Thread Starter

Icanmakeit67

Joined Sep 23, 2018
11
Please help. Considering building a vacuum tube tester from an old magazine design. Parts seem to be an issue because of the 1950’s era age of the design. I want to replace a Sarkes Tarsian Selinium voltage doubler Rectifier part # 78D. Is there a modern day equivalent to this part?
 

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nsaspook

Joined Aug 27, 2009
9,987
Do you have the transformers and filament switch? A modern solid-state diode (1N4005) will replace the individual Selenium devices and provide slightly higher DC voltages due to the reduced rectifier forward voltage drop.
 

Thread Starter

Icanmakeit67

Joined Sep 23, 2018
11
Do you have the transformers? A modern solid-state diode (1N4005) will replace the individual Selenium devices and provide slightly higher DC voltages due to the reduced rectifier forward voltage drop.
I found both online…..NOS. Different source for each one. Have not pulled the trigger yet because they are about $80 each. Looking through the parts list at each component to see if available. Appreciate your input. Any other words of wisdom is welcome. I’m an obvious armature in this realm. I can build anything from a good schematic but lack expertise in design so help with replacing parts with modern available ones is really helpful!!
 

Reloadron

Joined Jan 15, 2015
6,701
You may want to start here. Page 29 gives the data for the 78S which is just two rectifiers on one body. These were common years ago as full wave rectifiers using a transformer center tap transformer. A consideration here is the old selenium rectifiers had a much higher forward voltage drop than today's silicon. Note the 78S had a Vfwd drop of about 8 Volts. Today I would just replace SR1, SR2 and SR3 with 1N4004 diodes and be done with it. The original diodes were low current (Forward Current) and the 1N4000 series are 1 amp series diodes. I doubt it will matter. I see there have been responses since I type real slow. :)

Ron
 

nsaspook

Joined Aug 27, 2009
9,987
I found both online…..NOS. Different source for each one. Have not pulled the trigger yet because they are about $80 each. Looking through the parts list at each component to see if available. Appreciate your input. Any other words of wisdom is welcome. I’m an obvious armature in this realm. I can build anything from a good schematic but lack expertise in design so help with replacing parts with modern available ones is really helpful!!
Great!
A tube tester is a fairly dangerous project with possibly lethal high voltage from that rectifier section. Be careful if you build it and have the proper test equipment to verify safe operation.
 

Thread Starter

Icanmakeit67

Joined Sep 23, 2018
11
Great!
A tube tester is a fairly dangerous project with possibly lethal high voltage from that rectifier section. Be careful if you build it and have the proper test equipment to verify safe operation.
Super helpful… can I infringe on your knowledge once more? If you look at the schematic/parts list the is a choke specified that I did find but it’s very old and considered collectible of coarse and they want a small fortune. Is there a modern equivalent? I’m not building a nostalgic project, but yes a useful one. Modern parts are all ok. The 5K 25watt potentiometer might also be a problem too.
 

MisterBill2

Joined Jan 23, 2018
12,402
If the goal is to create a working tube tester that is one thing, but if you want to produce a copy that will be a much harder job. A modern power diode with better performance, like the 1N4005, is a good example of a much better part then the original.
You can probably locate a older tube tester that you can buy for much less than the switches alone needed to make a copy.
so let us know more about the purpose of creating the tube tester.

If you only need to test a few tube types there are circuits that let you check performance very well.
 

Thread Starter

Icanmakeit67

Joined Sep 23, 2018
11
If the goal is to create a working tube tester that is one thing, but if you want to produce a copy that will be a much harder job. A modern power diode with better performance, like the 1N4005, is a good example of a much better part then the original.
You can probably locate a older tube tester that you can buy for much less than the switches alone needed to make a copy.
so let us know more about the purpose of creating the tube tester.

If you only need to test a few tube types there are circuits that let you check performance very well.
Well I want to refurbish a couple vintage radios. I have acquired two AM radios from the fifties/sixties. So your right a few of the most common tubes I’ll need to test. If there is a project I can build aside from a tube tester that works too. Your input is welcome Thank You
 

MisterBill2

Joined Jan 23, 2018
12,402
OK, AM radios from that era were either "high end" radios with power transformers or series string AC/DC radios with five tubes, three 12 volt heater tubes, a 35 volt heater rectifier tube, and a 50 volt heater audio output tube. I know that Zenith made a six tube AM radio aimed at farmers, as it had an RF stage in addition to the standard circuit. Those tubes mostly lasted forever until the heater burned out. So for checking those tubes a heater continuity tester is what you need first. One failed tube and the whole string is dark. After that the next things to fail are capacitors, either power supply filter caps or the audio coupling capacitors.
 

Thread Starter

Icanmakeit67

Joined Sep 23, 2018
11
OK, AM radios from that era were either "high end" radios with power transformers or series string AC/DC radios with five tubes, three 12 volt heater tubes, a 35 volt heater rectifier tube, and a 50 volt heater audio output tube. I know that Zenith made a six tube AM radio aimed at farmers, as it had an RF stage in addition to the standard circuit. Those tubes mostly lasted forever until the heater burned out. So for checking those tubes a heater continuity tester is what you need first. One failed tube and the whole string is dark. After that the next things to fail are capacitors, either power supply filter caps or the audio coupling capacitors.
Thank You
 

MisterBill2

Joined Jan 23, 2018
12,402
All of the common 7-pin tubes in AC/DC radios have the heater connections on pins 3 and 4. So a single 7-pin socket plus an ohm meter and one can safely check for heater continuity for all of them.
 

Thread Starter

Icanmakeit67

Joined Sep 23, 2018
11
All of the common 7-pin tubes in AC/DC radios have the heater connections on pins 3 and 4. So a single 7-pin socket plus an ohm meter and one can safely check for heater continuity for all of them.
I was looking at pin outs of several tubes and see what you mean. My first non working radio I got is a Silvertone AM. Was able to find schematic before I purchased the radio.
 

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ApacheKid

Joined Jan 12, 2015
720
Please help. Considering building a vacuum tube tester from an old magazine design. Parts seem to be an issue because of the 1950’s era age of the design. I want to replace a Sarkes Tarsian Selinium voltage doubler Rectifier part # 78D. Is there a modern day equivalent to this part?
I'm curious what would be the interest in making a tube tester, not that I'm criticizing I am not, just curious.

If it is to help test old tubes then you might be better of looking for an old tube tester and perhaps repairing that rather than making from scratch.

https://www.ebay.com/itm/234582464551?hash=item369e345827:g:TyUAAOSwH5Nin9UM

I have several old tube receivers here, not even powered them up and one day I plan to power them up and repair if they need it. One is incredible, a "neutrodyne" design almost 100 years old, it still has tubes in it and they might even be original. The top lifts open (it has a piano hinge) and the interior looks pristine. I'm in Arizona and bought it at an "antique mall" for all I know its been in AZ all these years so very low humidity/corrosion.

Its identical to this one:

1654967634512.png
 
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MisterBill2

Joined Jan 23, 2018
12,402
Those tubes are expensive to find and replace, so treat them very gently and carefully. And caution with poweriing it up as well. Some of the radios had variable resistors to set the filament (heater) voltage and color. Avoid excess heater voltage. That radio does look like a beauty.

In the Silvertone radio those octal tube heater connections are usually on pins #2 and #7., except this radio has an exception. Check the circuit schematic.
And a note for great caution, which is that in many of those radios one side of the mains connection is tied directly to the chassis. This creates a serious shock hazard.
 
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MrChips

Joined Oct 2, 2009
26,485
I would begin with the assumption that I do not have access to a tube tester.
The first tools I would use are my senses: look, smell, touch.

1. Examine condition of all components. Look for signs of stressed/worn/burnt components, damaged wiring, connections and solder joints. With power applied, look for filaments glowing.

2. With power applied, look and smell for anything burning.

3. Since there are HV (high voltages) here, save the touch test for another occasion.

The second tool is a DMM (Digital Multimeter) or any voltmeter.
With the DMM set to measure 700VDC or 1000VDC, measure HV from the power supply or on the plate of any tube.

The third essential tool is an oscilloscope. Since not everyone has access to an oscilloscope we learn to be resourceful. What would be useful in this case is an audio amplifier to be used as a signal tracer. A powered PC loudspeaker would make an excellent signal tracer.
 

MrSalts

Joined Apr 2, 2020
1,832
The market for replacement tubes is pretty tough right now. I think the only tube manufacturing has been in Russia for the past 20-years or so. In the last 3.5 months, none have been coming into the western world. It doesn't look like any will coming any time soon either.

Western Electric just announced that they will be making a range of guitar amp tubes in the US to supply companies like ElectroHarmonix. It may take a few months to get started.
 

SamR

Joined Mar 19, 2019
4,292
I wish I could put my hands on my tube tester. I bought a very nice and compact in a case Mil-Surplus one at a ham fest a few decades ago and it is lost in the garage or at the storage center somewhere... I would think there are still a lot of good used ones for sale on eBay or at a local Ham Fest or swap meet. Much cheaper than trying to build one, troubleshoot, and calibrate it.
 
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