PreAmp Cathode bias Capacitor

[Chuck H]

I don't understand what you are trying to say.

Don't care. As long as the OP and others do, as it seems is the case. My goal was to correct your misinformation for the OP's benefit. It desn't matter to me if you understand it. I tried to be very clear. Beyond that I can only encourage you to think better.

Changing the cap (increasing the value) will in fact increase the gain across the entire frequency spectrum.

Just so we're all clear... This is exactly what I was disagreeing with... And I understood you perfectly. Not because you were more clear than I was, but because I'm less antagonistic.

Test it yourself it works.

"not covered by the original cap" WTF

Any frequencies bypassed by a capacitor cannot be more bypassed by a larger value capacitor. "covered" is a word people use as a metaphor to explain that something or someone is under the effective control of another something or someone. As in:

Bob-"Hey Chuck, tell soundguruman he's full of crap."

Chuck- "Got it covered."

[big_teee]

I do appreciate all the clarification, but the Pissing contest accomplishes nothing.
I don't like Dissention!
I ended up reading about the ByPass Caps on the Merlin Site.
I had forgotten I had the Merlin book tucked away in a drawer.
So all that is left for me on this thread is to get some different Caps of different sizes and Test.
I will report back with the Results.
Thanks again Guys!
BigT

[Tom Phillips]

...I ended up reading about the ByPass Caps on the Merlin Site. I had forgotten I had the Merlin book tucked away in a drawer...

Cool. Figure 1.19 in the book will show you a graphical view of how the cathode bypass cap affects the frequency response and gain of an individual stage and your listening test will give you first hand evaluation of the resulting sound.

[J M Fahey]

That said, I´m *almost* sure Ken Bran (who presumably was still designing for Jim) didn´t do much Math (nor read Merlin´s book ) but must have found the amp mushy or even worse, blocking somewhat (at the "everything on 10" setting) and lowered the cathode bypass cap value, "by ear" until he was satisfied with the sound.
That must also have been the origin of the very small volume control coupling cap, in the "High Treble" (Lead) channel.

[mooreamps]

In my next build I'm going to try out LED biasing for the input stage so I can have effectively full bypass without resorting to using a huge electrolytic.

are you planning to use "merlins" led by-pass cap???? I'm just do confused... "Would that be Full Bypass or Partial Bypass" ?????

Gee, what will I ever do to figure this one ?????




ps... "don't bother writing back to me on this one"....

-g

[J M Fahey]

Well, you deserve an answer , why not?
In this case the Led is both biasing (like a resistor) *and* bypassing, if you want to see it that way, because its internal impedance is *way* lower than the value of any sensible resistor you would use there.
So, strictly as an analogy, it might be considered "full bypass" ... and flat at any audible frequency and then some.
In fact, flat down to DC.

[Chuck H]

... and flat at any audible frequency and then some.
In fact, flat down to DC.

And that's the way Gary likes it.

[Enzo]

REminds me of a young woman I knew in our nation's capital...

[Chuck H]

Enzo... Your not suggesting that Gary smokes cigars!?!

[J M Fahey]

REminds me of a young woman I knew in our nation's capital...

Dear Enzo.
I *HOPE* , for your own good, that you refer to Mrs Wenzo by this.
Or I guess you´ll have to cook your own breakfast and wash your own clothes for the next couple of months
Not to mention being given the "cold look"

[Enzo]

Well, I had 40 years of life behind me when I met the missus. She knows I wasn't sitting in a church pew.

Besides, she knows "flat, down in DC" couldn't refer to her. Now if I had said "sizeable butt" then....

[Chuck H]

Wenzo never reads the forum over hubby's shoulder I can see.

[big_teee]

Recap!
I did finally get some additional Caps to try.
I tried a .68uf at Cathode of pin 3 of V2A.
It didn't pass enough low end, but does give that tightened up marshall sound.
I tried a 25uf bipolar, it was ok, but to much low end.
I tried a 4.7uf cap, the only thing I could find in that value was a radial electrolytic.
It didn't sound right.
I ended up putting 2 of the large mojo djon .68uf Caps in parallel.(takes a lot of space)
Still gave the marshall crunch but allowed more bass that just one .68uf did.
Then I increased the PI coupling caps from .022 to .047uf.
The .1uf were too much, so I stuck with the .047uf.
I tried bypassing the 10k cathode resistor of the low input side of Pin 3 of V1.
It was too much.
I may try changing the 10k to a 8.2k later on.
About to achieve all the gain one could hope for in a jcm800 2204, and still clean up and sound good.
Thanks,
Terry

[Gingertube]

When ever you reduce this cap the bottom end will be reduced and the amp will sound brighter as the frequency balance across the spectrum has been slanted more toward the top end.
You want to do this (reduce that cap) in 2nd, 3rd, 4th etc. gain stages in a high gain preamp so as to limit the bass energy being passed to the next stage, else the sound becomes muddy.

There is also an arguement about suppressing sub sonic intermodulation products - I don't know how valid that arguement is but it seems to make sense.

Cheers,
Ian
CAUTION: Ian's posts (mine) are based on my own preferences which don't include Marshall Amps (horrible shrill nasty things).

[mooreamps]

Well, you deserve an answer , why not?
In this case the Led is both biasing (like a resistor) *and* bypassing, if you want to see it that way, because its internal impedance is *way* lower than the value of any sensible resistor you would use there.
So, strictly as an analogy, it might be considered "full bypass" ... and flat at any audible frequency and then some.
In fact, flat down to DC.

because the a/c impedance of a led in this configuration is so low, any size or type of "merlin by-pass cap" has no effect.
don't take my word for it. Try it yourself...

-g