Cathode bypass cap and freq roll off

[johnsonad]

@Brad, you will have to come down and listen to the 755's. They are not the 19's but still sound great and are easier to PCS with

@Xavier, could it be PS noise you are hearing? Hopefully PJ will chime in. Mine are essentially Paramounts but the layout is much different and I have zero noise when removing the bypass cap.

 

[xcortes]

Mine are 2a3 Paramounts. Took them off again. Nice! Yes hopefully PJ can comment because the noise is too intrusive (on 110dB horns). Same on both channels.

 

[Paul Joppa]

The unbypassed cathode resistor raises the effective plate impedance of the tube - quite a lot in the case of the direct-coupled amps because that cathode resistor is much larger. That means the power supply noise isolation provided by the plate choke is greatly reduced. A much better power supply filter would be the only good answer - CLC instead of CRC, bigger caps, etc...  Ultimately, a shunt regulated supply?  :^)

 

[johnsonad]

Xavier, I'm not sure how much current that design draws but the Paramount stock plate choke makes an excellent PS choke in CLC (I use two per channel). With your top mounted OPT there should be plenty of room for one under the hood of a stock chassis plate.

 

[xcortes]

Thanks. I already have a clc ps. I asked my friend Jorge who has my sr45s with horns to try and report.

 

[johnsonad]

Are you using the Triad choke in your PS? If so, the BH choke has more Henries and better filtering.

 

[xcortes]

Mq bcp  something

 

[2wo]

I run my SR-45 without a cathode cap and didn't notice an increase in noise. My speakers are only ~95db though...John 

 

[johnsonad]

That may be my situation too John. Xavier's horns are much more sensitive. Have you tried it on the WE's?

 

[xcortes]

My speakers are 110dB efficient. With 93/95 db speakers it wouldn't be an issue. And according to PJ the problem exacerbates with dc amps which neither John's nor Aaron's are.

I need those new SR45s!

 

[johnsonad]

I second that! PJ is teasing us with SR! Hopefully this summer

 

[Paul Joppa]

I actually sent off some parts orders yesterday, to test out thepower supplies (there are four per channel) and  time delay circuitry. Progress!

 

[xcortes]

Good Paul!

In the meantime I'll replace the electrolytics with some Solen film.