I was fixing up the amp section of a Pioneer SM-Q300 receiver. It's a typical stereo 6BQ5 PP using a 6AN8 driver. I bypassed the tuner and preamp section and used just the amp section, after replacing all the coupling caps, and it sounds wonderfully sweet. It may not have the full bass like some 6BQ5 amps with bigger output transformers but the sweet midrange and delicate treble more than make up for it. A pleasant surprise.
One thing I noticed in the schematic is a curious way of doing feedback that I never saw before. The 6BQ5 input resistors are tied together and route back to the input tube's cathode, instead of to ground. Any idea why and what about this arrangement?
Simplified amp section schematic:
Full receiver schematic:
One thing I noticed in the schematic is a curious way of doing feedback that I never saw before. The 6BQ5 input resistors are tied together and route back to the input tube's cathode, instead of to ground. Any idea why and what about this arrangement?
Simplified amp section schematic:
Full receiver schematic:
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I would suspect that too, schematic errors didn't always get sorted out by the time the service or owners manual went to press.
A schematic bug seems the most likely explanation. But if you have the unit, check and see. The mid point of the input resistors would nominally be an AC ground, so would not seem to be used for a feedback source, but the feedback from the OT secondary to that point might be intended to drive the point, to give a little SE 2nd harmonic effect? Never know, some sound tweek might have been put in. Or it could just be a convenient point to pick up some bias DC for the output stage. (seems to be the wrong polarity though, unless they wanted a higher value cathode resistor for the outputs)
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Yes, curious to know if there is an actual link to the NFB.
I did a similar "refurbishment" on the Pioneer ER420 receiver. ( 12AX7 driving 6BQ5's ) and it has the grid leak resistors tied together and grounded as per normal.
I did a similar "refurbishment" on the Pioneer ER420 receiver. ( 12AX7 driving 6BQ5's ) and it has the grid leak resistors tied together and grounded as per normal.
I do have the unit and it's wired as shown in the schematic. No oscillation, no problem. And it sounds good. A real head-scratcher!
A head-scratcher for sure.. I suspect a spice analysis or experiment would reveal the resulting effect on performance to be quite small..
I cant see anything 'odd' about that feedback.
The source of FB is from OPT and instead of going just to the first gain stage, its also wrapped round the O/P stage.
Its probably giving a little LF stability, The SM-Q 300B has rather Small OPT....
BTW--Anyone got a dial-glass/tuning scale for one....?🙂
The source of FB is from OPT and instead of going just to the first gain stage, its also wrapped round the O/P stage.
Its probably giving a little LF stability, The SM-Q 300B has rather Small OPT....
BTW--Anyone got a dial-glass/tuning scale for one....?🙂
Except that this would be NFB for one output tube and PFB for the other. (hence my earlier comment about a little SE effect being introduced )
An intriguing connection at least, now that it is confirmed.
An intriguing connection at least, now that it is confirmed.
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not that small...... whimper
Googling pictures of the Pioneer SM-Q300B receiver leads me to believe it shares the same OPT as the ER420, ( AT6123 ) As said, the ER420 doesn't have this link.
It indeed uses the AT6123 output transformers. Small but adequate. I've seen smaller 6BQ5 transformers before like in those Motorola amps.
It's very sweet sounding. Delicate midrange and airy high and taut bass. Bear in mind, I am using only the amp section with all the preamp, tone controls, and switches bypassed so it's very transparent. As an amp, it's no slouch compare to other more coveted 6BQ5 amps. This is a very good way to get a decent tube amp on a budget. I let the rich boys to chase all the Marantz and McIntosh gears.
I got one of these off a friend the other day that is pretty slaughtered. In the schematic, what is the pot between the 5.5V windings for? Hum pot? Does it reference the cathode of the EL84 just for ground reference? I see it feeds the filaments for a few 12AX7s.
In planning to simply build another amp and replicate the finals with a different front end. So, that's why I am curious about the pot that references the cathodes. If it has a function to the finals or if its just a ground reference.
Why is it 5.5V vs. 6.3V?
Thanks!
Blair
In planning to simply build another amp and replicate the finals with a different front end. So, that's why I am curious about the pot that references the cathodes. If it has a function to the finals or if its just a ground reference.
Why is it 5.5V vs. 6.3V?
Thanks!
Blair
It's a mistake that appears in the SX-82 diagrams as well. The pot is probably a hum balance.
In addition to the post above, this 'mistake?' appears on the SX-110 diagram, which is roughly the same unit as the SX-82, but has a slightly different PSU and uses 7868s, but it also appears on the SX-34, SX-800 and SX-800A/2000 digrams, which are drawn differently, but shown feeding the same tubes, (a preamp tube and the two driver splitters), 12AX7's and 6an8s, respectively, which are all 6.3 tubes.
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