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Old 29th March 2005, 09:57 AM   #1
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Default Tube amplifier mods

Hello all! I have modded one of my tubeamps by removing a resistor and adding a 6AK5 gain stage and it sounds more beautiful than ever.

Heres the schematic, i dident include the power supply as its still the same and can be seen in other schematics.
Click the image to open in full size.
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Old 29th March 2005, 10:17 AM   #2
Giaime is offline Giaime  Italy
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Hello! Good! It seems a good amp.

Is it an american amp? That's because of the 6l6 outputs...
How do these sound in hi-fi? I only know them in guitar amps...
And that input pentode stays quiet or it is microphonic like many small pentodes?
You also have some strange capacitors values (235uF? )...

What resistor have you changed?
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Old 29th March 2005, 11:41 AM   #3
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6L6GC is a very good tube in HIFI, actually it was designed for PA amps tho. The resistor i removed was between the 120µF and 100µF cap on the schem, why some of the cap values it odd is because its 470µF caps in series. I built this amp from scratch using chassis, OPT and PT from a old organ amplifier, the guys on geek.scorpiorising.ca helped me with the design of the amp. The input 6AK5 is microphonic, you hear *dinnnnggg* in the speaker if you tap it, you even hear some faint clicks with a echo effect when you turn B+ on from the cathode and grids when they heat up.
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Old 29th March 2005, 12:52 PM   #4
Giaime is offline Giaime  Italy
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Oh yes, that resistor is just useless (if not bad) when there's already a choke to isolate the driver stage from the outputs. Good design!

I bet that every pentode you tried in the input stage was microphonic... I remember a design somewere that used a power pentode as an input, maybe a 6V6 or similar might work... they are less microphonic (but worse sounding)... just to try...

I didn't know that 6L6 were used in hi-fi (other than in american designs). I supposed that beam power tetrodes just don't cut it in high-fidelty outputs... What are their sonic characteristics against well known outputs (EL34, EL84, KT88, 6550...).


Edit: oh sorry actually some of the tubes I mentioned as regulars hi-fi are in fact beam pentodes...
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Old 29th March 2005, 01:21 PM   #5
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Microphonics is not a problem unless you place the amp on something that vibrates a lot, like a sub or if the gain is very high.
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Old 29th March 2005, 04:26 PM   #6
Giaime is offline Giaime  Italy
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Quote:
Originally posted by Tekko
Microphonics is not a problem unless you place the amp on something that vibrates a lot, like a sub or if the gain is very high.
Yes, and you might like the increased "airyness" that microphonic tubes add to your sound.
Keep posting! Maybe some pics of the amp?
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Old 29th March 2005, 05:08 PM   #7
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Heres some pics of the thing:

Notice the cable to the metal screen on the 6AK5:
Click the image to open in full size.

The amp from above:
Click the image to open in full size.
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Old 29th March 2005, 05:46 PM   #8
Giaime is offline Giaime  Italy
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Wow! An old and rusty amp! I love these things!
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Old 29th March 2005, 06:12 PM   #9
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It may be old but its not rusty
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Old 30th March 2005, 12:51 AM   #10
anatech is offline anatech  Canada
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Hi Giaime,
A KT-66 / 7581A is a 35W 6L6GC (30W). I'm sure you've seen those. I really like 6L6GC's in a good amp.

-Chris

P.S. Nothing like a 6DJ8 for microphonics!
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