SR45 amplifier
- Thread starter debk
- Start date
johnsonad
New member
Doesn't the PF design significantly minimize the effect of the PS cap on the sound, effectively shifting the effect of the last PS cap to the PF cap? For that reason I'm sticking with lytics in the PS and the sheer size of the caps.
In my build I focused on the PS ripple instead of the make up of the cap and with a CLC dropped it from 0.1V to around 0.4mV in sims.
http://www.bottlehead.com/smf/index.php/topic,4953.0.html
In my build I focused on the PS ripple instead of the make up of the cap and with a CLC dropped it from 0.1V to around 0.4mV in sims.
http://www.bottlehead.com/smf/index.php/topic,4953.0.html
take this with a large grain of salt- based on a foggy memory 10yrs old- but I think I went with a large good-quality electrolytic for cathode bypass in my Paramours. 350or 450uF, or something like that. I think I calculated at the time that this effectively took it out of the circuit, because it never really got enough of a charge to activate it, but it served as a "holding tank" or a spring for the charges that did come in, but they dissipated inside it.
Paul Joppa
Moderator
In the midband (i.e. where the plate choke impedance is large relative to the tube's plate resistance) and with the output transformer connected to the cathode, the power supply and cathode bypass caps are out of the signal current loop. However, the choke impedance is no longer large in the deep bass, so some signal current does flow through the two caps. For this reason, the cathode bypass cap does improve the deep bass response - but is not necessary in a tweeter amp, and is also optional if the plate choke is really huge.
For example - 45 plate resistance is 1700 ohms. With a mu of 3.5 and a cathode resistor of 1500 ohms, the plate resistance of the tube plus unbypassed cap is (if I remember the formula right) 5450, or 3.2 times the resistance with a bypassed resistor. So if a 40-henry choke is used with the capacitor, then you'd get the same bass extension with a 128 henry plate choke and unbypassed resistor.
For example - 45 plate resistance is 1700 ohms. With a mu of 3.5 and a cathode resistor of 1500 ohms, the plate resistance of the tube plus unbypassed cap is (if I remember the formula right) 5450, or 3.2 times the resistance with a bypassed resistor. So if a 40-henry choke is used with the capacitor, then you'd get the same bass extension with a 128 henry plate choke and unbypassed resistor.
debk
New member
I did an experiment and paralleled the 100uf cathode bypass capacitor with a 0.68uf Auricap. I picked the value because I happened to have a couple of them sitting around.
I didn't like it at all. The sound sounded smeared, especially with vocals. Was the paralleled capacitor causing a phase issue?
I removed 0.68uf and the sound was much better with the same material playing.
Interesting experiment, I don't know if the results would be the same with different value parallel capacitors.
On one hand I am very happy with the sound of the amps as they are now, but on the other hand it is a lot of fun and educational to experiment.
Deb
I didn't like it at all. The sound sounded smeared, especially with vocals. Was the paralleled capacitor causing a phase issue?
I removed 0.68uf and the sound was much better with the same material playing.
Interesting experiment, I don't know if the results would be the same with different value parallel capacitors.
On one hand I am very happy with the sound of the amps as they are now, but on the other hand it is a lot of fun and educational to experiment.
Deb
Grainger49
New member
I was told years and years ago that the cathode bypass cap adds gain.
I believe this is because it changes the audio path to ground. Bottlehead hasn't used a cathode bypass cap in any of the Foreplay preamps nor the Seduction and Eros (and probably not Reduction). I think there might be one in the Quickie. Might not. I'm not as familiar with it.
I believe this is because it changes the audio path to ground. Bottlehead hasn't used a cathode bypass cap in any of the Foreplay preamps nor the Seduction and Eros (and probably not Reduction). I think there might be one in the Quickie. Might not. I'm not as familiar with it.
johnsonad
New member
Paul Joppa said:
So PJ, a similar line of thought and a question. I plan on using a pair of 300b amps for 80Hz and up (drivers have little below 70 Hz). 5k OPT primary, 1500 Ohm cathode resistor, 80 H plate choke. Can I get away without a bypass capacitor and have quality bass that low?
...John Grainger49
New member
One last suggestion. Try a well broken in KK Teflon cap as a cathode bypass, bypass. Still just use jumpers. I have done this in my Paramours and my Ack! dAck!. Those stayed in.
galyons
New member
I bypassed the 220uf cathode cap with a 2.2uf K75, (mylar, oiled paper,aluminum foil), in my Paramour 45's, EXO-45, BCP-15's. They are staying in. The bypass removed a level of grain and grunge that just made the amps cleaner with truer timbre.
Hard to imagine a bypass quality cap, in this application, making the sonic's worse.
Of course, YMMV...
Cheers,
Geary
Hard to imagine a bypass quality cap, in this application, making the sonic's worse.
Of course, YMMV...
Cheers,
Geary
Grainger49
New member
Deb,
The 0.47uF KK Teflon is big enough. I would get by with 0.1uF or 0.22uF.
How many hours does the KK Teflon have on it? If less than 100 it would sound bad by itself. If less than 400 it isn't completely broken in.
The 0.47uF KK Teflon is big enough. I would get by with 0.1uF or 0.22uF.
How many hours does the KK Teflon have on it? If less than 100 it would sound bad by itself. If less than 400 it isn't completely broken in.
Paul Joppa
Moderator
"get away with"? sure. But that's quantity bass!johnsonad said:
"quality bass"? there is no substitute for trying it and listening.
Seriously - I can't see a predicted level difference that would matter, but there will be phase differences and other more subtle effects (i.e. those that are hard to measure but easy to hear - there are uncountably many of those!).
