[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

[Patrick Turner]

On Jun 20, 5:40*am, Roger Jones wrote:
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

LF pole = 159,000 / ( R x CuF )

Its easy to remember 0.1uF and 100k give a LF pole at 16 Hz.
so 50k gives 32Hz, and 50k + 0.05uF gives 64Hz.

So how come you got 133Hz?

I always use 0.47 caps on CF output tubes which have Rout = 1 / gm,
so a typical 1/2 12AU7 has Rout = 600 ohms.
If SS amp Rin = 10k, the 600 ohms has little effect, but 0.47uF gives
pole at 34Hz.
Many ppl like to use 1uF, don't be bashful, capacitance is cheap, and
2 parallel 0.47uF are fine.
But when you have 1uF, and pole is at 1.6Hz with Rin 50k, then the
tube turn on behaviour can upset the SS crap and cause it to ****
itself and trigger its protection circuits when the tube conducts and
cathode quickly rises to say 120Volts. Such a thing won't worry tube
amps much, maybe get a plop at speaker.
First turn on the source gear, Second turn on preamps, Third turn on
power amps. But ppl are impatient before sitting down for hours to
listen or watch media.

With phono amps, its very easy to make one with very good bass
response, and if its too good, its a curse and you'll see the bass
speaker cone wandering in and out and having a 32Hz pole somewhere
usually stops it and makes the music better.

Patrick Turner.

[Roger Jones]

On Jun 23, 7:38*pm, Patrick Turner wrote:
On Jun 20, 5:40*am, Roger Jones wrote:

Hi, Vacuumlanders,
This is pretty obvious but often overlooked...

(snip)

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.

(snip)

Its easy to remember 0.1uF and 100k give a LF pole at 16 Hz.
so 50k gives 32Hz, and 50k + 0.05uF gives 64Hz.

So how come you got 133Hz?

(snip)

Patrick Turner.

CF output impedance 500 ohms (you had 600, about the same.)
S/S amplifier input resistance 47Kohms per channel, or 23500 if
paralleled for mono (my tuner.)
Thus, total load 24Kohms, round number.
Then, f(-3dB) = 10^6/(2.pi.0.05.24000) = 132.69Hz, i.e. 133 Hz.
Or, using your reference point, i.e. 0.1 uF and 100K gives 16 Hz:
Reduce the 100K to 25K, gives 64Hz
Reduce 0.1 uf to 0,05 uF gives 128 Hz.
About the same as the 133 Hz (for 24Kohms.)
I agree that 1 uF coupling is too large with 50K input resistance, not
good for feeding a s/s unit.
Cheers,
Roger