Tweed 5e3 Coupling and Bypass Cap Mod Question

[BluesDude]

I have been looking closely at the 6g3 design considering my next steps.

I am seriously considering a 6g3 vol/tone mod. Along with that mod I am going to modifying some bypass and coupling caps to reduce some of the bass.

I noticed that the 6g3 changes the coupling caps on both the normal and bright channel out of the 1st preamp stage.

My question is about the coupling caps in that circuit and how it might related to changes I make. It seems that the coupling caps off of the 1st preamp stage are .02 uf. That should roll off some of the bass. The bypass cap is a 25 uf like the tweed circuit.

If I modify the coupling caps off of the first preamp, why do I need to modify the bypass caps and coupling caps of other preamp and power amp stages? It would seem that the first stage would take care of reducing the bass right up front. Why would the other coupling caps and bypass caps be modified? Is it that... lowering the 1st stage coupling caps will not quite do the job? What am I missing. There is something I do not understand. How does modifying the bypass caps and further stage coupling caps make a difference? Why would the first stage coupling caps not do the job (or in some cases not be adequate)?


Thanks

[pdf64]

With the 6G3, a 0.02uF coupling cap into a 1M pot won't roll off any bass - the breakpoint will be about 8Hz (unless the control is turned up high, in which case other parallel paths to ground become significant).
It does the job with a 5E3 because the coupling cap feeds a variable resistor, rather than a pot (though a virtual potential divider is formed, with the plate resistance of the 12AY7 forming the top section, and the vol control wiper and cold track end forming the other section.
You would need to go to 4n7 or 2n2 with 6G3 before you get much noticeable roll off.

[BluesDude]

Thanks pdf64.

It sounds like the circuit is a little different and the 1st coupling cap feeding the vol/tone circuit (on the 6g3 design) would need to be reduced guitar bass frequencies. That information will be helpful.


But I am still a little confused about the differences in tone shaping between coupling cap mods and bypass cap mods. Not to mention making the mod at different stages in the amplifier.

Why does it matter if I adjust the tone window using a coupling cap versus a bypass cap? What is the difference? And why would I choose to shape the tone by just making a coupling cap mod or just making a bypass cap (one versus the other)? What would be the reason I might decide to modify both?

Sorry for so many questions. I lack experience and I am just trying to learn a little along the way.

Thanks.

[pdf64]

In short, coupling caps roll off bass at 6dB/octave (ie output will be about 12dB down 2 octaves below the breakpoint frequency, and so on), whereas cathode bypass caps are shelf filters with about 6dB difference between the 2 shelves.
See
Designing Common-Cathode Amplifiers

[Dave H]

But I am still a little confused about the differences in tone shaping between coupling cap mods and bypass cap mods. Not to mention making the mod at different stages in the amplifier.

The gain difference between a bypassed and unbypassed cathode for a 12AX7 is about 6dB (x2) therefore it can only give a max bass reduction of 6db before it levels off. A coupling cap into a resistor will give a gain reduction of 6dB/octave, go down a couple of octaves it’s -12dB etc. If you have several stages with the same RC time constant the bass roll off will be 6dB/octave/stage. e.g. two stages will be -12dB/octave or -24dB if you go down two octaves.

As pdf64 said 22n coupling caps are more than enough bass for guitar in this circuit. When the vol pot is changed to a potential divider the first coupling cap needs to be drastically reduced for a noticeable bass reduction. I have 22n coupling caps with a 1n cap feeding the 500k vol pot and it still has enough bass for me.

Dave H.

[Bruce / Mission Amps]

For players who are using fat HB guitars exclusively, I recommend using something at or less then .015uF for plate load coupling to the stock volume control. I like the way a .0068uF cap sounds with my fake LP (EPI) using Seymour Duncan 59 neck and JB bridge pickups.
But a killer setup is a good old Tele or Strat with .022uF to .047uF coupling caps and 4.7uF bypass caps.

[BluesDude]

Ah hah! Its beginning to make some sense to me.



Thanks alot fellas. I appreciate the help.

[CoffeeTones]

After scouring the web and reading this post with the same questions as the OP in mind, some of his original question remains unanswered.

Concerning the .1uF coupling, 22uF bypass caps between the phase inverter and power stage and the .022uF in preamp #2. Is there a need to adjust thier values if you have reduced values in the EQ and preamp #1? If so what is the suggestion?

What are the recommended coupling cap and split cathode bypass cap values to stop, boomy bass with a Hot Tele and the 6g3 tone / volume mod?

Everything is great except the low E and A string response.

[BluesDude]

After scouring the web and reading this post with the same questions as the OP in mind, some of his original question remains unanswered.

Concerning the .1uF coupling, 22uF bypass caps between the phase inverter and power stage and the .022uF in preamp #2. Is there a need to adjust thier values if you have reduced values in the EQ and preamp #1? If so what is the suggestion?
...

I Think I have this correct... for the human ear to perceive a difference... and increase in 3db or decease of -3db is needed.

Pdf and Dave covered it. Reread their posts. If you roll off about -3db per in the first stage with a certain RC filter... you can roll it off further using an additional one in the following stages using the same value caps.

The question I have is: Can one effectively shape the tone a bit by altering the cap values of different stages (a little) (i.e., stair step at different frequencies)? For example roll off at 90 hz stage 1, then at say 120 hz stage 2 (or the other way around). Or is that just a waste of time?

[CoffeeTones]

For players who are using fat HB guitars exclusively, I recommend using something at or less then .015uF for plate load coupling to the stock volume control. I like the way a .0068uF cap sounds with my fake LP (EPI) using Seymour Duncan 59 neck and JB bridge pickups.
But a killer setup is a good old Tele or Strat with .022uF to .047uF coupling caps and 4.7uF bypass caps.

Bruce, first of all thanks. Secondly, I am only interested in playing classic rock, country and blues with Teles and Strats through my 5E3, no beefy humbuckers.

With the tone / volume mod, exactly which positions do I need to put the values you suggested for that "Killer Setup"? Do you suggest changing bypass and coupling cap values in the PI, preamp #2 or power stage for this application?

I realize this is somewhat subjective. However, I'd like the amp to be more focused, but full with "stockish" gain and the bass to be just before the edge of the stock response / breakup. Thanks