UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Reply
 
Thread Tools
Old 16th May 2021, 5:16 pm   #1
WaveyDipole
Nonode
 
WaveyDipole's Avatar
 
Join Date: Mar 2013
Location: Leicester, UK
Posts: 2,031
Default What is this bulb component?

This is inside a Levell Transistor Decade Oscillator model TG66A. Its about 2/5ths of an inch in diameter and 1.25in long. At first, on seeing the two electrodes, I thought it might be a vintage neon. However, one electrode is about half the length of the other and the longer one has a ring attached at the top. They seem spaced too far apart for a neon lamp. There is also a fine filament attached to both electrodes at the height of the shorter one and terminated on a bead of material stuck to the inside of the glass envelope.

It has just the two connecting wires with yellow sleeves going to it and appears to be marked ITT R54 (or possibly D54). I am thinking that it may have something to do with stabilisation or temperature compensation of the oscillator and might therefore be some type of thermal device, but am curious as to what it actually is and was hoping someone might have some specification data on it?
Attached Thumbnails
Click image for larger version

Name:	bulb-01.jpg
Views:	200
Size:	70.9 KB
ID:	234221   Click image for larger version

Name:	bulb-02.jpg
Views:	208
Size:	45.4 KB
ID:	234222  
WaveyDipole is offline   Reply With Quote
Old 16th May 2021, 5:22 pm   #2
snowman_al
Heptode
 
snowman_al's Avatar
 
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
Posts: 822
Default Re: What is this bulb component?

You are correct it is a temperature measuring device, it's a thermistor.

https://sound-au.com/project179.htm
__________________
Never Leave Well Enough Alone...
snowman_al is offline   Reply With Quote
Old 16th May 2021, 5:45 pm   #3
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 5,775
Default Re: What is this bulb component?

Absolutely. Though in this case it's not measuring an external temperature, it's responding to its own self-heating due to the current passing through it, and changing its resistance accordingly.
kalee20 is online now   Reply With Quote
Old 16th May 2021, 6:13 pm   #4
kan_turk
Hexode
 
kan_turk's Avatar
 
Join Date: Mar 2014
Location: Dublin, Ireland
Posts: 338
Default Re: What is this bulb component?

Some info here - page 5
Attached Files
File Type: pdf STC_Thermistor-Data_OCR.pdf (2.53 MB, 39 views)
kan_turk is online now   Reply With Quote
Old 16th May 2021, 6:47 pm   #5
WaveyDipole
Nonode
 
WaveyDipole's Avatar
 
Join Date: Mar 2013
Location: Leicester, UK
Posts: 2,031
Default Re: What is this bulb component?

Thank you for the link and the data sheet.

The linked article is interesting. I have previously come across an oscillator circuit that used a filament bulb as a stabilising device.

This TG66A instrument has some instability which appears as a rhythmic bounce at the lowest frequency end but otherwise seems to work well. There is one electrolytic cap that could possibly do with being replaced but I note the article mentions that noisy pots can cause instability. However, I would have expected that to have been a rather random effect and did wonder whether the ageing thermistor might be a contributing factor. Curious that the guy in the article ended up with two failed thermistors.

Anyway, the replies confirm that it is a thermistor and kan_turl has kindly provided a datasheet so that answers both of my queries. Thanks.

Last edited by WaveyDipole; 16th May 2021 at 6:52 pm.
WaveyDipole is offline   Reply With Quote
Old 16th May 2021, 7:20 pm   #6
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 17,869
Default Re: What is this bulb component?

Treat it gently. Check circuitry is OK before trying it with the thermistor in place.

Level bounce (especially at lower frequencies) is quite normal with thermistor stabilised Wien bridge oscillators, set off when you change frequency, but it should settle down when left alone.

These things are getting hard to find and are commanding quickly rising prices.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 16th May 2021, 9:12 pm   #7
Silicon
Octode
 
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 1,365
Default Re: What is this bulb component?

Some of those thermistors have a maximum Power dissipation of 3 milliWatts.

If my calculations are correct, the 50k thermistor has a current limit of 250 micro Amps.

Even digital multimeters may damage them when measuring their resistance.
Silicon is online now   Reply With Quote
Old 17th May 2021, 8:15 am   #8
WaveyDipole
Nonode
 
WaveyDipole's Avatar
 
Join Date: Mar 2013
Location: Leicester, UK
Posts: 2,031
Default Re: What is this bulb component?

Quote:
Originally Posted by Radio Wrangler View Post
Treat it gently. Check circuitry is OK before trying it with the thermistor in place.

Level bounce (especially at lower frequencies) is quite normal with thermistor stabilised Wien bridge oscillators, set off when you change frequency, but it should settle down when left alone.

These things are getting hard to find and are commanding quickly rising prices.

David
Quote:
Originally Posted by Silicon View Post
Some of those thermistors have a maximum Power dissipation of 3 milliWatts.

If my calculations are correct, the 50k thermistor has a current limit of 250 micro Amps.

Even digital multimeters may damage them when measuring their resistance.
Thank you. All points noted.

So perhaps the bounce is normal behaviour then. It only seems to become a problem when down to a handful of Hz. I will check whether it settles down after a while.

I will also avoid trying to measure its resistance of the thermistor with a DMM let alone an AVO!
WaveyDipole is offline   Reply With Quote
Old 17th May 2021, 9:06 am   #9
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 17,869
Default Re: What is this bulb component?

When you get down to low frequencies, the temperature of the little bead starts to follow the signal waveform and the stability of the level controlling feedback mechanism degrades and the bounce effect worsens.

Essentially it sets a limit on how low you can make a thermistor stabilised oscillator go.

Of course a higher thermal mass could be used to slow the response, but then you have an oscillator which takes much longer to settle, at all frequencies.

The thermistor stabilisation trick was invented by Bill Hewlett, and was the product which put Bill and Dave in business. One customer wanted a greater number than they ever thought they'd sell, and very early on in starting a business in the Packard garage. So this customer provided help and advice for them getting going. A Mr Walt Disney! He wanted the oscillators for testing sound systems in cinemas where he was going to release his new film, Fantasia. They also got used for electronic music effects.

Some years later, a chap came to see Bill and Dave with a new circuit he'd invented, hoping to do a deal, but H & P were too busy with a government contract, so they in turn advised him to get going in his garage, and they passed on the sort of help Disney gave them. Howard Vollum called his firm 'Tektronix'

It's quite strange to think of companies being generous and helping each other rather than trying to tear each other's throats out.

It was a different age.

A circuit with a bit of history!

David

The Baofeng is with the Scouts, ready for the next group to reach the level for their foundation licences. I bought another two, so that a group of three youngsters can go on the air at once. Thanks for the inspiration and the generosity!
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 17th May 2021, 12:07 pm   #10
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 3,263
Default Re: What is this bulb component?

There was quite a warm relationship between Vollum, Hewlett and Packard. Colleagues in competition.

Story was they used to meet up from time to time and share ideas. It is no accident that the distributed amplifier ended up in both Tektronix and Hewlett Packard oscilloscopes at the same time.

Thanks kan_turk for the thermistor datasheet - that is a very useful reference.

Craig.
Craig Sawyers is offline   Reply With Quote
Old 17th May 2021, 2:29 pm   #11
WaveyDipole
Nonode
 
WaveyDipole's Avatar
 
Join Date: Mar 2013
Location: Leicester, UK
Posts: 2,031
Default Re: What is this bulb component?

Quote:
Originally Posted by Radio Wrangler View Post
When you get down to low frequencies, the temperature of the little bead starts to follow the signal waveform and the stability of the level controlling feedback mechanism degrades and the bounce effect worsens.

Essentially it sets a limit on how low you can make a thermistor stabilised oscillator go.

Of course a higher thermal mass could be used to slow the response, but then you have an oscillator which takes much longer to settle, at all frequencies.
That all makes sense and thank you for the clarification. It was mentioned in the linked article but it didn't quite 'click' with me. Your comment here makes it much clearer.

Quote:
Originally Posted by Radio Wrangler View Post
It's quite strange to think of companies being generous and helping each other rather than trying to tear each other's throats out.
Quote:
Originally Posted by Craig Sawyers View Post
There was quite a warm relationship between Vollum, Hewlett and Packard. Colleagues in competition.

Story was they used to meet up from time to time and share ideas. It is no accident that the distributed amplifier ended up in both Tektronix and Hewlett Packard oscilloscopes at the same time.
Yes, I believe that would be practically unheard of today unless there was some commercial advantage for the collaborating companies involved.

Quote:
Originally Posted by Radio Wrangler View Post
The Baofeng is with the Scouts, ready for the next group to reach the level for their foundation licences. I bought another two, so that a group of three youngsters can go on the air at once. Thanks for the inspiration and the generosity!
Glad to hear it has been put to good use along with two additional radios.
WaveyDipole is offline   Reply With Quote
Reply

Thread Tools



All times are GMT. The time now is 8:51 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Copyright ©2002 - 2021, Paul Stenning.