cathode switching as a standby

[jimipage]

In Kevin O'Connor's "The Ultimate Tone" he talks about switching the cathode of the output tubes as a standby rather than the typical standby found in most amps. This seems to be a very simple and advantageous method, but my question is: in setting it up, would you need to separate the cathode ground path from the ground path of the filter caps, or would you leave them all connected and switch the entire cathode/cap ground? I'm almost done with a build and would like to incorporate this.
(I hope this question makes sense)

[Vince]

In Kevin O'Connor's "The Ultimate Tone" he talks about switching the cathode of the output tubes as a standby rather than the typical standby found in most amps. This seems to be a very simple and advantageous method, but my question is: in setting it up, would you need to separate the cathode ground path from the ground path of the filter caps, or would you leave them all connected and switch the entire cathode/cap ground? I'm almost done with a build and would like to incorporate this.
(I hope this question makes sense)

You wire one side of stdby switch to ground (at filter caps)
and second side to power tubes cathodes - if amp's fixed bias,
or Rk/Ck - if amp's cathode bias.

V.

[Ron]

One disadvantage is that you cannot quickly disconnect the B+ from the amp when smoke starts to spew from the amp. Even with a 500ma fuse in the B+ line, lots of things can turn to crispy critters with no B+ disconnect switch.

The only advantage I see is that the audio is muted more quickly with the cathode bypass/standby switch arrangement.

What does KO opine about such a switch?

[IaXy]

What does KO opine about such a switch?

KOC whole idea about this switch is that it takes the wear and tear off of the standby switch, therefore the switch will last longer.
Thats basically it in a nutshell.

I don't know about you but I have never replaced a standby switch because it was bad due to arcing, or any other reason for that matter.

Bob

[IaXy]

He has even gone as far as using a mosfet to lift the cathodes.

[MarkB]

Kevin loves slick switching.

[FUCHSAUDIO]

Not a great idea to leave the B+ on, no big load on it (from the power tubes) and letting it rise to high levels that might kill a filter cap (or someone inside the amp). Buy a "real" switch, put a.01 600-V disc across it, and wire a normal standby switch in there. No offense intended to Kevin, but this has worked for years in most every amp on earht with as standby switch. Even Fenders (who's standby switches are the same as their power switches, and unrated as standby switches) have worked fine. You want to do something innovative ? Buy a spst switch and wire it to drain off B+ in standby. Call it a safety switch !

[David Root]

"Mr. Fuchs" is right about that use of a cathode switch; I have considered using it as a tone stack defeat switch, in effect a gain boost switch, which I think is a safe use of it.

On my last amp, I used an interesting switch from Westlabs, a DPTT Carling, as an off/(on/standby)/play, all in one switch. Position 1 (down) is off, position 2 (middle) on/standby, position 3 (up) is DC on. Big honking 20 amp switch, no issues there. Puts two switches in one.

Only caveat I can think of is you cannot shut down the AC power with the DC still connected, to drain the filter caps, because of the switch's design. In practice however I found that it takes only a few seconds for all filter caps to discharge below 10 VDC going into standby from play. I wire my DC switch from the PT centertap directly to the ground of the rectifier filter cap. Perhaps "Mr. Fuchs" could comment on this switch and my wiring of it?

[FUCHSAUDIO]

I wire my DC switch from the PT centertap directly to the ground of the rectifier filter cap. Perhaps "Mr. Fuchs" could comment on this switch and my wiring of it?

The only "problem" with lifting the center tap of a power transformer from ground is that it does wacky things to the bias voltage. If you draw the bias from a tap on that winding, or through a resistor off that winding, the bias voltages go to some funny places when you lift that center tap.

Now, a zener string (slightly above the bias voltage, not limiting it during normal use) would fix it, but again, the traditional standby switch is simpler and avoids the problem entirely.

An spst switch wired to discharge the filters through a resistor (not a short) is the easiest method. While that three position switch is a nice solution (I buy them from David Allen sometimes when space is at a premium on mods), it does make the bleed function a non-option.

[David Root]

I omitted to say that my amp in question uses cathode bias, not fixed. I "borrowed" the HT switching of the PT CT from a Top Hat schematic I ran across on Schematicheaven, which was also a cathode biased design.

I had not thought about the consequences of its use in a fixed bias scenario, though, so I have learned something useful here. Thank you. Could you perhaps elaborate on that?

[David Root]

I omitted to say that my amp in question uses cathode bias, not fixed. I "borrowed" the HT switching of the PT CT from a Top Hat schematic I ran across on Schematicheaven, which was also a cathode biased design.

I had not thought about the consequences of its use in a fixed bias scenario, though, so I have learned something useful here. Thank you. Could you perhaps elaborate on that?

[David Root]

I omitted to say that my amp in question uses cathode bias, not fixed. I "borrowed" the HT switching of the PT CT from a Top Hat schematic I ran across on Schematicheaven, which was also a cathode biased design.

I had not thought about the consequences of its use in a fixed bias scenario, though, so I have learned something useful here. Thank you. Could you perhaps elaborate on that?

[FUCHSAUDIO]

Basically, when you lift the CT, the only path to ground is the bias supply. It can go from (normal) negative 50 to 75 volts to negative 150 or 200 volts when you lift the tap. A zener diode string above the average bias voltage (say 80 or 90 volts) would "cap" this runaway voltage, if you needed to. Just pointing it out. If you did it and were unaware, you might end up with an exploding bias cap.

In a cathode bias amp (no negative supply) it's fine. Ampeg used it in many amps, and I'm sure others did as well.

[David Root]

Fast reply! Thanx very much. Find the alternative path to ground, and that contains the consequences.