Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 12AX7 on January 05, 2013, 02:54:19 pm
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I had the first 2 gain stages configured with 100k plates and 1.5k cathode resistors for the longest time. Usually small marshall values on the bypass caps. The gain available was typical classic rock amount like that of say a peavey classic 30's gain channel. I would use it with gain set somewhere around 1-2 oclock and hit the front with a clean boost for sustain for days tone. Always worked great. Recently however i tried 2.2k cathode resistors on both V1 and 2 in place of the 1.5k's, partially because i wanted a bit less gain and also because when i tried it the tone because more to my liking. But what i didn't count on and makes no sense to me from a gain standpoint is that with the lesser gain of the 2.2k resistors i was actually getting the beginnings of blocking distortion when i use the clean boost. Can someone explain to me why this is? And note that without changing anything else including settings this happens with the 2.2's vs the 1.5's.
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I think that interaction between your cathode bypass cap and 2.2 k resistor results in more bass being passed than with a 1.5k. I would be curious to hear how the next size smaller cap affects the blocking distortion. Or better yet, try it with no bypass cap on the second stage.
--Craig
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But what i didn't count on and makes no sense to me from a gain standpoint is that with the lesser gain of the 2.2k resistors i was actually getting the beginnings of blocking distortion when i use the clean boost. Can someone explain to me why this is?
A boost pedal? If the output of the pedal is big enough, it can drive any input stage into distortion.
How can you know if it's the input stage (or 2) distorting, or if it's a later stage pushed into distortion due to a bigger output signal from the first stages?
We can go through an example of how to analyze the 2 versions of the stage if you'd like. But without using something like a listening amp or an o'scope to see where the distortion is happening, I'm not sure if you'll get your answer.