Ampeg SB12 - Schematic Question

dawsonaudio · Jul 3, 2015

I'm building an Ampeg SB12 bass amplifier and have a question about the bass input connection. There is a 100k resistor tied to the 1/4 connector. Is that a switching jack or is the 100k hard wired to the same point as the 120k resistor? I've seen a switching jack symbol and this isn't what it looked like. I've got everything else wired up, but I'm not sure about this part.

Thanks for the help here.
Nate

mjrippe · Jul 3, 2015

Switching jack.

PRR · Jul 3, 2015

> Is that a switching jack

Yes; and it looks sorta normal to me (awful compact and simplified).

In Bass jack it makes no difference.

In Instrument jack (and nothing in Bass), it forms a 120K:220K voltage divider for ~~3:2 loss and 340K input impedance. I dunno why.

When using both jacks the signals are mixed equally.

dawsonaudio · Jul 4, 2015

I sort of figured it was a switching jack. Thanks for taking a look at it and confirming.

mjrippe · Jul 4, 2015

PRR said:
In Instrument jack (and nothing in Bass), it forms a 120K:220K voltage divider for ~~3:2 loss and 340K input impedance. I dunno why.

I was wondering that too since they have the 5.6M as well, which would seem like a reasonable load for passive pickups. I guess the simple answer is - Use the Bass input!

CJ · Jul 5, 2015

that 5.6 meg will not mean anything when the git/bass is plugged in as the pickup DCR, probably about 8K, will dominate the 5.6 M,

nice amp, weird output tubes, have to try those,

some newer amps use a cap in series with the grid so that the pickup DCR is nullified, interesting to see if there are any sonic differences when doing this,

Ampeg SB12 - Schematic Question

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Ampeg SB12 - Schematic Question

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