Tube Amps / Music Electronics
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|9/9/2000 3:36 PM|
||Using 6EY6 in a Champ|
I have a '64 VC that is in great untouched condition if you overlook the fact that the speaker is quite useless at this point. I run it through a 2-12 cab.
This one has "the tone". I do not wish to do anything to the amp within reason. Even a cap job makes me hesitate.
However, I have tried a 6EY6 in the amp for short periods and after comparing with *many* other 6V6s I find I prefer the tone of these. I like it so much that I might consider minor adjustments to allow running one safely full time.
What can I do, if anything, to run a 6EY6 safely in this amp?
|9/15/2000 12:21 AM|
Using a handy tube manual, make sure you don't exceed any spec like zero signal plate current. I'd also compare the heater voltage with a 6V6 installed. If it drops with a 6EY6, I'd keep the original tube.
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|9/15/2000 12:45 AM|
Barry, I've run 6EY6s in champs, deluxes, trems and every other 6V6 amp I can think of and after proper bias adjustments, I've never had a failure of anything in them.
They want to idle at a little higher plate current and use a little more filament current too but other then that they seem to run just fine.
|9/15/2000 1:24 AM|
Thanks Bruce. That was more or less what I was after. I recall some concern over the ability of the power supply of the Champ to provide adequate levels of current using a 6EY6 for extended periods of time without blowing something in the power supply.
Now, how do I go about adjusting the bias on this puppy? I know how it works (and do it regularly) with my Holland (think Bassman) but this looks like a different circuit and it is not obvious to me. More importantly, how do I do that without hurting the original condition of the amp?
I have been running the 6EY6 for 20-30 minutes at a time. Although I do not notice any problems like excess heat and the like, I can't 'see' what negative effect this may be having on the amp so I am hesitant to leave it on for very long.
Thanks and Regards,
|9/15/2000 9:40 AM|
Just locate the cathode resistor on the power tube socket and unsolder one end from the board.
Now measure the REAL DC resistance of the cathode
bias resistor.. it could measure something like 450 to 495 ohms... then solder it back down in place.
Fire up the amp and let it warm up.
Then measure the DC voltage from chassis ground to the cathode pin 8 of the power tube and jot it down.
Remove all the leads and now with your negative lead off ground, put it on the cathode pin 8 and CAREFULLY measure the high voltage from pin 3 to pin 8.
Say it' about 365vdc
Now you have everything you need.
Say the cathode voltage measured about 21vdc across that 495 ohms you checked... or whatever you measured...
21v/495ohms= abt 42.4ma. That's the idle current.
Plate to cathode voltage is 365vdc.
365vdc X .0425a = around 15.5 watts.
Under ideal conditions, you'd want the idle power to be about 14 watts for a 6V6GTA and maybe 12-13 for a 6EY6.
15.5 watts is kinda hot for a 6V6GTA too but OK.
15.5 watts for a 6EY6 really is a little too high but the tubes are cheaper then 6V6GTAs and have a very cool tonal breakup.
Adjusting the bias is a simple as raising and lowering the cathdoe resistor value a bit and raising or lowering the plate and or screen voltage a bit to get the tube sitting at the right power level at idle.
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