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Adding a choke to an amp?

 
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azrael



Joined: 29 Jun 2009
Posts: 40

PostPosted: Thu Apr 01, 2010 12:36 am    Post subject: Adding a choke to an amp? Reply with quote

here's what was said in another forum about adding a choke:
Quote:
I once asked Peavey why they did not use a choke in their tube amplifiers - the reply was that it was cheaper to use a high wattage resistor and maximized their profit margin.
Sad really, because the choke plays a MAJOR influence in the clarity of a tube amplifier - they are literally shooting themselves in the foot by this cost cutting.
The old timers (a lot older than me - lol) always knew the value of the choke - it's ability to completely block any AC ripple on the supply line. In fact if one looks at the HiFi designs of the early 40s and 50s it is quite commonly used as an input filter directly after the tube rectifier. When used like this it is a considerably larger beast to cope with the plate load supply of the output tubes.
Vintage tube guitar amplifier designs are a compromise - the output tube plates are connected to the power supply before the series choke filter - this allows a much smaller size choke to be utilised.

Consider this - an amplifier with no choke filter will always supply a small AC ripple voltage down through the series resistor droppers right to the plates of the input stage tubes. With modern amplifiers the electrolytic filter capacitors hanging off this chain are considerably larger than they were 50 years ago and at low signal levels attenuate this AC ripple to almost zero - BUT when the amplifier is required to respond very quickly to a sudden transient ( when "biting down hard" on the guitar for example) these capacitors are soon emptied of charge and the AC ripple increases rapidly. This AC ripple voltage is then being fed directly to the input stage and modulates the signal which is consequently amplified through each stage until when the signal and modulated ripple voltage reach the output tubes.



Is this true? I'm considering adding a choke to my Peavey VTM.

Will it yield worthwhile tonal improvements?
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Firestorm



Joined: 25 Jan 2008
Posts: 2609
Location: Connecticut

PostPosted: Thu Apr 01, 2010 4:18 am    Post subject: Reply with quote

It's kind of an overstatement: sounds like it was written by the president of the Choke Manufacturing Workers of America. Chokes don't actually suppress all AC ripple. They do smooth it a lot, but so does a string or resistors and caps. The "old timers" used choke input filters because the power grid of the day was pretty unstable and a choke provided really good voltage regulation. If you used an input choke in a Peavey VTM, it would be bigger than your OT and would cut your voltage almost in half. If you put one in the conventional guitar amp location between the plate supply and the screen supply, you will get more stable screen voltage -- a good thing, but I'm not sure you'd hear the difference. It could also help prevent the tendency of loud transients to deplete the preamp supply. The problem with that isn't really that AC ripple suddenly appears at the preamps, it's that the voltage drop creates a little compression, which some people like and some people don't. You can cure that without a choke: just stick a diode in the dropping string to isolate the preamps from the power amp. Caveat: if you do that, the bleeder resistors (if you have them) will no longer drain the preamp caps. Don't get zapped.
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azrael



Joined: 29 Jun 2009
Posts: 40

PostPosted: Thu Apr 01, 2010 5:03 am    Post subject: Reply with quote

Okay, cool.

I was planning using a Hammond JCM800 choke, fwiw, since the VTM is a JCM800 copy.

That quote was taken from a thread where someone added a choke to a Peavey Windsor. He also upgraded the output transformer. He reported a noticeable improvement in tone, so I wanted to see what other modders and builders thought before going forward with this mod haha.

So not worthwhile?

How do I look for bleeder resistors, btw? I ask because when I went to drain the caps, it didn't seem that I had any stored voltage there.
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Firestorm



Joined: 25 Jan 2008
Posts: 2609
Location: Connecticut

PostPosted: Thu Apr 01, 2010 1:40 pm    Post subject: Reply with quote

Worthwhile or not? Hard to tell; I've never A/Bed the same amp with and without a choke. In the case of the posted quote, you say the guy also upgraded his OT. THAT can make a huge difference and if Peavey couldn't afford a choke, they likely couldn't afford good output iron. Go ahead and do both if you want. It sure won't hurt, but the new iron will be worth almost as much as the rest of the amp.

Um, if you don't know how to find your bleeder resistors, you want to be REALLY CAREFUL doing anything inside the amp. There may be electricity stored in there than can kill you dead.
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iknowjohnny



Joined: 24 Apr 2008
Posts: 1034
Location: los angeles

PostPosted: Thu Apr 01, 2010 2:15 pm    Post subject: Reply with quote

I've tried choke vs resistor many times, and in some amps it doesn't seem to matter. In a marshall design it does, but i wouldn't necessarily say it's better. It is to me, tho not as huge thing. Mostly feel. But i think as cost cutting measures go it's not all that bad considering the savings compared to many other cost cutting things. Most manufacturers cost cut things that compromise an amps ability to remain working properly for more than a few months or a year in order to save pennies. A choke could actually save them a few dollars with no compromise to the amp's reliability. Theres a marshall w/o a choke and i like it more than many mashalls WITH.
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Andy Le Blanc



Joined: 22 Dec 2007
Posts: 2418
Location: central Maine

PostPosted: Thu Apr 01, 2010 3:53 pm    Post subject: Reply with quote

A Choke input filter will change the power supply too much.
The regulation comes at the cost of voltage, if the amp wasn't designed for it
don't do it.... Adding a choke after the first filter cap is ok, but make sure the
DC resistance isn't too far off from the resistor you're replacing, say 20%.
Or at least be aware of what the resistance is and that it will change the PS voltages.
don't forget to think about how your gonna mount the extra trann'y before
you jump into it too.

And again be SAFE, it is the powers supply your getting into.

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azrael



Joined: 29 Jun 2009
Posts: 40

PostPosted: Thu Apr 01, 2010 4:20 pm    Post subject: Reply with quote

Firestorm: I just started modding amps, so I'm not all that familiar with every bit of tube amp circuitry. It just seemed that I didn't have much voltage stored up, as measuring the caps didn't show significant charges. I still drained them with a clip + 100K resistor grounded to the chassis, of course. One cap did read some voltage, maybe 50V.

While that guy replaced both output transformer and choke, I have read other reports where people just added the choke, and they seemed happy with the results. A choke is only like 30 bucks, but still, I'd only do it if it yielded benefits, haha.
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tubeswell



Joined: 27 Mar 2008
Posts: 1643
Location: Wellington. NZ

PostPosted: Thu Apr 01, 2010 4:39 pm    Post subject: Reply with quote

+1 what the others have said about a choke input filter needing to be real big, and stealing your power rail voltage.

On the other hand, a CLC filter (note this is not the same thing as a choke input filter - rather it is a capacitor input filter with a choke) between your plate and screen nodes will raise the B+ voltages in the pre-amp slightly - because of the low DC resistance in the choke. This will make the power rail a bit cleaner overall (because of the higher HT and greater headroom in each pre-amp stage). This will also take the screens to within a volt or so of the plate voltages, and if your amp doesn't already have screen grid resistors, then you will want them if you change to a CLC filter. And the higher pre-amp and screen voltages can make the amp sound noticeably 'chimier', so you might want to up the supply resistors for the pre-amp to lose a bit of the chime - a matter of personal taste.

It will also take any residual ripple right out of the power rail. You might not notice much less hum in a PP amp (because most of the ripple riding on the signal gets cancelled out in the opposite sides of the OT primary anyway), but it makes a noticeable difference to the general humminess of SE amps.

The choke has to be sized to cope with the expected maximum current draw at that point in the power rail. If your CLC filter is between the plate and screen nodes, it doesn't need to be huge, and a small 50mA 3-5H choke will work in most amps that have 2 or 3 pre amp tubes and a couple of 6L6s. If the CLC filter is placed between the rectifier and the plate nodes (like in a tweed Super or Pro or Bandmaster), then the choke needs to be rated at least the same current draw as the PT's High Tension winding.
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Firestorm



Joined: 25 Jan 2008
Posts: 2609
Location: Connecticut

PostPosted: Thu Apr 01, 2010 6:06 pm    Post subject: Reply with quote

I think the stock JCM 800 choke is 40H at 50mA -- sort of overkill in terms of inductance since the Plexis only used something around 7H (and Fender around 4H).
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Badside



Joined: 24 Mar 2010
Posts: 59
Location: Montreal, Canada

PostPosted: Thu Apr 01, 2010 7:37 pm    Post subject: Reply with quote

Firestorm wrote:
I think the stock JCM 800 choke is 40H at 50mA -- sort of overkill in terms of inductance since the Plexis only used something around 7H (and Fender around 4H).


Is that really what it was?

I know that's what Hammond replacement choke is. That seemed overkill so I bought a 7H part instead (159Q I think) from their generic lines.

Same thing when I built an AC30, their replacement was 30H and pretty hefty. I bought a cheaper 15H part.

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Firestorm



Joined: 25 Jan 2008
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Location: Connecticut

PostPosted: Thu Apr 01, 2010 8:01 pm    Post subject: Reply with quote

It might have been the Hammond specs I was thinking of. Some of the originals apparently had Drakes and some had Dagnalls so the aftermarket is confused. Even MM doesn't seem to have one consistent "clone" value. Goes to show there's a lot of wiggle room with a choke.
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Badside



Joined: 24 Mar 2010
Posts: 59
Location: Montreal, Canada

PostPosted: Thu Apr 01, 2010 9:06 pm    Post subject: Reply with quote

Firestorm wrote:
It might have been the Hammond specs I was thinking of. Some of the originals apparently had Drakes and some had Dagnalls so the aftermarket is confused. Even MM doesn't seem to have one consistent "clone" value. Goes to show there's a lot of wiggle room with a choke.


When I built my JCM800 2204, depending on the source it was anything from 5 to 10, so I figured 7 would be good. Also, the one I used has ridiculous current handling (150mA) and really low DC resistance (110ohm I think). I have stupidly high screen voltage, that thing is loud Very Happy

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