Marshall 1987 Choke

[Littlewyan]

Hello everyone

I'm going to be building myself a Marshall 1987 and am looking at what choke to use. I plan on using all Hammond Transformers btw. I believe I'd like to use a choke with a high resistance (200ohms Min) to lower the screen voltage and give it some sag to improve valve life. I also think at least 5H would be good, however I'm unsure of the Current Consumption in this amp. Looking at the Chokes available a lot of them are speced for 100Ma but the JCM800 Choke can only take 50Ma which seems a bit low to me. Can someone point me in the right direction?

So far I've added the 193C, the 159M and 158M Chokes to the list.

[Stevem]

Well each screen of a EL34 can handle a max of 8 ma, and each section of the three ax7s with those plate voltages needs no more than 1.8 ma so a 50 ma rated choke should do fine.

[Littlewyan]

Do the screens not draw more current when the amp is under load?

[Roe]

choke should handle at least 75mA if you're using RFT/siemens el34s

[Littlewyan]

The Choke I'm now looking at can handle 70mA, sort of borderline really.

What difference will the different size Chokes make to the sound? I know that if I use a lower Henries Choke then I might not get away with low PS Filtering like in earlier amps, but how will they affect the sound? I was looking at a 20H Choke as I know the first JTM45s had these.

[Roe]

for an authentic marshall sound you really should try a 3H choke, although 5H isn't too bad either.

[Littlewyan]

Well the very early 1987s actually used 20H Chokes, but all amps later than 68/69 used 3H Chokes. I want the 20H Choke as it has a higher DC Resistance so will give the Screen Grids a bit of sag and won't let them draw as much current due to the higher voltage drop. I'll just go for it I think, worst that will happen is the inductance will decrease as the current goes past 70mA.

[Roe]

Well the very early 1987s actually used 20H Chokes, but all amps later than 68/69 used 3H Chokes. I want the 20H Choke as it has a higher DC Resistance so will give the Screen Grids a bit of sag and won't let them draw as much current due to the higher voltage drop. I'll just go for it I think, worst that will happen is the inductance will decrease as the current goes past 70mA.

this is hardly true. actuall amps nearly always used 3h chokes, although schematics show 20h chokes. even 1966 amps used 3h chokes, and virtually all el34 1987s used 3h drake or dagnall chokes speced for 110ohms 3h.

1987 have lower voltage than 1959s and a 110ohm choke is fine if you use appropriate screen grid resistors or a flying resistor like the jtm45

[Roe]

Well the very early 1987s actually used 20H Chokes, but all amps later than 68/69 used 3H Chokes. I want the 20H Choke as it has a higher DC Resistance so will give the Screen Grids a bit of sag and won't let them draw as much current due to the higher voltage drop. I'll just go for it I think, worst that will happen is the inductance will decrease as the current goes past 70mA.

dbl post

[Roe]

Btw, a 3h choke will give that familiar raw marshall sound. A 20h rs choke will give a cleaner and crisper tone

[Littlewyan]

Btw, a 3h choke will give that familiar raw marshall sound. A 20h rs choke will give a cleaner and crisper tone

Thats why I want to use one, should give better bass response as well. Worst case I'll change it if I don't like it. Would I be right in saying that a 20H Choke would give the amp a softer tone?

[Roe]

yes, attack will be noticably softer unless you raise the voltages significantly. the bass will be better, as well. I prefer the 20h RS choke in high voltage 45/100s

[Littlewyan]

Hmm well basically I didn't like my reissue Marshall which had all 3 Filter Caps as 50uf + 50uf, was too harsh. So I was thinking that having the 20H Choke with 50+50 for the OT and two 32+32 for Screens and Pre amp might do the trick. What do you think?

Edit: Maybe I should perhaps go for a 3H - 5H Choke as the lowered filter caps might just do it for me. I'm just looking for the softness you get from a 67-68 50W.

[Roe]

For softness, try lower voltages, good filter caps,tubes and signal caps. try around 420v b+, F&T filter caps (16+16 for preamp; 32+32 for screens and PI; and either 50+50 or 32+32 for mains). Try sozos or mustards

[Littlewyan]

But go for a 3H Choke?