6CA4 and Input Capacitance

[Daver]

According to the data sheets a 6CA4 has a maximum input capacitance of 50uf. I'm working on a Twilighter (Magnatone 440) that uses 90uf for the first capacitor. 40uf in the can cap and a 50uf axial in parallel with it. The schematic shows this as well. So how does the little 6CA4 live with almost twice the rated input capacitance?

[R.G.]

It lives.

Just a shorter time. The reason capacitance is limited for rectifier tubes is that the high current peaks with a big capacitor deplete all the electrons the cathode can supply, leaving the cathode unshielded by an electron cloud. Ion bombardment and degradation of the cathode's emitting surface make emission get lower, until it has insufficient electron emission.

Reading that back, I'm aware how much it sounds like made-up-on-the-spot mumbo jumbo. But in this case at least, it's accurate. Too big a cap degrades the cap faster. As a suggestion, put a resistance between the 40uF that's there and the 50uF tacked onto it. The resistor will tame the current pulses and make the rectifier live longer.

Alternatively, replace the 6CA4 with a solid state rectifier and a resistance to drop the voltage back to where it is with the 6CA4. The SS diode doesn't much care if it's 90uF or 900uF. This is one reason rectifier tubes were abandoned by the industry before amplifying tubes.

[Daver]

Thanks R.G.,
The amp needs to be recapped. I was contemplating just using a 47uf cap and forgetting about making it 100% stock per the schematic. I'd think that would be sufficient for a first filter for this amp.
Dave

[Chuck H]

I'm not familiarwith that amp. But it was unusual at that time for a guitar amp to have more than 50uf of filtering at the first node. Since it's indicated as stock on the schem I think the designer must have had trouble with ripple. May be a good idea to stick with the original value. You can try 47uf, but you may find out why the designer added another cap to the circuit. Then again, if you pick a cap with good ripple rejection you may do just as well with a new 47uf as the original designer did with 90uf.

[Daver]

Chuck,
I'm going to try it with just 47uf and see how it goes. I can always add another cap if there are problems. Thanks!
Dave

[Daver]

Well I went with just the 47uf and it seems fine. No extra noise or hum. Just for fun I clipped in another 47uf just to hear what it would change. Very little. It's a bit stiffer and that's about all. So, the single 47uf stays and the rectifier can live a little longer and happier. Thanks!
Dave

[tubeswell]

There's a suggestion here about effective series resistance for that type of tube EZ81 @ The National Valve Museum