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OK, this isn't about guitar amps, but it is an interesting branch of tube theory which might have at least a little relevance.
I have a tube Grundig table radio; it has three OPTs and a pair of 6AQ5s in push pull. (See schematic for a similar setup.) The three OPTS are all wired in series. The "middle" OPT has a centertap which goes to B+. Each OPT drives a speaker. (One small speaker on each side and a bass speaker in the middle.) When it's in "mono" mode the tubes work push-pull. When it's in stereo mode, the phase inverter is bypassed and the output tubes work as two separate single ended amps. The middle OPT electrically sums the bass to make it mono. The "conjunctive filter" caps across the middle OPT keep the treble out of the central bass speaker.
That's all fine and well; I understand how it works and it might be kind of a cool project to build a guitar amp which could do stereo vibrato using only a pair of tubes and two output transformers or work in higher powered mono push-pull mode.
Here's the thing. I just acquired another Grundig console radio. This one has a similar setup with two small speakers on each side and a mono central bass speaker. Two EL95s in push pull. It does indeed have stereo separation, but it only has A SINGLE OUTPUT TRANSFORMER. I don't have a schematic for it, but I can't for the life of me figure out how they could have possibly accomplished stereo separation with only one transformer. It's a normal looking transformer, nothing potted that might be hiding secrets. I checked the speakers and they are all low impedance speakers, so it's no clever partial OTL circuit or anything like that. There are no "hidden" OPTS mounted on the speaker frames or anything. Does anybody have a clue how they could have possibly accomplished this? I can't understand how the core of the OPT wouldn't magnetically couple and cancel any stereo separation out.
Nathan
I have a tube Grundig table radio; it has three OPTs and a pair of 6AQ5s in push pull. (See schematic for a similar setup.) The three OPTS are all wired in series. The "middle" OPT has a centertap which goes to B+. Each OPT drives a speaker. (One small speaker on each side and a bass speaker in the middle.) When it's in "mono" mode the tubes work push-pull. When it's in stereo mode, the phase inverter is bypassed and the output tubes work as two separate single ended amps. The middle OPT electrically sums the bass to make it mono. The "conjunctive filter" caps across the middle OPT keep the treble out of the central bass speaker.
That's all fine and well; I understand how it works and it might be kind of a cool project to build a guitar amp which could do stereo vibrato using only a pair of tubes and two output transformers or work in higher powered mono push-pull mode.
Here's the thing. I just acquired another Grundig console radio. This one has a similar setup with two small speakers on each side and a mono central bass speaker. Two EL95s in push pull. It does indeed have stereo separation, but it only has A SINGLE OUTPUT TRANSFORMER. I don't have a schematic for it, but I can't for the life of me figure out how they could have possibly accomplished stereo separation with only one transformer. It's a normal looking transformer, nothing potted that might be hiding secrets. I checked the speakers and they are all low impedance speakers, so it's no clever partial OTL circuit or anything like that. There are no "hidden" OPTS mounted on the speaker frames or anything. Does anybody have a clue how they could have possibly accomplished this? I can't understand how the core of the OPT wouldn't magnetically couple and cancel any stereo separation out.
Nathan
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