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  #1  
Old 9th October 2010, 07:52 PM
ColinA's Avatar
ColinA ColinA is offline
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Default WAD 2A3 bias reduction

My question is how do I reduce the Bias & plate voltage on my 2A3.
I am running the amp with single valves and the 2A3s are run quite hard.

I have got some 1940s original 2A3s to try & I would like to lower the operating point to a safer area.
Could anyone give an idiots guide please?
Many thanks
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Old 10th October 2010, 12:02 AM
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Default Re: WAD 2A3 bias reduction

You need to reduce the HT - isn't it at something like 390volts? - can't remember ! that's way too high. And you need to reduce the cathode resistor. In the typical 2A3 amplifier a cathode resistor is 750 ohms. But the HT should be down at 300volts or so with 750 ohmsSE. And you are using the wrong output transformer impedance. If you have (can't 'member) nominal 8 ohm speakers and the output transformers have a 4 ohm output - then use this one. It will cut the distortion that you may be getting by using only one valve instead of two.
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Old 12th October 2010, 11:57 AM
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Default Re: WAD 2A3 bias reduction

Out of interest here are the measurements on my amp.
OPT are 2K/8R (no 4R tap)
These I believe are the early type & match the valve curves. (2.5K reccomended)
Anode voltage 283v
Cathode voltage 50v
Cathode resistor 1K
So the amp does not seem to be driving the valves out of spec.
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Old 12th October 2010, 12:56 PM
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Default Re: WAD 2A3 bias reduction

okay! I would agree but ... that transformer is wound for two 2A3s in parallel and now it only has one. Of course this isn't impossible or particulalrly damaging. In theory there is more distortion but you might not be able to hear it. At those measurements I can't see why the cathode resistor is 1K instead of 750ohms, but there you go.
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