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Old 18th February 2008, 02:50 PM   #1
diyAudio Member
Join Date: Feb 2008
Default Need help with Sugden A-51

Hi everybody

I'm trying to improve my old Sugden C-51 A-51 preamp/amp combo. Then i realized it is not a class A amp as i expected. It runs cold and there 2 pairs of mj12955 and 2n3055 mouted on those big heatsinks. Then i open the top cover and i saw 3 big 2500uf 40v capacitors for each sides. Two of them in series for 0v-35v-70v and one before the speaker output to filter the 35V.

Question 1
Why it's not a class A amp as expected? (no apparent mods)

Question 2
Someone have a schematic of that version of the amp?

Question 3
I will put 5400uf 63v caps instead of those 2500uf for the supply. What kind of capacitors should i use for the output? I've heard there is some audiophile-approved ones...

Question 4
I've order some transistors to try in place of those 3055 2955, Mj15003 Mj15004 and MJ21193 Mj21194. Is it dangerous to try them? How can i adjust them? My boards have 2 potentiometers.

Thanks in advance

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Old 18th February 2008, 03:08 PM   #2
Geoff is offline Geoff  United Kingdom
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Join Date: Apr 2001
Location: UK
I have some information on the C51/A51, along with the schematic and parts list for the A51. Send me an email and I will forward a copy of the files.
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Old 18th February 2008, 03:41 PM   #3
Geoff is offline Geoff  United Kingdom
diyAudio Member
Join Date: Apr 2001
Location: UK
Whilst transferring the Sugden files from my old pc to my current one I had a look at the schematics etc.

The C51 control unit was used with both the A51 and the P51 power amps.

The A51 is a Class-A design but it uses only NPN transistors (2N4914) in the output stage and has a 55V supply rail.

The P51 is a Class-AB design with a complementary NPN-PNP output stage and a 70V supply rail.

From the information you have supplied it would appear that you have a P51 not an A51. I have a schematic for the P51 but not the parts list (the transistor types and component values are not given on the schematic).

When you have emailed me I will send you what I have for both power amps.
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Old 18th February 2008, 04:04 PM   #4
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Join Date: Feb 2008
So it must be a P-51 inside a A-51 housing... Strange because it does'nt looks modified. But i have to look at your schematic to confirm that.


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