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Old 11th September 2004, 01:39 PM   #11
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Just adjust your bias for 25-30% max Pa(idle).
I don't understand what do you mean.Can you make it clearly for me?

If I go with 420 to 450V Va there will be changes to anode voltages at the first stage and the phase splitter.I thing that this will effect the charectistics of these stages.What do you thing?

Sorry for the newbie questions.But I'd like to learn.
Thanks
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Old 11th September 2004, 11:05 PM   #12
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No problem! We all began somewhere

Pa is "Anode dissipation" Pa(idle) means "Anode dissipation with zero signal input". This is the vacuum tube equivalent to "Quiescent" current or power in solid-state amps (Iq, Pq).

Biasing your amps for a certain power just means multiplying your anode current by the anode voltage. So, for 25% Pa @ 450V, you get:

(25/4)/450V = ~14mA per tube at idle.

However, do experiment with values. The figure I gave you is a guesstimate for your setup and is book-standard for AB1. Try biasing the tubes as high as 60% Pa (which is almost pure class-A) and see what sound you like.

As for your driver section, yes, it'll slightly change the operating conditions, hence sound. Your 10K resistor decoupling the section is a good place to start, but try higher values for the sound you want. Even though the 12AT7 is rated 330V maximum Va, several friends and I have run them up to +520V with only a 20% degredation in tube life.

Here's also a short reference for you as to audio classes:

Class-A: Tube conducts 100% of cycle, plate current does not vary from minimum to maximum power. Pa(idle) is usually run at 90% of maximum. Suitable for single-ended as well as push-pull amps.

Class-AB: Tube conducts less than 100% of cycle, but more than 50%. Plate current varies widely from minimum to maximum power. Pa(idle) is usually run anywhere from 25 to 50% of maximum. Suitable for push-pull amps, or single-ended units with creative NFB techniques.

Class-B: Tube conducts 50% of cycle. Plate current varies widely from minimum to maximum power. Pa(idle) is usually run at less than 15% of maximum (depends on tube type). Suitable for push-pull amps only.

There are also numbers that can appear after the classes, 1 or 2. 1 means no grid current is drawn and no power is needed from the driver. 2 means grid current is drawn and power is needed from the driver. While grid current adds distortion, it does increase the efficiency of an amplifier and NFB usually cancells out the distortion.

Hope this helps
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Old 13th September 2004, 01:05 AM   #13
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@Geek

Thank you for the detailed explanation!

I donít know if Iíll use 6L6,yet ( What do you say about EL34? I donít know which tube to choose,EL34 or 6L6).

If I choose 6L6 Iíll try ultra linear AB1 at 450V. But Iíd like to ask you one more question. Go with self bias or fixed bias? And why?

What about NFB? How much NFB to try?
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Old 13th September 2004, 06:47 AM   #14
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Originally posted by resident
Thank you for the detailed explanation!
Yer welcome! That's what we're here for


Quote:
I donít know if Iíll use 6L6,yet ( What do you say about EL34? I donít know which tube to choose,EL34 or 6L6).
Both are great sounding tubes. Some like EL34 better, others 6L6. I like both.


Quote:
Go with self bias or fixed bias? And why?
I like fixed bias for PP because of in operation flexibility and you don't need perfectly matched tubes. You also can squeak a little more power out of the tubes too.

Self-bias is also great, especially for SE amps. The circuit simplicity also lends itself for more reliability.


Quote:
What about NFB? How much NFB to try?
Audiophoolery metaphysics aside, try your amp with no NFB. If it's stable (no oscillations, especially ultrasonic ones) and you like the sound, go for it.

I like a little NFB myself to "mellow" out the superfast transient responses. Rule-of-thumb is maximum 10 to 12 dB of NFB. I say use what % needed for you to enjoy the sound, since it's you that'll be listening to it all day
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