Current rating in a fixed/cathode swicht

[chocopower]

Hi there!

What should be the current rating for a fixed/cathode switch in a 2x6L6 amp?

2A?

6A?

thanks in advance!

[Enzo]

COnsidering that the total current through a pair of 6L6s is not even 500ma most likely, ANY switch will work fine.

[jrfrond]

Enzo is correct. Just to be safe, I'd rate the switch at least 4x the current draw. Also, even though it is pricier, you might want to invest in a 4PDT toggle and wire each connection (i.e. bias and cathodes) with parallel, redundant switch sections, just in case to play it safe. The last thing you would want when playing is a no-bias condition.

[Enzo]

I normally use more switch than I need simply for mechanical strength. That 15A toggle will be more sturdy and reliable that a tiny toggle. Electrically it doesn't matter, but you are a lot less likely to break the heavy switch.

And another aspect of switches is that there is how much the contacts can carry then there is how much they can switch. They can usually carry more than they can switch. In other words it is easier for a switch to carry 1 amp through its contacts than to break the path of 1 amp of current. Breaking such a path can arc the contacts.

[chocopower]

One more question?

Is safe switch betwen fixed and cathode bias with the amp on?
or should i put it in Standby mode?


Thanks in advance!

[Bruce / Mission Amps]

One more question?

Is safe switch betwen fixed and cathode bias with the amp on?
or should i put it in Standby mode?


Thanks in advance!

MY first impression would be to say put the amp in standby first, then switch the biasing.

The cathode biasing will take a couple-few seconds to develop so the power tubes will be running for a moment with very low bias but at least not run away.

As far as the fixed bias goes, it depends on how you switch it, where the bias voltage comes from and how much impedance the fixed bias supply portion has.
If it takes a long time for the bias voltage to come back up (high resistance and large bias supply filter caps = slow charge up time) the power tubes are going to be running with none to low bias voltage for a few moments and they will draw lots current until the bias voltage is fully developed.
But, if you use a switch where the fixed bias voltage is always there but just not applied to the power tube's grids, then other then a pop when you flip the switch, I doubt it will cause any serious concern.

Actually, as long as the switching always works right, I don't think either scenario would cause undo harm to the tubes, just make them run a little hard for a few moments every time you switch it... that might make them wear out a fraction of their life span sooner.

[chocopower]

One side of the DPDT is used to bypass to ground the cathode bias arrangement and the other side, ground the neg. voltage in the bias suply.

I used the London power layout

[tubeswell]

I did a similar thing in my recent 5G9 build. In cathode bias mode, the bias mode switch breaks the circuit to the fixed bias supply and grounds the trem depth pot ground at the same time as connecting the cathode resistor/cap, and vice versa for the fixed bias mode. Seems to work fine. I've never tried switching it when its running. ( I deliberately 'hid' the switch on teh bottom of the chassis).

The way I have it running in cathode-bias mode I am using a 330R (measures 327R to each cathode including the 1R measurement resistors) and getting about 355V on the Plate and 358V on the screens on one tube and slightly lower on the other with 24V on the cathode giving 36mA of tube current (so maybe I can go to 270R and run it a little hotter?). The trem doesn't work nearly as well in this mode, and maybe the amp doesn't sound as good as it might it I could get the voltages lower still and use an OT with a higher load resistance (at present its 5k). But if I lower the B+ with a zener or two, it will adversely affect the beautiful cleans and trem in fixed bias mode. I guess I can't have it both ways.