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Old 15th May 2006, 07:10 AM   #41
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Location: Eire
The beauty of using a cheap slip bobbin transformer in a parafeed setup, and then trickling in DC to saturate it - is that any cheap transformer will do. You have totally removed the element of sizing the transformer because core size isn't the critical thing in forcing saturation. I would go for something around 10VA to start with. The step down ratio will be dependant on the final output of your valve preamp stage. You will want something like 1V to drive your gainclone.
Go look in your junk box. If the first transformer you try doesn't work well, try something slightly bigger.

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Old 15th May 2006, 08:05 AM   #42
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how do you calculate the R//C network in the parafeed?

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Old 15th May 2006, 08:23 AM   #43
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Love your signature! - I'm a big Lensman fan
Nigel Goodwin
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Old 15th May 2006, 05:28 PM   #44
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"how do you calculate the R//C network in the parafeed?"

Trial and error usually. There are formulas availble (search at the bottlehead forum) but they are much contested and seem only to give approximations. In the case of normal parafeed output stages the rule of thumb would by 4uf plus. In our case, since we want to use the resonance, it would need to be lower. Simply breadboarding the circuit and working back from the 2uf point would be the easiest way.

I though some more about the use of controlled DC to achieve saturation. My original thought was to simply bridge the parafeed cap with a a pot and series resistor. However having thought about this some more, this would alter the plate voltage as the pot was altered. This may or may not be a significant issue, but a better solution would be to reference the pot and series resistor to the positive supply rail, and tie it to the primary winding after the cap.This would still alter the operating point to some degree, but likely much less so than the other arrangement.

At the moment this is simple a thought experiment which needs someone to test it out. I am currently working on a valve amp- so it wont be me!

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