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EL84s Class A in a 6L6 type Fender


 :
3/25/1998 7:16 PM
Cosmik Debris
EL84s Class A in a 6L6 type Fender
You can buy these converter thingies (Black Jacks and Yellow Jackets) that allow you to plug in an EL84 into a 6L6 socket. They are cathode biassed and run in Class A.  
 
I would like to build my own. How do you drop the plate and screen voltage down from 490V down to something around 300 that is safe for an EL84? I was wondering whether is is possible to use a huge cathode resistor to lift the cathode up 180V or so rather than using something to drop the plate voltage down. Do you have to worry about the cathode to heater voltage.
 
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3/25/1998 10:50 PM
stephen delft

Do you have to worry about the cathode to heater  
voltage.
 
 
Yes ...  
 
- Stephen
 
3/30/1998 8:41 AM
Dave H

Here is my guess as to the contents of a Yellow Jacket.  
 
Heaters connected straight across from 6L6 skt to EL84 skt.  
A 150V (15W or more) zener between 6L6 plate and EL84 plate.  
A 160V (5W or more) zener between 6L6 screen grid and EL84 screen grid.  
A 0.1u cap between 6L6 control grid and EL84 control grid.  
A 470k resistor between EL84 control grid and 6L6 cathode.  
A 200 ohm 2W resistor between EL84 cathode and 6L6 cathode.  
A 22u 25V cap across the 200 ohm resistor.  
 
Connect the speaker to an OT tap of half the speaker impedance.  
 
The amp should now run in cathode biased class A similar to an AC15/30 but it would be a lot easier to modify the amp with one big zener and one common cathode resistor.  
 
Dave
 
3/30/1998 2:02 PM
Cosmik Debris

>Here is my guess as to the contents of a >Yellow Jacket.  
 
That's similar to what I had in mind but I thought it would be more convenient to lift the cathode up. I see now that you can't do this because of the heater to cathode voltage.
 

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