bias switching

[shooter]

I got how the switching between PA bias types is done, what I can’t figure is when?  Can you do it “on the fly”?

thanks for the insight

[HotBluePlates]

I wouldn't; I'd switch when the amp is off or on standby.

If there's a significant difference between the bias voltage, idle current between the two settings, there will be a transient when you switch the tubes from one d.c. level to the other. That might create a big thump in the speaker, but how loud depends on the amount of change.

Pops like that would be worse if you were switching something like that in the preamp, where the transient gets amplified by the rest of the amp's circuitry.

All this said, the pop may not be bad, especially if your two bias settings aren't very different. You could try it and see what you think.

[shooter]

thanks HBP;
I was worried more the "unknown state" between switching what effect that would have on the tube but figured the cathode open is safe, no current, but the grid floating, i wasn't sure.  I even started working on a "lock-out" logic circuit that wouldn't let you switch bias if B+ was on the plate.

I still have to coble up the fixed bias circuit and I did figure I'd try it live, just to see.

[HotBluePlates]

I was worried more the "unknown state" between switching ... but figured the cathode open is safe, no current ...

Smart thinking.

You have a milli-seconds moment where the cathode might be open, which is no-plate-current and can't be bad for the tube.

Unless of course you switch the cathode by shorting the cathode directly to ground and bypassing the existing cathode resistor. No plate current disruption there. The other pole of your double-pole double-throw switch will be switching the non-grid-end of your output tube grid reference resistors from ground to the negative bias supply.

Worst-possible case in that scenario is the grids are still at ground while the cathode drops to ground potential, giving 0v grid-to-cathode and max plate current. But being milli-seconds at most (and probably less time) you won't hurt the tube. You simply have the chance for popping like I mentioned, which may or may not be objectionable.

If it really worries you, move over to solid-state switching, with JFETs handling the connection for the grid reference resistors, and JFETs controlling MOSFETs for the cathode-to-ground connection (cathode resistor is always physically connected, but is simply shorted in fixed-bias mode; MOSFETs may be overkill, but you won't have to worry about current ratings when selecting them). One common switching voltage turns on/off each solid-state device as appropriate (though you may have to add extra solid-state components to invert switching polarity depending on which parts you use in each location), and switching time is likely in the nano-second range.

You don't have to worry about whether 2 mechanical parts do their work at exactly the same time because you can use a simple SPST switch to control a single voltage switching both circuits simultaneously. Then again, maybe your concerns about the mechanical switch aren't severe enough to lurch down the path of sand-state complexity. Really, it's a simple circuit in transistor-land, but takes a bunch of parts to replace a simple mechanical DPDT switch. The upside is the solid-state stuff will likely last forever, and the mechanical switch becomes much smaller, cheaper and easy to remotely control from a footswitch if you choose.

[shooter]

I made my living in the sand box, so my brain wanders into 1mS being a huge amount of time.

[sluckey]

I made my living in the sand box, so my brain wanders into 1mS being a huge amount of time.

It is! That's over 80 radar miles! But that's all relative. It's not so big in the audio world. It's only 1000Hz.

I did a switchable bias circuit in one of my earlier builds (November). I was jazzing with all the bells and whistles floating on the net. Heck, my rear panel was just as complicated as my front panel. Anyway, I must have flipped that switch a hundred times the first week I had it, just fine tuning and trying to hear a noticeable difference in sound. I removed the switch the second week and have never done another switchable bias circuit. I predict you may have a similar experience.   

But by all means, build it, and enjoy it. Just don't let anyone be flippin' back and forth while you be jammin'!

[kagliostro]

BTW, Steve, which position was your definitive one ?

Franco

[sluckey]

up

[octal]

 

[shooter]

up

Thanks steve, I'm working on the layout and will probably incorporate the "future" option since I have the room.  Most of what I've read the word "slight" difference seems the consensus.

The radar system I worked on only had a missile with a 30mi kill zone but we tweaked the radar so we could watch our threat sneak up way before that

[kagliostro]

UP .....       

looking here is difficult to say what is what 



Franco

[sluckey]

I was just checking to see how well you understand my Engrish. You passed.

Now the real answer is fixed.

Oh, and that rear panel... Left to right is preamp out, power amp in, bias adjust, bias test points, bias select switch, pentode/triode switch, speaker. Lately my rear panels are looking much simpler. And now I own a cheap DYMO so I don't have to break out a schematic to twist a knob or flip a switch!   

[kagliostro]

Simply I cheer for the cathodic polarization and I was curious if you too have preferred it to the fixed bias     

We cheer for different teams

---

So this is the old way



and this the new way



 

Franco

[sluckey]

We are on the same team. I have more cathode bias amps than fixed bias. And I really appreciate that wooden cab. That's a woodworker's dream. But the amp chassis has to be a nightmare. Just reading the front panel looks intimidating.   

[shooter]

That is beautiful!  Course Italy has a 1,000yr head-start on us

Just reading the front panel looks intimidating.

most of the guitar guys I know would give up after the 1st 3 knobs 

[kagliostro]

I linked an image I've found on the net, but I've friends who have amplifiers with a faceplate like that

this is one example



despite that, the inside is not a spaghetti fest



Few time ago Steve told that building his AC15 was enervating, I think he likes the Brian May version of Vox that is on the second image

of the previous post much more than the amp in the first image 

Franco

I apologize with Shooter for the hijack of his tread