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In Reply to: My SE KT66 is making music !!! posted by danlaudionut on April 10, 2007 at 08:03:19:
I would suggest doing two things.1. Put a 200uf bypass on your cathode resistor for the KT66. Putting a 2.7uf won't give you a knee like in a crossover, it will slope way up in the frequency range. You basically put in the Vox "Twang" switch on full twang.
2. If you want to limit the bass, use an input cap and input resistor to provide a first order highpass filter. A 0.02uf cap and a 100K resistor will give you an 80hz cuttoff
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Long Live Dr.Gizmo
Follow Ups:
Chris> > Put a 200uf bypass on your cathode resistor for the KT66.
I have yet to hear a 200uFd cap sound
better than totally horrendous.> > Putting a 2.7uf won't give you a knee like in a
> > crossover, it will slope way up in the frequency range.Why ?
At 200Hz the OPT inductance is out of the picture.
At 200Hz the OPT capacitance isn't an issue.
So what reactance are you saying interacts with
the 2.7uFd cap that would give a slow slope?
Not the power supply cap or the tube,
so what else is there?It's a moot point now anyway as
it's starting to sound better already.
Not as shrill more body.
If you do not want to do the 200uF cathode bypass, perhaps you could try the WE connection from cathode to B+. You can then use smaller value (higher voltage req tho) higher quality oils.
Well, I would say that just about every commercial amp that utilizes cathode bias has a cap of near this value. The sound that you describe your amp making would seem to fit into the "horrendous" catagory. All I would say is take the 5 minuets and give it a shot. The other thing to try is to not bypass the cap to ground, but rather to the B+ feeding the output transformer, monkey style. If you have a scope, I would be interested in your amps frequency response. I suspect it will not be ruler flat.
_______________________________
Long Live Dr.Gizmo
HiI'd not though of bypassing the cathode to the B+ rather than ground.
What is the thinking behind this method?. I'd have to buy some 47u 500volt blackgates to do this
AlThis is called Ultrapath or WE(Western Electric).
It removes the last PS cap from the circuit but
you may have hum unless you have a very quiet PS.
The reason for the 'funny slope' of the RC network formed by the cathode resistor and it's bypass cap, as compared to the RC network formed by the coupling cap and the grid resistor, can be explained as follows. At a very low frequency, the attenuation of the RC blocking network at the grid is close to infinite; not true of the RC cathode bypass network, because the cathode resistor has a very finite resistance at DC. So at very low frequencies, the gain of the output tube is reduced by the fact that the cathode impedance is equal to the cathode resistor, but it does NOT go to 'zero'.Take the cathode bypass cap away and the amp will still work; take the coupling cap away and it will not make a sound.
So if you goal is to 'roll off' the low frequency response, a suitable coupling RC network will be much more effective than the cathode bypass network.
Mikey> > but it does NOT go to 'zero'
I realize that at very low frequencies
the bypass cap is out of the circuit
and therefore the lower frequencies are muted
as well as having less dampening.> So if you goal is to 'roll off' the low frequency response
Attenuate maybe more apt than 'roll off'.
Less bass means less chance of clipping
on transients which will have sonic effects
occurring higher in frequency.I may increase the cap a little after
the amp is fully broken in but
I will still keep it as small as I can.
I will never put a 200uFd in there.
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