45 Parafeed cap,what value?

[najo49]

I have a 45 amp built by doc that I bought recently. In the original schematic from valve archives it gives the value of 10uf, for the parafeed cap but It has a 5uf sidereal cap . Any thoughts about this. I just sent for a 5uf mundorf supreme cap to try in  the sidereals place. Anyone have experience with this cap? also I am using the exo 45 opt,with the proper choke for this opt. Jan Ollsen

[Grainger49]

My opinion is to break in the Mundorf before making any decisions.  They should get 100 hours, about 4 days + some, before installing.

Here is my post on Painless Break In:

Painless Break In Post

"Painless" because you don't listen to the cap going from Ok, to not so Ok, to good.

[johnsonad]

That sidereal is a great cap! It was the first version of the auricaps. If you find something better, I'll give those a home

[Paul Joppa]

Got some references? I size parafeed caps by the plate choke inductance and OPT impedance, so I'd need both values to comment usefully. I know the EXO-45 is 5K, so I just need to know what the plate choke is. If you have the VALVE reference, that would help as well - might give a hint whether this was an exact copy of the published amp, or a variation with possibly different iron.

[Doc B.]

I don't remember this amp, and I sure don't remember the caps. It is quite possible that the 5uF caps were the best thing I had on hand when it was built - if I did actually build it. There are times when it is worth getting wrapped up in precision values for parts, like when you have to choose a resistor value to set a very precise amount of current through a component. But sizing parafeed caps to the last 1% is not necessarily one of those times. If the value is kept within a reasonable range, like 2:1 as opposed to 10:1, it simply shifts the LF rolloff few Hz.

[Natural Sound]

Got some references? I size parafeed caps by the plate choke inductance and OPT impedance, so I'd need both values to comment usefully. I know the EXO-45 is 5K, so I just need to know what the plate choke is. If you have the VALVE reference, that would help as well - might give a hint whether this was an exact copy of the published amp, or a variation with possibly different iron.

Hi Paul, I'm wondering what size parafeed cap you would recommend when using an EXO-45 that is 5K and a DOWDY that is 125H?

(Paramour II using 45 output tubes. Not shunt regulated.)

[Paul Joppa]

Hi Paul, I'm wondering what size parafeed cap you would recommend when using an EXO-45 that is 5K and a DOWDY that is 125H?

(Paramour II using 45 output tubes. Not shunt regulated.)

10uF, or - as Doc says - anything from 5uF to 20uF. 

[najo49]

paul, the plate choke is a bcp 15 the opt and exo 45 .

[Paul Joppa]

paul, the plate choke is a bcp 15 the opt and exo 45 .

The formula is 2 time inductance, divided by impedance squared.

BCP-15 comes with two different gaps and inductances, 40 or 50 henries at 50 or 40mA respectively. EXO-45 is a 5K primary. So 3.2 or 4uF - anything from half to twice is good enough.

[najo49]

Grainger, Thanks for the reply on burn in. Is there any reason why you could not do 2 caps or even 3 in a string. I see not tech. reason why not?. j.olsen

[Grainger49]

If you put the caps in series the crossover gets higher and higher.  So no bass through them.  If you put them in parallel they get half the current. 

Parallel is better but will take twice or three times as long depending on how many you parallel.