Fender bias cap question

[Chuck H]

Clarification and accuracy are great. Not really seen as "pedantic" (at least by me most often). Helmholtz, you are one of the cornerstones here for clarification on principals that are too often ignored or misunderstood elsewhere on line. Just so you know that you're not actually pedantic in the strictest definition. Soooo...

Since a constant current source is employed to maintain current I suppose I viewed it as a current purposed circuit.

And then there's the ever present reminders from everywhere telling guys like me that the bias supply does not provide current. And it actually doesn't provide current to the power tube grids (or at least not significantly) because surrounding circuits like grid loads and coupling caps create an insurmountable time constant. So the bias circuits service is not a current purposed circuit. Even though it requires current to make it adjustable.

Now that's just pedastic! (pedantic+fantastic )

And I'm not disagreeing with you. Rather, I'm just poking and hoping for more insightful discussion.

[Justin Thomas]

Sorry, Randall Smith beat you to it. And knowing him it's probably patented

He'll never find me in my living room & I don't "brand" my amps if I sell em. Usually they carry the brand of whatever carcass I built it into. He can go after Baldwin, Thomas, Sony, RCA, Silvertone, Hammond, & a bunch of no-name discount makers if he wants to.

Plus, I'll come up with such a ridiculous implementation that any serious legal challenge would write me off as a harmless incompetent doofus. I'll wire up an air raid siren powered by my bias circuit for the power tubes. Wired to a footswitch for that one guy that yells Freebird at every gig. Or blinky lights or some other silly thing. Maybe a fan that sucks instead of blows the hot air out. Because negative voltage.

Jusrin

[Chuck H]

He'll never find me in my living room & I don't "brand" my amps if I sell em. Usually they carry the brand of whatever carcass I built it into. He can go after Baldwin, Thomas, Sony, RCA, Silvertone, Hammond, & a bunch of no-name discount makers if he wants to.

Plus, I'll come up with such a ridiculous implementation that any serious legal challenge would write me off as a harmless incompetent doofus. I'll wire up an air raid siren powered by my bias circuit for the power tubes. Wired to a footswitch for that one guy that yells Freebird at every gig. Or blinky lights or some other silly thing. Maybe a fan that sucks instead of blows the hot air out. Because negative voltage.

Jusrin

Well I don't think Randall actually chases these pantents in the DIY or mod arenas. Though I have read about a couple of small time builders/sellers that Mesa contacted regarding patent infringement

Many of his patents are actually wrongly approved and there is ample precedent for their circuit and function. I don't think it would be hard to avoid conviction. So it might be fun to drop Mesa an email each time one of us uses a circuit he's patented Neener neener

[Tom Phillips]

...Many of his [Randall's] patents are actually wrongly approved...

I'll second that ! The patent office has been doing a mediocre job of awarding patents over recent decades.

[Chuck H]

I'll second that ! The patent office has been doing a mediocre job of awarding patents over recent decades.

To be fair technologies have been on a fast track. Not to mention ideologies specific to a genre like ours. I'm sure the old codgers who researched Randall's patents were scholarly enough but there was no one there with credentials AND familiarity with the genre like you or Helmholtz to call B.S.

I remember that Dan Torres used to sell what he called a "patended midrange control" for guitars. He never clarified that the patent had long expired and that it wasn't HIS patent, but Gibson's

[Helmholtz]

And then there's the ever present reminders from everywhere telling guys like me that the bias supply does not provide current. And it actually doesn't provide current to the power tube grids (or at least not significantly) because surrounding circuits like grid loads and coupling caps create an insurmountable time constant. So the bias circuits service is not a current purposed circuit. Even though it requires current to make it adjustable.

Yes, I took the chance to be pedantic here to remind of the influence of the shunt resistor value/current on bias voltage ripple.
When both series resistor and shunt resistor values are increased, ripple will be lowered or a lower value filter cap could be used.

[Chuck H]

Yes, I took the chance to be pedantic here to remind of the influence of the shunt resistor value/current on bias voltage ripple.
When both series resistor and shunt resistor values are increased, ripple will be lowered or a lower value filter cap could be used.

And this is a really important consideration too. And actually supports Justin's position of looking at these circuits like power supplies So it's possible I was inappropriately pedantic when I "corrected" him This is a good discussion.

[Helmholtz]

Since a constant current source is employed to maintain current I suppose I viewed it as a current purposed circuit.

To explain better I would need to elaborate on current sources and LTPI theory but not sure if this is the right place here.

[Justin Thomas]

And actually supports Justin's position of looking at these circuits like power supplies

Literally all I did was "hey, this part of the schematic looks like this other part of the schematic, so it must be pretty much the same..." And nobody called me a doofus for it.

Geez this is so much better than Farcebook... There's a reason I don't send people over here.

Jusrin

[Chuck H]

Literally all I did was "hey, this part of the schematic looks like this other part of the schematic, so it must be pretty much the same..." And nobody called me a doofus for it.

Geez this is so much better than Farcebook... There's a reason I don't send people over here.

Jusrin

Yep. I love being in over my head here. But only because the brain trust continues to tolerate my limitations I think most people would rather have an on line fantasy hovel where they can play big shot occasionally. It's rare that I meet someone that I think will get on here and enjoy it. But this is where the best information is. We have A LOT of lurking members that are just here for the info but choose not to interact.

Not to take away from ALL the other forums. I peruse a lot of sites in the course of research and I do gather useful information elsewhere. But this is my first stop and it's the only forum where I participate.

[Leo_Gnardo]

In recent videos over the past year Mister Lyle of Memphis, aka Psionic, has recommended adding a second, smaller filter cap at the adjustment pot wiper in the standard run of Fenders and similar amps built with a single bias filter. I agree, because occasionally I'd been doing that in previous decades to minimize hum from the bias supply. Typically it's a 4.7 or 10 uF I'll install. Those who "tune by ear" may select the smallest cap that provides an acceptable reduction of hum.

[Helmholtz]

When power tubes are well balanced the bias ripple can be higher before it gets audible.
Thus better filtering might not be noticeable unless there's some unbalance.
Reason is that bias ripple is a common-mode signal which is suppressed when the tubes have equal gain.

[35L6]

I do actually read all the time without logging in but I have a question . What happens to the bias when the power tube grids are driven close to 0V and the impedance drops ? Does it load down the bias making it less negative ?

[Helmholtz]

I do actually read all the time without logging in but I have a question . What happens to the bias when the power tube grids are driven close to 0V and the impedance drops ? Does it load down the bias making it less negative ?

The bias supply will hardly notice grid conduction as it's "decoupled" by the large value feed resistors.

[trem]

I do actually read all the time without logging in but I have a question . What happens to the bias when the power tube grids are driven close to 0V and the impedance drops ? Does it load down the bias making it less negative ?

I am not ANY kind of tech, so i might be way off here, but........ i do not see how it could.
There is no bias current at the grids, it is just a voltage tap.

The bias voltage does not change does it..... just the AC which rides the fixed neg bias at the grid.?

I would think the only danger would be if the cats and plates could start conducting Before the neg bias circuit had time to play.?