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Old 13th Aug 2019, 1:17 am   #1
Bazz4CQJ
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Default Heathkit GDO using a Tunnel Diode

For some reason, the design of this GDO is something that I come back to at periodic intervals. It was made in the early 60's, used a tunnel diode (TD) with no useful ID as the oscillator and 3 Ge transistors as a meter amp, all running from single AA battery.

Although TD's went out production a long time ago in the West, there are lots of Russian TD's for sale on line. Most of these are described as 'switching" diodes, a few as 'amplifier' diodes and just a small number are described as 'oscillator' diodes. I'm unsure as what significance to put on these designations; what would be the major difference between them?

One parameter that would come in to play in an oscillator circuit would be the capacitance of the diode and quoted values of 10-20pF seem common. That value will obviously limit the upper frequency limit.

Anyone got any insights on this?

B
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Old 13th Aug 2019, 6:41 pm   #2
m0cemdave
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Default Re: Heathkit GDO using a Tunnel Diode

The GE Tunnel Diode Manual was once considered to be the standard text on the subject of TD's, and is well worth a read.

It's now available online:
w140.com/Ge1961TunnelDiodeManual.pdf
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Old 13th Aug 2019, 10:12 pm   #3
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Default Re: Heathkit GDO using a Tunnel Diode

The Wikipaedia page on tunnel diodes has an aura of accuracy about it (not always the case)
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Old 13th Aug 2019, 11:39 pm   #4
Bazz4CQJ
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Default Re: Heathkit GDO using a Tunnel Diode

Thanks for the GE manual Dave, I seem to have missed that.

Re the Wiki page, I see that there is a lot of data on this page http://w140.com/tekwiki/wiki/Russian_tunnel_diodes.

When you compare the specs of the switching, amplifier and oscillator diodes, the latter seem to be running appreciably higher currents, and have somewhat lower capacitance.

All that said, looking at some web pages today, the recurring story seemed to be that Heathkit's Tunnel Diode GDO was not very good (though very neatly designed and assembled). People seemed content with the valve and transistor Heathkit GDO's but the TDO is typically described as not being very sensitive.

Consistent with previous threads on this Forum, there are aways lots of very favourable comments to be found about the valve GDO's made by Millen and Measurements Inc., the latter using an Acorn valve.

B
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Old 14th Aug 2019, 9:17 am   #5
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Default Re: Heathkit GDO using a Tunnel Diode

I know someone who has a heathkit tunnel dipper. Ill ask him to post about it here. Not sure he will as hes allergic to his PC. AFAIK his dipper doesnt work. This might be in line with your point about them not being very good.

Ive had a large number of tektronix scopes with good tunnel diodes in them go through my hands over the last couple of years. I have played with some surplus tunnel diodes on occasion and found them to be tetchy and difficult to drive. My experiments were mostly limited to attempting to use them as pulse generators however and plotting the curve of one. Ill leave the latter here as it was interesting:

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Old 14th Aug 2019, 4:46 pm   #6
Bazz4CQJ
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Default Re: Heathkit GDO using a Tunnel Diode

There's a Youtube video which discusses the Heathkit TDO (model number is HM-10A). That and other reports suggest that all of the old composite resistors have drifted in value and must be changed. The three transistors in the meter amplifier are mounted in sockets, so can easily be removed and checked. The Youtube vid suggests one of them may be best replaced (very leaky) with something more modern to improve performance.

The need to have three transistors in the meter amp maybe suggests that the oscillator is not really up to the job. Makes you wonder about the particular tunnel diode that was selected; could another component do better? Of course this leads to the thought that perhaps a better tunnel diode (there lots of Russian ones to consider), together with a modern amplifier chip for the meter, could be an interesting project... or may be not .

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Old 14th Aug 2019, 5:28 pm   #7
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Default Re: Heathkit GDO using a Tunnel Diode

If the intent is to have a play with tunnel diodes, then why not? Have some fun!

If the intent is to create a GDO, then don't bother. The heathkit ones proved that though it could be done, the end result's performance was seriously disappointing, and not good enough for most day to day uses. On the other hand, there are much superior dippers using valves, bipolars, FETs and even the triode part of a magic eye has been pressed into use. Take your pick.

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Old 14th Aug 2019, 6:07 pm   #8
Bazz4CQJ
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Default Re: Heathkit GDO using a Tunnel Diode

If you recall the thread we had ~2 years ago asking what is the best GDO, the consensus was that the Megacycle Meter made by Measurements Inc was very hard to beat.

Subsequently, I started to make a copy of that unit and got as far as making up a pcb ready to accept an Acorn valve and with a variable capacitor mounted right up against it.... but then I found another variable that I though was a better choice, so some re-work needs doing, but finishing that is high on the 'To-do' list for this winter. I also have a small solid-state vibrator board which I'm hoping will allow it to be run from a 12V SLA if it needs to be used outside.

That would compliment/supersede my existing FET GDO, which I built at least 40 years ago. I also recently bought an old Japanese GDO with a bipolar transistor; not too good >50MHz, but really quite fair below 30MHz.

So, that's my route to a 'super' GDO. The TDO is more of a curiosity which breaks out like a rash now and then .

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Old Today, 9:28 pm   #9
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Default Re: Heathkit GDO using a Tunnel Diode

Someone has just offered a Trio DM801 dip meter at our local radio society, he's asking 50
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