Will these caps work?

[Jameson]

I've never rolled in new caps, so please excuse the ignorant question. I'd read in an earlier thread that changing out the 3.3uf and 0.1uf caps in the circuit were upgrades people recommended so I went out and bought a set of Solens (film caps). Then I looked at the manual to see where they went and realized the stock 0.1uf caps were a different style than what I bought. Will these work, or do I need to find the same style as what I'm replacing (metallized polypropylene)?

 

[Paul Birkeland]

Those are what I tend to use in that position.

 

[Jameson]

Sweet, thanks!

 

[Tom-s]

As long as you put in 0.1uf for the interstage (de)coupling cap with a minimum 600V rating. For another capacitance value, you should also change a resistor.

For the parafeed cap, you can go for anything from 1uf - 4uf for 2a3 and 3.3-15uf for 45 or so.
As long as you have the 600V minimum rating.

Here's the formula for normal parafeed cap size:

C = 2*L / (R*R)

C is the capacitance in farads, L is the plate choke inductance in henries, and R is the load impedance seen by the plate (i.e. transformer primary impedance) in ohms. If you use kOhms the C is in microfarads.

This value gives the best bass extension while keeping the impedance seen by the tube's plate relatively constant and resistive to the lowest possible frequency. The value is not especially critical, anything from half to twice this value will work pretty well. For speakers with especially deep bass there may be some interaction with the speaker impedance fluctuations, and experimenting within this range of values can sometimes prove fruitful.

Edit: To save money i went with 2.2uf caps for output coupling for 2A3.
Edit 2: PB posted while i was typing.

 

[Jameson]

Yeah, the 0.1uf is at 630v and the Solen Silver 3.3uf I got is at 700v, but that formula is a great reference. Thank you!

 

[Deke609]

As long as you put in 0.1uf for the interstage (de)coupling cap with a minimum 600V rating. For another capacitance value, you should also change a resistor.

I'm assuming this is the "Rstop" resistor between the coupling cap and the grid? Is there a formula or rule of thumb for determining the value of the resistor?  I used 0.22 uF in place of the 0.1 uF in my SII-2A3 (retailer was out of the CuTF in 0.1), but made no changes to the resistor.


cheers and thanks,


Derek

 

[Tom-s]

https://bottlehead.com/smf/index.php?topic=1950.0

 

[Paul Birkeland]

I'm assuming this is the "Rstop" resistor between the coupling cap and the grid? Is there a formula or rule of thumb for determining the value of the resistor?

The rule of thumb is about 200-1000 ohms of resistance with the body of resistor reasonably close to the tube socket pins.

 

[Paul Joppa]

I do not think any resistors need to be changed.

 

[Deke609]

Jameson - apologies. I didn't mean to hijack your thread. But it is at least a closely related topic, so I hope you don't mind.


Tom, PB and PJ: many thanks.  "Time constants" and "cathode stripping"! Sheesh. Glad I'm ok with the 0.22 uF's.


Derek

 

[Jameson]

All good! I got my question answered plus a lot more.

 

[2wo]

I used the Solen silver as parafeed caps in one of my own amps and have been very happy with them...John

 

[Jameson]

Threw these in before work this morning. The biggest difference I noticed is that the amp is now smoking and sparking. Still sounds pretty good.

Kidding. Everything went as smoothly as you'd expect for such an easy tweak.

I haven't spent enough time listening to have impressions yet. Thanks for everybody's help and input.