MagneQuest/Peerless Forum

RE: PP EL34 amplifier with UL driver, Part 2

65.13.183.123

In Reply to: RE: PP EL34 amplifier with UL driver, Part 2 posted by KevinC on June 12, 2008 at 16:36:29

I know most of you know this, but I wanted to point out... in the power
supply, you note a 100uf/500V cap as the input filter from the 5AR4.
I note here that all spec sheets on the 5AR4 dictate a _maximum_ capacitor
input filter of 40uf. (I think someone dug up one that said 50uf once...)
I note that 100uf is a little over twice as much. This will do two things to
you. It will really decrease the average life-span of a 5ar4 (and in some cases
kill it pretty early on, depending on the manuf. of the tube). Secondly, you
have too much capacitance for the 5AR4 to charge in a reasonable time-span.
It will draw too much current (leading to the decrease in life-span) both
instantaneously and over a small delta t right after the tube starts conducting.
This will also happen during musical peaks at high volume.

Suggestion for "fixing" that and having a better responding amp (probably, IMHO)
is to put a total pi filter in front of the CT. Go with say a 30uf input cap
filter, then hit a choke. Looking at your schematic and interpolating I
would use a 10H at 150mA. I bet 10H at 100mA would be ok, but still.
Conversely you could beg Mike for a 4H 200 (or was it 250?)mA choke.
Recall, as the current passes the rated value, inductance falls, this is slightly
true in the other direction also, so a lower H bigger I will do you here too.
After that go to a 30 or 40uf cap. If you put a cap too large at the CT you
can end up with an amp that has a "compressing" effect on musical peaks,
a phenomenon I suspect you would have if you use a single 100uf input
filter as you have shown.

Just my $0.02, but I have built a lot of PP power amps with 5AR4 rectifiers. :)

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