Cathode & screen resistor value in 6L6 driven 5F2A?

[funkgang49]

What would be the appropriate Cathode & Screen resistor value for cathode biased 6L6 in 5F2A circuit to get maximum clean headroom? The HT is 400v at Power tube anode.

[Structo]

The early tweed amps didn't generally use screen resistors.

But tubes were much more robust back then and fairly cheap so I guess most people didn't care.

But you could try a 470R screen resistor and a 470R cathode resistor.

Measure the voltage across the cathode resistor and divide by the value to get the cathode current. (that will include the screen current)

The 470R cathode may be too much so you could start lowering it until you come up with around 50ma.
That should be about 20 watts.

[tubeswell]

Yes you could start with Rk = 470R and see where it gets you with biasing. If its too cold, then try 330R, 270R 250R or 220R, depending on the plate voltage you get. With SE Class A operation you can safely go to 100% plate dissipation with the right load resistance. And use a 10W resistor for the Rk.

[funkgang49]

What is the relationship between the screen resistor and cathode resistor? What effect, if any, does the screen resistor value have on the power tube performance?

[tubeswell]

The purpose of screen grid resistors is to prevent the screen over-dissipating when the screen current peaks. If the screen grid resistors are bigger than about 500R, they start to function more like screen dropper resistors.

If they are unbypassed, screen grid/dropper resistors cause screen compression (if they are large enough). Under this condition, the control grid curves become more bunched together (thus the transconductance is decreased). Gain is decreased, but the attack is softened giving a 'spongier' feel.

Cathode resistors (in cathode-biased output stages) are there to provide appropriate grid-to-cathode voltage in order to bias the stage, so that the tube operates within an optimal/desired set of parameters. A lower cathode resistance makes the tube run hotter (giving a 'blusier' attack). Higher cathode resistance makes the tube run cooler, with less distortion (to a point).

[funkgang49]

Thanks guys.

Would the same cathode/screen resistor values, such as suggested by Tom (470r/470r), work for EL34/KT77?

[tubeswell]

The Mullard EL34 datasheet suggests 470R is okay for Rg2, and somewhere between 100R-200R for Rk in Class A mode if the plate dissipation is right. But these values vary depending on the operating conditions. It is not a simple matter of just picking some arbitrary value of resistors for the job.

http://scottbecker.net/tube/sheets/129/e/EL34.pdf

[martin manning]

If you are going to put 400V on the anode, then for EL34 I'd recommend a 6k6 primary, 470R screen and 470 Rk. This should bias at about 65 mA with 30.5V on the cathode.

[Leo_Gnardo]

Thanks guys.

Would the same cathode/screen resistor values, such as suggested by Tom (470r/470r), work for EL34/KT77?

Just a little reminder to connect your EL34 pin 1 (G3 = suppressor grid) to ground if you choose to experiment with them. Don't find out the hard way - I lost a nice pair of Siemens because I didn't know, many years ago.

[JazzGuitarGimp]

Thanks guys.

Would the same cathode/screen resistor values, such as suggested by Tom (470r/470r), work for EL34/KT77?

Just a little reminder to connect your EL34 pin 1 (G3 = suppressor grid) to ground if you choose to experiment with them. Don't find out the hard way - I lost a nice pair of Siemens because I didn't know, many years ago.

In a self-biased amp, should G3 still tied to GND, or is the cathode a more appropriate place to connect G3?

[funkgang49]

If you are going to put 400V on the anode, then for EL34 I'd recommend a 6k6 primary, 470R screen and 470 Rk. This should bias at about 65 mA with 30.5V on the cathode.

The OT is a SE-3K from Merc. Mag. with 4/8/16 ohm taps. Right now and for testing purposes I'm driving a single 12" speaker at 8 ohms. Would dropping to the 4 ohm tap achieve 6k ohm on the primary?

[cbass]

............

[martin manning]

An 8 ohm load on the 4 ohm tap will double the primary impedance to 6k.

[Leo_Gnardo]

Thanks guys.

Would the same cathode/screen resistor values, such as suggested by Tom (470r/470r), work for EL34/KT77?

Just a little reminder to connect your EL34 pin 1 (G3 = suppressor grid) to ground if you choose to experiment with them. Don't find out the hard way - I lost a nice pair of Siemens because I didn't know, many years ago.

In a self-biased amp, should G3 still tied to GND, or is the cathode a more appropriate place to connect G3?

Good question. And considering old Traynors connect their G3's to the first filter on the bias supply, I'm guessing anything close to (or under) ground will work. What is important, is that G3 isn't just "left hanging in space" or as I did, just left a Fender's pin 1 connexion on it - which is a jumper to pin 5 = G1. My EL34's lasted about 20 minutes then faded to no gain. See, even attached to the drive signal, negative bias & all, the mighty Siemens came to a bad end. Just to be safe, I'd choose ground rather than cathode potential for G3.

Others who may know better, sound off! We'd like to hear from ya!

[tubeswell]

g3 can be connected to the cathode