Re: Power Supply Filtering: Is More Better ?

Claudio wrote:
Doggone wrote:

Chief_Billy@xxxxxxxxxxx wrote in
news:1152721510.017136.192300@xxxxxxxxxxxxxxxxxxxxxxxxxxx:


I'm building a Marshall JTM-45 clone. The BOM calls out 16uF filter
caps
at the pre-amp C- and D-feeds (to PI and pre-amp, respectively) from
the PS. I'm thinking of bumping these up to 40uF, same as the B-feed
(to screen grids) . I'm thinking that this will give me a tighter
bottom end and a more lively tone. I'm using a GZ34 tube rectifier.

Would increasing the filter , as I plan to do, be a 'good thing' ? Any
detriments ?


You can increase the value of these caps without concern for
stressing the recto tube. The reason being that between the recto
and the cap, you have an 8.2K resistor. Then yet another 10K resistor
before the second cap. (Folks, look at schematic diagrams before
offering canned advice).

That is not the case for the first cap after the recto and, to a
lesser degree, the cap after the choke. They can be a significant
load on the recto. But for the C and D feed, fill your boots.

Changing the valus of these won't affect the sound appreciably, if
at all. The first pair of preamps (feed D) are operating in class A
and out of phase, The PI pair (feed C) are also out of phase.
The result is the current in the droping resistors is almost
allways constant. When pushed hard, as when clipping occurs, there
will be a change on the power supply load, albeit small.

Using an AB switch will help you hear the difference, if any.

--
If it ain't broken, fix it till it is.


I agree with Doggone, but in general, the problem between caps and
rects is *time*.
When a cap is empty (no electrical charge on it) it behaves as a short
circuit to the electrical flow. When current start to go thru it, it
starts to charge and then goes from a short circuit situation to a open
circuit one. That happens when you turn on the amp. The time it takes
to go from short to open depends on the cap value (the greater value,
the more time it takes). The problems is that when the cap start to
charge, the first moments are critical, because it's near to the short
circuit situation and current can be **very** high. If cap is
oversized, these first moments could last enough to stress the tube to
the point of damage. It's funny to see this on silicon rects... they
pop like corn =D Not too funny with tubes...


Good post. Thanks.
.

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