Anybody considered this?

[rooster]

In my express builds, I was always wanting to hear what the amp sounded like with a smaller choke. But I still wanted the stock choke available.

So I installed a 125VAC SP/ST switch, the type used to power on a Fender amp, considered the 25 watt resistor value I would need in parallel with the 1K stock resistor and created a switchable choke. My end choke values are 1K ohms and 180 ohms. I chose the 180 value because I had read of some obscure/early Marshall amp that used a 180 ohm choke, and not the traditional 100-112 ohm Plexi choke.

The results. With the smaller choke value the amp is tighter and louder. The noise floor does not increase, and the amp is similar but - for lack of a better description - more Marshally, more headroom. I've had guys play the amp who were saying that the stock Express was too soft, guys that pick fairly hard, suddenly find their happy place as we switched to the smaller choke. I make the switch on the fly, BTW, with amp running. (Yes the mA current increases on the plates, but since I run my Mullards at 28 mA this is not a problem for me. If you are running your tubes at 40 mA, you might want to check your plates and bias point with either choke and make a change.)

Anyway, just an idea. The install was simple, the switch is installed on the top of the chassis, placed near the OT and towards the back of the amp. It's easily reached and labeled. I been using this for two years without issue and using vintage Mullard XF2 power tubes. One amp has a LarMar master volume on it, the other does not.

[fusionbear]

That sounds great. My only disagreement with you is to switch it on the fly. You risk arcing inside the switch that could be catastrophic. I always tell my clients, power down completely, wait 30 seconds, switch, and then restore power. YMMV...

[RockinRocket]

When you replace the resistor with a choke I thought the voltage go up? Wouldn't that account for the more headroom/tighter?

[Colossal]

When you replace the resistor with a choke I thought the voltage go up? Wouldn't that account for the more headroom/tighter?

I think Rooster just meant "choke" resistor. Not an actual choke, but replacing the 1k 25W with an 180R, the rough equivalent resistance of say a 3H choke. At least that's what I got from the post (Rooster?). Lowering the value of that resistor would certainly tighten the amp but greatly reduce the screen compression effect and maybe affect the clean to mean equal volume phenomenon (but maybe not). But a resistor affects all frequencies equally where a choke does not. A choke is going to greatly reduce the AC ripple.

[rooster]

All good comments.

First the idea of arcing within the switch. Since the 1000 ohm resistor is always in the circuit, and the switch is either adding or removing a smaller resistor in place, the worst that could happen is that the parallel (lower)resistance value would be constant. In the case where the switch became open, then the resistance would return to 1000 ohms.

Also, consider that we are talking about 400 VDC and this type switch (capable technically or not) has switched on/off this kind of voltage/current in Fender amps for over 50 years. (I love the word 'catastrophic', BTW.)

As to the value of the parallel resister, let's do the math: a 1000 ohm resistor in parallel with a 100 ohm resistor would = 91 ohms. So my parallel resistor value is easy to figure: 220 ohms.

As to the sonic effect of an actual wire wound choke vs. a 25 watt wire wound resistor, sure, there has to be a difference, but I doubt the ripple would be that much different. Ha, since nobody makes a 1000 ohm wire wound (typical style) choke we may never know.

[ToneMerc]

As to the sonic effect of an actual wire wound choke vs. a 25 watt wire wound resistor, sure, there has to be a difference, but I doubt the ripple would be that much different. Ha, since nobody makes a 1000 ohm wire wound (typical style) choke we may never know.

In the general sense let's talk apples to apples and oranges to oranges; a resistor serves as part of an RC filter and the ckoke an LC filter, you would not need a choke with a DCR of 1K to makes a comparison to the 1K RC filter, just use the equivalent LC circuit. The DCR of a choke really doesn't matter that much as it's the inductance characteristic that we are concerned about.

If the choke doesn't make much difference in filtering ripple then why did all the early amps have more expensive chokes for hum reduction when a cheaper resistor supposedly could have worked? The choke is a superior choice for filtering AC sub harmonics, especially when filter capacitor values are lower. There is no better example of tthis than some of the Marshall circuits when can be prone to ghosting.

TM

[Littlewyan]

This is true ToneMerc and when experimenting with my JTM50 I did replace the 3H choke with a resistor (330R I think) and noticed the noise level rose by a noticeable amount. However, I was using lower filtering at the time (32uf Mains, 32uF Screens) whereas the Express has 80uF Mains and 40uF Screens plus a lower HT. My Marshall had a high HT at 470V so it will require even more filtering.

I think what Rooster was experimenting with was the sag of the screens node. Obviously with a choke having a much lower DC Resistance it will have far less sag than a 1K resistor. In the case of the Express having slightly higher filtering and a lower HT he has obviously got away with the lower filtering from using a smaller resistor. I know for a fact that a 1K Resistor hasn't got anywhere near as much filtering capability as a 3H Choke. I read an article on it once, I'll try and dig it out.

[Littlewyan]

From the Aiken Site

Q: One intriguing thing about some amps is the 1K resistor used for the screen supply filter instead of an inductor. Does it really work like a small choke? How much inductance would such a resistor really have? Would this inductance be enough to significantly reduce power supply ripple (compared to e.g. a metal film resistor of the same resistance)?
A: The inductance of the resistor is negligible at the frequencies of interest in the power supply. For example, using a typical inductance of 0.04mH for a 1K wirewound resistor, you would get an inductive reactance of 0.03 ohms at 120hz. This, in conjunction with the usual 50uF filter cap, which has a reactance of 26.53 ohms at 120Hz, would give an attenuation of around -0.01dB at 120Hz (the power supply ripple is twice the 60Hz mains frequency because it is full-wave rectified).

By contrast, the 1K resistor itself, which has a reactance of 1K at 120Hz, gives an attenuation of around -32dB at 120Hz in conjunction with the 50uF capacitor, which would reduce the residual 120Hz hum by a factor of around 39 times.

Okay, you might ask, why use a choke? Well, a 6H choke would have a typical resistance of of only 100 ohms or so, so it would drop less voltage than the 1K resistor, for DC. However, for the 120Hz AC ripple riding on the DC voltage, the inductive reactance would make it look like a whopping 4.5k resistor. This 4.5K, along with the 50uF cap, would result in an attenuation of -44dB at 120Hz (ignoring the effect of the 100 ohm choke resistance, which would be negligible compared to the 4.5K reactance of the inductor, but it does help to dampen the overshoot at the corner frequency of 9.2Hz). So, by using a 6H choke, you get 12dB more attenuation of hum (a factor of 4 times less ripple) along with less voltage drop to the screens (more power output) and less preamp hum.

The only thing a large resistor buys you is screen supply sag, which you may or may not like. You can always do both - use a choke and add a largish series resistor for better ripple rejection and screen supply sag.

[rooster]

Littlewyan, thanks for the reference.

So the breakdown is this:

1K wirewound resistor = -32db of 120Hz with a 50uf cap

6H 100ohm choke = -44db of 120 Hz with a 50uf cap

Also Aikens does not address in this discussion the effect of the 1K wire wound resistors on the screens? This has to be of some impact, less ripple on the screens if nothing else, haha. And, as was pointed out by someone besides Aiken, Ken used 80uf which I will assume reduces the ripple a bit more. Bottom line, honestly, I'm not experiencing audible ripple in my Express builds, using either value resistance, 1K or 180 ohm.

Tone Merc, I'm thinking of the tweed Deluxe (or a BF Princeton Reverb with a 1K CC resistor?) with that 10K ohm (or 4.7K) carbon resistor right now. It's interesting reading the Aikens documentation, eh? And really, as to why Fender used chokes, I think the answer is that this was dependent on the amp model and cost at first, and then ultimately the need for more power. Speakers that could handle the power are in this mix somewhere too, of course. It's a grab bag, really.

[Littlewyan]

There would be less rippple with a 1K resistor than a 180Ohm resistor. However there would be less ripple with a 6H Choke 180Ohm than a 1K Resistor.

I believe the Express has 40uF on the screens and 80uF on the mains. I suspect the noise floor in my Marshall increased when I stuck a 330Ohm Resistor in place of the 3H Choke as the mains filtering was only 32uF so there was more ripple for the screens filter to get rid of and the HT was much higher.

[rooster]

"There would be less rippple with a 1K resistor than a 180 ohm resistor. However there would be less ripple with a 6H Choke @ 180 ohm than a 1K Resistor." True. And also, note that Aikens did not use an example using a 3H choke, but instead a 6H choke. Would the 1K wire wound would outperform the 3H choke? Hm.

Also, too, in my use of the 220 ohm and 1K resistors in parallel, I am not clear if the ripple is in fact different than if I used a single 180 ohm resistor? Perhaps it is based solely on the resistive value -as Aikens says - but I'm not so sure.

Also after reading the Aikens article I have momentarily considered that I could replace the 220 ohm resistor with a 220 ohm typical choke. It might be worth a try....but then I consider that I'm not hearing audible ripple.

[rooster]

double post

[martin manning]

"There would be less rippple with a 1K resistor than a 180 ohm resistor. However there would be less ripple with a 6H Choke @ 180 ohm than a 1K Resistor." True. And also, note that Aikens did not use an example using a 3H choke, but instead a 6H choke. Would the 1K wire wound would outperform the 3H choke? Hm.

No, it wouldn't. A 3H choke looks like a 2.26k resistor at 120 Hz so the ripple is halved.

Also, too, in my use of the 220 ohm and 1K resistors in parallel, I am not clear if the ripple is in fact different than if I used a single 180 ohm resistor? Perhaps it is based solely on the resistive value -as Aikens says - but I'm not so sure.

Since the inductance of the resistors is negligible, 2x negligible wouldn't make any practical difference.

[Littlewyan]

At the end of the day rooster, if you didn't notice the noise floor increasing and your happy, then thats job done in my book. I was simply pointing out that the choke is better than the resistor at filtering out AC Ripple.

[rooster]

Martin... wait a minute. Do you know what the resultant 120 Hz ripple loss would be after passing through the 3H choke? Aiken said the 6H choke 120 Hz ripple reduction was -44db. My point is, if the 1K resistor reduces the 120 Hz ripple by -32db, what would we see in reduction using a 3H choke? It would have to be less than what we see with the 6H choke, no doubt. But how much less, that was my question, with my point being it would be much closer to -32db. Do you have a number? -34db?

At the end of the article, Aiken concludes that you are basically using the 1K resistor for the sag factor. Which, when taken in context with the entire Aikens response, tells me that the impact on 120 Hz ripple - in a guitar amp - between the two devices - a choke or a 1K wire wound resistor - is not that significant. Did I misinterpret his words?