POWER SUPPLY QUESTION

[Colas LeGrippa]

Hi my friends !

I have modded an amplifier by changing the ss rectifier to a 5Y3 tube.  Following the new tube specs, I have swapped the 50uF reservoir cap for a 20uF , but I kept in place the 50uFl follwing the choke ( 5H ). Then I have:  5Y3, 20uF, choke, 50uF, resistor and 20uF for the only one preamp tube.  My question is: is the 50uF cap after the choke is «seen» by the 5Y3, in other words could that cap  eventually destroy the recto tube? In that particular amp, there is no big current involved. A 6L6 drawing .085 at 300V and a 12 ax7, which total is below the max of the 5Y3 rating. But those values are idle values. At full volume for a long period of time, would that 50uF cap could harm the rectifier ? I can easily replace it, it's no big deal. But I am just curious knowing if it's acceptable. I must add that the 50uF cap after the choke supply the power tube screen grid. Gracias, amigos !

Colas

[Paul1453]

Have you tried the PSU program?

That is the only way I'd have a clue.

I think I did try sticking in a lower value cap before the choke on a tube rectified design.
I thought I might be able to trick it and still use my 50/50uF cap can after the choke.
I seem to recall PSU warning me about still exceeding my GZ34's values.   

[HotBluePlates]

The choke will tend to limit the current drawn by the 50µF cap which follows it. And the screens & preamp don't draw that much current anyway.

The real answer for how big a cap you can get away with is a complicated calculation, involving the resistance of the PT primary & secondary, the winding voltage of the PT, the amount of current drawn by the amp's stages, and the choke/etc connected downstream of the 1st filter cap. It's painful enough that no one bothers to calculate it and uses a program instead.

There are ways to be able to increase the value of the 1st filter cap when using a 5Y3, but they will add to voltage sag under load (beyond what the 5Y3 will already do), and so the value is debatable (i.e., "... if you wanted more sag, why did you try to stiffen the power supply by using bigger caps?").

[Colas LeGrippa]

Yeah sure ! The final question is: how much sag do you want ? And the answer is to design the power supply accordingly.
But understanding what we're doin' in an amp is always useful. When a mod is for a client, doin' exactly what is needed to be done is important. First, it lowers the cost of the repair/mod, and ensures that what has been done will not damage the amp in the long run. A repair under warranty is always a pain in the.......neck. For both the tech and client.

Thanx buddies

Colas

[HotBluePlates]

You asked:

I have modded an amplifier by changing the ss rectifier to a 5Y3 tube.  ... I have:  5Y3, 20uF, choke, 50uF, resistor and 20uF for the only one preamp tube.  My question is: is the 50uF cap after the choke is «seen» by the 5Y3, in other words could that cap  eventually destroy the recto tube? ...

And I said:

The choke will tend to limit the current drawn by the 50µF cap which follows it. And the screens & preamp don't draw that much current anyway.

I might not have been clear, as I took a detour about how much filter a 5Y3 can support. So more clearly: a spike in current draw from the 50µF after the choke will be minimized by the effect of the choke itself.

Inductors (chokes) tend to oppose changes in the current passing through them.

[Colas LeGrippa]

Thanks fpr the precision HBP, but I had understood ! ( though sometimes I seriously doubt of my understanding when I read your posts )

Have a good day. 

Colas