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Old 24th December 2003, 11:12 PM   #1
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Join Date: May 2003
Location: USA
Default Finally got a tube amp circuit to work!!! (mild debug help needed)

Hello all-

Today I set to work with 6 D cell batteries and 16 AA batteries... Since it is Christmas Eve and RadioShack, etc. are closed, I have no battery holders, so everything is taped together. heh The amp is breadboarded (yes... on a plug board... no solder anywhere...). I used 4 D's in series for the heater (6V@~600mA continuous) and 16 AA in series for 24V@25mA for the plate. Much to my surprise after working on this for a while, it worked!!! And it soudns pretty darn good. Values for schematic are:

B1- 6V (4x D cell)
B2- 24V (16x AA)
C1- 2.2uF (cheap film)
C2- 470uF, 16V Rubycon
C3, C4- 36,000uF Sprague Powerlytic
C5- 470uF, 25V Rubycon
C6- 1000uF, 25V Rubycon
R2- 100k
R3- 1k
R4- 10k
R5- 100k
V1- 6GM8

The output with and without the cathode bypass cap was WAY too high, so I added the 100k resistor on the output. This is essentially a preamp... and it will be running to my headphone amp. So, the amp already can stand alone without a preamp, but I want the tube there for the more tubey sound (my amp is just a buffered amp with OPA2132 and BUF634 right now).

I removed the cathode bypass cap, but I do believe the amp sounded better with the cap in place. What else could I do to reduce the overall output? With the gain from the tube (about 14) before my amp's gain of 11, it is rather loud. I tried a resistor on the input, but it got muffled sounding. Does anyone have suggestions?

The schematic is attached... thanks to anyone who can provide some tips.
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File Type: jpg 6gm8 battery.jpg (24.9 KB, 236 views)
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Old 26th December 2003, 01:20 AM   #2
rkbh1 is offline rkbh1  United States
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Default Have you tried an input level control?

Congratulations on getting it to work. You may want to try a 100K logarithmic potentiometer between the input and the bottom (B-) rail, with the wiper connected to C1 input and the two ends connected across the output of your device feeding this circuit. It will work just like a regular volume control. Your problem now is that you are getting the full output of the circuit, so you need some sort of level control on the input.
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