Balancing the Difference

[Tone Lover]

Im greatly interested, in that you can dial it in.
To get those pleasing harmonics we all like but cant always get out of the design. Cant wait to try it.
Bill
PS. after I read this a few more times of course.

[Structo]

My 100w D clone is my main amp.

I have 500R resistors on each screen grid of the four power tubes. (6L6GC)

The Dumble power supply (Classic) feeds the screens after the choke.

If I measure plate and screen voltage at idle, the plates and screens seem to sit at the same voltage.

Now we know inductors oppose a change in current but does this drop the screen voltage enough when being played?

I've seen amps that use a 300R or so resistor to imitate a choke.

[matt h]

(deleted)

[Structo]

Thanks Matt.

I probably shouldn't have wrote 'imitating', but to substitute a resistive load for the choke.

The Brown Note D'lite is a good example of an amp without a choke.
I believe it was an option but they used a 300-500 ohm 10 watt resistor in place of the choke between plate and screen node.

I built a 2x 6L6 D'lite (D'lite 44) that started out with the resistor then upgraded to a choke.

I tweaked that amp a lot so I can't remember if I heard or felt a difference after the choke was installed.

[matt h]

(deleted)

[Andy Le Blanc]

I've digging around looking at an evolution of how screen grid have been dressed in amp over time.

With field coil rigs, you see the coil as a pi filter before the ct of the OPT and the screens, there's no other resistors.
Its the grid alignment in the valve, the screens are in the shadow of the grids, that prevents screen current.

You see this carried over as the speaker field coil is eliminated, the power supply is simplified to R-C filtering

Then you see the insertion of a resistor between the screens and opt, or the screen are sourced after a resistance on the power supply, still without screen grid resistors. This puts the screen voltage under the plate voltage.

Early HI-FI amps use a voltage divider, and you begin to see chokes in the PS after the OPT source, these approaches introduce some regulation for the screens.

Over time you see the screens are often positive, then very negative (tweed era), and then with a filter choke, the screen get closer to plate voltage.

I'm beginning to think that screen grid resistors allow the P-P circuit to self-balance around the tube character. Whether or not the screens are tied or separated, changes the tone and response.

With bleeders or a voltage divider you introduce a little regulation, and control over screens voltage. It falls to where you want them to ride.

I had nice experience with a client recently, he wanted to use KT66 in his rig.
The KT66 genalex data sheet gives a design center screen voltage of 400V and suggests a screens voltage less than 400, less than fender values

I was able to set this condition with the selection of one resistance value in the screens circuit, at the bias point selected for the amp, selecting the screens voltage is simple and easy, and can be done to the clients taste.

[shane]

Hi Andy

After all the interesting research you've done, what conclusions have you come to regarding the screen supply set up?

What do think is the best way to set up the screens?

Cheers
Shane

[Andy Le Blanc]

so far it depends upon what your after.

you can go several ways, but the thing is that it can be approached the same
as any other operational point that you set up.

you choose the load, operation point, bias supply and voltage, and screens supply and voltage.

As a DIY builder, you can design for what you want, the screens don't have to be a mystery.