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Old 8th November 2013, 09:14 PM   #1
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Default Gu-50 SE Amp , quick question...

Hello, just wanted to ask a quick question. I am building the below gu-50 SE but what amount of max current should the b+ transformer be rated for? And I was looking at some Edcor transformers that had to 6.3 volt filament windings, can these be wired in series so the the center tap becomes 6.3 volts and the screen cond 12.6 volts.

Sorry for the basic questions, new to doing homebrew tube equipment and wanted to make sure that I am purchasing the right transformer.

Thanks
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Old 9th November 2013, 01:27 AM   #2
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The max current rating for B+ is around 200mA for this schematic (100mA/channel).
Yes, you can connect 2 separate 6.3V to get 12.6V. However, care & some basis measurement should be taken to make sure the same winding directions for both.

Some words about the schematic: Good design with direct coupled (no capacitors in signal path). Signal feed directly to G2 of power tube is worth for trying. However the dynamic and detail of sound is need more discussion . But it is realy worth for trying.
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Old 9th November 2013, 02:24 AM   #3
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Thanks, so this will be a mono single channel amp, so I will only need 100ma.

This is a slightly differnt schematic... Thoughts?

Again Thanks
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Old 9th November 2013, 09:12 AM   #4
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Hi Fox,
the difference between the schematics is not slight but dramatic.
the first one is screendrivin the GU50 and in the second one its in the normal pentode mode. Both have their pros and cons. think it will be interesting to build both and see how it performs.
Good luck.

Greetings,
Hilmar
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Old 9th November 2013, 02:47 PM   #5
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Thanks for the reply, now that I got the power transformer sorted out. Just need to decide on a output transformer.

Most of what I read said that triode mode would give about 8 watts peak and pentode mode would give about 12-18 watts peak?

I was looking at the 10, 15, and 25 watts output transformers by Edcor.


FoxWhiskey
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Old 9th November 2013, 02:59 PM   #6
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You should try both schemas to taste the differences. Pls note that C3 and C4 must have voltage rating 450V or higher.
Good luck.
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Old 9th November 2013, 03:41 PM   #7
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I am aware of the cap voltage ratings, just wanted to get a sufficient output transformer for he project.


Fox
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Old 9th November 2013, 04:16 PM   #8
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I got the 15 watt from Edcor. The amp will do about 15 watts peak, if at idle the GU50s are using 40 watts. Mine is doing about 31 watts at idle.
Here's a pic of mine: Click the image to open in full size.

I don't think the first schematic will amplify enough to drive the screens. The GU-50 isn't a very good tube to screen drive, because of the 250 - 300 volt screen voltage. It needs to be more like under 200 volts to do well with screen drive.

You can run the GU-50 ultra linear, if the B+ is 300 or under. Or in triode if the B+ is 350 or under. Screen voltage for pentode would be better if it's regulated.

Last edited by DeathRex; 9th November 2013 at 04:30 PM.
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Old 9th November 2013, 04:46 PM   #9
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Ok, I am in the process of finding the resisters I need, the ones that are not marked for wattage should be 1/2 watt rated? Any paticular type of resistor prefured? And the 100uf caps, what voltage should these be? 50v?

I just ordered the power and output transfomers from edcor.


Fox
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Old 9th November 2013, 06:44 PM   #10
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Use 100uF/50V is OK.
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