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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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27th Feb 2019, 6:07 pm | #1 |
Dekatron
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High Vceo audio transistors
I'm trying to fix an audio amp which uses MJL4302/4281 pairs. Vceo is 350V and the amp requires 'selected' 410V transistors. The OEM no longer supplies them. AN Other has fitted stock transistors and they failed.
Just wondering about the possible solutions?
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27th Feb 2019, 6:08 pm | #2 |
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Re: High Vceo audio transistors
Sounds like a pretty silly design to me.
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27th Feb 2019, 6:13 pm | #3 |
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Re: High Vceo audio transistors
I'm with Paul here, maybe buy a dozen and test? Or reduce the HT voltage and use standard ones. Are there any other 450V types out there?
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27th Feb 2019, 6:18 pm | #4 |
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Re: High Vceo audio transistors
It does sound a strange design but there you go. I can't just see anything easily available, which is maybe why the OEM used selected ones, it happens sometimes in audio amps. How do I diy-select for high-Vceo ?
I don't think reducing the voltage is easy as it's 240 in, transformer-rectifier-capacitor supply out. The amp is a 'transformerless' 100V line amp, driving the 100V line directly.
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Kevin |
27th Feb 2019, 7:30 pm | #5 |
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Re: High Vceo audio transistors
What about horizontal output transistors like 2SD1555? Burly and within spec by some margin ...
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Al |
27th Feb 2019, 7:52 pm | #6 |
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Re: High Vceo audio transistors
I have used some 700V ones in the past (PHE13009), but they are principly designed for switching. I successfully used them in high voltage capacitance multipliers, but their gain is very low. Nowadays with more use of FETs I suspect choice maybe limted.
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27th Feb 2019, 8:21 pm | #7 |
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Re: High Vceo audio transistors
Motorola made some: in the past I've used the likes of the MJE340 in a classic 'single-ended' Class-A output-stage with a couple of hundred volts of HT.
There were similar single-ended class-A outputs used in a number of 1970s TVs, clearly designed around the "what can we cheaply fit in place of the beam-tetrode?" make-this-TV-transistorised-at-lowest-possible-cost basis. |
27th Feb 2019, 9:09 pm | #8 |
Heptode
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Re: High Vceo audio transistors
Hi Kevin,
Cloud Electronics CXV425 amplifier uses MJL4302/4281 transistors selected for high voltage. I bought some from the in October last year, they come as part of a repair kit for the amp.
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Martin BVWS member GQRP Club |
27th Feb 2019, 9:29 pm | #9 |
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Re: High Vceo audio transistors
Grundig used the MJE340 in the sound output of the 1970 model 717, they are rated at 300v. I used to replace with BD410, my book givs 500v for those, so worth a try.
I probably still have some if you are stuck. Les. Last edited by MotorBikeLes; 27th Feb 2019 at 9:30 pm. Reason: Punctuation and sp. |
27th Feb 2019, 9:43 pm | #10 |
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Re: High Vceo audio transistors
The two numbers given by the OP are a complementary pair, ie. both NPN and PNP types.
The MJL4281 being NPN and MJL4302 being its PNP counterpart. Edit: see here: https://www.onsemi.com/pub/Collateral/MJL4281A-D.PDF He also needs specially selected 410V Vceo types or a suitable substitute for another complementary pair. Post#8 by Sortedradio might help.
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27th Feb 2019, 11:24 pm | #11 |
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Re: High Vceo audio transistors
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28th Feb 2019, 12:56 am | #12 | |
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Re: High Vceo audio transistors
Quote:
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28th Feb 2019, 2:42 am | #13 |
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Re: High Vceo audio transistors
Not very helpful but should it not be using valves with this HT?
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28th Feb 2019, 3:53 am | #14 |
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Re: High Vceo audio transistors
There is a compliment for IRF840 and many others.
They go up to 800V. It would require some modification but would cut the cost of repairs. |
28th Feb 2019, 8:00 am | #15 |
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Re: High Vceo audio transistors
Digikey show over 5 pages of BJT's with a collector/emitter breakdown of 450v, several similar parts to the MJL4302, like the MJE18004, with a ceo of 450v - https://www.onsemi.com/pub/Collateral/MJE18004-D.PDF, surely you'll be able to find a replacement or am I missing something?
Andy.
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28th Feb 2019, 9:58 am | #16 |
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Re: High Vceo audio transistors
You could try Full-Volume in Sheffield (0114 244 8891) who distribute Cloud kit. They have been helpful in the past.
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28th Feb 2019, 10:29 am | #17 |
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Re: High Vceo audio transistors
Ah, it's a high power 100v line driver.... presumably using high voltage transistors to dodge the use of a transformer on the output.
Understandable, I suppose, but as the speakers are going to be connected using transformers, maybe one at the drive end isn't quite so devastating. At least there's a reason for those high voltage transistors, though no real excuse for using specially selected ones. David
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28th Feb 2019, 10:44 am | #18 |
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Re: High Vceo audio transistors
Here's the beastie, each amplifier has 4 channels, 2 in each 'heatsink tunnel', the right hand module has been removed. Everything's a bit tight.
I'm not confident fitting non-audio specific transistor equivalents, given its history, and I'm supposed to be the one to effect a permanent 'fix'.! Most high voltage, high current transistors in this range seem to be designed for switching use, and dont easily track/load share in the linear mode. Sometimes these things like to oscillate as well.
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Kevin |
28th Feb 2019, 10:58 am | #19 |
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Re: High Vceo audio transistors
"at extended mains voltages (eg. 260v) those transistors can be swinging just shy of 400v. We test each transistor, and only use those which are capable of at least 405v." - Cloud Electronics
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28th Feb 2019, 11:01 am | #20 |
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Re: High Vceo audio transistors
It is not always realised that the breakdown voltage can be measured (non-destructively) by using a variable power supply feeding the C/E via a high value resistor to limit the current. As the voltage increases, a meter across the device will show the voltage level off at the breakdown voltage (a bit like a zener diode).
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