[HAM] Best 12AU7

[David Anderson]

Scott,

You can use a tube tester for both purposes.

You can, however, use the amplifier itself to do this as well.

Leslie driver tube sections share a cathode resistor, but if (and  
only if) you're sure that the 56k plate resistors are well-matched to  
each other, you can tell by the plate voltages if both sections are  
drawing equal bias currents.

And if you have an oscilloscope that can sum the readings for two  
channels, you can test the transconductance balance of the 12AU7 by  
measuring the signals at the twin triode plates. If you sum the 180  
degree out of phase signals, you should get zero or close to it. The  
same goes for a 147 circuit, though the 12AU7 is also acting as a  
phase splitter here. (If you wanted to, you could implement AC  
balance in a 147 amp by installing a pot to vary the voltage dividing  
point where the inverted signal is fed back to the 2nd 12AU7 section. )

You could theoretically do something similar with output tubes, but  
the plate voltage swings of an output tube are usually too large to  
read directly with most scopes unless you protect them somehow. You  
can test for balanced output tubes by running the amp into a 16 Ohm  
load resistor and looking for symmetrical clipping.

David

On Mar 11, 2008, at 11:19 AM, Scott Hawthorn wrote:

> At 02:22 AM 3/10/2008, Keith H Clark wrote:
>> Scott,
>>      Check ebay, that's where I got my Hickok 539C. Refurbed and  
>> calibrated
>> it. Seems like prices have spiked in the last couple of years  
>> though...I'd
>> send you mine but it gets too much use :>)
>
> Good idea, thanks. So far, I have gotten around this simply by  
> checking
> voltages around tubes. Would it be safe to sum up the advantages of  
> using a
> tubes tester as
> 1. Checking output tubes for transconductance (?)
> 2. Checking input tubes for balance?