Need help selecting choke size please

[leftu2]

Hey fellas, it's the peavey vtm 120 modder again....lol.
I think I want to try a choke in place of the resistor in this amp but not sure what choke I need.
Here's a link to the schematic:
https://www.thetubestore.com/peavey-schematics

P.S. last time I tried to post a pdf of this schematic folks said they had issues try to see it (blurry)

[shooter]

i'm painting so;


here's an oldey that Marshall used a choke in, quick look says it should be close, it's T3 in the schematic, maybe surf up some data 


https://el34world.com/charts/Schematics/files/Marshall/Marshall_1959_jcm800_super_lead_power_amp_100watt_year_1988.gif

[jjasilli]

Plagiarism by way of similar amp is one way.  To spec a choke the factors are: i) current handling; ii) DC resistance; and iii) Henries.  Presumably this is the choke to the screens.  The only current that matters is screen current @ max signal.  You can get this online from the tube charts for your power tubes.  Note whether the chart is giving you current draw info for 1 or for 2 tubes.  Then apply that to your number of tubes.

(Power tube plate current doesn't matter, because this choke will not be feeding the plates.  Downstream in the B+ line, preamp tube current draw doesn't matter either, because it's too small - not even a rounding error.)

Generally, DC resistance should not exceed about 100 Ohms or so.  But some people use a choke with a B+ dropping resistor, and/or screen resistors.  In that case a choke with more DC resistance could be used.  Note that the choke is feeding all the screens, so the amount of the choke's stated (or measured) DC resistance will, in effect, be multiplied by the number of screens (Ohm's Law).

The amount of Henries delves into the art of design.  Fender uses a few Henries; Marshal uses about 20 Henries.

[sluckey]

Note that the choke is feeding all the screens, so the amount of the choke's stated (or measured) DC resistance will, in effect, be multiplied by the number of screens (Ohm's Law)

Say what? 

[tubeswell]

what shooter said


http://www.classictone.net/40-18058.html


https://www.amplifiedparts.com/products/choke-hammond-replacement-marshall-amps


etc

[shooter]

 
to be fair JJ did say to plagiarize, which is the best i could do after 2hrs @100F on a ladder   

[leftu2]

Thanks guys,

The amount of Henries delves into the art of design.

Am I right in that the henrie # is the amount of filtering you get with 3 being less and 20 being more?
The one's tubewell suggested were 3H and the jjasilli said marshall used around 20H.
Will higher filtering make the amp stiffer? Just wondering what the affects would be lower vs higher....

[sluckey]

Am I right in that the henrie # is the amount of filtering you get with 3 being less and 20 being more?
 
Will higher filtering make the amp stiffer?

yes and yes

[leftu2]

Am I right in that the henrie # is the amount of filtering you get with 3 being less and 20 being more?
 
Will higher filtering make the amp stiffer?

yes and yes

On the ones tubewell suggested  , i can change just the henrie # as long as the other specs stay the same, correct?

[shooter]

yep
current is a don't change Down parameter, the bigger the Henry the more opposition to fast current change

[leftu2]

Cool, thanks guys

[Williamblake]

If filtering is the goal, why not use two chokes less than half the size and have even better filtering? I know, nobody does that ...

[tubeswell]

Thanks guys,

The amount of Henries delves into the art of design.

Am I right in that the henrie # is the amount of filtering you get with 3 being less and 20 being more?
The one's tubewell suggested were 3H and the jjasilli said marshall used around 20H.
Will higher filtering make the amp stiffer? Just wondering what the affects would be lower vs higher....

Yes - 20H will resist changes in current at higher frequencies more than 3H.

However, in a CLC filter, the smoothing also depends on the capacitance.

In geetar-amp speak, you can get the practically the same result with a 3H choke if you have higher capacitance in the filter.

See formula about 1/2-way down the page in this link: http://www.valvewizard.co.uk/smoothing.html