Choke?

[GUITARmole]

Just curious....why does the AC30/15 use a choke...and they bigger Marshalls (50-100W) use chokes...but there's no choke on the 36W "Marshall'?

What effect does the choke have on actual tone or feel? It just smooths the power supply and reduces ripple, right?

[zaphod_phil]

It actually has the effect of making an amp feel a little softer and more compressed. Without the choke feeding the screen grids (ie using more capacitive filtering) you get more of tighter, punchier, faster response. For example the big difference between a big Marshall and an equivalent Hiwatt's power stage is the choke. Jimmy Page's Hiwatts had chokes fitted to soften their feel to be a bit more Marshall like. And I guess you already know the difference between the tone and feel of a Marshall 18/36W and an AC 15/30. I suppose that with these smaller cathode-biased amps, not using a choke probably helps give them some more punch, like a bigger fixed-biased Marshall....

[GUITARmole]

Thanks!

[Shottky]

That's a burst of knowledge! I was under the impression that a choke made an amp tighter and more punchy, whereas the lack of a choke made the amp more compressed...

[zaphod_phil]

I was under the impression that a choke made an amp tighter and more punchy, whereas the lack of a choke made the amp more compressed...

Yes, I've come across that idea myself a few times, and I'm not really sure how it arose. It's wrong. A choke reduces transients by generating internal back EMF, and this has the effect of reducing punch.

Purely for illustation, here's a post from Trinity's forum, where Tripower455 (also an 18watt.com member) talks about the effect of the switchable choke in one of his self-built Hiwatts.

It's a Jimmy Page/SAP preamp mated to a DR504 power section, with a switchable choke.... With the choke engaged, it'll do the Marshally thing... It breaks up a little sooner and has a noticeably softer attack. It is overall a much looser feel than the using the 100R resistor. I like it, but it really isn't my cup of tea for everything or my style of playing. I prefer the tighter, punchier Hiwatt type output section

[GUITARmole]

So hypothetically speaking if a person had two Lite IIb amps that they just built...and they wanted to convert one to an AC15 with an EF86 pre...and they were going for more of a clean high headroom punchy jangly Country/Surf kind of a sound...would they be better off leaving the existing Lite IIb power supply intact with 32uF caps (instead of the AC15's 16uF) and no choke?

I had always thought like Shottky, that the choke made the amp tighter and less compressed...great info, thanks again!

[krx]

There's different ways to use a choke though... What if we filter the WHOLE power supply through the choke (cap input supply) a la the 5F4/5E7/etc.? Those seem like pretty punchy amps to me despite their fairly small power supply caps. Would they be punchier without the choke?

Or you could do a choke-input supply, which I've never messed around with myself.

[zaphod_phil]

So hypothetically speaking if a person had two Lite IIb amps that they just built...and they wanted to convert one to an AC15 with an EF86 pre...and they were going for more of a clean high headroom punchy jangly Country/Surf kind of a sound...would they be better off leaving the existing Lite IIb power supply intact with 32uF caps (instead of the AC15's 16uF) and no choke?

For more punch, then yes, leave out the choke and keep the bigger caps.

There's different ways to use a choke though... What if we filter the WHOLE power supply through the choke (cap input supply) a la the 5F4/5E7/etc.? Those seem like pretty punchy amps to me despite their fairly small power supply caps. Would they be punchier without the choke?Or you could do a choke-input supply, which I've never messed around with myself.

Very true. Obviously, we've been talking about using a choke to feed the screen grids, as in many vintage amps. To use a choke to filter the whole power supply requires a bigger choke, which works out more expensive, so amps like that are much less common. And the choke is usually part of a pi filter, with a cap each side, which is probably more why the amps are punchy, plus the fact that in amps like this the screen grids are always resistively fed. The thing to remember about chokes is that they always react against fluctuations and transients in current, while caps react against voltage changes. In other words, to current transients an ideal choke looks like an infinitely high impedance. So voltages after a choke will show strong fluctuations in response to swings in current draw, unless there's also a sufficiently large cap there to counteract them. Choke input filters are in interesting case, since they produce much lower DC voltages than the common capacitive filters. They can be very useful if you want to use a PT with a secondary voltage that's otherwise too high for the kind of amp you're building.