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Old 20th December 2011, 06:28 PM   #1
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Default 6U8 driving 45

Anyone familiar with using 6U8 driving 45. A friend gave me this schematics and want to know if 6U8 would be a good tube to drive 45. Thanks.
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Old 20th December 2011, 06:41 PM   #2
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Originally Posted by xecluded View Post
Anyone familiar with using 6U8 driving 45. A friend gave me this schematics and want to know if 6U8 would be a good tube to drive 45. Thanks.
It looks fairly good, and so long as you stick with Class A1, a cathode follower will make a good grid driver.

I would, however, suggest DC coupling with a negative rail. That helps eliminate that coupling capacitor, and the resulting blocking distortion on signal peaks. I would also recommend at least some gNFB to help clean up the output. (NFB is not the "Spawn of Satan" that a lot of audiophools think it is. )
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Old 20th December 2011, 06:42 PM   #3
DF96 is online now DF96  England
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The cathode follower seems unable to decide which bias method to use, so it uses both DC coupling and cathode resistor bias. Either will do. It is daft to use both.
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Old 20th December 2011, 06:52 PM   #4
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Thanks for the suggestion. I am contemplating between using 6U8 or 417 driving 45. Which design do you think would be best for the 45. Here is a schematics for 417 driving 45.
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Old 21st December 2011, 12:08 AM   #5
kmaier is offline kmaier  United States
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You could also try this:

Looking for a 45 SE Schematic

Regards, KM
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Old 21st December 2011, 03:40 PM   #6
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Thanks kmaier. I've received the PDF file of your 12AT7 drive 45 design.
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Old 21st December 2011, 04:55 PM   #7
kevinkr is offline kevinkr  United States
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The 417A/5842 makes an excellent driver for the 45 and 2A3. Consider fixed bias on the grid of the 45..
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Old 22nd December 2011, 02:02 AM   #8
kmaier is offline kmaier  United States
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Xecluded, you're welcome... the gain of the amp is moderate... 775mv input will yield 1-watt output with good tubes. The input/driver section has a gain of ~50 volts/volt. With a 2V pk-pk 10kHz square wave input, the signal at the 45 grid is 100V pk-pk with no ringing or slew limiting. I've tested with OPTs from Hammond, Magnequest, Electra-Print, James and Transcendar and none have any notable issues but some of the better ones certainly have the performance edge over the Hammonds. If you get some really good 45 tubes, you can realize s/n ratio better than 80dB (referenced to 1-watt) via the AC hum balance.

Regards, KM
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Old 22nd December 2011, 02:32 PM   #9
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Thanks KM. I plan to use/recycle output transformers from vintage single ended 6v6 amp. It all is well, I will try the more expensive Electraprint transformers.
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Old 23rd December 2011, 10:24 AM   #10
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I would wire that tube with a 1k gris stopper going into a 220k resistor to ground. a 240 ohm resistor on pin7, remove the zener, connect the 1 uf cap to pin 3 and 7. a 270k from pin 3 to B+. remove 500k resistor. change cathode pin 8 circuit to a 47k with a 2.2 uf coupling cap to the 45. change the 100k to 220K on the 45 grid circuit. get a 2.5 VCT transformer and install the cathode circuit on the center tap of the heater transformer. try it see how it is some people like the ones I come up with off the top of my head.

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