Independent tube biasing?

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Independent tube biasing?

Postby fcervone » Wed May 04, 2016 4:52 pm

i hope this is not a stupid question but here goes. Why dont more amp manufacturers use independent tube biasing? I had a maven peal amp that you could set the bias on each tube independently. No need for matched tubes.

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Re: Independent tube biasing?

Postby battradio » Wed May 04, 2016 7:31 pm

Cost and most users would never bother to set it correctly .
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Re: Independent tube biasing?

Postby Geek » Thu May 05, 2016 1:04 am

Depends on who the amp is target marketed toward. If the masses, the above is painfully true.

For higher end builds, they exist with detailed instructions (Rogue Audio) or many others have free biasing (you supply the tubes) from their authorized depots.

-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Re: Independent tube biasing?

Postby burnedfingers » Fri May 06, 2016 5:44 am

If you remember years ago cars had wing vents in the driver and passenger doors. A bean counter figured that Ford motor company could save aproximately .78 per unit by getting rid of those vents. The result was two fold. Not only did they save money on each unit out the door but the people buying these cars now were purchasing AC instead of going without it. BAck to independant biasing.. who will loose? The tube suppliers won't be selling you matched pairs at inflated prices. The amplifier manufacturer will loose profit because of adding additional pots and resistors. The bottom line is usually a money factor. There are some of us that will take the time to modify our circuits to have individual tube bias because we see the merit behind the idea.
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Re: Independent tube biasing?

Postby paart » Sat May 07, 2016 9:37 am

Being able to independently set a DC bias point for two output tubes doesn’t eliminate the need for matched tubes. While doing so is beneficial in reducing the DC component in the output transformer primary at idle, it doesn’t ensure that the two tubes will conduct identically at 25, 50 or 100 percent power output.
This can be thought of as the AC balance of the output pair. While some designs incorporate an AC balance control for the phase splitter, it ends at that point. If the output tubes do not have matching curves so that the “AC” balance is maintained in that stage, then increased distortion will result.

Due to production tolerances, all tubes deviate from the published plate or transfer characteristic curves in a number of ways. The published curve is an ideal, or average, and individual tubes will deviate from this. Setting the grid bias at idle sets the relationship between grid voltage and plate (and screen grid, if applicable) current at one point only. During operation this relationship will vary from tube to tube, as the grid voltage is varied over a wide range.

The same thought applies to trying to match tubes on a tube tester. A tube tester only measures the transconductance of the tube at one point on the curve. Two tubes that show the same reading on a tester at the specified grid voltage (sometime labeled “bias”) may show considerably different results at a different grid voltage.

Be careful where you buy your “matched tubes”. True tube matching requires special equipment and knowledgeable people!
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Re: Independent tube biasing?

Postby dcriner » Sat May 28, 2016 4:46 pm

When a seller claims that tubes are matched, it is likely that just their emission is close - or maybe nothing at all. Even if perfectly balanced, tubes' performance change with age.

For my ST-70, I aspire to reset the bias every so often, but often just wait until next year.
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