Cathode Bypass Caps or Coupling Caps?

[zendragon63]

I hope I can ask this where it makes some sense. So if you choose a larger preamp stage cathode bypass cap and a correspondingly smaller coupling cap, is the resultant (rolled-off) total signal content the same as it gets to grid of the next stage?

Maybe another way: If you use a larger bypass capacitor in a gain stage, are you amplifying a larger bandwidth--presumably with more harmonic content down to say 80 hz or below--and then limiting it with the coupling cap and grid leak resistor the following stage? Many of the members favor the smaller bypass cap and the more standard .01 to .02 uf coupling cap though lately I have begun experimenting the opposite. Just trying to understand something below the surface.

I can't definitive measure this either way with the analyzer I have. If this has been beat to death somewhere in the forum, send me on my way but if not, any experiences, thoughts or theories would be much appreciated. TIA. Regards

dennis

[FYL]

You're dealing with a form of cascaded filters. Cut-off frequency is set by the dominant pole - say RkCk are set for a 1 Hz response and Cp for 100 Hz, F3 will be at 100 Hz.

Slopes add - two first order (6 dB/octave) stages give a second order (12 db/octave) filter. Here they'll only add below 1 Hz.

Oh, and there's strictly no harmonic content below fundamentals - H2 is at twice F. Too much BW in the low-end can lead to intermodulation and other unwanted distos.

[zendragon63]

Thanks FYL; I needed to try get my mind over the hump there. Much to try to learn. I think that Merlin walks through progressive poles in his chapter on feedback in his first book. The key is the progressive nature of it i.e. first order, second order, etc.

I often wondered why the earlier Marshalls used these honkin huge bypass caps on the first stage. Initially I just trust that the designers had their reasons. Whether there are sub harmonics generated and then captured only to be limited at some point later on is likely fodder for the theorists--I just didn't want to miss something subtle if indeed it was there to be had, even if it isn't practical to chase or pleasant to hear. Again, many thanks. Regards

dennis

[sluckey]

I often wondered why the earlier Marshalls used these honkin huge bypass caps on the first stage. Initially I just trust that the designers had their reasons.

I think it's because the first Marshall was a copy of the Fender 5F6A Bassman wich was designed for a bass guitar.

[JayB]

You have to think about the grid leak resistor and it's interaction with the coupling cap also. Any voltage divider after the coupling cap will determine the frequency response.

Why not use both ways to tailor the tone even more? Who said you had to use the standard .02 coupling cap?

[Fresh_Start]

IIRC Merlin wrote something about a large filter cap on the first stage helping to reduce heater noise from the cathode.  But I may not be recalling correctly...

In any event, sluckey's right for sure - the Bassman is/was a bass amp.

Cheers,

Chip

[jjasilli]

Yes, "Designing Tube Preamps for Guitar & Bass" - Merlin Blencowe

[FYL]

I think it's because the first Marshall was a copy of the Fender 5F6A Bassman wich was designed for a bass guitar.

+1

[HotBluePlates]

IIRC Merlin wrote something about a large filter cap on the first stage helping to reduce heater noise from the cathode.  But I may not be recalling correctly...

In any event, sluckey's right for sure - the Bassman is/was a bass amp.

Cheers,

Chip

I don't know if I subconciously plagiarized, but that was something I pointed out a few times on the forum.

It was an effect I think I discussed with PRR a few years back, after I was talking with a well-known amp maker. Anyway, it was noted that if you remove the bypass cap on a stage (generally the first stage), gain is lowered due to feedback but sometimes hiss and hum also increases. I/we haven't run tests to determine the exact cause, but there are a couple of mechanisms that can act to cause increased noise in a gain stage with an unbypassed cathode resistor.

Ultimately, I/we settled on the theory that Fender used the big cap because it was a bass amp, which sets the first bass roll-off low enough to still remain below the bass range after subsequent roll-offs. A second effect was to have a stout bypass on the first stage to kill any filamne thum leaked by that tube; note Fender used a 12AY7, which was marketed in the 50's as a special low-noise tube for audio use.

[jjasilli]

Yes, I remember that post, but not when.  Merlin's book is copyrighted 2009

[Fresh_Start]

Thanks HBP - I didn't know that about the 12AY7 tube.

Cheers,

Chip

[HotBluePlates]

I don't own Merlin's books, and the conversation with the amp maker was before I went back into the military in 2007. I'm thinking I might have cribbed it from older tube books.