0,4 and 8 aren't balanced.

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In Reply to: cathode feedback for PP amp posted by dhsettim on September 21, 2005 at 17:59:41:

Don't forget that balanced output requires equal but opposite voltages on the two "active" lines. The 4 and 8 ohm taps are at a ratio of 1 : Sqrt2 so if you use the 4 ohm tap as a "centre" point the two voltages are not 1 : 1 they are 1: 0.414.
If you had 0, 4 and 16 ohms you'd be OK, as long as the 0 ohm isn't grounded inside the amp. If it is, grounding the 4 ohm tap would create a shorted secondary so you'll find out pretty fast. The impedance the output transformer sees is the total impedance across its windings, so if you connect the speaker between the zero and 16 ohm taps and it's an 8 ohm speaker you will be reflecting half the expected load into the primary. This might work with the extra feedback or it might not.
To work out which feed goes where I would simply observe the phase of the OPT primaries and the secondary on a scope. If you can't do this, observe which plate voltage sums with the output voltage and which one subtracts. For negative feedback you need the subtractive connections.

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Mark Kelly

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Follow Ups Full Thread

Topic - cathode feedback for PP amp - dhsettim 17:59:41 09/21/05 (4)

0,4 and 8 aren't balanced. - Mark Kelly 18:56:05 09/21/05 (3)

Thanks, Mark - dhsettim 14:39:35 09/23/05 (2)

still can do it... Re: Thanks, Mark - wlee 07:58:40 09/24/05 (1)

Re: still can do it... Re: Thanks, Mark - dhsettim 00:22:07 09/26/05 (0)

Follow Ups

0,4 and 8 aren't balanced. - Mark Kelly 18:56:05 09/21/05 (3)

Thanks, Mark - dhsettim 14:39:35 09/23/05 (2)

still can do it... Re: Thanks, Mark - wlee 07:58:40 09/24/05 (1)

Re: still can do it... Re: Thanks, Mark - dhsettim 00:22:07 09/26/05 (0)

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