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 often done on a bench s/s 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

Hi!!!
Another tip for a use for your tube FM tuner, is that it makes a GREAT input device for your iPod, MP3, or other portable audio... These modern devices usually match poorly to your normal system inputs, and sound not so good...

I tried using one of those cheapo FM transmitters (Belkin) used for connecting an iPod to your radio, and through my old Fisher and Sherwood tube tuners it sounded vastly better!!!

The sound was much fuller, with a great midrange, and much smoother highs, and even started to "fluff-out" the space and stereo of the music!!! I was surprised... It made these sources MUCH more listenable and less fatiguing to hear!!!

I still have this set up on my electronics bench for an extra input and long listening device... When It's 2 AM on a Sunday morning, and I'm STILL at the bench....

Steve K

You can always change that .05uF cap to a .47uF device. That yields a 90% reduction in overall lossiness and LF phase distortion.

Fred
owner
Classic Audio Repair

For cathode follower outputs feeding solid state devices my preference is to use at least a 1 uF capacitor or larger.

I agree, the more capacitance coupling into the 47k solid-state input the less the overall loss and LF phase shift. Low-frequency phase shift is plainly audible to most of my bona fide audiophile customers. And Jordan, who has magic ears and is my #1 tech, simply can't stand the way LF phase shift muddies up the sound. We'll be auditioning a receiver -- say, a Marantz -- at the customer counter, and some tin-ear owner will hit the LOUDNESS button, which as you know boosts the bass, but of course the imaging goes all to hell, and often I'll hear from the depths of the store "Turn that off!"

Fred
owner of (or owned by!)
Classic Audio Repair

Appreciate the tips, guys. They deal with a problem that is never on my menu, but it's good to know all the same, just in case.

Larry

My suggested 0.22 uF gives -3 dB at 30 Hz into 24K (two s/s inputs in parallel) at a 45 degree phase lead - but this is mono, no imaging issues. Of course, for stereo, at 47K per input, that would be at 15 Hz for -3 dB each channel. But there's no program material is down there... and, if there were, it's mono. Anyway, use 0.47 uF if they are to hand.
My main message: get rid of the 0.05's! 0.22 uF is a minimum.
Cheers,
Roger

I believe I blew the input stage on a solid-state tape deck once when I was taping LP records via a tube pre-amp (with CF output).

The output of a tube pre-amp or tuner, etc., may swing around a bit until warmed up, especially anode followers. Coupling capacitors have to charge up. etc.

Be careful.

Charlie

Good point, Charlie. A too fat cap might allow too much current to flow in the low-Z s/s input. I think I'd limit the cap to 0.47 uF.
Cheers,
Roger

Another option is to use a delay relay circuit which will leave the output coupling caps connected to a resistor for maybe 30 seconds and then switch to the output jacks.

I saw something similar in a Conrad Johnson preamp I worked on once.

I like "pre-charging" the output coupling capacitor via a delay relay and resistor idea best, then the coupling capacitance is less likely to be an issue, as far as "throwing an initial charge" into a following transistor or IC input stage.

Charlie