does this beard amp has at7,ax7au7 front stage. I repaired a beard amp that had 6el84 in pp and in parallel. I set the bias acroding to the manual presented. The tubes were also change, i too had some problem trying to get them to the rating. I think the bias what as -18volts i think. The bias sets all entire 6 el84. What i did was set the bias to the manual voltage then turn it down with respect to the highest tube dissapating the most current so that the particular tube is within safety figures. This however push some of the valve to a lower value dissaption. I install a semi match unit. Get 6 tubes to match is really hard. Also does yours has the led to tell you whether the tube is overbiased?
Dumb question, since I don't know the circuitry here: usually, idle current (the thing that bias controls) is determined by measuring the voltage across some sort of cathode resistor. Is that how this is set up? If that's the case here, you need to see what that cathode resistor is, measure the voltage across it, then back calculate what the idle current is.
If you know anode voltage (caution: this is a measurement of DEADLY voltages!), then it's easy to figure out what the max acceptable idle current is, knowing the max plate dissipation. How that will affect your tube life is, as Frank says, something pretty empirical.
"Idle current" versus "bias" is one of those bits of sloppy terminology which can confuse the heck out of the inexperienced. I'm on a personal crusade to make sure everyone uses the terms correctly
If you know anode voltage (caution: this is a measurement of DEADLY voltages!), then it's easy to figure out what the max acceptable idle current is, knowing the max plate dissipation. How that will affect your tube life is, as Frank says, something pretty empirical.
"Idle current" versus "bias" is one of those bits of sloppy terminology which can confuse the heck out of the inexperienced. I'm on a personal crusade to make sure everyone uses the terms correctly
Hi,
Actually it was pretty simple.
Very much along the lines of using the obiquitous 7818/7918 as a current reg.
Throw in a resistor and a LED that lights up when a certain amount of current is exceeded and you're done.
They did claim full Class A operation indeed.
Cheers,
Actually it was pretty simple.
Very much along the lines of using the obiquitous 7818/7918 as a current reg.
Throw in a resistor and a LED that lights up when a certain amount of current is exceeded and you're done.
They did claim full Class A operation indeed.
Cheers,
Attachments
EL84 specs output 12w max
anode voltage 300v max (would u beleive it)
screen grid voltage 300v max
isnt it strange that most amp engineers exceed the max
on designs.
I find a bios of -10v is perfect for my amp but its a guitar amp
ac30 so this may not help you.
as was said erlier u have no chance of setting a suitable voltage
for bias as long as your valves are unmatched unless u want to do every one individualy which is not the best solution but probly
the only 1 without getting matched valves.
stormy
anode voltage 300v max (would u beleive it)
screen grid voltage 300v max
isnt it strange that most amp engineers exceed the max
on designs.
I find a bios of -10v is perfect for my amp but its a guitar amp
ac30 so this may not help you.
as was said erlier u have no chance of setting a suitable voltage
for bias as long as your valves are unmatched unless u want to do every one individualy which is not the best solution but probly
the only 1 without getting matched valves.
stormy
stormy said:
In my school, we were told that the power dissipation was a more important concern than voltage rating.
In other words, one could put 375 volts on the plates, as long as idle current did not cause the plate's power dissipation to exceed 12 watts.
So for 375 volts, one would need to idle no more than 32 milliamps. This is why fixed bias is used, and that the fixed bias is very close to cut-off. Also, tube dissipation will never be exceeded in this case because the current across the tube is in inverse proportion to the voltage across the tube. In other words, when the tube sees a full 375 volts across it, it sees zero current. So dissipation is zero. When full current is flowing through the tube, zero volts is seen across it, so power again is zero... all this is theoretical of course. It never really gets to zero.
About 14-15 volts is typical for that. Of course, with that tight a tolerance, sound is not as sweet. Why is a mystery to me. The 6BQ5 in push-pull sounds best and balanced to me at about 12-13 volts bias.
At 10 volts, it is practically all class A P-P.
Gabe
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