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Old 26th September 2007, 12:34 AM   #1
dscrobe is offline dscrobe  United States
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Default Matching Impedances

I want to build a push-pull stereo integrated 40 Watt tube amplifier. I found from Hammond a 40 watt push-pull tube output transformer. The nominal primary impedance is listed at 6600 ohms. When selecting the 2 push-pull tubes, do I need to make sure that each tube's nominal plate resistance matches the output transformer primary impedance when matching impedances? Or is it the combination of both plate resistances that should match the primary winding?
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Old 26th September 2007, 01:39 AM   #2
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With pentodes, plate resistance is irrelevant. Look at the data sheets under "Typical operating conditions" and look for an operating point giving 40 watts or more with a load impedance of 6600 Ohms (or close to that). This will probably be in AB1. EL34 comes to mind, as does the 6L6GC. The pin-outs are close enough that you could use either if you have adjustable bias.
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Old 26th September 2007, 03:52 AM   #3
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Does the 6.6k OPT have primary taps for ultra-linear operation? If so, it is suitable for EL34s in UL-mode. For triode-mode, EL34 ideally requires 5k P-P (6.6K will give less power and less distortion) For pentode mode class AB1 with fixed bias and B+ at 400v, it should be 3.5k P-P. That's fairly critical; pentodes behave poorly if the load impedance is too high or too low.
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Old 26th September 2007, 06:49 AM   #4
dscrobe is offline dscrobe  United States
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Yes, the OPT has connections for ultra-linear operation. I have checked a typical op (NJ7P tube database) for a 6L6GC. Tom, it looks like this should be close enough. The typical op doesn't call for ultra-linear mode. I will connect to the center tap. How do you determine the size of resistors to use for connection to the No. 2 grids of the power pentodes?

Push Pull Class AB1
Plate Voltage 400 V
Grid No. 2 Voltage 300 V
Grid No. 1 Voltage -25 V
Load Resistance 6.6Kohm
Power Output (approx) 34Watts
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