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Old 25th September 2012, 03:33 PM   #21
SY is offline SY  United States
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Originally Posted by pilovis View Post

Anyway, I increased the load resistor of 6G6G and the feedback resistor to have a little more gain...
Problem is, you need a LOT more gain. This circuit really needs to be rethought from the ground up.
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Old 25th September 2012, 03:39 PM   #22
pilovis is offline pilovis  Italy
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No, not significantly. The cap can only do bad things there, if it does anything.
I don't think a cap of 100uF in parallel to 7-10 ohm (internal resistance of the two diodes) can do any bad thing, maybe it can do nothing.
What about all other caps in parallel to "much bigger" cathode resistors for self biasing? Do they do terrible disasters?

Last edited by pilovis; 25th September 2012 at 03:41 PM.
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Old 25th September 2012, 03:40 PM   #23
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No, not significantly. The cap can only do bad things there, if it does anything.
You get rid of about 10ohm but i agree with you that the setbacks are not worth it.

Pilovis find a high gain pentode and use plate to grid feedback.

That way you get descent power with nice distorsion figures.
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Old 25th September 2012, 03:41 PM   #24
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You should believe SY when it comes to LED biasing. Maybe read some of his articles on the subject. If you use LEDs, you don't need a cap.
Gain is your main issue here, anyway.
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Old 25th September 2012, 03:49 PM   #25
pilovis is offline pilovis  Italy
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You should believe SY when it comes to LED biasing. Maybe read some of his articles on the subject. If you use LEDs, you don't need a cap.
Gain is your main issue here, anyway.
You're right but I don't need very much power from this amplifier, 3 W output on 8 ohm load is enough for me.
I will use it with a separate preamplifier, this is just the final stage.
Also, I choosed the 6G6G as a driver mainly for its cost (very cheap, NOS = 2 euro each here in Italy) and its low heater's current (0.15A).

Last edited by pilovis; 25th September 2012 at 03:53 PM.
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Old 25th September 2012, 04:43 PM   #26
TheGimp is online now TheGimp  United States
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The bias circuit won't work. You only get about 9V max depending on the R & C values.

In order go get it to generate the level you seek, you would need to tie the positive rectifier output to ground instead of the CT to gnd. With the CT to gnd, you only really are using one half the transformer in a half wave rectification topology. The other half does nothing.

If you tie the +rectifier output to ground, you may risk greater hum from the heaters.
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Old 25th September 2012, 05:17 PM   #27
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You'll have to have some major preamplification, your gain is only 0.15 or so.

A cathode resistor in a self biased circuit is a different situation than LEDs; because of impedance (and how that affects gain and output impedance), it usually requires a capacitor bypass. LEDs are much lower impedance and operate essentially as voltage sources.

Seriously, you have some fundamental problems in the circuit and you're only picking at the edges.

edit: my bad, the gain is better than that but still much too low
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Old 25th September 2012, 05:44 PM   #28
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Wow! Mu of 9.5 when triode strapped.

Add another gain stage if you really want to use that tube.
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Old 26th September 2012, 02:06 AM   #29
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Problem is, you need a LOT more gain. This circuit really needs to be rethought from the ground up.
How about a 6SJ7 in place of the 6g6g?
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Old 26th September 2012, 02:37 AM   #30
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6CB6 strikes me as a better choice- you want transconductance, I would think, allowing you to increase the closed loop gain while keeping the source impedance to the transformer low.
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