Fender Champ 5F1 - v1a bypass capacitor question

[RadioComm]

Hello Everyone,

Build this amp a long time ago (15 years +?). Bought the unfinished cabinet and chassis off of eBay, made the board from a piece of garolite, and the rest either from Tubes&More or what I had hanging around.

Read one of Robinette's page:

https://robrobinette.com/How_Amps_Work.htm

And decided to add the v1a bypass cap for extra gain, an wow, one can really hear the difference. I personally love it.

Anyway, in his article, he states:

"Adding a bypass capacitor around the cathode resistor reduces this negative feedback because the bypass capacitor acts as an electron reservoir. When the tube's grid goes positive and flows more electrons through the tube it's much easier to pull electrons from the bypass cap reservoir than pull them through the cathode resistor. The freer flow leads to more electrons flowing through the tube so more gain is generated."

Since I'm perfectly happy with the way it currently sounds, this question is just out of curiosity:

Is there a correlation between the amount of gain and the capacitance of the cap? If so, how much of an increase in capacitance is needed to hear an increase in gain? Same question,
 for less gain.

Thanks

[sluckey]

I can hear a BIG difference in gain between no cap and a 25µF cap for all frequencies. I can hear a big difference in gain between a 5µF and a 25µF, but only with the lower frequencies. Using a 25µF on the preamp tube increases the low freq gain so much that many people feel the amp is too boomy sounding. A popular mod to decrease the boominess is to replace that 25µF with something smaller. Marshall was very fond of using a .68µF

[SILVERGUN]

I found this chart from Merlin very helpful
https://www.valvewizard.co.uk/OtherStuff.html

[RadioComm]

I can hear a BIG difference in gain between no cap and a 25µF cap for all frequencies. I can hear a big difference in gain between a 5µF and a 25µF, but only with the lower frequencies. Using a 25µF on the preamp tube increases the low freq gain so much that many people feel the amp is too boomy sounding. A popular mod to decrease the boominess is to replace that 25µF with something smaller. Marshall was very fond of using a .68µF

Understood. Thanks so much for the informative answer.

I found this chart from Merlin very helpful
https://www.valvewizard.co.uk/OtherStuff.html

Thanks for the link. Great reference.

[Esquirefreak]

I built a 5E2-ish amp a while back. To beef up the gain I put caps in parallell with the v1b cathode resistor. However I wired the caps in series with a B5k pot (25μF to ground on one side and 470nF to ground on the other). That way I can boost either highs or lows. Works really well.

[AlNewman]

I love bypass caps.  They are really the only tool we have in most amps to actively increase frequencies and flavour distortion, mostly we cut frequencies through coupling caps, or tone stacks, or grid resistors. 

They are mainly reactive with the cathode resistor, so there isn't really a one size fits all, you need to tailor them dependent on what each gain stage is doing.  You can also add a snubber cap after the gain stage to create a band pass filter, or a frequency hump. 

I think on paper the amount of gain the stage has is consistent no matter the value of the bypass cap, but it seems to me like the tube seems to work harder amplifying all frequencies, so if you dial back the frequencies a bit with your bypass cap, it seems to give it a little more push.  To my ears anyways.

In the fender hot rod series they use switchable bypass caps as bright switches, fat switches, more drive switches, and it creates so many opportunities to flavour the sound, they are a lot of fun to modify.

[PRR]

If you bias the tube by magic, you get a set gain.

If you bias with a cathode resistor, you get less gain. In most cases, half gain.

If you bypass the resistor with a cap, you get half-gain at low freq and full gain at high freq.

https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/

[RadioComm]

If you bias the tube by magic, you get a set gain.

If you bias with a cathode resistor, you get less gain. In most cases, half gain.

If you bypass the resistor with a cap, you get half-gain at low freq and full gain at high freq.

https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/

Thanks PRR. Great site! Love the calculator.