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In Reply to: Unbalanced double triode creates unbalanced speaker output? posted by anthony v on September 05, 2003 at 19:08:35:
Anthony,Sometimes, it's not that simple. The manufacturer could have designed one-half of the duo-triode as an input tube and use the other half for another function, in the same channel.
So, you may not hear a difference between left and right channels.
It's kinda nice to have strong, symmetric, and quiet tubes throughout the unit---but, I've found that the one spot it does make a difference is the phase inverter postition.
Follow Ups:
Steve Oda wrote..."It's kinda nice to have strong, symmetric, and quiet tubes throughout the unit---but, I've found that the one spot it does make a difference is the phase inverter postition."
Explain please, if you would, what difference can you attribute to it? Thanks Steve.
Jim,As I recall, the long-tailed Fender style phase inverters use one-half of the duo-triode to drive the upper part of the sine wave. The other half to drive the lower part of the sine wave. As in the Deluxe Reverb AB868 circuit. But, many of Fender circuits use similar layout.
I try dressing the PI with tighter tolerance resistors and caps to get a more symmetric sine wave off matched power tubes. For guitar amps this gives a browner clipping tone.
This is another common misconception - that the "phase splitter" actually divides the signal. Not true! Actually, both tube sections drive the entire sine wave - one just does it 180 degrees out of phase to the other. Here's a quote from noted amp designer Randall Aiken:"Common misconception #1: A phase splitter splits the signal into two halves, each half feeding one output tube so each output tube amplifies only half the signal.
The truth: A phase splitter produces (ideally) two equal amplitude sine waves, each inverted with respect to the other, not two "half sines". Both power tubes get driven with a full, complete sine wave at all points of the input cycle in any class of operation."
The long-talied pair (due to the "tail" resistor) tends to be self-balancing also.
Keep in mind too, that the gain is different between the two triode sections because while one half of the tube has signal applied to the grid, the second half signal is applied to the tube's cathode. This means the gain will be different by the nature of the circuits.
Given all this, it's hard to see how anything but gross mismatching (one section normal, one section near death) could have any effect.
This is another common misconception - that the "phase splitter" actually divides the signal. Not true! Actually, both tube sections drive the entire sine wave - one just does it 180 degrees out of phase to the other. Here's a quote from noted amp designer Randall Aiken:"Common misconception #1: A phase splitter splits the signal into two halves, each half feeding one output tube so each output tube amplifies only half the signal.
The truth: A phase splitter produces (ideally) two equal amplitude sine waves, each inverted with respect to the other, not two "half sines". Both power tubes get driven with a full, complete sine wave at all points of the input cycle in any class of operation."
The long-talied pair (due to the "tail" resistor) tends to be self-balancing also.
Keep in mind too, that the gain is different between the two triode sections because while one half of the tube has signal applied to the grid, the second half signal is applied to the tube's cathode. This means the gain will be different by the nature of the circuits.
Given all this, it's hard to see how anything but gross mismatching (one section normal, one section near death) could have any effect.
"The long-talied pair (due to the "tail" resistor) tends to be self-balancing also"If it was operated with a theoretical constant current source in the cathode, and if the Miller effects were insignificant, then yes- otherwise , the 2 sides end up with unequal gain, hence the balancing schemes usually employed vary the plate resistor of one side to balance them, or use fixed calculated resistors (82k & 100k) as a cure all like in the Fender and amps.
This assumes of course that the tube in question has equal gain and transconductance for both halves, which is what Steve's point was- that in his example, since the circuit is fixed with no adjustment like a Citation or VTL has, one would be advised to use a tube with equal halves to ensure balance if that's what one seeks.
Jim,Yes, you are correct. I reduced my explanation too simplistic. However, I find that guitar amps tend to sound better (maybe it's the clipping characteristics) with a symmetric phase inverter.
Really, non-symmetric tubes make for a completely different tone. Some folks actually like it. Like having non-matched power tubes.
You know how some guitar nuts hunt for that unique total harmonic distortion induced tone.
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