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Author Topic: Parallel Push Pull amplifiers  (Read 1632 times)

nodz

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Parallel Push Pull amplifiers
« on: May 06, 2013, 06:29:52 PM »
I was wondering if anyone could help me.  I have a question regarding drawing load lines for biasing  of tubes in an amplifier when there are four tubes in parallel, e.g a fender 100 watt amp.
If two tubes have a plate to plate resistance of 10k ohm and we have an OPT with a primary of 10k, then the load line that we need to draw on the tube/plate characteristics curve is 1/4 of this, or 2500 ohms.  If using 250V at plate, load line should pass through the points  250V, 0ma and 0V, 100mA.  Now if we add a second pair of tubes.  If using the same 10K primary OPT, should a 1250 ohm load line now be used, passing through 250V, 0mA  and 0V, 200mA?

J M Fahey

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Re: Parallel Push Pull amplifiers
« Reply #1 on: May 06, 2013, 06:40:25 PM »
No.

1) if you add more tubes but keep the transformer and load the same, the load is still 10K PP or 2500 ohms per side , because that's the load reflected into the primary and you changed nothing in that respect.

The (combined) plates will *still* swing from , following your example, 250V/0mA to 0V/100mA .

2) as of what tube "sees", now it has to provide same voltage swing as before but half the current, because the other one is helping it.

So each tube will go from 250V/0mA to 0V/50mA and each will behave as if load were *double* , not half, so each will behave as having a 5K load.
Draw your curves accordingly.

nodz

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Re: Parallel Push Pull amplifiers
« Reply #2 on: May 06, 2013, 09:04:45 PM »
Sweet, thank you very much.

 

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