303 Output stage transistor replacement

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12-Nov-2011 06:07 PM

Xerxes75

Xerxes75

Posts: 42

On this moment i am shopping around for some parts to complete my revision. Currently there are 38494 transistors placed in the output. In the site i found multiple solutions to replace them.

My question is which are best? i am leaning towards the MJ15003. But against the other options they have a different voltage? must i do something to fix that?

xerxes

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12-Nov-2011 11:55 PM

EJP

EJP

Posts: 1712

I used to use 2N3055 as a cheaper option but these days I would definitely use MJ15003. The higher voltage rating just gives you more reliability, nothing you have to do about it.

Dada Electronics Australia
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13-Nov-2011 04:54 PM

MJ

MJ

Posts: 55


I see no season to replace good working transistors.

But if you have the right tools you can measure the leaking current. After all, don't fix it if it ain't broke....

 

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13-Nov-2011 11:33 PM

EJP

EJP

Posts: 1712

I read that the 38494 is a 2N3055 selected for wide safe operating area (SOA). Another reason to stop using 2N3055s.

If you buy the correct version of the MJ15003 it has several times the gain of the 38494 and therefore a fraction of the distortion, as well as much greater voltage tolerance and SOA rating. That would be a reason for replacing 38494s.

EJP
Dada Electronics Australia
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14-Nov-2011 07:52 PM

MJ

MJ

Posts: 55

A standard Q303 triplet gains let say 100*100*35

change it tro 100*100*70 and there's not much of a change

maybe harmonic distorstion goes from 0.002 to 0.0015 but that's not audible at all.

 

As mentionent don't 'upgrade'  good working transistors. It's good for labour and  manufacteurs but  Quad music lovers will hear no difference at all.

 

 

 

 

 

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14-Nov-2011 11:47 PM

EJP

EJP

Posts: 1712

Well an MJ15003 has hFE=150 so there is more change than that, 5-6 times as much O/L gain. I have measured THD reductions of 2/3, from 0.03% to 0.01%, changing 38494s to MJ15003s. However I have to agree that I never change power transistors in 303s at all unless blown.  Too risky for one thing ;-)

EJP
Dada Electronics Australia
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15-Nov-2011 01:26 AM

Stefaan

Stefaan

Posts: 628

For the High-end boards we recommend 2x MJ15003 or MJ15003 + MJ15004 in the NPN/PNP version.

For the power-supply we recommend MJ802.

Stefaan

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11-Feb-2020 02:35 PM

Grog

Grog

Posts: 7

sorry for dragging up and old post, but i couldn't find an answer anywhere!

I've fully refurbished my 303, replacing all components on the driver, PSU boards, the bridge rectifier and main capacitors, 

I have changed the PSU regulator transistor to an MJ802, 

this 303 is a very early one from the first year of production, the board layout is earlier and different to all those shown in the service documents available.

I have updated to the later basing arrangement, (sub board) and doubled up the current limiting diodes as I don't use ESL's

it's sounding great, but I keep thinking of replacing the output 38494's with MJ15003's 

I know the output devices I have should last a fair while longer, but would the thought of better performance and longer lifetime is still tempting me...

if i change to MJ15003 do I need to add any compensation capacitors to prevent hf instability?

i've not seen it mentioned anywhere but looking at the internal wiring methods used, i guess the extended bandwidth of the newer devices must lead to come risk of oscillation unless compensated?

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11-Feb-2020 05:33 PM

MJ

MJ

Posts: 55

The first version of the 303 schematic is on the site. I simply use the original transistors on my endamps. They work ok, never change a winning horse.

This transistors from the sixties uses a good epitaxial proces. Later transistors uses another, cheaper proces and causes from second break-down issues

So better use the original ones instaed of paying good money for less hardware. 

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