Amp voicing: Bypass caps vs coupling caps.

[jordan86]

I've been looking at doing a mix of the two in my Princeton build. Wanted to see if there are a strong preference for decreasing bypass caps vs coupling caps when it comes to trimming bass. I've read a good bit on which caps and which values to experiment with. I'm more so wondering if the two accomplish the same thing or if there is any difference in the approach. Or maybe more specifically, is one more "destructive" than the other when it comes to gain and EQ?

I think of it like at a mixing console. You could adjust certain frequencies with compression, or with EQ. But EQ tends to be a little more destructive in that you are removing frequencies for good down the line.

Is there a more noticeable response in gain and EQ going with like 5uf bypass caps vs .02 coupling caps?
 

[vampwizzard]

Go play with this to see what effect you can expect on changing cathode bypass caps: https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/

If you want more bass in the circuit, change your bypass caps up to 0.047 or 0.1. If you want to decrease the bass in the circuit reduce to 0.01. There is a reason the values in the schematic were settled on. Have a goal on what you want it to do and the folks here can help you out better. My understanding is that the cathode bypass cap gives you a slight bump in gain on the 12ax7 and the value you use places the frequency shelf for that boost. The effect is to tighten or loosen the overall gain whereas the bypass caps are going to effect EQ more. https://www.youtube.com/watch?v=HrkYUPmEUhk

[shooter]

like at a mixing console

and just like a console, it's tweaked by the operator(s ears)
once you get to amp tweaking, it's a YES/NO logic tree, but my tree and yours look noting alike 

gator clip in an identical cap that's "in play", was that better or not,

[tubeswell]

IMO its better to have the input stage cathode resistor fully bypassed (to minimise the S:N ratio at the start of the signal chain*), and then deal with reducing bass frequencies in the subsequent filters. YMMV


*V1 is where the amp gets its 'best shot' at achieving a low S:N ratio (because everything after that amplifies any baseline noise** that is present in the signal at V1). So the more 'signal' you can boost at V1, the less noise you will have in proportion to signal.


**And there are all kinds of 'noise' (Gaussian Noise, Johnson Noise, Shot Noise, Flicker Noise).

[jordan86]

IMO its better to have the input stage cathode resistor fully bypassed (to minimise the S:N ratio at the start of the signal chain*), and then deal with reducing bass frequencies in the subsequent filters. YMMV

*V1 is where the amp gets its 'best shot' at achieving a low S:N ratio (because everything after that amplifies any baseline noise** that is present in the signal at V1). So the more 'signal' you can boost at V1, the less noise you will have in proportion to signal.

When you say "fully bypassed", that would mean like be like a standard 25uf bypass cap? I was thinking of trying a 4.7uf or 2.2uf cap at V1 to tighten and clean up bass just a touch before the tone stack. Are you saying best S:N would be with a 25uf at v1 and then clean things up in the later stages?

[tubenit]

There are a bunch of Princeton builds including Princeton Reverbs.  Which one?

Can you post an accurate schematic of your amp in it's current state, please?  It would help us in helping you.

With respect, Tubenit

[jordan86]

It's the Mojotone blackface PR kit. Here....
https://923962.app.netsuite.com/core/media/media.nl?id=3393932&c=923962&h=1a2d186b30f62b14658d&_xt=.pdf&vid=SvxoPyNqAoUdFB5T&chrole=17&ck=zx7sO2KuAgq8ENaz&cktime=175724

[tubeswell]

IMO its better to have the input stage cathode resistor fully bypassed (to minimise the S:N ratio at the start of the signal chain*), and then deal with reducing bass frequencies in the subsequent filters. YMMV

*V1 is where the amp gets its 'best shot' at achieving a low S:N ratio (because everything after that amplifies any baseline noise** that is present in the signal at V1). So the more 'signal' you can boost at V1, the less noise you will have in proportion to signal.

When you say "fully bypassed", that would mean like be like a standard 25uf bypass cap? I was thinking of trying a 4.7uf or 2.2uf cap at V1 to tighten and clean up bass just a touch before the tone stack. Are you saying best S:N would be with a 25uf at v1 and then clean things up in the later stages?

Yes

[d95err]

If you’re aimong for a bit more ”Marshally” flavor, you should go a bit more radical on the cathodes. Try the typical 2k7/0.68u combination on the first and second stages.

[nandrewjackson]

From what I've heard in my builds:


12ax7 cathode caps at 5uF or greater give a gain boost to the whole audio spectrum at that triode, compared to no cathode cap.


12ax7 cathode caps like 1uF, 0.68uf, 0.47uF give a treble boost at that stage, compared to no cathode cap.


Coupling caps at the 12ax7 plate;


0.01uF or 0.02uF are a good starting point.
0.047uF will let more signal past them into the next stage.


A single 0.006uF (or 0.003, or 0.004) somewhere in the amp can eliminate any flub in the low end when playing at high volume.  But an amp with all low numbers like 0.006uF will probably be lacking in character. 


Anything as large as 0.1uF is , from what I've seen commonly used, and from what I've built, usually reserved for after the last 12ax7 triode, before the phase inverter /driver tube. And 0.1uF is sometimes used after a phase inverter, prior to the power tube grids. At that point in the amp, the *tone* has been shaped and it's time for the power tubes to amplify it for the OT and speakers 

[Tone Junkie]

Low string on a guitar resonates at 82hrtz /2K7 and .68 cap is at 82 so is a 820 ohm 2.2 uf  there's a formula to know were the cross over frequency is at . if your doing low gain amps . you can put the cross over frequency way below and not have any problems just like fender did .That's why fender amps have a fuller sound as compared to the bright channel on a Marshall. if your doing a higher gain amp you don't want to dip to low below that threshold. gives it farty bass doesn't sound very good.

[tubenit]

This is what I would try.

With respect, Tubenit

[jordan86]

Thank you all. Not trying to turn my PR into a Marshall by any means. Just want to tighten up the lows a touch. I don’t plan to crank it up too much, but when I do it’d be nice to keep things intact.

I did get an Allen TO20B so that will be a part of the build, and will help some too, no doubt. I do have some 2.2 and 5uf bypass caps ordered as well. I think I will stick with the stock coupling caps for now and see how she sounds “stock” before tweaking.

The pragmatist in me wants to mod as I go and just do the work once, but I am realizing that’s not how voicing an amp works.

Appreciate everyone’s input.