Spitfire: choke vs resistor

[kkregsg]

I'm building a Spitfire clone, and in looking over several different layouts/ schematics I see most use a choke, whereas the Watts layout substitutes a resistor. How would the resistor affect the amp's voicing?

I did see Chuck H's schematic for "here's the beef" parts substitutions Thanks, Chuck. You were very helpful in suggesting transformers. If anyone has some helpful tips on this amp, please feel free to share.

[kleuck]

Usually, chokes give a better filtering, a stiffer sound (less voltage drop and sag) so slightly more power (depending on the choke obviously)

[Dave H]

I don't think you'll notice much if any difference in voicing. I have a 470R resistor for the choke, 1k screen resistors and 47u capacitors and it still sounds great.

[Alex R]

Yes I put a choke replacement resistor in a plexi-type build, gives a little more compression and softness I feel, and the customer was happy.

[kkregsg]

Thanks for the quick responses. I learn something new every day, whether I need to or not! Ultimately I will try both to get a first hand feel for the difference. Experimentation is the mother of the huge screw up or the quantum leap forward. And I get to solder more stuff.

[bob p]

Interesting thread. I've been thinking about exactly this problem -- whether or not an AC-15 type of build really needs a big choke or not.

Chokes tend to be most helpful in circuits that have a lot of variation in load. I'm not sure whether the choke would make a huge difference in a low-powered circuit like this one. There's a reason that Fender started off building low-powered amps like the Tweed Princeton with chokes, and then stopped using chokes in favor of resistors. I think it's because the bean counters thought it was a diminishing-returns type of situation.

Another way to look for the answer to this problem is to do the math. If you're interested in reading, I'd recommend one of the old tube design books by Norman Crowhurst. You can find them on Pete Millet's site. There's a chapter on PSU design that includes a choke selection nomogram which I've found to be incredibly helpful for choke sizing. If you do the math, you'll find that many times the chokes for musical instrument amps aren't optimally sized. When I looked at the AC-15 schematic I thought that the choke seemed kind of large, but I haven't gotten around to doing the math yet.

[bob p]

I did see Chuck H's schematic for "here's the beef" parts substitutions Thanks, Chuck.

Since I'm thinking about a similar build, I'd be interested in seeing that if you could provide a link. Thx.

[kkregsg]

Bob, thanks for your info. I will definitely read up on the subject.
Here's the thread with Chuck H's annotated schematic with his suggestions to tame some of the treble in the Spifire:
http://music-electronics-forum.com/t31023/

[bob p]

Here's that choke sizing nomogram that I was talking about.




If you're not sure how to use the nomogram, just ask. Here's a link to the original text:

High Fidelity Circuit Design by Crowhurst & Cooper from 1956.

[Dave H]

I tried the Spitfire circuit but it had too much treble for me because of the ‘bright’ cap on the volume pot (I’ve never liked those). I tamed it as in J20a.pdf below. If you use a 500k 10% log pot for the treble control wired for max treble at the cw end (not like a VOX cut control) the treble curve will be flat with the pot in its mid position. If you prefer a bass pot to the switch it can be done like J20b but it doesn’t really need any tone controls. I gigged it for years with the only tone shaping being a 1n cap feeding the 1M volume control.

J20a.pdf
J20b.pdf