What does C7 & C13 do?

MTS module area including module modifications

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NuSkoolTone
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What does C7 & C13 do?

Post by NuSkoolTone » Thu Dec 27, 2007 11:39 am

I know they can be used to get a little more gain cookin, but is that all they do? Do they affect EQ of the module at all?

I'm guessing C7 affects V1 of the module, and C13 affects V2 of the module.

Am I on track?

Sometimes I've noticed adding or deleting caps here can make a nice difference for gain/clarity/headroom etc.

Any insight? I'd like to do these mods more intelligently and not based on trail and error.

Thanks!
This is where you write stuff no one reads anyway.

NuSkoolTone
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Post by NuSkoolTone » Mon Dec 31, 2007 11:06 am

Not ONE person has a clue about this???
This is where you write stuff no one reads anyway.

kc2eeb
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Post by kc2eeb » Mon Dec 31, 2007 6:40 pm

If they are connected to the cathode(s) of the tube(s), and are electrolytic, i.e. polarized, they are bypass caps and affect both the gain AND tone of the particular triode section. The cathode resistor in combination with the bypass cap set the gain and tonal response of the tube. The lower the cathode resistor value, the greater the gain. Lower capacitor values pass more highs than lows, .1 gives mostly treble, .68 more upper mids, 25 mfd lets most frequencies thru. The cathode resistor can be found anywhere from 820 to 10K. The lower the resistor value, the higher the gain, but the less effect the bypass cap will have in relation to frequencies "bypassed." This does not mean, "OK, lets put a 820 cathode resistor in and bypass it, and I'll have high gain nirvava." You will have high gain feedback, and an unplayable amp. Best thing to do, if you want to learn more, is get a good book on tube amp design. I'm not going to name any specific one, but if you do a little google search, you'll find plenty. Do some reading, get a grasp on how tubes operate, and you'll get much better results, even if it means you would rather not do it yourself, and have a competent tech do the work for you.

NuSkoolTone
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Post by NuSkoolTone » Wed Jan 02, 2008 9:06 am

Thanks kc2eeb,

That gives me a great starting point regarding the Gain and Tone and how the caps work.

Is there any info regarding which cap affects what? I've seen a schematic, though my skills for that are minimal.
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kc2eeb
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Modules

Post by kc2eeb » Wed Jan 02, 2008 8:40 pm

C7 affects ONE cathode of V1 and C13 affects ONE cathode of V2. From looking at the modules (I have not modified any module myself, yet) sometimes only half of one tube is used, sometimes both. Not using part of a tube in a preamp is one way of shaping the gain character of the preamp. The way each triod is coupled to the next, and whatever filtration, i.e. "tone control," is put in between, is one of the many ways tones are created. Many "clean" preamp stages do not use a bypass cap and use a relatively high value cathode resistor to maintain a flatter cleaner frequency response. You really need to get a good book regarding amp design, especially so you don't do things like allow the plate voltage to go where it's not supposed to and short out a power transformer. EXPENSIVE. PM me and I'll recommend a few.

Gee Donner
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Post by Gee Donner » Sun Jan 06, 2008 4:38 pm

Nuskool,

On one or more places in RM100 modules you'll find high pass filters and low pass filters. If you know what frequency you are looking for you can use mathematics. Of course its better to use your ear to decide what values, than to figure it out mathematically. But here is the theory behind it.

If you have a cathode resistor connected in parallel with a capacitor following formula applies.

F=1
------------- = Cut off frequency
2pi * R * C


EDIT: Using a 2K7 resistor in combination with a 0.68uF gives us:

F=1
----------------------------= 87Hz
6.28 * 2700 * .00000068


This means that in this case, all the frequencies above 87Hz will run on full gain.
Last edited by Gee Donner on Mon Jan 07, 2008 2:21 am, edited 1 time in total.

SacredGroove
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Post by SacredGroove » Mon Jan 07, 2008 12:53 am

Does .00000068 exist? Can you convert this to another value format?

Scott
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JKD
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Post by JKD » Mon Jan 07, 2008 1:35 am

SacredGroove wrote:Does .00000068 exist? Can you convert this to another value format?

Scott
0.68 x 10^-6 ? i.e. u or micro

680 x 10^-9 ? i.e. nano

SacredGroove
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Post by SacredGroove » Mon Jan 07, 2008 1:40 am

JKD wrote:
SacredGroove wrote:Does .00000068 exist? Can you convert this to another value format?

Scott
0.68 x 10^-6 ? i.e. u or micro

680 x 10^-9 ? i.e. nano
That's what I was looking for.

Thanks!
Scott
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Gee Donner
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Post by Gee Donner » Mon Jan 07, 2008 2:21 am

Yeah, Sorry I edited my instruction now. Maybe its easier to understand...

///GT

SacredGroove
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Post by SacredGroove » Mon Jan 07, 2008 8:12 am

Gee Donner wrote:Nuskool,

On one or more places in RM100 modules you'll find high pass filters and low pass filters. If you know what frequency you are looking for you can use mathematics. Of course its better to use your ear to decide what values, than to figure it out mathematically. But here is the theory behind it.

If you have a cathode resistor connected in parallel with a capacitor following formula applies.

F=1
------------- = Cut off frequency
2pi * R * C


EDIT: Using a 2K7 resistor in combination with a 0.68uF gives us:

F=1
----------------------------= 87Hz
6.28 * 2700 * .00000068


This means that in this case, all the frequencies above 87Hz will run on full gain.
Is this partly the reason Ultra-UltraXL have more gain?

F=1
-----------------------= 59Hz
6.28 * 2700(R16) * .000001(C6)

Scott
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Gee Donner
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Post by Gee Donner » Tue Jan 08, 2008 1:46 am

By putting capacitors in parallel with the cathode resistors you do boost the gain, but only in different frequency ranges. That causes more gain. So it contributes a little to the amount of gain. But the main reason why the ultra has so much gain is a factor of how the circuit is designed and how much impedance between each gain stages it has.

///GT

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