Choke Question

[rockford]

Heres a general amp question. If the AC30 used the same radio spares output trans. as the marshall jtm 45, why does jtm (can) use a 3H choke, and AC30 use a 20H ? Can somebody shed some light on this for me ? Much apprecited.

[jsn]

I think the fact that both amps used output transformers that they could get thier hands on cheaply says it all - they used whatever they could get! (not that there is anything wrong with that)

In the power supply, the difference between a 3H and a 20H choke will be kinda like the difference between 8uF and 20uF capacitors - subtle.

But I sorta think the real question is which would sound better. And that is one of those mojo things where half of the people will tell you to go bigger and half to go smaller. I personally think it is often best to use as little power supply filtering as you can get away with. I think it helps the dynamics. A low impedance power supply is a good thing. But I am more of a hi fi guy than a guitar guy so there might be more going on there - like maybe you would prefer a big choke power supply for it's "sag" effects.

[jsn]

Another thought on the sag thing. The AC-30 is a bit more class A than the Marshall right? That would mean the current draw would be more consistent, and make the likely higher DCR of a big choke less audible. Maybe that is a real reason other than "what they could get thier hands on", which I admit is not giving these guys enough credit :)

[Funkalicousgroove]

First off, A JTM45 used a RS, an AC30 used a Haddon, or Albion.

Now the reason for the 20 H choke in the vox is because of the 16uf first stage, and the overall lack of filtering, the large choke partially makes up for this.....partially.

Secondly, the JTM45 used either a 7.5H or 10H choke, Marshall didn't start using smaller chokes until they increased the filtering substantially.

Bottom line: Bigger choke you need less filtering.

[rockford]

So is it always a trade off between filtering and choke ? If somebody put in
higher filtering , would it "cancel out" the feel or sound influence of a lower rated choke, say dropping from 20H to 10H or 7 or 5 ?

[Funkalicousgroove]

Yes and no,

It depends on the PT and the current drain of the circuit.

Generally a larger choke allows for less filtering, but potentially more sag,

If you use enough filtering you don't even need a choke, but it definitely helps to smooth the DC. Too much filtering and a large choke makes an amp so stiff and unforgiving it's almost unplayable.

The AC30 is a fairly unique circuit in that the supply rail goes 16uf, 20H choke, 32uf, and everything else comes off that 32uf in paralell instead of continuing down the line in series with resistors and decoupling caps, so the large choke is almost necessary. With 32UF or more on the plates the Large choke becomes unnecessary and could be potentially dropped to 10 or 5 H.


The choke is something that can be used to fine-tune your amp, you can go bigger or smaller to change the feel and response.

[rockford]

Interesting stuff. I'm making notes on this, and will probably try some rec. tube swaps to take the "hardness" out of it.

[drz400]

best thing to do is put it on a scope and look at ripple at full power.
I like to see very little personally cause I cant stand the double note thing.
Just remember that most of this only matters when driving the amp to a decent volume.

[rockford]

I think my experience of this topic came when I switched a Mullard / RCA 5AR4 for the stock jj. There was a day or two in between, and I just noticed the amp seemed harder to play. Did'nt put 2 & 2 together at first, then
put jj rectifier back. NIght and Day difference. If the amp had been rebiased,
would the amp have "felt " more playable ?

[David Root]

Funk, in a tweed or old Gibson supply that feeds both the screens and the plates off the choke in a CLC configuration, the choke is usually on the small side, 3-5H, and the caps either side of the choke are pretty small by modern standards, eg 16 uF or so.

Most recommendations for new builds in that mode I've seen are to make the caps a bit bigger, especially the second one after the choke that is directly feeding the plates and screens, and not mess with the choke.

What's your take on this?

I'm starting both a tweed Bandmaster, modded for harp, (cathode bias) which I'll probably use the new JJ KT66s on, and a Gibson EH-150, modded to run 380-400V on plates/screens instead of 270V and running a pair of '40s JAN 6L6GAs for 25W or so.