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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. |
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29th Jun 2017, 10:39 am | #21 |
Octode
Join Date: Nov 2004
Location: Hebden Bridge, West Yorkshire, UK.
Posts: 1,885
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Re: "Old-style" transistor audio amplifiers.
If you want to use Germanium and want some consistency, cheapness and availability there are plenty of ex-soviet transistors around. They can be had cheaply from the Ukraine (I've never had any bother with customs charges etc). I quite enjoy finding the datasheets then translating them from the Russian
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29th Jun 2017, 11:18 am | #22 | |
Heptode
Join Date: Dec 2014
Location: Lincoln, Lincolnshire, UK.
Posts: 583
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Re: "Old-style" transistor audio amplifiers.
Quote:
I think I have seen circuits in which a transistor is used for bias regulation, although have none to hand. I do have some BA156 diodes which were designed for bias regulation although I have had them so long that I do not know their exact use. |
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29th Jun 2017, 3:19 pm | #23 | |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,005
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Re: "Old-style" transistor audio amplifiers.
Quote:
I've got some genuine Siemens/Intermetall AC153 here but they're the type that are in a case that's a bit smaller than the old TO-1 envelope that OC81s and AC128s come in - meaning they won't fit the classic copper-clip-type heatsinks unless I squash them a bit.... I also have a couple of AC107 from the same stable but they're really low-noise preamp transistors rather than finding a natural home as a driver-stage. I've got no underlying visceral desire to go with Ge devices though, what with all the issues of variable leakage current, gain-matching and thermal runaway they bring. Silicon's a lot easier to tame. Whatever, I think I'll be able to bash together something that will work as well as if not better than the original Ge designs. I'm not after hifi here after all [hearing damage means I've got major roll-off of response below about 200Hz... I like to describe it as my 'organic hum-filter']. |
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29th Jun 2017, 3:56 pm | #24 | |
No Longer a Member
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
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Re: "Old-style" transistor audio amplifiers.
Quote:
http://www.worldphaco.com/uploads/Gretschdoc.pdf On the other hand in the UK, the transistors used for this treble boost application were the OC44 and OC71. Brian May for example had one with an OC44 and the Rangemaster unit that T.Rex used had an OC44 or OC71. Why the AC107 wasn't selected I don't know, it was either overlooked or too expensive. On the other hand, Toshiba in Japan were making a range of low noise germanium transistors in that era for Japanese tape recorders. The Japanese excelled in battery operated tape recorders. There were at least six type of low noise transistor. The common ones are the 2SB439 and 2SB440, which were used together in the front ends of many tape recorders and 8 track tape players. |
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29th Jun 2017, 4:21 pm | #25 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,005
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Re: "Old-style" transistor audio amplifiers.
Fascinating story on the AC107 and equivalents: I've only ever come across them 'in real-life' being used as preamp for a ribbon-microphone.
Wonder if anyone ever tried wiring two in parallel? Multiple paralleled transistors allegedly reduce intrinsic device-generated noise by 1/n where n is the number of paralleled devices. |
29th Jun 2017, 7:43 pm | #26 | |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,213
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Re: "Old-style" transistor audio amplifiers.
Quote:
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29th Jun 2017, 10:13 pm | #27 |
Dekatron
Join Date: Jun 2013
Location: East Sussex, UK.
Posts: 3,326
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Re: "Old-style" transistor audio amplifiers.
AC107's were used in the 1st Leak transistor amplifier in the front end of the pre-amp.
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30th Jun 2017, 12:18 am | #28 |
Heptode
Join Date: Jul 2009
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 719
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Re: "Old-style" transistor audio amplifiers.
There was a Wireless World article by AR Bailey (November-December '66) which described a (then) pretty decent 20W amplifier using a home made driver transformer and DC coupling to the loudspeaker.
It's available on the Americanradiohistory website. |
30th Jun 2017, 3:48 am | #29 |
No Longer a Member
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
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Re: "Old-style" transistor audio amplifiers.
Funny you should mention a device which used a transistor in the front end, with valves. I have attached a circuit that nobody on this forum will have ever seen before. This is because it was designed by my brother in 1973 and only deployed in NZ as far as I know and not published elsewhere. It boosts the gain of an audio amplifier input (typically using an EF86, EF37A, or 5879) where the gain was not enough overall for a magnetic phono cartridge. This transistor (npn germanium) was added to the front end of Juke box amplifiers to get the gain up for this application, without having to add another valve stage. I hope it surprises and delights, see attached.
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30th Jun 2017, 9:51 am | #30 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,087
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Re: "Old-style" transistor audio amplifiers.
No I've not seen that circuit, though I have come across the idea of driving the transistor into the cathode of an earthed-grid valve, cascade-style.
Interesting! I like the feedback from anode to base, to define the gain. I'm a bit worried about the DC conditions, as it is totally dependent on Vbe for the OC139, which is smaller than for silicon hence proportionally more temperature dependent! |
30th Jun 2017, 10:11 am | #31 |
No Longer a Member
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
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Re: "Old-style" transistor audio amplifiers.
Yes, with the emitter grounded the and the bias sourced from the valve's anode, the negative feedback stabilizes it, despite the temperature dependence of the Vbe, not dissimilar to the scenario you see when a transistor's base is biased from its collector with its emitter grounded. But it would be less thermally stable than a silicon device I agree. It might be better with a small value emitter resistor added, which could be bypassed to AC too. Apparently though it worked fine as it was. The idea behind it is that the valve is driven both at its cathode & grid.
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30th Jun 2017, 8:57 pm | #32 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,195
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Re: "Old-style" transistor audio amplifiers.
Hi Danger Man, yes the Bailey amp was built en-mass by the engineers at Reyrolle on Tyneside. A pretty good performer it was too.
Mine is still about and still functioning. We had the advantage of being able to get Radiometal lams and trifilar wire for the driver transformers. Ed |