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EL84 -> 6L6 Mod

2/14/2004 4:58 PM
EL84 -> 6L6 Mod
Hey All,  
Ok, I have a Carvin Vintage Series 50W 1x12 Amp--the power section is built on 4 EL84s in PP (modeled after an old VOX I believe). I'm not a big fan of EL84s, though, and I prefer the sound of 6L6s. I'm thinking about keeping the existing preamp, but tearing out the existing power amp section, and replacing it with a copy of 50W Blackface power amp. The OT is 4/8/16 Ohm switchable. I'm sure that this swap would affect the load presented to each tube, but I can't quite figure out how--I suspect that each of the 2 6L6s would see half the load that each of the 4 EL84s did (assuming that the same secondaries were tapped on the OT)--does that sound about right?  
I know that most (if not all) of you are smarter than me, so please let me know any other potential complications that you may see, and generally if you think that this mod is doable.  
Yes, I know this is a MAJOR mod, and yes I know that I may be a little bit insane :). Any help, thoughts, ideas, etc... would be very much appreciated.  
2/18/2004 4:05 AM
Don Symes

One thing not to overlook is the heater current -  
the EL84 wants 760mA per tube and the 6L6 wants 900mA each.  
Actually, that looks OK.  
Think about going with a "correct" OT - the one you have should work OK, but the OT is a big part of the sound.
2/18/2004 2:32 PM
Wild Bill

Think of it this way, Jeff!  
I have an OT with an 8k primary load, using 2 tubes. If I use 4 tubes I need a primary of 4k, half of what I started with.  
Now, if the OT was designed to run from 4 EL 84's it probably was designed at around 4k. Running a pair of 6L6's at 4k is not likely to be seriously out of whack. Besides, how do we define "out of whack"? All tubes work at all loads, it's just that at too low a load they tend to butn up and at too high a load they put out less power and maybe unstable.  
If you look at the data sheets they usually have a curve of plate load vs power output and distortion. The curve is pretty broad around what works ok, if not optimum.  
Still, you could sleep easier if you actually knew the specs of your OT. This is easy to measure. You need a low voltage AC supply and a multimeter. A train set trannie or one of those wall wart battery eliminators (make sure it's AC out and not DC!) is used to feed something between 6 and 20 volts AC into a speaker tap. R G Keen has a great FAQ that tells you all about it. If you know the input voltage you can measure how much the OT steps it up to the primary and you know the voltage ratio. A little math and you then know the impedance ratio.  
Good luck!  
---Wild Bill
2/21/2004 3:40 AM

Thanks for the info guys. I'm going to test the impedance ratio on the OT; if its too far off and will need to be replaced, I may just forget about the mod and go ahead and build myself a Bassman head copy :)  
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