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PP bias balance pot
PP bias balance pot
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Old 28th November 2007, 05:41 AM   #1
fred76 is offline fred76  Philippines
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Join Date: Jan 2005
Location: Manila
Default PP bias balance pot


I just want to ask knowledgeable folks here about this biasing scheme and its application for using an IT (or in this example parafeed L-inverter w/ 2 CT connections)...

This was posted in another forum by Kuei Yang Wang a few years ago:

Click the image to open in full size.

To quote him:

"Drawn here is the output stage with hum balance pot and bias balance...

Balance the voltages for the Grids... allows the halves to be separated to inject a few volt positive (in my example with the Amp balanced with perfectly matched valves around 2.4V into each grid) into the grid circuit so that varying this voltage balanced the current."


1.) How do you determine that the DC Bias is correctly adjusted?

2.) If I am to use a panel meter on the chassis how should it be wired/configured in the cirucuit and with least sonic penalty?

A friend mentioned I need a Milliameter in between the Plates and the OPT. I could bypass the meter using a switch. Is this the only way? Thank you.
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Old 29th November 2007, 06:34 AM   #2
Serge66 is offline Serge66  Switzerland
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Join Date: Nov 2007
Location: Canton of Jura
Hi Fred,

A mA-meter seems the solution.
With a switch that allows to measure the current in each plate. And to switch off the measurement circuit when not in use.

In my amp, I use indirect heated tubes (KT66) and I inserted a 1 ohm resistor in series with each cathode. The mV across these resistors give me the current in mA. It is a floating measurement. The cathodes are at 40V!!!

I am now surrounded by autistic people drooling on their mobile phones, tablets... sad
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