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Old 16th October 2008, 06:37 PM   #1
Green77 is offline Green77  Sweden
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Red face Need help...again :)

Hi all.

Got some, well, stupied questions again...

I have built a el34 pp amp showed below..

http://crude.axing.se/images/hfta-amp.jpg

works really greate but i need help...

If i want to "put in" a active tone control, could that been done like the picture below? Just build it and put it in "serie" before my amp?

Thanks. //Daniel
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Old 16th October 2008, 07:24 PM   #2
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OT question: does bias adjustment work as drawn?
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Old 16th October 2008, 10:05 PM   #3
kevinkr is offline kevinkr  United States
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I've seen this topology used before, and it does work.. No issue with tubes going super nova if the wiper opens up, the C- actually will increase to the maximum value allowed by the resistive divider network that the pot is part of. It's a little trickier to tweak than some others, but not much. It's pretty fail safe.. (Even total failure of the pot, shorted or open cannot result in a 0 bias condition.) I'd even go so far as to say clever, and commendable. The U.S. military liked this sort of approach.

The schematic is not the best drawn instance of this circuit I have seen.
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Old 16th October 2008, 10:14 PM   #4
kevinkr is offline kevinkr  United States
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Default Re: Need help...again :)

Quote:
Originally posted by Green77
Hi all.

Got some, well, stupied questions again...

I have built a el34 pp amp showed below..

http://crude.axing.se/images/hfta-amp.jpg

works really greate but i need help...

If i want to "put in" a active tone control, could that been done like the picture below? Just build it and put it in "serie" before my amp?

Thanks. //Daniel
What you've drawn should work, but somewhere along the way you ought to have a volume control as well. Provided that the tone control circuitry has enough head room it would be better to run it at normal line levels to assure good snr and attenuate as needed after it.
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Old 16th October 2008, 10:58 PM   #5
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Quote:
Originally posted by kevinkr
I've seen this topology used before, and it does work.. No issue with tubes going super nova if the wiper opens up, the C- actually will increase to the maximum value allowed by the resistive divider network that the pot is part of. It's a little trickier to tweak than some others, but not much. It's pretty fail safe.. (Even total failure of the pot, shorted or open cannot result in a 0 bias condition.) I'd even go so far as to say clever, and commendable. The U.S. military liked this sort of approach.

The schematic is not the best drawn instance of this circuit I have seen.
I did not look carefully and thought that it is a completely self-biased, now I understand that "ground" signs were used as points where powering voltages are applied. Yes, such arrangements of pots is good, I use it myself.

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