V1 Cathode Bypass Cap - Changing Value

[vjf]

Has anybody tried a different values for the V1 cathode bypass cap other than the standard 25 MFD...something like .68 MFD (Marshall Plexi) or no cap?

If so, what did you find? Any noticeable changes?

[tyru007]

I changed the 25uf cap to a .68 cap on V1 (forst gain stage) of a BF champ to roll off some of the bass (rolls off about 6 db of gain below 200 Hz). I found the original circuit a little too boomy into a 12" speaker. I have the 25uf on a switch in parallel with the .68 to give the circuit a bass boost at low volumes. Most of the time I use it with the .68 and the boost off.

[vjf]

Thanks tyru

I read that a 25uf cap would provide a Fat boost while a 0.68uf cap could provide a High boost.

Still curious if anyone tried this in a 5E3 circuit though.

[tubeswell]

Yes different RC combinations will pass different bands of frequencies (like a tone control). Smaller bypass caps will result in more bass roll-off. Why not make a switchable cathode bypass cap? I've done this in a few geetar amps. Or you can also do a variable half-boost with say a 100nF fully bypassing the cathode resistor, and then in parallel with that again - a 22uF in series with a (say 5k) pot (on the ground side of the 22uF cap). There are good explanations of cathode bypass on the Valve Wizard site (towards the end of Ch 1 of Merlin B's book from here: http://www.freewebs.com/valvewizard1...Gain_Stage.pdf ).

Another good mod for 5E3 is to replace both the first stages' .1uF coupling caps with .022uF. This reduces a lot of the flabby bottom end and makes it sound nice with lower-powered speakers

[vjf]

Thanks for the link tubeswell...good read

[big-daddy-59]

You might also want to read this site:

Tales From The Tone Lounge:Mods and Odds!

lots of good info there.

[hasserl]

My preferred value for the cathode bypass cap on V1 of a 5E3 is 4.7uf, this value will cut some of the boomyness that the stock 25uf cap produces, but is fuller and not as shrill as .68. A .68uf cap will have a bottom end roll off around 431hz when used with an 820 ohm rK. This makes the bottom end too weak, IMO. But this is a personal preference thing, so try different values and see what you think.

FYI, I also prefer .022 coupling caps combined with the 4.7uf bypass cap. That will greatly reduce the flubbiness of the stock circuit values.

[vjf]

Some good stuff on that site thanks.

I don't have the kit yet but I'm going to build it to spec first then maybe play around with values down the road. Sounds like some of these changes do make an audible difference.

[MWJB]

Also consider the 2nd stage bypass cap, 25uf/25v at V1 with 0.68uf at V2 works for some.

+1 on the 0.022uf coupling caps in the preamp.