Matchless DC30 screen grid and bias resistor values?

[wpaulvogel]

You can also run Russian 6P14P tubes. They are claimed to be more durable with voltage and dissipation.

[pdf64]

Increasing the resistance between the OT CT and screen grid HT supply nodes will reduce screen grid voltage and hence anode current. So anode dissipation should come down.

[Ledvedder5150]

Increasing the resistance between the OT CT and screen grid HT supply nodes will reduce screen grid voltage and hence anode current. So anode dissipation should come down.

So I would put the resistor where I've marked up this drawing? The OT CT comes into point R.

Screenshot_20221004_234200_Gallery.jpg

[Roe]

Increasing the resistance between the OT CT and screen grid HT supply nodes will reduce screen grid voltage and hence anode current. So anode dissipation should come down.

So I would put the resistor where I've marked up this drawing? The OT CT comes into point R. Screenshot_20221004_234200_Gallery.jpg

yes, or on the other side of the choke (doesn't matter)

[Ledvedder5150]

I just wanted to give an update. I installed a 330 ohm resistor in series with the choke. Now, this is what I'm getting for bias.

Screenshot_20221016_192649_Chrome.jpg

[roberto]

Hi, I'd suggest 82R on the cathodes to avoid pushing too much the EL84.
Screen grids resistors depend on the voltage at which the amp runs and the loadline too (with lower loads you can lower this value because when g1 reaches zero, the anode is at a higher voltage compared to amps with higher Raa, and secondary emission is lower), but I usually stay around 270 Ohm in pentode mode (or UL at 23%).

[pdf64]

That calculator may tend to underestimate screen grid dissipation for EL84 and EL34, as it assumes that 5.5% of cathode current flows via the screen grid, when around 10% would be more realistic.
So anode dissipation may actually be lower than it shows.

[Roe]

just try and see if the tubes redplate. measure voltages for cathode, screens and plate

[Ledvedder5150]

just try and see if the tubes redplate. measure voltages for cathode, screens and plate

Cathode is 11.1v
Screen is 334v
Plate is 342v

[Roe]

just try and see if the tubes redplate. measure voltages for cathode, screens and plate

Cathode is 11.1v
Screen is 334v
Plate is 342v

11.1V is too much, 10v is ok with 50R, given good tubes
screens and plates are a little high. try increasing screen resistance (e.g. 470R 3-5w). check for redplating.

[Ledvedder5150]

just try and see if the tubes redplate. measure voltages for cathode, screens and plate

Cathode is 11.1v
Screen is 334v
Plate is 342v

11.1V is too much, 10v is ok with 50R, given good tubes
screens and plates are a little high. try increasing screen resistance (e.g. 470R 3-5w). check for redplating.

I'm pretty sure that what I was also getting before adding the 330r at the choke too.

[Roe]

Cathode is 11.1v
Screen is 334v
Plate is 342v

11.1V is too much, 10v is ok with 50R, given good tubes
screens and plates are a little high. try increasing screen resistance (e.g. 470R 3-5w). check for redplating.

I'm pretty sure that what I was also getting before adding the 330r at the choke too.

Perhaps the wall voltage is a little high?

Would a more inefficient rectifier tube help?

with 345v plate voltage, vox used 780R screen resistance in the 70s.