Pop engaging standby question

[Luthierwnc]

Hi All,

I'm working on an amp that is giving me a loud pop when I turn the Standby switch from hot to standby. The amp is loosely based on the 102/124 build with the B+1 filters on the rectifier side of the standby switch (2X220uf and 2X270k). The switch sends juice to the OT center-tap and the choke.

Most of what I've read (with not much common agreement) is that a high-voltage .047 cap across the switch will reduce that but I've only seen that used when all of the filter caps are on the rail-side of the SB.

Any thoughts on how to get rid of this? FWIW, the voltages themselves are fine and there is a tail resistor at the end of the rail to compensate for the lack of a FET.

Cheers, Skip

[martin manning]

Is this the same amp that had the ground loop and the gremlin? If I understand correctly you're grounding the CT and connecting the choke to the B+1 filter at the same time?

[Luthierwnc]

Hi Martin,

Yes, same amp. The "ground loop" was a lead-dress issue. The gremlin was a broken pin, so, operator error on both.

The center-tap of the PT is always grounded. B+ goes to the pole of a heavy-duty switch. That is also the tie point to the B+1 filter cap circuit. The CT and the B+1 filter circuit are grounded on one of the power transformer bolts.

The switch throw is connected to the OT center-tap and the choke. The B+2, B+3 and loop grounds are attached to a lug on a V6 socket bolt. I pull B+2 off pin 6 of that tube for the DC to elevator the heaters. That circuit is grounded on a lug on a V5 socket bolt. That is also where the cathodes for the push and pull side of the power tubes go. The DC is 63 volts, IIRC. AC is 3.34 volts per side.

I've attached a close-up of the amp as it stands right now showing the ground points. The radial cap on V5 is a 47uf @ 100v across a 47k resistor to ground. From left to right, the first red arrow shows:

the B+1,
PT center-tap
PT shield and
relay circuit grounds.

The second arrow is the cathode ground for:

V7 and V6
the bias circuit and
the loop

Next is the cathode ground for:

V5 and V4
the DC heater elevation circuit and
B+2 and B+3

The last red arrow is the ground wire for the B+2 and B+3 caps.

B+4 and 5 are grounded on the bus which is bolted to the chassis near the input socket. 14 gauge Romex -- nothing fancy. That is also where the tail resistor on the rail is grounded. Except for the loop, the grounds are like the 124 example in the TAG archives.

I've had other amps I didn't make with popping standbys. It isn't a deal-killer. I'd just like to know if it represents something else that could be handled better.

Thanks, Skip

[martin manning]

The ground scheme seems reasonable. The only thing that you might try there is moving the B+3 and loop grounds over to the loop jacks and away from the power tube cathode and B+2 grounds. The cap across the standby will help reduce contact arcing regardless of where the switch is in the string.

[Luthierwnc]

Running a .047 cap across the standby switch cleaned it right up. Thanks!

I'll give some thought to moving B+3 closer to the loop jacks. Those are isolated Neutriks. The loop grounds on the board runs to a small bus wire that connects the send jack, send trimmer and return tack. That then goes to the power tube lug.

I would need to separate B+2 and B+3 under the filter cap board. Doable but a lot like making a ship in a bottle. sh

[martin manning]

Re the grounds, if it's quiet then it ain't broke... I wondered if the big transient that is induced when the standby is switched wasn't somhow getting into the audio at the loop.