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cathode caps and resistors


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8/3/2000 3:49 AM
Gord cathode caps and resistors
Hi,  
 
On V1 of many amps (eg. 5F6-A), the cathode resistor is common to both cathodes, with a cap across it. I believe the Bassman values to be 250uf and 820 Ohm. If the cathodes are tied to ground separately, then:  
 
-what are the advantages from both electrical and tonal standpoints?  
-would the values have to be doubled for both the cap and the resistor in order to keep from altering the tone?  
 
I heard that the current Vintage Guitar magazine has an article regarding this.  
 
Thanks  
 
Gord
 
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8/3/2000 5:59 AM
Brian

quote:
"-what are the advantages from both electrical and tonal standpoints?"
 
 
Don't think that there would be too much tonal difference in making the switch, but the leakage from one triode to the other would be minimized. This helps eliminate channel crosstalk.  
 
quote:
"-would the values have to be doubled for both the cap and the resistor in order to keep from altering the tone?"
 
 
The resistor needs to be doubled. The cap value is usually 25uF I believe, and this is large enough that not doubling it won't really affect the tone.  
I usually just use the 1k5/25uF combination on each triode section.  
 
BT
 
8/3/2000 4:25 PM
Gord
Hi  
 
Is is 25uf or 250uf?  
 
Gord
 
8/3/2000 4:54 PM
M Cook

I've been wrestling with this for the last couple of days. Please correct me if I'm wrong as I'm just a hobbyist.  
 
If you took a shared cathode resistor and wanted to split it to separate cathode resistors, wouldn't you have to halve the value to get the same sound?  
 
The 5f6-a uses a resistor value of 820R and a cap of 250u. The schem. says there is a cathode voltage of 2.5V. Using V=IR, the current across this resistor is ~3.05mA. Now doesn't the current effect the sound of the triode? If you split the cathode and wanted to get this amount of current from half of the triode, you would have to use a resistor of ~410R (voltage at 1.25V per half). If you use a 820R on each cathode the current would be less at ~1.52mA per side. If you double the value and use a 1K5 on each cathode, the current would be much less at ~0.834mA per side.  
 
I know that when the cathodes are shared, they both contribute to the current but changing the amount of current has got to affect the tone & gain from the triode, doesn't it?  
 
My Marshall 1986 used a shared cathode on the first tube and when I split the cathodes with 820Rs, I noticed a drop (though slight) in gain. (Tone was harder to determine as I did not use identical values of caps.)  
 
As far as the value of the cap is concerned, doesn't the cap just function as a high pass to a specified frequency? To get the same sound you would just have to use the same value of cap on each cathode.  
 
My $0.02 worth of opinion as well as confusion. Can anyone clear this up?
 
8/3/2000 6:30 PM
Reverb

Greetings,  
 
In your message you used the correct formula, but your reasoning was not straight.  
 
As always, V = I x R  
 
You say you have 2.5V (call this Vold) measured across 820 ohms (call this Rold), which does give you 3mA (call this Iold) according to V = I x R.  
 
Now, you want to split the current evenly between two separated triode stages. This means you want 1.5mA per stage (call this Inew), with the voltage to remain at 2.5V bias level per stage (same Vold as before). Now take a look at the equations:  
 
V = Inew x Rnew = Iold x Rold  
 
Inew = 1/2Iold, so:  
 
V = 1/2Iold x Rnew  
 
Rearranging, you get:  
 
Rnew = V/(1/2Iold) = 2 x V/Iold  
 
Substitute in know values:  
 
Rnew = 2 x (2.5V/3mA) = 1.66k (double 820)  
 
As you know, 1.5k is used since it is a standard value and gets you close enough.  
 
Hope this helps and doesn't confuse,  
 
Reverb
 
8/3/2000 8:07 PM
M Cook

Thanks Reverb! :)  
 
I haven't seen this formula. Something new to think about.
 
8/5/2000 6:08 AM
Steve A.

M.  
 
My Marshall 1986 used a shared cathode on the first tube and when I split the cathodes with 820Rs, I noticed a drop (though slight) in gain. (Tone was harder to determine as I did not use identical values of caps.)  
 
    "R" covered the other issues but I thought I'd try tackling this oneó perhaps there was some coupling through the shared cathode components that made your amp sound louder...? You also mentioned using different valued caps here so that might have something to do with it, too.  
 
    In some cases I try to figure out the math, but to split shared cathode components you multiply Rk by two and divide Ck by two... although dividing Ck by two isn't really necessary. If you want to make a very noticeable change in a Ck cap, go up or down by a factor of 10... like going from .22uF to 2.2uF to 22uF to 220uF (after 220uF the "knee" of the frequency response curve is well into the subsonic range so most people would not try a 2200uF Ck cap...)  
 
Steve Ahola
 

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