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35 watts cathode bias 6L6 5E3?

9/7/2004 7:17 AM
Rick 35 watts cathode bias 6L6 5E3?
Ive just built a rack mount amp based on the 5E3 And have it cathode biased but intend to change to fixed bias.Just out of curiosity I ran it and did a few measurements.27 volts across the 270 cathode resistor with a B+ of 390-400 vdc.If my calculations are correct thats .1 of an amp x 363 volts = 36.3 watts Is this correct.I have a 40 watt 4k O/T and a very substancial solid state power supply.Any comments.Also I only have -28 volts available for the fixed bias at around 400vdc is that enough to get 30-35 watts out of this rig.Any help would be gratefully recieved.Thanks all
9/7/2004 10:58 AM
Bruce /Mission Amps

Your audio calcs are not correct however, my cathode biased Super is about the same at 100ma and 410vdc.  
With a real 25 watt OT, (very slightly larger then a Pro Reverb OT) it can make about 24-26 watts, just at the onset of obvious clipping as viewed on my O'scope across a dummy load.  
9/8/2004 6:37 AM
Bruce,not quite sure how this works but I'm measuring 382 vdc on the 6L6 plates and about 53 milli amps on each tube (using a weber bias rite) if I subtract the 27 volts across the cathode resistor I get 355vdc.I thought if you multiply that by .1 amps (approx) it would give the power out for the 6L6s (P=VxI) which gives 35.5 watts.Where am I going wrong here.Sorry to be pesky but I just wanna get it right for future calculations.Thanks for your help.Rick
9/8/2004 7:19 AM
Wild Bill

Rick, power drawn is NOT power out!  
Nothing is perfect except maybe Dolly Parton's profile! Those tubes draw DC power to do their job of converting that energy into AC audio output power.  
Different output classes like A, ABx, B, C or whatever have design efficiency factors. Then there are losses in the OT and other parts involved.  
In the real world you'd be lucky to get 30% of that DC power you calculated transferred as audio power to the speaker.  
Hope this answers your question...  
---Wild Bill
9/8/2004 9:27 AM
I see,I get what your saying,I knew from other discussions that cathode bias 6L6 5E3s seem to max out at 18 or 20 watts but didn't understand why the figures didn't seem to add up.You've put me on the right track.Thanks to Wild Bill and Bruce.
9/9/2004 2:20 PM
OMG, Wild Bill!
"Nothing is perfect except maybe Dolly Parton's profile!"
Now you know you're going to get my tubes running hot talkin' like that, Bill!;)  
9/8/2004 9:20 AM
Bruce /Mission Amps
Re: 35 watts cathode bias 6L6 5E3?
Audio output power is AC and all those measurements you made were DC input measurements.  
You need all that DC info for other reasons but it's static information giving you NO audio output info.  
Sending an AC signal into the amp's input and measuring the PA's AC clean undistorted output across a known impedance, at a given frequency, is the only way I know of to make a meaningful measurement of audio output.  
As an example, if you were to send a clean 100Hz@100mvac signal into the amp and measured a 100Hz output of 12vac (clean) from the amp across a fixed resistor of say 4 ohms, then you can come up with a reasonable measurement of of power output.  
In that scenario, 12v^2/4ohms = 36watts.  

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