Hi - I need some help with selecting a choke.

I have a Carvin SX-100 which regularly blows power transistors, so I was thinking of replacing the power stage with a couple of EL34's.
I thought I might use a similar layout as a marshall 1963 or 1964. The schematic for these shows a choke, but with no other specification.
I'm going to keep the Carvin PSU for the preamp stage, and build another PSU for the EL34's.
Do I need a choke? If so, how do I work out what sort of choke I need...
And, what do chokes actually do, anyway?

Hey - thanks

i think chokes are mostly just for filtering. a filter cap under load still drains a bit, so you don't get perfect (straight-line) dc, so they use chokes to get it closer to perfect since inductors try to counter changes in the current. that's my reasoning anyway. i could be only half-right or totally wrong.
You don't know what a choke is or what it does, but yet you're gonna build a tube power section for your amp?

I think you're in over your head a bit, but anyways, here's a link to a 50W PP EL34 driven power amp schematic. - http://www.ax84.com/static/corepoweramps/50W_PP/AX84_50W_PP_Poweramp_Schematic.pdf

Edit: Jim was pretty much spot on with his explanation of what a choke does.
Last edited by Matt420740 at Nov 19, 2009,
Forget it, not worth the effort or substantial expense. You'll require a new power transformer, an output transformer, a choke, tube bases, output tubes, driver tubes, plus sundry components including new tagstrip or equivalent layout method etc.
And, as previous post hints, it's likely you'll electrocute yoursrelf (= death, yours) before you finish the project.
Thanks for the replies... surely it's up to me to decide whether this conversion is worth it?
I already have all the necessary components, with the exception of the choke.

Can anyone explain what the critical factors are when selecting a choke?
Chokes will usually just be listed by inductance, though you should also take into account the current & voltage handling as well as the DC impedance.

The "proper" choke for the 1963 design is a 20H choke, though you can use a different value if you like. A higher inductance will give you more filtering. The choke will need to be rated for the amount of current the screens will draw, so watch for that. Lower DC resistance is better in a choke; I think most fall under 50 ohms for guitar amps.
Sorry to assume where you are at, get some dumb questions in here .
Anywhere between 8 to 15 H is usual in instrument amps and should be fine, 20 H more like hi fi territory I'm surprised that is the '63 spec. Bigger = more expensive.
Hammond chokes are plus minus 15% from memory.
As above current rating is primary concern. They also have a voltage rating, potted designs generally having higher specs. Higher inductance and voltage current specs mean more expensive.
The choke filters the screen supply, the push pull amp which is more sensitive to noise in screen supply than the plate/anode supply. Check data sheets for ball-park figures of peak screen supply draw (grid no 2 current) when operating in similar conditions (class of operation and voltage) - I'd expect the two tubes to draw less than 60mA total at max signal. Adding safety factor, a say 12H 100plus mA choke shouldn't be hard to source. http://www.hammondmfg.com/153.htm
If you placed the choke before the plates you'd be looking at large currents and therefore a beefy choke so it's seldom done except for med to low power single ended designs.
^The 63 was originally a PA design, hence the 20H choke. 8-12 is probably a better spec for the job in this case.