Cyrus 1 power transistors

[Fleetwood Max]

Hi All,
The power transistors blew on my Cyrus1 quite some time ago. Mission told me the ones to use are BUV28a (at £6.00 ea!). I've also seen somewhere on this forum that BUV28 and BUV48 are used.
Could anyone give me a pointer towards which to use, I'm not wanting to turn it into a speaker eater, just to get it singing again!

Thanks in advance!

Max

[nigelwright7557]

I would advise finding out why the amp blew in the first place before simply putting in new expensive transistors !

In my early days of amp repair I made the mistake of just replacing output transistors only to find they blew again immediately or the first time they were stretched with volume.

Problems with the bias circuitry are classic problems.

[RocketScientist]

Are you sure those are the right part numbers? Those are TO220 switching transistors, which are not what I'd expect to find as output transistors in a normal Class B amp. They're also under $3 each US.

I'm not familiar with the Cyrus 1, but if it has more than one pair of outputs per channel, you also might need to worry about gain matching the sets. Otherwise the transistors may not share the current equally and the ones doing most of the work can die an early death (often taking other parts with them as Nigel suggests).

If they really did design the amp around switching transistors I'd be reluctant to substitute something else. I'm not sure what the design reasons were for using those transistors.

[Fleetwood Max]

Hi Nigel & Rocket Scientist,
Thanks for your replies.
The BUV28a was a part number given to by Mission as a part number to replace obsolete originals.
Here is a section of the email from Mission:
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The output transistors for your version of Cyrus 1 were PT7. These are obsolete, but we have a direct replacement available, which is the BUV28a. These cost £6.00 each, plus carriage of £1.06 and VAT, which gives a total of £22.41. It would be more usual to replace all four output transistors, and this would cost £29.45, if you decide to do that.
----------------------------------------------------------------------------------
I gave the amp to a mate who does that sort of thing, and he diagnosed the output transistors.
They blew a few years ago, and as I had a Quad 34/405 pre & power amp I just used that.
I think it gave up because i really used to cane it, back in the days when I was a batchelor and the next door neighbour was a fairly deaf understanding sort of bloke!

Regards,

Max

[jaycee]

"PT7" was just their factory code for BUV28. Get them from any supplier such as Farnell or CPC.

Yes the Cyrus is not really meant to be "caned" with only those TO-220 sized output devices. I belive someone on this board did explore replacing the BUV28 with more robust TO-247 devices although I can't remember which ones and whether it required much circuit alteration to use them.

[RocketScientist]

It's also really odd they're *switching* transistors which typically are vastly inferior for audio use. The two applications are totally different. But, as I said, hopefully the designer(s) had a good reason.

[nigelwright7557]

Originally posted by RocketScientist
It's also really odd they're *switching* transistors which typically are vastly inferior for audio use. The two applications are totally different. But, as I said, hopefully the designer(s) had a good reason.

I have had good results with IRFP240/9240 switching MOSFET's.

They are cheap, plentifull and high current.

[RocketScientist]

Originally posted by nigelwright7557

I have had good results with IRFP240/9240 switching MOSFET's.

They are cheap, plentifull and high current.

Yeah, MOSFETs are an entirely different beast and certainly won't work in the Cyrus 1 without a re-design. There are lots of bipolar transistors designed for audio and the advantages over their switching cousins are substantial.

And even among MOSFETs the IRFP240/9240 are generally considered signficiantly inferior to both the 2SK1530/2SJ201 and 2SK1058/2SJ162 which are only slightly more expensive and designed for audio use. The double die MagnaTec audio MOSFETs (i.e. BUZ901DP) also work well but they *are* expensive.