.0022uf vs .022uf phase inverter input cap

[Steveo]

Hello, i have 2204 type circuit that im tuning. Has anybody tried a .0022uf pi input coupling cap instead of a .022uf in their marshall? If so have you noticed much difference in tone and if so what?
Thanks

[pdf64]

I’ve not tried that on such an amp, but you may not hear much difference unless turned up high enough to push the power amp into heavy overdrive (phase splitter grid conduction).

[Roe]

1n5 is sufficient, giving a -3db point at 53hz as Randall Aiken showed. But with negative feedback, even lower values give sufficient bass. Fender used 500pf and 1nf - the former cuts bass clearly, the latter is more subtle until you remove the negative feedback or until the power amp distorts seriously.

I tend to use 1nf-3nf. Instead of removing bass in the phase inverter, it is much better to remove bass in the first two gainstages

[Steveo]

My amp has quite a bit of low end at the moment. I think i might just try a .0022uf there. I was thinking it might feel a little tighter also. Thanks

[wpaulvogel]

The heavy bass is coming from the previous stages. If you want to give that amp something, solder a.082 in parallel with the .022. Make the input capacitor to the phase inverter .1uF. This is not a typo 100nF. The amp will come alive. Just turn the bass control full off or nearly full off and crank the mids.

[Steveo]

Well, i have a 1uf bypass cap on my third stage before the cf. I should probably just change it to a more normal .68uf first and see how that works. Everything sounds really good just kind of fine tuning at the moment. Seems like with my setup it would be around 3-4db less at low E using the online calculator with the .68uf compared to the 1uf

[wpaulvogel]

That would make a change for sure. While you’re in there give the large coupling cap recommendation a try and report back. Play it driving the phase inverter hard and see what you think.

[Steveo]

Ok thanks, ill have to get a .1 or a .082 before i can try it out. What tonal changes does it make, is it clipping the pi some?

[wpaulvogel]

As you probably know, the Trainwreck Express and Liverpool have these coupling capacitor values on the phase inverter. I decided to build a different amp with a totally different preamp circuit but to use the Trainwreck design of the phase inverter. The output section is a Marshall output transformer 3.6k with 6ca7. When testing the output of Trainwreck amps I noticed the output clipping was asymmetrical once driven hard and decided to investigate.
On the new build, it also forms the asymmetrical clipping and the rich harmonic content once it’s driven hard. The only common denominator in both circuits is the phase inverter design. I haven’t proven it with a third amp of different design to the other two (I had four Trainwreck Express that all reacted the same and the 50BMG amp), so it seems that the 100nF input coupling capacitor may be the secret sauce. One of the two output tubes (I can’t remember) is worked more than the other. I’m interested to see if you get similar results.

[Steveo]

I installed a .1uf cap parallel with the .022uf on a switch but havent had a chance to turn it up yet. Hopefully tomorow, not much difference at low volumes at least.

Ive been able to turn it up but still not much difference that i can detect at the moment. Ill continue to keep it on the switch though. I havent been able to do an a b comparison really quickly because it really pops if i dont put it on standby first. Maybe with a scope youd be able to see something more definite.

[alfi27]

Only seen this on the Friedman Dirty Shirley, in conjunction with a 470k resistor between the PI grid and the 10k. I'm assuming the two are counteracting each other to a degree. I did try that circuit in my old JMP, but of course a lot of other things were different from a stock 800 so hard to say exactly what it did. Generally speaking I much prefer to lower the couplings in the preamp to tighten up the low end, but for all I know this might have a particular effect?

[Steveo]

The main thing i notice is the amp feels slower with the .1uf cap engaged. Like almost sag, im more of a metal/rock guy so i prefer the faster feeling .022uf

Well after a while ive come to really like having this on a switch. It makes solos sound really nice and full, it would be great with a strat or single coil type guitar. Ive been playing it like this 90% of the time now. It is slower feeling which forces me to dig in more but for rock i think its great.

[Roe]

The main thing i notice is the amp feels slower with the .1uf cap engaged. Like almost sag, im more of a metal/rock guy so i prefer the faster feeling .022uf

yes, big caps give a slow amp. rolling of some low end really helps. if not, the recording engineer will probably use a 100hz high-pass filter in the studio