Choke question for Rocket - Will an AC30 choke work?

[fzfwyv]

Have a question regarding the choke for the Rocket. I have a MM Vox choke that is 20H -- 100mA. How much of a difference is it from the Rocket Hammond choke that is 12H -- 25 mA?

Sorry, but this is showing my ignorance because I'm not sure if that is a big difference, a little difference, and how what effect it would have on the amps tone and overall feel.

Thanks for any help!

-Rich

[Allynmey]

Rich, a 158L is the correct specs. 15H 75mA

[karczochrulez]

I have a choke: 8,1H 140mA 173 ohm
Will be enough to rocket?

[fzfwyv]

Thanks for the correction...I just checked price on this (I wish all components were this reasonable!)

-Rich

[FYL]

Have a question regarding the choke for the Rocket. I have a MM Vox choke that is 20H -- 100mA. How much of a difference is it from the Rocket Hammond choke that is 12H -- 25 mA?

KF used a Hammond 158L spec'ed at 15H, 75mA and 411R, the MM choke you mention can perfectly be used but why pay more?

The Rocket is a Vox AC-30 TB à la Trainwreck, JMI/Vox spec'ed "10-20 HY 100MA" for CH1, most app. 10H chokes will be fine.

[rooster]

FYI makes a good point. What I don't know is the DC resistance of the MM choke you have in hand. They don't list this spec on there site either. But FYI posted what the Hammond piece was (411 ohms) and if the MM piece is very close to this then I agree with him, use it.

My point being, I think KF really had a feel/plan for the voltage drop here, OT vs. screens, and I would probably honor it if you are trying to replicate something KF.

[FYL]

What I don't know is the DC resistance of the MM choke you have in hand. They don't list this spec on there site either.

Lemme see what I've got in my notebook. Here we are : MC10H 11,35H @ 100 Hz, 287R DC, JMI30C, 18,56H, 412R. Only one sample of each measured, so YMMV.

[RJ Guitars]

Rich,

When we did the Group Build for the Rocket amp, I researched chokes quite a bit. The VOX AC-30 choke is a 15 Henry 100mA choke. The Hammond 158L is 15 Henry 75mA choke.

I talked with Phil over at Heyboer for quite a while about chokes for the Rocket amp. His bottom line was that it was kinda idiot proof to wind them and he firmly believed that you would never hear a difference if you got anything close. This kinda validates the thoughts that anything that will handle the current and is rated between 8 and 20 Henries will work just fine.

Gerald Weber published almost exactly the same line in his first book. Randall Aiken has a pretty good write up on chokes and how they are used in guitar amp power supplies. Worth reading just because the guy is pretty sharp and knows his stuff.

The Hammond 158L is cheap and has the proven record. To me it doesn't look like what was used in the "Malou" Rocket, but I could be wrong and if any of these guys know their stuff, we'd never be able to tell anyway.

rj

[rooster]

FYL - (Umm, sorry about last letter there, BTW.) You seem to be a good guy to know if you want some important data, thanks and good to know. So you are saying that the MM choke has a DC resistance of 287 ohms, right? Well, in a world of percentages that's a difference of at least 25%. I am thinking this is signifigant but then I have only used the Hammond piece, I haven't used the MM version.

RJ makes a valid point from my experience regarding chokes in general, but then the DC resistance value issue doesn't come up? RJ, do you see the resistance value of the MM choke being a deal breaker in this build? If not, at what percentage difference would you say that it would? Kinda curious about your take on this, BTW.

[FYL]

To me it doesn't look like what was used in the "Malou" Rocket, but I could be wrong and if any of these guys know their stuff, we'd never be able to tell anyway.

The published pix of Malou show some kind of printing/markings on the bobbin (Hammond uses a sticker on top). Could be a Triad Magnetics C-series choke. Or a Stancor. Or...

Anyway, the Hammond 158L is perfect in a Rocket-style amp.

[FYL]

So you are saying that the MM choke has a DC resistance of 287 ohms, right?

The MC10H, yes.

Well, in a world of percentages that's a difference of at least 25%. I am thinking this is signifigant

The lower R choke will drop slightly less voltage. The PS will show a hgiher initial spike and will be less well damped.

[Zippy]

My eyes, hair, and mind are fading, help me recollect...

Was it Tubedogsmith that did one of the earlier Rocket builds - back before we knew as much as we do know? I recall his build as using a resistor in lieu of a choke and the reports were that it played quite well.

What does one hope to garner by using a choke in this build? More bass response? Less noise??

Playing RJ's Rocket, I hear plenty of bass - maybe more than one needs. Gain is lower than the rest of the 'Wreck family so using a choke to lower the noise floor probably isn't necessary either.

I corresponded with a ol' Vox factory guy some years ago who shared his thoughts on the matter: Use a resistor of value somewhere between 300 and 1000 ohms and tweak to taste regarding how much sag you want.

So, why do we need a choke in this application?

[tubeswell]

A choke is really good for cancelling residual ripple in the B+ (thereby avoiding undesirable hum). In most amps it is part of the filter in the B+ rail before the screen/pre-amp supply - where you only need a small (say 40mA) choke. (In most amps, the combined current draw of screens and pre-amp tubes is not high).

Also the resistance of the particular choke isn't critical to its function to filter out ripple - chokes drop less voltage (than any resistor that is big enough to work sufficiently in a filter), so if you want to keep the screens under the plate voltages you need to add more resistance in front of the screens (e.g.; screen grid resistors). However, if you use a (say) 1k-5k resistor instead (which you will need to do to take the ripple out - and even then it won't be as good as a choke at doing this), you will drop more voltage (and consequently have less to play with for the screen and plate supplies). What's more, if you use a resistor you will get more sag. The effect of a choke is to stiffen up the power supply more than a resistor.

The more-critical aspect of chokes is their inductance. The higher the inductance, the more effective the choke will be at cancelling out any hum caused by power supply ripple. Way back in the good-ole bad-ole days, chokes used to be used more at the 'front end' of the power supply (i.e. between the rectifier and the plate supply - look at the 5E7), where they need to be big enough to deal with plate current as well. But (arguably) the inductance of the OT primary is sufficient to remove residual ripple in the B+ that might otherwise affect the plates, and in PP amps hum is cancelled out anyway (due to the opposing swings on each side of the output stage).

[fzfwyv]

I measured the Merc Vox choke and its 466 ohms. I asked originally because I had the choke already from a previous stock up of parts when I was on an AC30/4 building kick.

Thanks for the info and discussion, it's helping me understand all the nuances!

-Rich

[Macshaft]

I used the Hammond 194E @ 30H . Amp sounds fantastic

Cam