Resistor in series with choke

[Paul P]

Amps with chokes, at least Fenders, end up with screen voltages very
close to plate voltages, maybe even slightly higher. Amps with a resistor
instead drop enough voltage across the resistor to have the screens at
much lower voltage.

Since the screens are supposed to be quite a bit lower than the plates why
not put a resistor in series with the choke to get the benefits of both choke
and resistor ? Wouldn't this be easier on the power tubes ?

Is there any difference between doing this and using sufficiently high-valued
screen resistors (which some say will adversely affect tone) ?

Paul P

[Bruce / Mission Amps]

Amps with chokes, at least Fenders, end up with screen voltages very
close to plate voltages, maybe even slightly higher. Amps with a resistor
instead drop enough voltage across the resistor to have the screens at
much lower voltage.

Since the screens are supposed to be quite a bit lower than the plates why
not put a resistor in series with the choke to get the benefits of both choke
and resistor ? Wouldn't this be easier on the power tubes ?

Is there any difference between doing this and using sufficiently high-valued
screen resistors (which some say will adversely affect tone) ?

Paul P

"...Since the screens are supposed to be quite a bit lower than the plates..."

Oops... you have leaped to an assumption that is not really "correct"...
I agree it is probably better to have screen voltages around the same as the plate, and only a little higher if they are higher, but there is no rule about being quite a bit lower.
You can use a resistor with a choke if you want but the idea is to not drop a bunch of voltage across a resistor when drawing current.
The big drop you see is because the DC resistance of the output transformer is high enough that when drawing lots and lots of plate current, there is additional drop through the OT but not so much through the choke and following stages, who's plates are being filtered and regulated with big caps.
I suggest if you are concerned about this, you can just Tee off the screen node after the choke with a power resistor to steer the B+ to the screens and their individual screen resistors.

[Enzo]

Paul, I think when you look at Fender circuits like that, it is worthwhile remembering that a lot of the amp circuits we talk about around here have been in existence for something like 50 years, and they have worked just fine. SO perhaps there is no particular need to change it because of "supposed to" sorts of things. Leo Fender also ran 6V6s at as much as 100v over their "maximum" plate voltage. Even though he wasn't supposed to.

And just thinking, wouldn't putting a resistor in series with the choke reduce the choke's effectiveness? In the manner a resistor in series with a cap reduces its action.

[mooreamps]

The screens should be a little lower than the plates, imo, since you really should want the plate to collect the free-space electrons, and not the screens.

-g

[Enzo]

Well, that is why they line up the screen grid wires with the control grid wires, so the screens exert their influence while hiding in the control grid shadow. That helps keep screen currents manageable.

[Arthur Dent]

And just thinking, wouldn't putting a resistor in series with the choke reduce the choke's effectiveness? In the manner a resistor in series with a cap reduces its action.

No. The higher the impedance of the choke, the better it will filter, whereas adding impedance to a cap (in parallel with the load) will decrease its filtering effectiveness. The choke is basically a big resistor that acts almost exclusively on AC. The resistor does the same thing, just impartially.

[Bruce / Mission Amps]

And just thinking, wouldn't putting a resistor in series with the choke reduce the choke's effectiveness? In the manner a resistor in series with a cap reduces its action.

Well, my gut feeling is that I would think no.
It probably would add slightly to the effectiveness as a whole, just as in parallel the opposite would be true.
In parallel with the choke it would probably detract from the node.... paralleling
it might be like a swamping resistor across an LC circuit lowers the Q...
I guess I'm thinking this out as the choke is an impedance at 60Hz to 120Hz and at DC it is only a resistance.. so the resistor would be a DC resistance.

[mooreamps]

Well, that is why they line up the screen grid wires with the control grid wires, so the screens exert their influence while hiding in the control grid shadow. That helps keep screen currents manageable.

This is "exactly" correct. Ideally, we would like to see screen grid current to be zero. However, if those wires on the screen become out-of-alignment with respect to the shadow of the control grid, then free-space electronics will strike those screen wires and flow out the grid. This is precisly the reason why I measure plate current, from the plate, and screen current, from the screen.

-g

[mooreamps]

And just thinking, wouldn't putting a resistor in series with the choke reduce the choke's effectiveness? In the manner a resistor in series with a cap reduces its action.

It would not decrease the "effectiveness" of the choke. But, it does give you poorer regulation because you're dropping the Q of the circuit.

-g

[J Martin]

Or you could go ultralinear for better screen control.

[mooreamps]

Or you could go ultralinear for better screen control.

I read from Kevin, he does not seem to believe guitar amps sound that good running in ultra-linear. You can try it if you want, and make your own judgements.

-g