Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jack2911guitarblues on February 05, 2015, 09:22:25 pm
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Hi there,
Wondering if anyone (or more!) can give me some information on theory of tube amps, and in particular cascade gain stages.
I have built a 1987x clone, and also just installed the "hotswitch" in it, whereby the 2 inputs are able to be run in series in the the first tube (V1 a & b).
I am slowly understanding how all this works and find it all immensely satisfying. But my question is, when the pre-amp is cascaded, the "hotswitch" schematic shows the signal coming from V1 A Plate, going to one side of the Vol. control, and then the signal goes from the wiper straight straight to pin 7 of the 2nd stage of V1, which I understand, no problems.
Why is that another type of cascade circuit I have seen, shows the signal FIRST going through a 470k resistor and paralleld with a 250pf cap, and THEN going on to pin 7 of the 2nd stage of V1?
Is the cap and resistor necessary when cascading the signal from the plate on V1a, to the input of V1b??
What does this Cap/resistor combination actually do as far as gain/tone do? and why is it used anyway??
I would appreciate some theory on this, whenever some one has the time or chance to answer my question! Hope my question is not too confusing!!
Jack :help:
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This might help you.
http://www.valvewizard.co.uk/gainstage.html (http://www.valvewizard.co.uk/gainstage.html)
Brad :icon_biggrin:
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What does this Cap/resistor combination actually do as far as gain/tone do? and why is it used anyway??
There are those on this board that can give all the math involved, and I am not that guy. One way to look at it is this. The resistor forms an attenuator that passes all frequencies equally, but at a slight reduction. The picofarad cap which bypasses it would only pass higher frequencies but with less attenuation than the resistor. This would be called a high pass filter.
One reason this might be used is that LOUD bass tones can rattle a speaker, cabinet, and circuiit, particularly a low cost speaker, etc. At higher gains it is common to try and bleed off some of the bass to help shape the final output.
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Here you can have a very interesting read on the matter, I think you'll find it very useful
http://www.aikenamps.com/index.php/grid-resistors-why-are-they-used (http://www.aikenamps.com/index.php/grid-resistors-why-are-they-used)
K