Module riser board cap.

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JKD
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Module riser board cap.

Post by JKD » Tue May 31, 2011 1:24 pm

All of this is from memory of when I modded my Egnater COD module so I'm sure someone will correct me if I got it wrong :-)

Not sure if this has been posted but if your module has a cap on the tube riser board.

It's an easy mod to change one of these to brighten or darken the gain in the later stages of the module. The cap is labelled c1.

Higher cap value = darker
Lower cap value = brighter

(I'm fairly sure the original was 1000pf and I doubled this to whatever is in there now.)

I've yet to go back in and clip the leads/install the cap properly in my module from when I experimented with values:

Image
Last edited by JKD on Wed Jun 01, 2011 1:57 pm, edited 1 time in total.
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Post by okstrat » Wed Jun 01, 2011 1:53 pm

That's not a coupling cap, it's in parallel with a plate resistor. Look at an old Mesa boogie amp, they put caps in parallel with plate resistors all the time to cut high frequencies because without them an old boogie would oscillate all over the place. Larger the cap, more high end attenuation there is.

Pete

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Post by JKD » Wed Jun 01, 2011 1:56 pm

Thanks Pete. :-)
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Post by okstrat » Wed Jun 01, 2011 2:00 pm

JKD wrote:Thanks Pete. :-)
NP. You can also do it with the other plate resistors on the module too if you want to pull a little hiss out or stabilize a module that is too gainy.

Pete

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Post by Shinozoku » Wed Jun 01, 2011 3:05 pm

So that resistor above it can have the value tweaked to tame gain/hiss without making anything kersplode?
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Post by okstrat » Wed Jun 01, 2011 3:18 pm

Shinozoku wrote:So that resistor above it can have the value tweaked to tame gain/hiss without making anything kersplode?
cap, not the resistor. As with anything else, don't go changing stuff randomly without knowing what it is.

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Post by Shinozoku » Wed Jun 01, 2011 3:29 pm

Ah, I just saw you say "you can do it with the other plate resistors on the module too" and took it as you saying you could change the resistor on the riser board (which appeared to be what the cap was in parallel with, though I don't have a module to look at right now).

So just play with that cap? What about adding one to modules that don't have one?
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Post by SacredGroove » Wed Jun 01, 2011 6:29 pm

Coupling caps come after the plate resistor, in series. Among other functions, the coupling cap blocks DC from entering into the next tube stage. Depending on cap value, inherently, the low end will be increased. The higher the value, the more low end. :wink:
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Post by Jaded Faith » Thu Jun 02, 2011 3:41 pm

Typical values are 100pF to 1000pF. Most modules with a cap there stock are around 100pF. Around 500pF will be a pretty big jump and 1000pF would be way smooth. That 2200pF in JKD's pic must be as warm and snuggly as a winter fireplace..... :lol:
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Post by JKD » Thu Jun 02, 2011 3:58 pm

Jaded Faith wrote:Typical values are 100pF to 1000pF. Most modules with a cap there stock are around 100pF. Around 500pF will be a pretty big jump and 1000pF would be way smooth. That 2200pF in JKD's pic must be as warm and snuggly as a winter fireplace..... :lol:
Honestly can't recall now if the stock was 500 or 1000pf now, I'm thinking 1000 but the COD module is very bright sounding one stock.

I loved the change.
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Module Riser Board Cap

Post by Graham Pearson » Thu Jun 02, 2011 4:05 pm

Only 2 of my stock modules have these capacitors. I can't read the value on my JTM and my 1086 has a 100p.

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Post by Jaded Faith » Thu Jun 02, 2011 4:23 pm

Good call. I didn't know it was a COD. I know they were quite bright and prone to oscillation.
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