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Re: 35 watts cathode bias 6L6 5E3?

9/8/2004 3:24 PM
Re: 35 watts cathode bias 6L6 5E3?
Bruce, it always bothered me that Fender rated output power in their amps without specifying distortion content (i.e. 15W @ 1% THD). Do you find that measuring output voltage on a scope just before clipping and calculating output power corresponds well with what they publish on the rear panel? In the RF world, we use broadband thermal power sensors to measure output power so all harmonic distortion gets factored in to the measurement as well. But usually, output power is given at a specific level of distortion so you have something to compare it against. I'm sure a 5E3 turned up all the way is putting out a lot more than 12W or whatever it's rated at.  
9/8/2004 6:39 PM
Bruce /Mission Amps

The back panels only say what is the lowest rating wattage speaker to use. I don't think there is any truth to the actual output power.  
As an example, a BF Pro Reverb will only hit about 32-36 watts output "clean" into a 4 ohm dummy load.  
A BF Super Reverb will do about 40-45 watts into a 2 ohm dummy load.  
Most BF Twin Reverb amps have a hard time hitting a clean 85 watts...  
However, a VOX AC100 will actually do about 105 to 115 watts output with virtually only a hint of distortion.  
Why? One look at the iron of a vintage English amp, compared to the cheaper iron in a vintage Fender built amp, should tell you.  
A 5E3 running with a 100mv to 150mv input, will only do about 9.0-10.5 watts *rms* (I hate saying rms watts) but I mean perfectly clean looking watts output.  
The rub is just what you said... as viewed with a scope across a known 8 ohm DCR... not an impedance... a good running 5E3 will only push about 8vac to 9vac rms.  
For testing, I use a 120Hz frequency for that power measurement as I think it's a reasonable compromise freq with this small of an OT.  
Now, the 5E3 will hit more power then that if I allow all kinds of distortion.  
So, as far as cranking the amp up to the ends of the earth... little 5E3 black book has written in it, flat out:  
about 10.6vac at 83Hz and around 11.8vac at 1000Hz.  
That would be:  
83Hz - 14 watts  
1000Hz - 17.4 watts.  
This is serious nasty power tube distortion levels and a little phase inverter distortion thrown in too.  
Ahh... but with the right guitar/speaker/tube combo, what great overdrive sound when riffing right under that level!!!  
9/9/2004 1:06 PM

Great info Bruce, thanks! Your point about the output transformer size, is that why a Princeton Reverb is only rated at 12W and a Deluxe Reverb is rated at 22W seeing that both sport 6V6's, are fixed biased, and have plate voltages of around 420V? Not sure if the power transformer is smaller too, I guess that would be another limiting factor. Thanks again.  
9/9/2004 2:51 PM
Chris @ CMW amps

sorry for highjacking.....  
There are more differences between the PR and the DR causing a difference in power ( and sound ):  
- LTP PI ( more balanced signal to outputtubes ) versus CathodyneSplitter  
- one more gainstage in a DR so more voltage to the PI/ouput  
- 12" versus 10" speaker, 12" is louder most of the time  
But putting a DR OT in a PR will certainly make the PR louder with some more low-end  
Hope this helps,  

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