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Author Topic: Low Gain in the Tube settings only, SS is fine  (Read 9504 times)

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MarshallJMP

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Re: Low Gain in the Tube settings only, SS is fine
« Reply #15 on: Time Format »

Ah nice,you fixed it,must say i only had this happen once and a long time ago that one of the anode resistors failed.
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rnolan

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Re: Low Gain in the Tube settings only, SS is fine
« Reply #16 on: Time Format »

Hey MJMP, I know your generally a man of few words (antithesis of me LoL and my long raves and diatribes). So R2 on the tube board failed (and this is very rare), it's an Anode resistor (= resistor in the Anode part of the tube circuit (to ensure the correct voltage is applied to the Anode in the tube ??)), so (if your up for it, I'm interested to learn more) could you please explain what that meant for the circuit ?
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MarshallJMP

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Re: Low Gain in the Tube settings only, SS is fine
« Reply #17 on: Time Format »

It's rare in a MP-1 but it can happen,but it happends more in tube heads,usually with low wattage resistors they use now and I see it a lot in older marshall's and fenders where they use carbon composite and these tend to go up in value after years of use.Last i worked on a 76 JMP and one shifted from 100k to almost 500k. :o

So if the Ra (anode resistor) goes up in value the current will drop and the gain will increase.But this will give less linearity and a higher output impedance,so for ECC83/12ax7 tubes the standard is 100k and 220k,higher then this and current will be to low and the sound becommes fizzy and will not be good anymore.You can see this when you start with drawning load lines (see pic).It's just ohms law to draw it,take the Va voltage and devide it with Ra.This will give you the max current.As you can see the 47k line crosses the lines where they are very straight (so lineair,BTW this is from the ECC83 datasheet Ia vs Va).
100k is lower but as you can see the lines it crosses isn't so lineair anymore and it's even worse for 220k.And from there you can choose the bias point so you can calculate Rk etc.... waaay to much to explain.Works for preamp and power tubes.But it can also be done with just plain formulas and calculations.Both will work but using the datasheet way (it's more geometry) isn't so easy.But i'm sure there's a lot of info about it on the net.
For instance here's an example on how to calculate a design.

http://www.aikenamps.com/index.php/designing-common-cathode-triode-amplifiers
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MarshallJMP

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Re: Low Gain in the Tube settings only, SS is fine
« Reply #18 on: Time Format »

http://www.ax84.com/static/p1x/p1-ex-theory.pdf

Here is a very good pdf on tube design.
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rnolan

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Re: Low Gain in the Tube settings only, SS is fine
« Reply #19 on: Time Format »

Hey MJMP, thanks my friend, very edifying  :thumb-up: I have a little reading to do LoL
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MarshallJMP

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Re: Low Gain in the Tube settings only, SS is fine
« Reply #20 on: Time Format »

No problem :thumb-up:
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