value of 1st cathode bypass cap super twin

[JC@]

Hello,
I have trouble identifying the value of the V1A cathode bypass cap in a Fender Super Twin (180 watts)
Is it 750uF/3volts? Why that big?
In case it is, would 470uF or 1000uF be ok as a substitute?

What schematic are you looking at? I don't think I have that one- just the 135 watt model...

Steve Ahola

[JC@]

Hi Steve.
This one: schematicheaven.com/fender.htm -->CBS 180W Super Twin (not the Super Twin Reverb version one which goes right after in the list)
Is not very clear though.
Thanks

[stokes]

Its clearer on the Super Twin Reverb schem,looks like it is 750uf/3v the 470uf or 1000uf would be okay if that is all you have.

[JC@]

Yes the schematic for the Reverb version is a lot clearer.
That high value still intrigues me.

[stokes]

The bypass caps value is selected by the capacitance reaction to the cathode resistor,without getting into the formulas,when you see a 25uf cap across a 1.5k cathode resistor like most Fender pre-amps you get a flat frequency response boost.The higher uf you see on this amp will boost more highs past that resistor.

[JC@]

That is the opposite to what a coupling cap between a tube's plate and the grid of the next stage does with frequency. is it?

[Bruce / Mission Amps]

The bypass caps value is selected by the capacitance reaction to the cathode resistor,without getting into the formulas,when you see a 25uf cap across a 1.5k cathode resistor like most Fender pre-amps you get a flat frequency response boost.The higher uf you see on this amp will boost more highs past that resistor.

The larger the cap, the more lows will be boosted... any size cap will pass the highs and a larger size cap passes the same highs too, but with a larger amount of capacitance, more of the lower freqs are bypassed for more gain in that stage at all frequencies above that -3dB frequency.

It has limitations but here's a chart I drew up on Randall's website:
http://www.aikenamps.com/RC-FHz.htm

[stokes]

Okay.I got it backwards,doing the math for the capacitive reactance I was using 80hz as my target low frequency,so when I came up with such a low cap.react. 1.3 as opposed to 80 with the 25uf cap,my mind kept thinking low.But the intended effect is to increase the gain of the stage,as Bruce says.

[Matt T.]

While we're on the subject: IIRC the response from a cathode bypass cap is more of a 'shelving' type of reponse as opposed to a 'roll-off' type of response like with HPF/LPF that we get from our RC combos (like a coupling cap and a resistor to ground). Here is a thread I found on kg's site about that.

EDIT: I just reread it (quickly) and it appears that there is a 'slope' but there are high and low limits where the response flattens out...so kind of a mix?

[hasserl]

Nice chart Bruce. You guys are too cool! 8)

[JC@]

Aha, this is getting more and more complicated. For reference I would prefer a little book all filled up with "thumb rules" like: bigger cap->more gain->more lows... something like that.

And I am still asking myself why that huge cap (750uF) in the Super Twin.

[Tom Phillips]

That 750uF so big that it's effect is way past the point of dimminishing returns. I'll bet that you can lower the value significantly until you hear any change in amp performance.

Has anyone actually seen a stock 750uF cap in one of these amps? Looking at the poor quality of the schematic that is posted I think that it's possible that what you are seeing is "spot on the scan" 50uF.

[Bruce / Mission Amps]

Right, the frequency response curve is so wide and the time constant is so freakin' slow that the 750uF cap is probably charging up fully to cathode voltage, acting like a 1.5v battery... which when on the cathode is kind of like a fixed bias in a class A preamp... kinda.

[Tom Phillips]

Its clearer on the Super Twin Reverb schem,looks like it is 750uf/3v ...

OK, I see that now and it is clearly written "750uF".
Still makes no sense to me. If it were my amp I'd chnage to 25uF and adjust down from there if necessary