SR45 amplifier

[debk]

What is the effect of removing the cathode bypass capacitor?

How low a value can be used?  The schematic calls for 100uf

Deb

[johnsonad]

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

[Matts]

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]

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.

[debk]

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

[2wo]

I an not using a cathode bypass cap, My speakers are not the last word in deep bass...John

[debk]

Will removing the cathode bypass capacitor cause a decrease in gain?

Deb

[Grainger49]

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.

[johnsonad]

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.

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?

[2wo]

I bent the leads into little "J" hooks and was hot swapping a 100uf cap in and out and no there was no apparent reduction of gain...John

Of course I could not recommend this  ...John 

[xcortes]

Last time i did that It was with the parafeed cap. The HT arced into my hand and i enjoyed 400+ volts. Ugliest "audio" experience.

[debk]

I bent the leads into little "J" hooks and was hot swapping a 100uf cap in and out and no there was no apparent reduction of gain...John

Of course I could not recommend this  ...John

What differences in the sound did you hear

Deb

[2wo]

I mostly use Hornshoppe Horns, they only go down to 60-70hz or so, then I go to a pair of subs. So I can't say about the deepest bass but all in all I think the upper end is a bit better without them...John 

[Grainger49]

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]

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