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RC constant question


 
10/27/2004 1:00 PM
bnwitt RC constant question
Bruce,  
I noticed that the 5E3X2 uses a 50uf cap across the 6V6's cathode resistor. I understand that the resistor value is halved from normal in order to maintain the same voltage drop for biasing with the doubled current. I also assume the cap goes from 22uf or 25 uf to 50uf to maintain the RC constant of that path.  
 
Do you think that part of the change in bass response between the V1 shared cathode and split cathode setups we talked about earlier might be partially due to the fact that the Hoffman layout does not change the cap value along with the resistor value(1.5k)and therefore changes the RC constant of the two triodes? I know that using lower values for the cap with the standard setup has been reported as a bass frequency fix by others and this would equate to a 12uf on an 820 ohm resistor wouldn't it?
 
10/27/2004 2:31 PM
Bruce /Mission Amps
email

In terms of reactance, if you ignore the fact that the cathode is in parallel with the tube to ground... sounds weird but the anode is also AC grounded through the filter caps,...  
then here's some quick -3dB numbers.  
 
1k54.7uf = 22.6Hz  
(I like this combo because I am trying to clean out some of the woofy bass response in the 5E3)  
 
1k522uF = 4.8Hz  
82022uF = 8.8Hz  
8204.7uF = 41.3Hz  
 
So yes there is a very small difference in tone but the real difference in this effect is caused by the shared cathode current.  
One of the side effects of a cathode bypass capacitor is to reduce this, but, when ever you modulate a cathode, you will also modulate the "apparent bias" and the subsequent plate voltage of that tube a bit.  
If the cathode voltages change due to drive on one side but both triodes are sharing the same cathode resistor then the unused channel's plate voltage will vary a bit also and the variance of DC voltage is "AC".  
We hear variable DC as AC or, audio, which is now coupled to the unused volume pot by the wiper of that channels volume control.  
That's why if you turn the unused channel's volume pot up, while the actual volume pot of the used channel is at zero, you'll still hear low freq audio coming out of the unused channel's triode and it will be amplified by the next stage, the 12AX7 after the volumepots.  
Splitting the cathodes will negate this effect.  
Remember, the volume pots are in parallel and they are allowing the first 12AX7 triode to operate nearly wide open, it is the wipers that are grounded when the channel is not in use.  
With the 6V6s sharing one cathode resistor, the value of the biasing resistor is reduced to lower number... hence as you have alluded the cathode bypass cap must be increased to keep the same "gain" in the power stage.  
Again ignoring the reactance as viewed by the cathodes of the four parallel 6V6s,...  
 
25022uF = 28.9Hz  
15022uF = 48.2Hz  
15050uF = 21.2Hz  
Not a very big difference but it's there.  
 
Bruce
 
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