|Author:||Zozobra (registered user: 239 posts )|
|Date:||Tue, Feb 28th, 2012 @ 11:15 ( . )|
I've had a little time to revisit a KT88 poweramp/supply design thats been sitting on my hard disk since November now that I'm back home. I've now added the bias supply and heater elevation circuits to it which has thrown up a few uncertainties for me.
Here is the design:
Firstly I'm a little bit unsure of the bias supply and the reservoir caps as I'm not using a transformer with a CT. Are R20 & R21 poorly chosen? Looking at Merlins power supply book it suggests that the bleeder resistance should be less than 10xR27 and I've over shot that! Does the fact that I'm using series/parallel caps change anything? Would 100k resistors be more suitable? Would this be detrimental to their function in balancing the caps? Do I need a pair for each pair of caps?
Next the heater elevation. Assuming I have something like 420V idle at A then I should get something like 70V elevation. does that look sensibly arranged?
Finally the preamp I'm thinking of using has an EF86 as the second stage which requires a lower voltage than the rest of the preamp and looks something like this:
It looks like splitting the power supply into two branches to feed the input and EF86 stages is a sensible way to do this but I'm slightly concerned about the grounding scheme. I'm using a local star bus layout so should the ground also split too? I'd guess so otherwise it could form a loop?
Thoughts, opinions and your patients appreciated as ever!
--* Poweramp/supply design
2/28/2012 @ 12:43--Paul Fawcett
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