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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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It is currently 10 Apr 2020, 01:54

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PostPosted: 04 Apr 2020, 11:09 
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Joined: 14 Feb 2010, 13:13
Posts: 665
Location: Lisbon, Portugal
Hi!

I hope you´re all ok regarding this worldwide pandemic situation.

I´m currently building an amplifier, but already placed hardware and wired up heaters and some other things.

I was going to build the classic Williamson 6550 with two 6SN7GT tubes per channel, but I decided to stop, as I find the schematic to complex.

I wonder if any of you can help me choosing a relative simple schematic in UL PP mode? I have already one EL34 Oddwatt and would like to try another, specially avoiding power potentiometers which I don´t have, maybe some transistor/regulator circuit for the Bias?


Cheers,

Miguel


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PostPosted: 04 Apr 2020, 16:17 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi Miguel, The pots do not need to be large. Doing the math the current through a pair of KT88s is only 0.180 amps I squared times R = worst case of about 0.8 watts. A one watt is sufficient as you ought to never have the pot set at the extreme end. 2 watt would be more than enough. If you are using other tubes it is pretty easy to figure out as well. There is an alternate way to balance the tubes see the type "D" Poddwatts. Just be sure that the control voltage is fairly low. A range of about 2-3 VDC is all that is usually needed. It does increase the cathode votlage by a like amount and if too high will put the LM317HV at risk. You will probably have to juggle the resistor values to get it that low. In the Poddwatts since the voltage on the cathodes is already low the dc voltage is set a bit higher to allow operation of the LM317 further away from about 4 volts in the negative swing direction as they become non linear there. In the bigger amps this is not so much of an issue as they don't normally swing that low.

Good listening
Bruce
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Type D.jpg


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PostPosted: 05 Apr 2020, 10:07 
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Joined: 14 Feb 2010, 13:13
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Location: Lisbon, Portugal
Hi Bruce!
After several readings and looking at the D schematic I understood your good explanation.
I only have regular 0.2W potentiometers so I'm out of business unless I find some sort of transistor circuit with small trim-pots...

Stay healthy!


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PostPosted: 05 Apr 2020, 10:27 
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Joined: 04 Jun 2008, 20:59
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Hi, For the type "D" use that size is fine. I use 1/4 watt ones there all the time the actual dissipation is nil. Roughly 1 milli watt.

Good listening
Bruce

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