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6L6 in a Champ

2/15/1998 5:59 PM
michael crown
6L6 in a Champ
How hard is it to modify a Champ for a 6L6GC or 5881? Can the power supply handle it without upgrade?
2/16/1998 6:29 AM

This type of question has been floating around for a while, and I'm really surprised nobody has addressed the issue of load resistance. 6V6's and 6L6's have differnt load resistances. These values are critical when matching to the output tranny. If you are going to change the tube, I'd suggest changing the ohmage of your speaker.  
2/16/1998 8:40 AM

I have a silver panel Champ that runs with a (TungSol) 5881 tube and the stock 5Y3 rectifier with no problems. That is, the power trans does not overheat with the added current draw.  
The 6L6 type will draw more heater current than a 6V6, and more cathode current. I wasn't comfortable with the 1 watt 250 ohm carbon Rk, so I replaced it with a 270 ohm 5w wirewound resistor.  
I don't know what primary current the stock output trans is rated for, so it could possibly be overheated if the 6L6 is just "plugged in" without monitoring the resultant cathode current to see where you are.  
Yes, the power supply (on a SF) can handle the mod, but I don't know if this is safe for the older tweed model.  
The primary load impedance is close enough to work well with either a 6V6 or 6L6 in this circuit when using the 4 ohm speaker.
2/16/1998 12:41 PM
What a great site! I am getting exactly the info I am looking for here. Please bear with me as I am somewhat of a novice. How does this mod sound?
2/16/1998 2:29 PM

With the 6L6 or 5881 the sound is a little stronger but cleaner. Be careful of running it wide open, because that's how your stock speaker's voice coil will end up. I don't turn mine all the way up unless an auxiliary speaker is substituted.
2/17/1998 1:16 AM
michael crown

I have a Kendrick Black Frame. Do you think it can handle the extra power? Did you have to re-bias your amp for the 5881? Thanks, Mike.
2/17/1998 7:34 AM

The Kendrick speaker should handle the power easily. Modern speakers have better heat dissipation capability than ones made 30 years ago. Since you didn't state anything about a speaker, I assumed it was the stock one, which would be abused with additional power.  
In circuits such as this, it's the cathode resistor which sets the bias. You need to take voltage measurements and make a few calculations using Ohm's Law to find the operating point of the tube. Since this is a Class A output stage, the tube can be run at or near maximum allowable dissipation. Knowing the cathode current and the plate-to-cathode voltage, you must compare their product to the tube mfr.'s maximum wattage rating to see where it's running, and possibly have to adjust the cathode resistor's value to keep within the maximum ratings.  
The old 5881 I used has a lower maximum rating than the currently manufactured Sovtek version, the 5881/6L6WGC, so I changed the cathode resistor. There's no magic value I can give you and say "just change it to this", because it's dependent on the actual voltages in your amp, the tube you're going to use, and whether or not you're expecting to make one value work for both 6V6 and 6L6 types.  
After you've made the change to 6L6, you might miss that singing overdrive/sustain that you had with the 6V6.  

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