forward biased PN junction. The minority carrier in the base controls the operation of the device and both carriers are participating, thus bipolar operation. For the unipolar transistor, the input is a reverse biased PN junction and the electrical field at this junction controls conductivity of the channel which connects input and output. No minority carriers are involved. It is a majority carrier device and it is commonly called field effect transistor, FET.

Because the input of the bipolar device is a forward biased PN junction, its impedance is low. On the other hand, the input of the junction FET is a reverse biased PN junction, and its input impedance is very high. The only contribution for lowering the impedance is the junction leakage current and the gate capacitance. There is also the non-junction type FET called the Insulated Gate FET or IGFET. Actually the input of IGFET is a capacitor. The gate is sitting on an insulating material. If the gate is a metal and the insulating material is an oxide layer over a silicon, it is an MOS capacitor. This kind of unipolar device is known as MOS FET.

Unlike the vacuum tube, transistors have two genders, N and P type, based on the material as discussed above. For the bipolar transistor, they are the NPN and PNP. For the FET, they are the N channel and P channel. The FET is further classified by its operating condition (mode) as either an enhancement mode or a depletion mode. If the transistor is normally off with zero gate voltage, it is an enhancement (off) mode and if it is normally on with zero gate voltage, it is a depletion (on) mode.

The FET requires a conducting channel to pass current from the input (Source) to the output (Drain). For the enhancement mode, there are very few charges in the channel. To make it conduct, the gate voltage is increased to induce charges into the channel. The gate voltage at which the channel is about to start conducting or pass current, is the turn-on threshold voltage.

For the depletion mode, there are already lots of charges in the channel. To turn off the current in the depletion mode transistor, the charges should be depleted. The gate voltage which will deplete all the charges is called the turn-off voltage. However unlike enhancement mode, for the case of depletion mode, Idss ( the drain current when the gate voltage is zero) is used to characterize the FET. The initial charges in the channel are built in during the fabrication process, and it is not easy to control. Thus the Idss spec is made very loose. Very often different part numbers are assigned to cover the wide range of Idss.

2.2 Solid States Circuits

A depletion mode N-channel FET works in principle like a vacuum tube. It can be a MOS FET or a junction FET. Many FETs available for an amplifier application do not have high drain voltage. By teaming up with a high voltage bipolar NPN transistor, the drain voltage can be extended. Other important specs to consider are gain and Idss. They should match to the tube circuits. Because of loose Idss spec, some form of bias tweaking is needed to duplicate tube function.

A pentode tube can be considered as a triode and a buffer combination. The circuits shown below are starting point. A combination of one of these input circuits, and one of these output circuits will replace the tube function.

The transistors selected for this project are:

2N5484, N-channel depletion mode junction FET (Mouser Electronics p/n: 512-2N5484)
LND150N3, high voltage N-channel depletion mode MOS FET (Mouser: 689-LND150N3)
2N5551, high voltage NPN transistor (Mouser: 610-2N5551)
FQPF1N50, High voltage N-channel enhancement mode power MOS FET (Mouser: 512-FQPF1N50)
These are my selections from Mouser Electronics. There are many other choices you can make. When an external DC source pin with AC grounding is available, the base voltage can be stabilized by a Zener diode as shown in the output circuit (B) and the emitter terminal can supply a stable drain voltage to the preceding FET stage.


3.1 Tube Base – I

One obvious base source is the tube itself. At first I was very hesitant to destroy perfectly good tubes. The picture shows how I destroyed a tube to get the bottom part. The tube envelope near the bottom was scratched by a small grinding tool (1). This helped somewhat to avoid total destruction. Wrap the tube with vinyl film food wrap and gently hit the upper part of the tube with any small metal tool. Hold the pointed end of the tool when you use the kind of tool shown at (2). Make sure the inside pins are available for secure connection (not easy). Also external connection to the pins can be made.

3.2 Tube Base – II

Another way to make the base is using a tube socket. All the metal parts were removed from the socket. The right size pins are inserted from the top. The pins shown in the picture are from a D-sub connector (1). Only 1/4 inch thick socket (4) worked in this case because of the body length of the pins. When circuits are built on the base, make sure that it is working and the pins are properly mating to the socket on the radio. Then the pins are glued with an epoxy adhesive (5).

A third possibility is to try to find a source of 7-pin tube base sockets. I was told that they use to be plentiful, but are hard to find now.

3.3 100 khz Xtal Marker Oscillator, 6DC6

The LND150N3 and 2N5551 combination were used for 6DC6. No bias adjustment was needed for the input FET.

The output signal is not as strong as the vacuum tube counterpart, but I do not see why a strong signal is needed for a marker. It may be the case that the original designer could not find any capacitor less than 1 pico farad.


3.4 Main Tuning PTO, 6AU6/7543

Pin 6 of the PTO tube is AC grounded. This terminal is clamped at 14V with a 14V Zener diode. Any voltage from 8 to 20V will work. Use of the Zener diode helps to stabilize the PTO frequency especially for those earlier 75S- radios which do not have B+ voltage stabilization. Again no bias adjustment was needed for the 2N5484. For the right output voltage and frequency compensation of the PTO, please see June 2002 Album section, at the www.collinsra.com web site


3.5 CW BFO, 6DC6

The 2N5484 is a basic oscillator. The 2N5551 NPN transistor is an amplifying buffer and at the same time it extends drain voltage. Both transistors required bias adjustment for a proper operation. I used 20 K ohm variable resistors for the bias adjustments. I found that 10 K ohm at the 2N5484 source and 4.7K ohm at the 2N5551 emitter gave right output value. You may find slightly different resistor values for your circuit due to the transistor gain difference. The bias level should be set to produce 1.6v to 2.2v at product detector cathode. The 20 pico farad capacitor between pin6 and pin7 is to compensate frequency difference. If this capacitor is not used, the BFO frequency can still be adjusted as instructed in the factory manual.


3.6 Detectors and Audio Voltage Amplifier

For the 6AT6, the replacement is one to one. The gain of the FET amplifier seems a little bit low compared to the 6AT6. It can be easily compensated by the AF gain control. If you are using very old speaker, make sure that the speaker has adequate efficiency. Today many high efficiency speakers are available. You do not need a HIFI speaker.

The two diodes are Schottky diodes. Much smaller surface mount two-diode chip is also a good choice. I used 1N5819’s but any silicon Schottky signal diode will work.


3.7 Audio Power Amplifier

The original design of the AF power amplifier is shown in the February 2002 Album section of the www.collinsra.com web site. A few revisions are made here.

Much higher resistor values are used for the voltage divider from pin 6 to pin 2. This is to reduce the loading of the voltage amplifier drain. An un-bypassed 390 ohm resistor is added at the drain terminal for a small amount of negative feedback to improve linearity of the amplifier. A one ampere rated power MOSFET, FQPF1N50, is used to operate at a higher current density which improves linearity and gain. Also two 1W 10Kohm resistors are used instead of three 15Kohm resistors in the source bias circuit. The maximum output power is less than that of the original 6BF5. However, it is quite adequate for normal operation.

The picture below shows some of the finished Solid State tubes. (1) is the original 6BF5, and (2) and (3) are the SS AF power amp. They are about the size of 12AX7 tube. The bases (2) and (3) are Japanese origin and they are not available in the US. (4) is the audio voltage amplifier build on base II. (5) is the PTO oscillator built on base I. (6) is the 100Khz marker oscillator built also on base I.

The broken edge of the glass base is wrapped with a fish paper and glued with 5 minute epoxy adhesive.

Image8 Image9

4.8 Illumination Lamp

Two illumination lamps are replaced with high intensity light emitting diodes. I used green and red LEDs. The colors are not the best choices but I happened to have them in my junk box. The parts numbers or their manufactures are unknown. I adjusted the current to 15ma with R in the diagram. This is a 90% power reduction from the original lamps.

I used green LEDs for the dial and red for the S-meter. The green dial LEDs are aiming about 30 degrees off to cover wider area. The S-meter LEDs are looking in opposite directions because of the edge lighting. The pictures shown are my prototypes. Do not operate LEDs in the AC circuit without a high voltage rectifier in series. Many LEDs have very poor reverse characteristics. Also use in pair in opposite current flow to make it balanced full wave operation as shown in the diagram below.

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The light projection pattern can be modified by changing the LED’s plastic dome shape. By roughing the shiny surface, the light can be scattered. These LED lamps are installed in the 75S-3C as shown in the picture below.


LED lamps are installed in 75S-3C


The reduction in heater current as a result of using Solid State tubes has caused the heater voltage to go up to 6.9V. This condition will shorten the life of the vacuum tubes in the radio. The reduction of the power transformer load also increases the high voltage output. Later model 75S- has Zener diode regulated 140V, B+ supply. The series resistor (R86, 1 Kohm) and the Zener diode (CR6,1N3010A) will dissipate more power to maintain constant output voltage.

The graph below was plotted to see how the heater voltage changes with the AC line voltage. Ideally I want to keep heater voltage slightly below 6.3V. At 110VAC input, the heater voltage is 6.2V with SS tubes installed. The regulated B+ supply voltage is stable between 120VAC and 110VAC input and it starts to drop around 110VAC input. At 100VAC input, it is 128V on my Round Emblem 75S-3C. This means that below 110VAC, the Zener diode is not regulating.

I used a 60 watt variac for the voltage reduction. If your radio has a 140V regulated B+ supply, reduce the AC input to the radio with the variac until the heater voltage is 6.2VAC or the B+ starts to drop out of regulation- whichever comes first. For a 75S- with no B+ regulation adjust the AC input for a heater voltage of 6.2VAC. The optimum point of my radio was at 110VAC with the Solid State tube installed, and the total power consumed in the radio was 40 watts. With all the original tube installed and at nominal 120VAC input, the power consumption was 70 watts. This represents a 43% reduction in power consumed by the radio.


An alternative to the variac is to use an external power resistor (30 ohm/5w in my case) in series with the AC input line. When a small 12V filament transformer secondary is connected to the primary in series in phase, it becomes an auto transformer to boost or reduce the AC voltage by 10% depending on input/output wiring. If your AC line fluctuates, the solution becomes more complicated.

Reducing AC input voltage is not a new topic. If I find a simple way to reduce and stabilize the input AC voltage, it will be posted here in this Album section.

Please e-mail me at donkang@ieee.org for any suggestions or comments.