Choke for that Peavey Windsor?

[bobloblaws]

I recently completed the "Doug Hammond" mod for the Peavey Windsor which more or less "Marshallizes" it.

https://www.aronnelson.com/gallery/m...H/amp/windsor/

I did the basic preamp and phase inverter mods but as of yet I have not installed the recommended Hammond 194 C choke (40H, 50mA DC), although I have purchased one. I'm unsure whether I should go ahead and install a choke at all or whether this one is ideal or not. I read this article by Randall Aiken (https://www.aikenamps.com/index.php/chokes-explained) and I can't say I understand it completely, but it's left me wondering if 40H is too high inductance wise and whether one of the 100UF reservoir caps should be removed if the choke goes in. Also, is there an expected difference in tone or feel with the choke or is it only for filtering/hum reduction?

- Bobby

[Enzo]

It is just a guitar amp, don't overthink it. And also, it is not a hifi, so many things the audiophile community thinks are important really don't apply.'

You have the amp, and presumablyit now works? Play it, how does it sound? WHy would you not try the "new" amp out before throwing another mod at it?

As to the choke, what do you think it will accomplish? I mean what is your purpose in installing a choke? A choke is a power supply filter, OK, but is there a ripple problem? At the first node, the center tap of the output transformer, a few volts of ripple won't matter as they cancel. By the time we get to the second node - the screen node - there is darn little ripple left.

WHy would you want to reduce a filter cap there? Your link replaces the resistor with the choke. This means the voltages downstream will be higher. Higher voltages on the screens and higher in teh preamp. Is that desirable to you?

My own personal opinion is that chokes are largely pointless. I mean they work, they function as you'd expect, but what they replace is almost always already working fine.

Others may disagree.

[bobloblaws]

It is just a guitar amp, don't overthink it. And also, it is not a hifi, so many things the audiophile community thinks are important really don't apply.'

You have the amp, and presumablyit now works? Play it, how does it sound? WHy would you not try the "new" amp out before throwing another mod at it?

Yup, it works, and it sounds OK, better than when it was just a Windsor, not exactly like a Marshall, to be expected of course. So yes, I have tried it.

As to the choke, what do you think it will accomplish? I mean what is your purpose in installing a choke?

Maybe you are asking strictly rhetorically, but in any case my answer is I ordered the choke along with the other parts I didn't have in stock purely based on the documentation for the mods in question. Since then I came across a forum post where someone did these mods and put in a 15H choke saying he thought 40 was too high but he didn't elaborate.

WHy would you want to reduce a filter cap there?

I asked because there is in effect a 200 uF filter cap there which is twice that which Marshall uses and what I'm asking is whether there is such a thing as too much filtering. Intuitively the answer would be no, the quieter the better, but sometimes things are counter-intuitive. Also, Aiken says "Typically, 5-20 Henries is a good choice with the standard 32-50uF electrolytic capacitors." So he seems to suggest the filter cap value is a consideration when choosing inductance rating and I'm guessing vice versa.

Your link replaces the resistor with the choke. This means the voltages downstream will be higher. Higher voltages on the screens and higher in teh preamp. Is that desirable to you?

I don't know. Since I have the choke already it can't hurt to try it and see how it affects the downstream voltages and whether it sounds subjectively any different.

[Enzo]

No, won't hurt to try it.

200uf, nothing wrong with that. The silicon rectifiers will not be stressed. I notice your guy doesn't mention an expected benefit from the choke either.

[bobloblaws]

Another thing I'm curious about, Aiken implies that the series resistor after the rectifier has a filtering property in addition to voltage dropping. Is this correct? I always assumed that resistor was strictly for voltage dropping so that the screen voltage was slightly lower than the plate voltage.

[Enzo]

The resistor doesn't filter, strictly speaking, but it separates the nodes, so two caps with a resistor between filter better than two caps in parallel and no resistor.

[Helmholtz]

Another thing I'm curious about, Aiken implies that the series resistor after the rectifier has a filtering property in addition to voltage dropping. Is this correct? I always assumed that resistor was strictly for voltage dropping so that the screen voltage was slightly lower than the plate voltage.

Aiken is right. A capacitor by itself is no filter. A filter needs at least 2 components. The typical tube amp power supply arrangement contains a cascaded chain of RC or LC low pass filters. The filter effectiveness (meaning ripple attenuation) of an RC filter depends on the product R*C. So increasing the series resistance and/or capacitance improves filtering.
Replacing the series R by a choke is a way to further improve filtering, as an LC filter is a second order filter.
The DC voltage drop caused by a choke solely depends on its DCR.

The filter cap values (actually the RC or LC time constants) influence the dynamic behaviour of the amp. For my playing style I prefer relatively low value filter caps, which support a little compression. Higher capacitance makes the amp respond stiffer.

[Helmholtz]

the recommended Hammond 194 C choke (40H, 50mA DC)

Actually I have never seen a 40H choke in a Marshall. My two 100W plexis from 1968 and 1969 both use 5H/ ~100R chokes and the later 70s versions had Drake chokes measuring 5H/106R as well. Not sure about later JCM800s though.
So you might consider to get this: https://www.hammfg.com/files/parts/pdf/194F.pdf

The 40H choke will give better filtering with lower cap value and its higher DCR (420R) will drop around 6V more.

[Helmholtz]

BTW, it is no good idea to wire a (standby) switch in series with an inductor (choke) carrying DC. Opening the switch will induce a high flyback voltage and cause arcing. If you want to leave the position of the switch you should wire a flyback diode across the choke. Diode orientation: Cathode to input, anode to switch side.

[bobloblaws]

Actually I have never seen a 40H choke in a Marshall. My two 100W plexis from 1968 and 1969 both use 5H/ ~100R chokes and the later 70s versions had Drake chokes measuring 5H/106R as well. Not sure about later JCM800s though.
So you might consider to get this: https://www.hammfg.com/files/parts/pdf/194F.pdf

The 40H choke will give better filtering with lower cap value and its higher DCR (420R) will drop around 6V more.

So my takeaway is that with a choke I get additional filtering without the additional sag or compression that would result from a higher cap value. Did I get that right?

From Aiken "In general, the larger the better, within reason (larger inductances at low DC resistances mean larger chokes, which cost more money)." Since I've already spent the money, theoretically there would be no downside to installing it, with the exception of a potentially undesirable voltage increase, right? Is that 6V calculation based solely on the 20R increase between the existing resistor and the choke? I can't figure out where 6V came from in terms of ohms law.

How did you calculate the DCR for that choke? I don't see it listed as a specification anywhere and it does in fact show as 422R on my meter.

I also don't understand what the 50 mA specification actually represents. Obviously it's not a max current rating. Is it expected current given some theoretical condition? Please forgive my ignorance, I have no formal education in this stuff.

[Enzo]

I can't figure out where 6V came from in terms of ohms law.

Expected current through its 420 ohm resistance.compared to existing resistor, I'd assume.

[Helmholtz]

So my takeaway is that with a choke I get additional filtering without the additional sag

The choke replaces a series resistor. For the same filtering effect as the choke the resistor value would have to be so large (around 30k with the 40H choke) that you would lose all of your supply voltage by the voltage drop.

I can't figure out where 6V came from in terms of ohms law.

The 40H choke has a DCR of 420R (from the Hammond datasheet), so around 300R more than the 5H choke. The DC current through the choke should be roughly roughly 20mA ( preamp +screens). So around 6V additional drop.

The 50mA rating of the choke is the max DC current the choke can handle without overheating and without saturating. When saturation starts, inductance drops.

[Enzo]

I'd think it valuable to consider the context. A choke adds filtering action, in the same sense so does adding caps. SO in this amp, how much does filtration increase by changing resistor to choke? I mean we have a 200uf reservoir cap, would making that 400uf really reduce hum very much? I am making it a practical question rather than theoretical.

[Helmholtz]

I am making it a practical question rather than theoretical.

Sounds more like a rhetorical question to me .

I think we both agree that the amp doesn't need additional choke filtering with that high value filter caps.
OTOH, myself I would never use that much capacitance when cloning a Marshall (probably would go with 50µF). Some years ago I did extensive listening tests with one of my old Marshalls, adding additional capacitance in small steps. I didn't like any increase at all.

[bobloblaws]

The 40H choke has a DCR of 420R (from the Hammond datasheet),.

Oh crap, I see it now on the data sheet, I need glasses.

The choke replaces a series resistor. For the same filtering effect as the choke the resistor value would have to be so large (around 30k with the 40H choke) that you would lose all of your supply voltage by the voltage drop

Right, but I was talking about trading off between choke and capacitance for more filtering, not choke and resistance.

The 40H choke has a DCR of 420R (from the Hammond datasheet), so around 300R more than the 5H choke. The DC current through the choke should be roughly roughly 20mA ( preamp +screens). So around 6V additional drop.

Ok, I didn't realize you were talking in terms of the difference between 2 different chokes. 300R x .020A = 6V
I was thinking status quo vs installing the choke that I bought (420R). Which works out to virtually no change in voltage using 20mA as nominal current.

The 50mA max rating makes more sense to me now, I was thinking about it incorrectly and assuming a much larger current going to the preamp and screens.

Thanks, gentlemen, for enlightening me.