47uf for the phase inverter filter

[dscottguitars]

I have a good 47uf/500v cap and wondered if that can be used for the phase inverter instead of a 22uf?  What is the reason most schematics choose the 22uf for the preamp tubes?

Thanks,

Daniel

[dscottguitars]

I think I may have answered one of my questions by putting another 47uF/50v cap in series with it for a total of 22.5uf/550v.  Would that work?

[Willabe]

What is the reason most schematics choose the 22uf for the preamp tubes?

They went with as small a cap as they could that would still give acceptable results. The B+ dc supply voltage is in series so each stage is cleaner than the last stage as you go towards the 1'st preamp tube. So smaller values could be used as you go. Some amps used only 10uF for the 1'st preamp tubes.
 
Years ago filter caps were big and cost was high. So much so that it was cheaper to use a choke instead for more filtering. Guys just got used to using the old values, they work fine. But you can up the values in most places in the B+ string. You have to be careful with the 1'st cap/s, first B+ node if your amp has a tube rectifier. If you go too large there you can kill the recto tube with the caps pulling too much current at start up.

What is your amp that your asking about?

Nothing wrong with using the 47uF/500v if it will fit. Some like a little more B+ filtering. If your amp has lets say 47uF at the 1'st B+ node (plate) and 22uF at the 2'nd node (screen) I'd go with putting the 47uF at the 2'nd node and putting the 22uF at the 3'rd (PI).

So, 47uF, choke or R, 47uF, R, than 22uF at the PI.

When using filter caps in series you need to use voltage sharing R's in parallel with the caps, to make sure 1 of the 2 caps don't see the bulk of the voltage and self destruct because of over voltage. If the 2 caps are the same ratings (uF/V) the 2 R's will be the same value, like on a Fender AB763 they are 220K/1w.

But in your case the value of the 2 R's will be different. I don't know how to do the math to get the R values for you, sorry.




                    Brad        

[dscottguitars]

Thanks....

The amp is somewhat of my own design, close the the Bassman with a Marshall style preamp.  But I use two preamp tubes each wired as one big triode, in parallel.  The first is a 12AV7 and the second a 12AX7.  I have lower voltage on the AX7 to get gain with lower volume.  The cathode resistors and bypass caps are close what Marshall uses.  The B+ string is:  2x220uf(series for 110uF)-plates; 47uF-screens; 22uF-AT7 PI; 22uF-AV7; 22uF-AX7.  I only have 2-22uF's left and wondered if using a 47uF that I do have would change much on the PI.  I read that it affects the tone, but then no details are given that I can find.

Since the voltage values are so different on the idea above, I won't use the series setup on those 47's.  I forgot that was an issue.

[Willabe]

Hi Daniel,

The amp is somewhat of my own design, close the the Bassman with a Marshall style preamp.  But I use two preamp tubes each wired as one big triode, in parallel.

OK, sounds very cool buddy.    

I only have 2-22uF's left and wondered if using a 47uF that I do have would change much on the PI.  I read that it affects the tone, but then no details are given that I can find.

No, you don't have to change anything on the PI if you use a 47uF instead of a 22uF that feeds the PI's B+ filter node. I don't know who said/wrote or in what context as far as sound (maybe it was about distortion vs. headroom?) , but...

How about this, since it's very easy to try and you won't do any harm, try a 47uF instead of a 22uF in the PI B+ filter node. Then go back and put the 22uF in and let your ears decide which You like better? Please let us know what your ears hear.

Over all more filtering in the B+ dc supply will give a stiffer/tighter/cleaner bottom end and a quieter amp.

Will you hear the difference or would I in that position of the B+ supply, with the uF value change? Maybe yes, maybe no.    

But it's an easy thing to try at this point. Remember you will have to turn up the amp to fairly hear a potential difference by stressing the B+ PSU. I'd bet when you start to hear the difference is when you get to 5 to 6 to 7 to 8 on the amps volume control.


           Brad                

[SoundmasterG]

When you change filter cap sizes in RC coupled stages such as in the preamp or phase inverter, then the tone will change with larger caps giving more bass reponse, but it is a very subtle thing, especially with the values you are talking about. If you change the filter cap sizes for the main B+ filter, that isn't RC coupled in that location so you don't notice a tone change per se, but the amp will be quicker to respond to transients which means a quicker and tighter bass response.

I used a 47uF filter cap for the LTP in my Bogen build as I was initially using a different phase inverter circuit that benefitted from the larger cap size, and I didn't bother to change it when I switched to the LTP. It sounds fine and gives more stable performance and less hum so why not.

I'd just go ahead and use it or order up another 22uF cap, either way it will work fine...


Greg

[dscottguitars]

I decided to use an 8uf in my distortion stage tube based on the above responses about sag vs tight.  It seems to work well and I didn't notice any change in tone from my last amp I made.  I may just use this set up from now on...

Thanks all for your help.

[phsyconoodler]

Quote:"They went with as small a cap as they could that would still give acceptable results"

Partially true.
If you over-filter a preamp it sucks chime away from the amp.The feel changes dramatically and can be detrimental to great tone in some vintage amps.
  A Fender blacface amp does not like to be over-filtered.They sound dull with too much filtering.Same with a Vox AC30/AC15 and many Marshall amps.
  There is more afoot than 'the builders were cheap' It may have been accidental like many of the vintage amp 'holy grail' tones but I'm thinking not totally.

[Willabe]

If you over-filter a preamp it sucks chime away from the amp.The feel changes dramatically and can be detrimental to great tone in some vintage amps. A Fender blacface amp does not like to be over-filtered.They sound dull with too much filtering.

G. Weber has said this too, in 1 of his books. His slant was you can up the B+ plate/screen nodes filtering on Fenders (TW's, BRF and BF) but not the other stages. He liked to go with 110uF for the plate node and 40uF for the screens, to tighten up the notes on the low E string. But he made no comment on this;    

My question is do you think that to give each triode from input to PI it's own 22uF filter (split load and it's driver get 1 each, but LTPI's 2 triodes would only get a single filter cap), would that take some of the chime away?


                Thanks,   Brad