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Old 19th August 2011, 03:20 PM   #1
death is offline death  Iran
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Thumbs down How about paralleling output tubes

Hi , This is my first post and i am so glad joining this forum.
Recently i made a single ended tube amplifiere with 6ar5 , i want to know what if i add another 6ar5 parallel with the output tube, can i gain a better power ? what about the distorsion ?
thank you.
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Old 19th August 2011, 06:20 PM   #2
DF96 is offline DF96  England
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No, you would probably need a different output transformer.
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Old 19th August 2011, 06:23 PM   #3
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Death -- to realize the extra power the second tube could provide, you would need to replace the output transformer with one that has one half the primary impedance that the existing transformer represents -- otherwise, no power increase will be realized. The reduced impedance is required since the output stage would now have the ability to pull twice the current. The power supply would also need to be able to supply the additional quiescent current the second tube would draw.

Distortion as a percent of power output should remain unchanged (although the power output will be doubled), but the frequency response on the high end should be extended due to the lower winding capacitance that a transformer with less primary impedance can afford.

Dave
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Old 19th August 2011, 07:01 PM   #4
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I think there is a way to double the output power for paralled tubes without changing the OT primary Z.

Lets say the initial tube runs a 1xV and 1xI. Then with two tubes in parallel, use 1.4xV and .7xI on each tube (or 1.4xI for both). (current is reduced by lowering the idle current and reducding the drive level) Now the dissipation in each tube remains the same at 1.4xV times .7xI, but the output power increases to 1.4xV 1.4xI => 2x and the primary OT impedance stays the same at 1.4xV / 1.4xI => 1x. (note that the operating point of the tubes is changed now, with each tube seeing twice the original primary Z, likely lower distortion now)

So just use 40% higher B+ approx. and reduce the idle current (increase the bias and reduce the input AC signal, ie volume control) on the tubes. Input sensitivity will be increased due to twice the effective gm. Total current draw from the raised B+ will now be 40% higher too, so the power supply must be upgraded obviously.

The tubes must be rated to safely use the higher B+ of course. The OT will have to handle twice the power now though, so if it is not rated for this, then the Low Frequencey spec will have to be increased by 40% to handle the extra power. (ie, a 20 Hz low end spec will increase to 28 Hz for the now higher power level to avoid magnetic saturation in the OT) Going beyond doubled up tubes though may cause heating problems in the copper winding resistance of the OT, and poor damping factor from excessive winding resistance at that power level (if the OT is not rated for that power level).
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Last edited by smoking-amp; 19th August 2011 at 07:20 PM.
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Old 19th August 2011, 07:35 PM   #5
DF96 is offline DF96  England
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Also possible saturation in the OPT due to increased DC current. Basically, you can only do it if the OPT was being under-run with one valve.
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Old 19th August 2011, 08:24 PM   #6
tricomp is offline tricomp  Denmark
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...and what about moving the 8-Ohm speaker from the 8-Ohm transformer connection to a 16-Ohm connection if that's available.
That should reflect back on the plates with half the primary impedance, suitable for 2 tubes in parallel.

Of course the above reservations regarding the transformer and power-supply still holds.

Just a thought but easily implemented if the transformer is compatible.


rgds,

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