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Old 9th Feb 2014, 11:02 am   #39
fitchmr
Retired Dormant Member
 
Join Date: Oct 2009
Location: Felixstowe
Posts: 1
Default Re: Mullard 3-3 information required

Hello.

I have just built a 3-3 amplifier, but I noticed there are two different circuits. One was published by the Radio Constructor and the other by Mullard. I began with the RC one, and was dismayed to find it unstable. The DC feedback path from the EL84 cathode to the EF86 screen, which consists of a 1M resistor in parallel with a 0.1uF, means that a small amount of signal passes to the EF86 screen at low frequencies. Far better is the Mullard design that has a capacitor directly to ground from the EF86 screen. In fact I don't know how the Radio Constructor ones ever worked. Just thought I'd share that, as I spent quite a few hours tracking down the problem.
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