A few Ampeg SB12 Questions

[Steelwitch]

Hey guys,

I have my friends Ampeg SB12 and I am trying to figure out a possible input jack miswiring.. I know I am overlooking something extremely simple. The normal channel impedance is at the right level, from the tip to ground it reads 5.9M with the 100k, 47k, and 5.6M (R1, R2, and R4). But when the normal channel is shorted at the jack, the bright channel from tip to ground is reading only 147k, presumably from R1 and R2, and a 1v signal is down in the 100mv range at the input. So presumably, signal is being bled to ground. I believe according to the schematic that I have, that the bright channel should be the one with the shorting jack (in this case it is the normal channel). What is it that I'm missing here? Attached is a picture of the input jack wiring.

The other question I have is regarding the standby switch wiring. It is a design which grounds the PT center tap, but voltage still bleeds from the rectifier to ground through the bias supply (also putting 87v on the 100v bias cap in standby) and trying to figure a way to rewire so that it cuts off the B+ after the reservoir cap, and I believe it is possible by hardwiring the center tap to ground.. but I am currently not finding a great way to cut off B+ after rectification because it is all on the circuit board. Has anyone done this? The complaint was that signal still gets through even with standby on, which is a small complaint and maybe not even worth doing, but out of curiosity, wanted to get y'alls thoughts on this. Thank you!

[The Dude]

All of those circuit board designation numbers mean nothing without a schematic.

[Steelwitch]

So sorry, I meant to include it with my post! It can be found here
https://ampeg.com/support/files/Sche...Schematics.pdf

[Steelwitch]

I will say also that this schematic is different from the one on the fliptop base, as the position of the shorting jack is reversed between normal channel and bright channel.

[Helmholtz]

With a full wave rectifier the CT needs to grounded for normal operation.
So in standby the CT is lifted from ground and there shouldn't be a path for a current to charge the power supply caps.
Does the switch really open completely?

What is the B+ in standby?

[Steelwitch]

With a full wave rectifier the CT needs to grounded for normal operation.
So in standby the CT is lifted from ground and there shouldn't be a path for a current to charge the power supply caps.
Does the switch really open completely?

What is the B+ in standby?

I posted the schematic in a previous reply (but forgot in my original post, sorry!) it's here: https://ampeg.com/support/files/Sche...Schematics.pdf

Yes, that is how the stanby switch works in this amp. with standby off, B+ is 500v and with standby engaged the voltage drops to approx. 250v and starts draining slowly. is voltage floating with no path to ground with the CT disconnected? The -V on the bias cap rises to -80v and with the CT connected and operating properly it is -55v, which is basically normal.

[The Dude]

Your input jack wiring and readings correspond with the schematic you posted. As far as the standby: as Helmholtz says, "......in standby the CT is lifted from ground and there shouldn't be a path for a current to charge the power supply caps......", so I'm not sure what the problem is. Standby should work fine as is. Sure, you could wire a more traditional standby circuit, but it shouldn't be necessary.

[Helmholtz]

In standby there's no longer a supply for B+ and the residual voltage should just be the charge stored in the caps.
I'd expect it to be drained off after a couple of seconds.

[Steelwitch]

Your input jack wiring and readings correspond with the schematic you posted. As far as the standby: as Helmholtz says, "in standby the CT is lifted from ground and there shouldn't be a path for a current to charge the power supply caps", so I'm not sure what the problem is. Standby should work fine as is. Sure, you could wire a more traditional standby circuit, but it shouldn't be necessary.

So is the "bright" channel supposed to be significantly quieter than the "normal" channel? That is the issue that's presenting itself. I would imagine they should be similar volumes, unless the impedance difference has that effect.

What I think may be the path to ground is the bias supply. when the standby switch is open it reads 160k which is the path to ground from the bias network off of pin 4 of the rectifier tube.. does that make sense?

[Helmholtz]

What I think may be the path to ground is the bias supply. when the standby switch is open it reads 160k which is the path to ground from the bias network off of pin 4 of the rectifier tube.. does that make sense?

Need to think about it (too tired now).
But if so that arrangement could only supply a couple of mA's.

What happens if you pull the rectifier?

[The Dude]

The bright channel would have less overall signal due to more signal grounded through R2 via the switching jack. But, C1A would pass more high frequency around R1 giving you more highs.

[The Dude]

......What I think may be the path to ground is the bias supply. when the standby switch is open it reads 160k which is the path to ground from the bias network off of pin 4 of the rectifier tube.. does that make sense?

But, the transformer winding for the rectifier tube has it's CT completely disconnected from anything, so it shouldn't matter.

[g1]

Did anybody install some kind of arc suppressor across the standby switch (cap or maybe resistor) ?

[Helmholtz]

But, the transformer winding for the rectifier tube has it's CT completely disconnected from anything, so it shouldn't matter.

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?

[Steelwitch]

Did anybody install some kind of arc suppressor across the standby switch (cap or maybe resistor) ?

nothing like that no, this design is probably hard on the rectifier tube. the original I just replaced was in bad shape. filament rattle, and took almost 20 seconds to start delivering B+