High Vceo audio transistors

[McMurdo]

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?

[paulsherwin]

Sounds like a pretty silly design to me.

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?

[McMurdo]

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.

[Al (astral highway)]

What about horizontal output transistors like 2SD1555? Burly and within spec by some margin ...

[ionburn]

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.

[G6Tanuki]

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.

[sortedradio]

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.

[MotorBikeLes]

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.

[Red to black]

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.

[Herald1360]

https://electronics.stackexchange.co...-series/285259

[McMurdo]

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.

This is the amp. Cloud no longer supply them.

[Boater Sam]

Not very helpful but should it not be using valves with this HT?

[Refugee]

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.

[Diabolical Artificer]

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.

[sortedradio]

Originally Posted by McMurdo

This is the amp. Cloud no longer supply them.

You could try Full-Volume in Sheffield (0114 244 8891) who distribute Cloud kit. They have been helpful in the past.

[Radio Wrangler]

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

[McMurdo]

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.

[sortedradio]

"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

[lesmw0sec]

Originally Posted by McMurdo

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?

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).