Rectifier tubes and Filtering

[Structo]

There has been a lot of talk lately about tube rectifiers.

One thing I don't quite understand is that a particular rectifier tube has a maximum first filter cap capacitance.
Take for instance the 5U4GB tube that I am using in a 6V6 push/pull amp I just built.
In the data for that tube, it states a max. of 40uf for the first filter.

Why is that and what would happen if this value was exceeded?

[Andy Le Blanc]

The subtext in the RCA data states, "Higher values of capacitance than
indicated may be used, but the effective plate-supply impedance may have
to be increased to prevent exceeding the maximum rating for peak plate current".

[dehughes]

The subtext in the RCA data states, "Higher values of capacitance than
indicated may be used, but the effective plate-supply impedance may have
to be increased to prevent exceeding the maximum rating for peak plate current".

Bingo.

[FunkyE9th]

Why is that and what would happen if this value was exceeded?

When you 1st turn on the amp, the cap essentially acts as a short (i.e. the window when the voltage is going from zero to the max has a lot of high frequency content. The cap is a short high freq.) . So you have a lot of current in that very small window until you get to steady state and the cap is fully charged (i.e. DC) and acts as an open circuit. The bigger the cap the bigger the current at turn on. So if you exceed the max current of the tube, it's not good for the tube. As mentioned above by Andy, increasing the plate impedance will limit the current.

-FunkyE9th

[Andy Le Blanc]

I think you add resistance either before the rectifier plates or just before the
first filter cap. A smaller cap will draw a lower amplitude current pulse.
The current draw from a very large cap is very short high amplitude pulses.
it can exceed the rectifier peak ratings and cause ringing in the PT, creating more noise.

[Structo]

Thank you all for the explanations.

That makes a lot more sense now, especially the part about the rectifier seeing a short as the cap charges up.

In the amp that I am using the 5U4GB I have a cap can that has a 40uf for the first filter followed by three 20uf sections.
This tube's data says a 40uf is max for the first filter.
The 40uf is followed by a 4H choke.

I do have a bit of noise but the chassis is so small it's difficult to do much.

This is the amp that I used a 50 watt 130v Zener to drop the voltage.
I have the Zener on the center tap and also have the Zener bypassed by a .01uf ceramic cap in an effort to quell any switching noise.