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Old 20th June 2013, 09:41 PM   #1
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Default Latest project, 80 watt valve amplifier

Here is my latest project.

12ax7 gain stage, into tone control, another 12ax7 gain stage.
12ax7 phase splitter into class ab quad el34 output.

Bias current is currently 80ma per el34 but I intend to decrease it to nearer 40ma.

It sounds very good, very detailed sound.
Click the image to open in full size.
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Old 21st June 2013, 06:17 AM   #2
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Can you show the schematic ? Seeing it would make this much more interesting.
Is this for HiFi or quitar use ?
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Old 21st June 2013, 11:05 AM   #3
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It has a switch for high and low gain for hifi or guitar use.
The cct diagram is too big to show.
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Old 21st June 2013, 11:09 AM   #4
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what is your plate voltage? 80mA per tube seems rather high..
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Old 21st June 2013, 08:10 PM   #5
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Quote:
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what is your plate voltage? 80mA per tube seems rather high..
Its quite a low B+ at 340VDC.

Yes I had a look around at other designs and 80mA is high.
I have ordered some more high power cathode resistors to reduce it to around 40mA.
Wen I designed it I wasn't sure of the cathode voltage so the cathode resistor was a best guess.
The cathode voltage is about 20 volts so I need a larger cathode resistor.
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Old 21st June 2013, 08:15 PM   #6
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Quote:
Originally Posted by nigelwright7557 View Post
It has a switch for high and low gain for hifi or guitar use.
The cct diagram is too big to show.
Cool build! Very nice and clean PCB. Do you have more pics showing the PCB itself? But I'd expect a guitar amp to be voiced different from a hifi amp, not just a difference in gain. The EQ even more so.
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Old 21st June 2013, 08:17 PM   #7
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The 12ax7 and el34 are often used in guitar amplifiers.
There is no feedback so the valve sound isn't watered down.

The EQ is just a standard tone control with bass and treble controls.
I have a gain control as well as volume to alter the overdriving sound.
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Old 21st June 2013, 09:10 PM   #8
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Quote:
Originally Posted by nigelwright7557 View Post
The 12ax7 and el34 are often used in guitar amplifiers.
There is no feedback so the valve sound isn't watered down.

The EQ is just a standard tone control with bass and treble controls.
I have a gain control as well as volume to alter the overdriving sound.
I got the guitar part with the 12AX7 and EL34 , but am a bit reserved about the hifi part, since both tube types are rarely used in hifi, EQ center points differ from guitar and feedback is actually a good thing for hifi.
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Old 21st June 2013, 09:13 PM   #9
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I see what you mean now.
I use it with my guitar but also like the same valve sound when listening to my mp3 player through it.

I should have said listening to music not hifi lol
Its not hifi with the amount of distortion it has.
I like the warm sound of valves and the detail.
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Old 21st June 2013, 11:30 PM   #10
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Quote:
Originally Posted by nigelwright7557 View Post
Its quite a low B+ at 340VDC.

Yes I had a look around at other designs and 80mA is high.
I have ordered some more high power cathode resistors to reduce it to around 40mA.
Wen I designed it I wasn't sure of the cathode voltage so the cathode resistor was a best guess.
The cathode voltage is about 20 volts so I need a larger cathode resistor.
yes, you are running the tube about 27 watts, higher than the tube's rating of 24 watts, your tubes will not last long that way...

no wonder your B+ is low, you can bias each tube to say 70% or 16 watts or about 40+mA per tube.

good luck...
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