Coupling Capacitor Expertise Needed

[Rick K5IAR]

Here's the layout.  I have a first audio stage, two 1620's (actually 6K7's) feeding the modulator output, two 828's in push pull.  The coupling capacitors from the plates of the first audio to the control grids of the modulator output are .25 mfd @ 1000 vdcw.  These are the little rectangular cans that mount with flanges near the bottom.  One is testing pretty far out of tolerance and I don't have one.  However, I do have two NOS .1 @ 1000 vdcw that will bolt right in.  I think it would be just fine to use them in place of the .25 mfd caps, but I would sure like to get a little feedback from those more knowledgeable before I swap them out.  What do you guys say, do you see any problems with this?  Of course, I mean changing both of the .25 caps out to the .1 caps.

Thanks,
Rick/K5IAR

[Bacon, WA3WDR]

Caps that big probably won't be a problem if you have .25 on one side and .2 on the other.  What I guess will happen is that you will lose some of the push-pull at very low frequencies.  But for speech, I doubt that anyone would notice.  Maybe furnace rumble...  What are the grid leak resistors, like 47K or 100K?  0.2uF and 47K are -3dB at about 16 Hz; .25 might only be down 2 to 2.5 dB or some such, so there might be a slight level imbalance way, way down low.  Ain't gonna matter.  But you could look for a .05 and add it in there when you find it.

Oops I see you want to replace the 0.25 with 0.1.  Well, 0.1 and 47K are -3 dB at about 34 Hz, probably low enough that it won't matter for speech.  You might be down a dB at 60 though.  Not a real problem, I don't think, especially if the grid leaks are really 100K, because then it would be about half of those frequencies.

Is there global negative feedback around those capacitors?  If there is no bottom end motorboating or any frequency response bump, I wouldn't worry too much.

[k4kyv]

Or you could increase the  grid  leaks to 220K and that should make up for any loss of lows.

[Rick K5IAR]

Thanks guys, this is exactly what I needed to know.  The grid leak is 330k now, so I should be okay, don't you think?

Rick/K5IAR