A few Ampeg SB12 Questions

[Steelwitch]

I thought about the residual B+ in standby and think the OP has a point there.

Imagine a full wave rectifier circuit where instead of the CT one of the outer ends of the HT winding is grounded.
This will result in half-wave rectification of the full HT voltage (only one of the diodes left operative).

In the amp the grounding is provided by the high resistance bias circuit, which will drop several hundred volts and limit the available current to a few mA's.
A fraction of the current runs through the bias diode and charges the bias cap to some negative voltage.

I'm only wondering how a supply current of 3 to 5mA could produce any sound.

What is the voltage drop across P26 (probably should read R26) in standby?

in standby, there is -250v on one leg of R26 and -33 across it, so a 217v difference.

[Helmholtz]

in standby, there is -250v on one leg of R26 and -33 across it, so a 217v difference.

Not clear.
Voltage drop is the voltage across the resistor, measured from end to end.
Is it 33V or 217V?

[Steelwitch]

Not clear.
Voltage drop is the voltage across the resistor, measured from end to end.
Is it 33V or 217V?

Sorry for the ambiguity. It's 217v which should mean the current is 250mA. If my math is right.

[Helmholtz]

It's 217v which should mean the current is 250mA. If my math is right.

No, this means only 217/100k = 2.17mA, so even less than I assumed.

Remember 1/k = m, 1/m = k and 1/M = ΅, 1/΅ = M.

[J M Fahey]

Here΄s the schematic in a displayable graphic format:

Input matches your picture.

Main consideration, usually ignored, is that Ampeg pickups were *Crystal mikes* , think Piezos, so source is a very small value capacitor.





Any significant loading cuts Bass drastically, period.

You can call that "Bright" if you wish

Normal jack, normally shorted, opens and finds a 5M6 resistor to ground, so response is flt down to vrey low frequencies.

Bright jack finds 147k at low frequencies, which drastically cuts Bass, plus 100k resistor is bypassed by .005 cap which boosts highs, even with conventional magnetic pickups, so Bass is now slappy bright.

Of course, since this is an all passive setup, you lose signal, no free lunch.

EDIT: for some time now, Forum lost an important functionality.

In the old days you clicked on an image icon, and it offered a ton of possibilities, including uploading from ow computer or anywhere else, what happened?
Now we can "add an attachment", not the same functionality, at all.

[g1]

EDIT: for some time now, Forum lost an important functionality.

In the old days you clicked on an image icon, and it offered a ton of possibilities, including uploading from ow computer or anywhere else, what happened?
Now we can "add an attachment", not the same functionality, at all.

Do you have 'advanced editor' turned on? It's the A button (underscored) in upper right corner of post box. With it turned off you lose lot's of stuff.