Increasing gain

a DIY, modified Mullard 5-20 monoblock design

Increasing gain

Postby justmikey » Mon Aug 13, 2012 1:34 pm

I've finished my two Eiclones with 6SL7 front ends and KT88s. They sound good. I would like to increase the gain somewhat. Would it be reasonable to simply decrease the feedback? Suppose we changed the 2K R8 feedback resistor to something a little bigger. I have 3.3K on hand. Would that be a signicant change? I suppose C7 would have to be adjusted too, but I'm not sure what value to use. I am using Edcor 60W 4.3K - 8 OPTs and we know all OPTs are not the same with regards to tuning feedback. I do not have a scope on hand right now so I'm trying to use the world's original test equipment (my ears). I have rev B boards - wasn't the original rev A using less feedback? The sound quality right now is superb - very clean. If less feedback means a bit more gain and maybe some more "tubiness" then great! Thanks guys for your helpful comments. Building these was a lot of fun.
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Postby Geek » Mon Aug 13, 2012 4:11 pm

Preamp ;)
-= Gregg =-
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Postby Shannon Parks » Mon Aug 13, 2012 4:34 pm

Let me track down that 4.3K --> 2K feedback change and get back to you on that. It rings a bell. Also, we can add an electrolytic (polarized OK) across R3.

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I found the rev A feedback values

Postby justmikey » Tue Aug 14, 2012 3:29 am

In the manual for the rev B boards there is a sketch of the PCB layout with what appears to be the old rev A component values. It shows 4.3K for R8 and 560pF for C7. I might just have to give that a try...
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