Max input filter cap for EZ81?

[truth_hz]

So, I'm working on an old Knox amp. Tube compliment is 2x 12ax7, 2x 6bq5, and a 6ca4 recto.

Looking at the guts, I noticed that the first filter cap is 80uf, which is way bigger than I'm used to seeing. All the datasheets I've seen list the max input filter as 50uf.

I am re-capping the thing, should I go back down to something like 47uf? It seems odd that stock they would use such a high value, but the little bit that I heard from playing it, sounded nice.

as is, the power supply layout is:

recto--->80uf input filter which feeds the OT center tap--->2k2---->80uf which feed the screens and all the preamp tubes.

B+ is pretty low in this amp: 280v at the first filter node.

[tubeswell]

What I found out in my enquiries about filter cap ratings in old guitar amp schemes/designs, is that they used to have smaller caps because that was what was available in those days.

The reason that alot of tube manufacturers were conservative about reservoir cap ratings for rectifiers was that they were anticipating that their tubes would last longer if they woeren't worked too hard. In reality with guitar amps, there is no such thing as "too hard" right? I reckon 80uF would probably be okay. If you're worried about blowing up the rectifier tube one day, do R.G. Keen's immortal amp mod #1 to protect your PT

[truth_hz]

thanks for the reply. The whole amp looked like an oddity, which attracted me to it. I'll probably drop in a little smaller cap, just to keep things safe, and keep the higher value one downstream.

The R.G. mod, refresh my memory, diodes in series with the recto plates?

thanks for the input.

-morgan

[mooreamps]

Something else to consider. When those guys spec'ed those rectifier tubes, they were assuming zero ohms resistance between the output of the rectifier tube and the first filter cap. However, you could also run a 10 ohm 1 watt resistor up to a 100 ohm 5 watt resistor between the rectifier and the first filter cap, to help abate the surge current through the rectifier hitting that first cap.

-g

[tubeswell]

The R.G. mod, refresh my memory, diodes in series with the recto plates?

-morgan

Yep