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andyjevans 9th March 2004 09:35 AM

Input stages - to decouple B+ or not?
My present amp has a 12b4 voltage stage (no cathode bypass) DC coupled to a concertina (split-phase). Most designs I've seen don't decouple the B+ supply between these stages, so I haven't (e.g. Morgan Jones EL84 amp with 6922 first stages). However, a tube designer I know strongly recommended doing so. I'd welcome some ideas about decoupling between stages, and why doing so/not doing so might sound better or have other advantages. I can also use chokes for decoupling - I have a few 10H 100mA - and I'm open to any other method. Andy

fdegrove 9th March 2004 10:28 AM


In your case, direct coupled, I wouldn't decouple.
The direct coupling sets the grid bias of the next stage and relies on the difference between the voltage of the anode of the first stage and the grid voltage present on the next.

Since B+ is common to both any variation there will maintain the difference on both hence the bias is maintained.

Now if both stages were independent from each other, any change in B+ will upset this relation and correct bias will be lost momentarily.


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