18 watt 2xEL84 biasing?

[Skilpadda]

I have a dual 250ohm 5w cathode resistor(125ohm 10w...?) and a 100ohm shared screen resistor. 315v plate voltage and 304v screen voltage and 10.2v on the cathode. Calculations gove slightly above 12w plate + screen dissipation. It is slightly too close to max in data sheet for my comfort. I measure across the 100 ohm screen resistor and find 8 mA of current which is as expected per data sheet (7-22 for this application)
This gives a screen dissipation of 2.1w for the screens at idle. Data sheet say 2w max but can be temporary up to 4w for spech and audio...
On my wiew this idle is to high for comfort too...
But changing to a higher cathode resistor (2x270 or 2x300ohms) will pull down the dissipation of both the screens and the plates will it not....?
I am some surprised because at 270 - 0 - 270 volts the B+ voltage is fairly low even for an EL84 amp...

[sluckey]

I like to use 150Ω for shared cathode resistor. I also like to use two separate 100Ω for screens. I have one fixed bias EL84 amp with separate 1K screen resistors.

You may want to consider splitting the cathode resistors, ie, use one 250Ω (or 300Ω) for each cathode. You'll need to get one more 100µF bypass cap. I actually prefer using separate cathode resistors because you can then get a real power dissipation for each tube rather than just divide by two and assume both tubes are perfectly matched (they never are).

[d95err]

Vox gets away with 100 ohm cathode resistor with similar or even higher voltages, so you should be safe. Although Voxes are known to eat EL84s for breakfast...

[Skilpadda]

Thank you for the replies, very helpful. Amp started to blow fuses now so i worry one of the el84s have developed some kind of short Will it change tone much to bias slightly colder...?

[Skilpadda]

It is a short in the WCA4 rectifier i believe. Limiter is dim with it out of the amp but very bright with it in, even with nothing connected to pin 3.
I put in valve wizards protection circuit for the cathode follower. A resistor and a diode in series between grid and cathode. Is there a chance this can kill the rectifier...?

[sluckey]

It is a short in the WCA4 rectifier i believe. Limiter is dim with it out of the amp but very bright with it in, even with nothing connected to pin 3.
I put in valve wizards protection circuit for the cathode follower. A resistor and a diode in series between grid and cathode. Is there a chance this can kill the rectifier...?

Why not use an EZ81? It will drop the B+ a bit, maybe even take care of your bias concerns. That resistor/diode on the cathode follower did not blow the rectifier.
 

[Skilpadda]

I already ordered the ez81 tube. But B+ should be same. The wca4 emulates the voltage drop of the ez81 and even the slow startup, at least it is what it say on their website

[sluckey]

OK. The EZ81 filaments will connect to the filament string for the other tubes.

I know this is your Weber 6M18 amp but others might not make the connection to your other thread. It would be good if you mention that with a link to the schematic if you open another thread about this amp.

schematic...      https://www.tedweber.com/media/kits/6m18tmb_schem.jpg

[Skilpadda]

Ah yes sorry about that. I also borrowed some ideas from the JA TMB schematic.....

https://www.18watt.com/gallery/picture.php/1071/category/226

[Skilpadda]

If i decide to give each el84 its own 250ohm resistor will they not require a 50uF capacitor instead of the old shared 100uF to have the same tonal characteristics....? Or am i way off

[PRR]

They are in series to signal. A simple analysis says they should both be 200uFd.

HOWEVER. The cathode cap in a push-pull power is NOT a "bass cut" as it is in single-ended stages. It actually does very little. It mostly acts when the stage is OVER-driven, affecting transient action in ways we do not understand what the ear "likes".

Personally I'd use whatever I had a lot of.

Obsessive tone-seekers will want a selection of different caps, and styles and venues, to see what works best for the way they will use the amp.

[Skilpadda]

Aha, thank you for that. Then i was way off in the opposite direction. I will see what  i have lying around and use whatever is closest. But i think 50uF is what i have more likely...