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Old 11th September 2012, 12:50 AM   #1
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Default Chinese GU50 PP

I have a GU50 PP coming from China. $286 sans tubes plus postage of $178 to Australia so still cheaper than buying the iron locally.
Any suggestions for mods?
The schematic is here
http://www.bezdz.com/images3/T30B-50-%B8%BA%D1%B9.bmp
The build quality of their other amps look good so will be interested to see what it looks like on arrival. I have already built a Se GU50 and love the sound of this tube.
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Old 11th September 2012, 01:45 AM   #2
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It is quite strange amp: UL, 420V B+, Gu-50. Most probably they used Gu-50 tubes in the amp designed around EL34.
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Old 11th September 2012, 02:38 AM   #3
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Looks that way looking at their other EL34 circuits. Its all point to point wiring so easy to mod. Any suggestions? I am not a valve guru by any means but am handy with a soldering iron! Seems to be plenty of scope with the power supply voltages, perhaps a higher B+?
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Old 11th September 2012, 03:13 AM   #4
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No, you will fry screen grids by higher B+, and higher load impedances will be needed (different iron). I would better wait for this wonder and check how does it perform. Then try it with EL34 tubes if you have them, and compare. Gu-50 are nice pentodes, but they want twice more B+ and higher load impedance.
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Old 11th September 2012, 08:09 AM   #5
palmas is offline palmas  Portugal
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The 0.1uF cap seems a bit small for a 97k load and the 50k volume is a bit to low, but just my impression. The rest looks like a classic. You should try the russian GU50, they are very tough and sound beautifuly.
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Old 11th September 2012, 09:39 AM   #6
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You were right Wavebourne, found a very similar circuit while digging around on their site.
http://www.bezdz.com/images3/T1S-34-%20KT88.bmp
Will be interesting to see what it sounds like.
Will post pics and first impressions when it arrives.
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Old 11th September 2012, 02:15 PM   #7
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screen grid seeing 416 volts?
but the GU50s have a max 250volt screen grid.
operating waay over spec, are they ok with it?
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Old 11th September 2012, 02:29 PM   #8
palmas is offline palmas  Portugal
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Quote:
Originally Posted by ruben alstad View Post
screen grid seeing 416 volts?
but the GU50s have a max 250volt screen grid.
operating waay over spec, are they ok with it?
That is not a problem with russian GU50, even at 450V... never used chinese ones.
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Old 11th September 2012, 02:42 PM   #9
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Originally Posted by palmas View Post
That is not a problem with russian GU50, even at 450V... never used chinese ones.
then it shouldn't be much hassle to make a kt88 p-p Williamson style amp with 440volt, ultra linear/triode would it?
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Old 11th September 2012, 02:49 PM   #10
palmas is offline palmas  Portugal
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Quote:
Originally Posted by ruben alstad View Post
then it shouldn't be much hassle to make a kt88 p-p Williamson style amp with 440volt, ultra linear/triode would it?
KT88 is a different bugger, not that well built and prone to faillure
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