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Old 18th February 2013, 02:58 PM   #1
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Default My new GU50 SE (all pentode)

This is my new SE with GU50. Idea is "stolen" from Wavebourn. I tried 6J9P and 6J49P as input tube and I found that 6J49P sound better. I got around 12W.

schematic:

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amp:

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Old 18th February 2013, 03:00 PM   #2
TheGimp is offline TheGimp  United States
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Clean, very nice.
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Old 18th February 2013, 03:29 PM   #3
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Cathode resistor of GU50 not bypassed ?
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Old 18th February 2013, 03:32 PM   #4
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Originally Posted by artosalo View Post
Cathode resistor of GU50 not bypassed ?
No, with bypassed I have to much gain and no difference in sound. I must find the right NFB resistor, but right now I am out of 110K-1M resistors.
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Old 18th February 2013, 09:21 PM   #5
Ejam is offline Ejam  Australia
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Default Power Supply Details

Matejsirk

Can you give some power supply details, maybe a schematic would be nice.
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Old 18th February 2013, 09:30 PM   #6
Ketje is offline Ketje  Belgium
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Doesn't 01 on 100k cutoff the bass to high?
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Old 18th February 2013, 10:06 PM   #7
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Originally Posted by Ketje View Post
Doesn't 01 on 100k cutoff the bass to high?
Mona
-3db is: Solve[10^5 == (1/(2*Pi*f*100*10^-9)), f] - Wolfram|Alpha

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Originally Posted by matejsirk View Post
No, with bypassed I have to much gain and no difference in sound. I must find the right NFB resistor, but right now I am out of 110K-1M resistors.
"No difference".....interesting. Pentode without cathode decoupling yields gigantic output resistance. What speakers do you use ? Open baffle ? With my bassreflex speakers this would be impossible.
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Old 18th February 2013, 11:56 PM   #8
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Nice looking build, and I like the use of the regulated screen supplies. I am inclined to jump on the bandwagon suggesting a lower feedback resistor value instead of an unbypassed cathode resistor. By quick and dirty estimate, you are getting a plate impedance of around 4kOhm with the bypass and 10kOhm without it given the current feedback scheme.
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Old 19th February 2013, 09:13 AM   #9
45 is offline 45  Italy
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Quote:
Originally Posted by the_manta View Post
"No difference".....interesting. Pentode without cathode decoupling yields gigantic output resistance.
It doesn't mean anything. A constant current ampifier can work as good as constant voltage (a typical solid state amp) or (quasi) constant power amp (a typical zero feedback DHT).

Quote:
Originally Posted by the_manta View Post
What speakers do you use ? Open baffle ? With my bassreflex speakers this would be impossible.
It's not about bass reflex or dipole. It's more about drivers. A constant current amp + drivers with a good self damping (in the good sense) just work fine.
Have you ever heard of Ultrasound?

Sito Ultrasound Main Frame

Have a look at their bass reflex loudspeakers and amplifiers. Although I disagree with the multipole concept, their standard bass reflex speakers with their high Zout amps sound very nice.

Last edited by 45; 19th February 2013 at 09:27 AM.
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Old 19th February 2013, 02:22 PM   #10
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Today I add 220uF bypass cap on cathode on GU50 and change NFB resistor to 110K. Sensitivity is now 1V eff.

My speakers are SCHULZ KSP 215K + NEO CD3.0 in TL cabinet. Crossover 5500Hz only coil for KSP and cap for TW. Sensitivity around 94dB.
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