Mystery BT-1 transistor used in ITT-KT radios audio output - UK Vintage Radio Repair and Restoration Discussion Forum

audion_1908 · 30th Apr 2020, 5:20 pm

I am trying to repair a very water damaged ITT-KB junior radio from 1959 but it like meny ITT-KT radios used a mystery BT-1 transistor and mine has a hole rusted in it, it's used in meny ITT-KT radios audio output, and the internet has come up dry? Can any one help find a data sheet for the bt1 transistor? So I can find a good sub
Thanks

https://www.vintage-radio.net/forum/...d.php?t=166164

Herald1360 · 30th Apr 2020, 6:13 pm

Given that it's just a PNP silicon transistor used for biassing applications I would think that just about any small signal PNP silcon device would work just fine. Finding one in a suitable case might be the biggest problem....

audion_1908 · 30th Apr 2020, 11:03 pm

Some on this forum think it might be germanium?

paulsherwin · 30th Apr 2020, 11:15 pm

Yes, I'm not convinced it is silicon. It was common practice to use a PNP Ge transistor clamped to the output heatsink to control the bias on the output pair as the temperature changed, to stop thermal runaway. If it looks like an AC126 or AC128, then that's probably what it is. A replacement won't be critical in terms of transistor characteristics, but will need to fit the heatsink properly so that it's correctly thermally coupled.

See
https://www.markhennessy.co.uk/artic...sistors.htm#t2
https://www.radiomuseum.org/tubes/tube_t2_tr.html

audion_1908 · 3rd May 2020, 9:39 am

Thanks for helping me to solve the bt1 transistor mystery.

Herald1360 · 3rd May 2020, 10:46 am

The Si /Ge mystery could be settled by anyone with an original set and a multimeter. Used as it is in the bias chain it could be either though Ge would seem more logical.

vidjoman · 3rd May 2020, 11:32 am

Back then transistors had a very wide characteristic and radio manufacturers would buy them unmarked and select and mark for their own purpose. Thorn did this and some circuit diagrams would give a ‘house’ type and Thorn would supply a suitable tested replacement. Transistors were manufactured and marked in that way too.

tri-comp · 8th May 2020, 5:27 pm

Have a look:
https://www.radiomuseum.org/tubes/tube_bt1.html

rgds,

/Torben

Kala_12 · 9th May 2020, 6:06 pm

Hi,
When did silicon transistors really start? Surely it was after 1959, so it's got to be germanium hasn't it?
Stuart.

AC/HL · 9th May 2020, 6:12 pm

It has to be, to stabilise a germanium output stage. Silicon wouldn't work.

Herald1360 · 9th May 2020, 10:04 pm

That depends. Tempco is about the same though you would need to drop a bit more volts on emitter resistors than with Ge.

That said, Ge is definitely more suitable here.

30th Apr 2020, 5:20 pm	#1
audion_1908 Hexode Join Date: Nov 2009 Location: Chertsey, Surrey, UK. Posts: 456	Mystery BT-1 transistor used in ITT-KT radios audio output I am trying to repair a very water damaged ITT-KB junior radio from 1959 but it like meny ITT-KT radios used a mystery BT-1 transistor and mine has a hole rusted in it, it's used in meny ITT-KT radios audio output, and the internet has come up dry? Can any one help find a data sheet for the bt1 transistor? So I can find a good sub Thanks https://www.vintage-radio.net/forum/...d.php?t=166164

30th Apr 2020, 6:13 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Mystery BT-1 transistor used in ITT-KT radios audio output Given that it's just a PNP silicon transistor used for biassing applications I would think that just about any small signal PNP silcon device would work just fine. Finding one in a suitable case might be the biggest problem.... __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

30th Apr 2020, 11:03 pm	#3
audion_1908 Hexode Join Date: Nov 2009 Location: Chertsey, Surrey, UK. Posts: 456	Re: Mystery BT-1 transistor used in ITT-KT radios audio output Some on this forum think it might be germanium?

30th Apr 2020, 11:15 pm	#4
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,947	Re: Mystery BT-1 transistor used in ITT-KT radios audio output Yes, I'm not convinced it is silicon. It was common practice to use a PNP Ge transistor clamped to the output heatsink to control the bias on the output pair as the temperature changed, to stop thermal runaway. If it looks like an AC126 or AC128, then that's probably what it is. A replacement won't be critical in terms of transistor characteristics, but will need to fit the heatsink properly so that it's correctly thermally coupled. See https://www.markhennessy.co.uk/artic...sistors.htm#t2 https://www.radiomuseum.org/tubes/tube_t2_tr.html

3rd May 2020, 9:39 am	#5
audion_1908 Hexode Join Date: Nov 2009 Location: Chertsey, Surrey, UK. Posts: 456	Re: Mystery BT-1 transistor used in ITT-KT radios audio output Thanks for helping me to solve the bt1 transistor mystery.

3rd May 2020, 10:46 am	#6
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Mystery BT-1 transistor used in ITT-KT radios audio output The Si /Ge mystery could be settled by anyone with an original set and a multimeter. Used as it is in the bias chain it could be either though Ge would seem more logical. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

3rd May 2020, 11:32 am	#7
vidjoman Dekatron Join Date: Jun 2013 Location: East Sussex, UK. Posts: 3,326	Re: Mystery BT-1 transistor used in ITT-KT radios audio output Back then transistors had a very wide characteristic and radio manufacturers would buy them unmarked and select and mark for their own purpose. Thorn did this and some circuit diagrams would give a ‘house’ type and Thorn would supply a suitable tested replacement. Transistors were manufactured and marked in that way too.

8th May 2020, 5:27 pm	#8
tri-comp Heptode Join Date: Oct 2008 Location: Aalborg, Denmark Posts: 903	Re: Mystery BT-1 transistor used in ITT-KT radios audio output Have a look: https://www.radiomuseum.org/tubes/tube_bt1.html rgds, /Torben

9th May 2020, 6:06 pm	#9
Kala_12 Hexode Join Date: Dec 2012 Location: Carmarthen, Carmarthenshire, UK. Posts: 268	Re: Mystery BT-1 transistor used in ITT-KT radios audio output Hi, When did silicon transistors really start? Surely it was after 1959, so it's got to be germanium hasn't it? Stuart. __________________ Half my stuff is junk - luckily, my wife doesn't know which half!

9th May 2020, 6:12 pm	#10
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: Mystery BT-1 transistor used in ITT-KT radios audio output It has to be, to stabilise a germanium output stage. Silicon wouldn't work.

9th May 2020, 10:04 pm	#11
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Mystery BT-1 transistor used in ITT-KT radios audio output That depends. Tempco is about the same though you would need to drop a bit more volts on emitter resistors than with Ge. That said, Ge is definitely more suitable here. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O