Correct choke placement?

[Donkey]

Greetings! Looking for some opinions on choke placement for an amp I currently have (Randall RH50T). I am looking at placing a Hammond 155H (which is rated at 5H 400v/50ma and 270 ohms) in place of R142 which is located right after the rectifing diodes and between two sets of filtering capacitors. Any thoughts would be greatly appreciated! Thank you in advance!

[pompeiisneaks]

I don't think that will work.

The amp shows +444V at that point. that means the voltage rating is over spec for that choke

Also the choke says 50mA current. I believe the two el34's alone are going to pull more current through them than that already let alone any preamp tubes. You should get a sense of how much current is flowing through the amp's main supply and then overrate it a bit.

i.e. I'd shoot for a voltage rating of the sum of the voltage ratings of the caps in series after the rectifier. (C96 voltage rating + C79 Voltage rating) say each is a 250 VDC rating, then 500VDC is your mark.

A good guess on current is going to be the sum of the tubes. I just did a bias calculation for EL34 at 444 VDC and get about 40mA per tube at 70% so 80mA just for those, add in the 3 preamp tubes (if I read that right) which probably will pull around 5-6mA each, if not more, (3x6 = 18mA) I get at minimum 98mA so I'd overshoot and guess other parts may pull more, and you need 120mA or so to be safe for the rating of hte choke. One of the marshall ClassicTone plexi chokes says it's 250mA 3H

So something like that is probably a super safe bet.

~Phil

[sluckey]

Plate current does not flow through the choke. Only screen current. I'd still want a 90mA (or slightly higher) choke. The voltage rating doesn't scare me.

[Donkey]

Thank you for the replies gentlemen. I will look into a different choke option. As far as placement,would that be the best place to install it?

[pompeiisneaks]

Plate current does not flow through the choke. Only screen current. I'd still want a 90mA (or slightly higher) choke. The voltage rating doesn't scare me.

D'oh! I didn't see that they slid the rest of the preamp and plate off one node and the resistor based only to screens.

Does having a choke on a screen only node help like you'd want/expect>?

~Phil

[Roe]

around 75mA is sufficient. place the choke wherever it is quiet. Try moving it around before you cut the wires

[Donkey]

As far as placement in circuit, I am assuming that R142 is a good spot or is there a better place for it?

[sluckey]

As far as placement in circuit, I am assuming that R142 is a good spot or is there a better place for it?

That's the usual spot.

[pompeiisneaks]

As far as placement in circuit, I am assuming that R142 is a good spot or is there a better place for it?

That's the usual spot.

I asked in my last post. It's common to put the choke to only help the screens? It seems like it would benefit the rest of the power supply as well no?

Or as you stated that's normal?

~Phil

[sluckey]

You're right. I spoke too quickly. I change my answer to "I see no benefit to adding a choke to this fine amp."

[pompeiisneaks]

You're right. I spoke too quickly. I change my answer to "I see no benefit to adding a choke to this fine amp."

So when is a good decision point for when a choke becomes helpful for the preamp stages? or is it really a cost balance between choke vs just having enough filter capacitor filtering to do the job?

I.e. bump a 16uF to say 32 and call it a day

~Phil

[roberto]

I asked in my last post. It's common to put the choke to only help the screens? It seems like it would benefit the rest of the power supply as well no?

Or as you stated that's normal?

~Phil

In standard push pull guitar amps you'll find in 99,99% of the cases the choke for screens, PI and preamp.
This permit to have good ripple reduction with a reasonable cost (filtering everything would mean to have a huge choke with high current capability and very low Rdc, in order to avoid excessive dV across it).

Some single ended may filter the plates too, because noise rejection of SE amps is worse than PPs.

That said, there are very good spunding amps that have resistors instead of chokes (as this forum has demonstrated over the years).

[roberto]

So when is a good decision point for when a choke becomes helpful for the preamp stages? or is it really a cost balance between choke vs just having enough filter capacitor filtering to do the job.

That part of the amp defines also the dynamic behaviour of the amp by changing the voltage of the screens (so the gain of the pentodes) when pushing it. Mesa DynaWatt for example enhances this point by using resistors of high value and a low value capacitor with a specific RC. ...like a compressor.

Larry Novosibir used a rheostat between the main node to the screens to change the amp's behaviour.

The ripple us just part of the story.

[pompeiisneaks]

That makes sense, so basically putting the choke ONLY for the screens like it would be here is somewhat of a waste then.

If it's really wanted you'd also end up impacting the overall amp a bit by lowering voltages somewhat by adding the choke somewhere before the screen and the next B+ section that goes to the preamps as well, i.e. the preamp section woudl have some voltage drop across the choke.

Is that why you think it's no longer worth it in this amp sluckey? I guess I was thinking to just change the connections a bit so the choke hits both the screens AND the preamp section but not sure from there.

~Phil

[sluckey]

The PI and preamp get their B+ directly from the plate node via R96. The screens get their B+ from the plate node via R142. That's two parallel B+ paths. So if you're gonna choke it you need two chokes, one to replace R142, and another in series with R96. Not worth it IMO.

Or... You could move the PI and preamp B+ path from the plate node to the screen node and use a single choke like a lot of sensible amps. More better IMO.

Or... you could leave this fine amp alone.