[Roger Jones]

Hi, Vacuumlanders,
This is pretty obvious but often overlooked...
I've restored several AM and FM tube tuners over the years (BC-1A,
FM-4, FM-3 and Trio AF-350G come to mind.) Although destined for use
with various tube amplifiers, testing is done on a s/s bench stereo in
the workshop (and at least one has stayed on such.) Most of these
tuners have a low impedance output cathode follower (although I had to
add a CF stage to the Trio unit), but typically they only have a 0.05
uF output coupling capacitor.
This is OK to feed the 1/2 meg or so AUX or TUNER input of a tube pre-
amp, but nowhere near large enough to feed the typical 47Kohm input
impedance of a s/s amplifier. Also, if it's a mono tuner and you use
an RCA audio cable Y-adapter to feed the L+R inputs together, the load
on the tuner output is as low as 24Kohms.
For the low end -3dB point, Xc equals Rin (plus CF o/p impedance but,
at about 500 ohms, you can ignore this.) Thus, 10^6/(2.pi.F(Hz).C
(uF)) = 24000. Substituting C=0.05 uF and solving for F gives 133 Hz.
Far too high... no bass! You need C at least 3 times larger, say 0.15
uF. A convenient value is 0.22 uF. Using the same equation give F=30
Hz. Perfect! Just replace the original 0.05 with 0.22 uF.
When you put the tuners back on tube pre-amps just leave them as
modified... neither will object!
Yesterday, I zipped up a restored Heathkit FM-4 for mono use in the
bedroom on a paralleled pair of s/s powered speakers... no bass! Damn,
I'd forgotten the output cap... mea culpa! Just fixed it!
Cheers,
Roger