Bottlehead Forum

Bottlehead Kits => Legacy Kit Products => Topic started by: tongwi on March 26, 2018, 01:27:22 PM

Title: DC coupled 2A3 AfterGlow with PGP 8.1, soft start needed?
Post by: tongwi on March 26, 2018, 01:27:22 PM

Hello Paul, Doc,

I had upgraded pair of parmours with series feed TFA-204, modified with 6N1P input tube.
I bought PGP 8.1 power transformer and plan to convert it to AfterGlow very long time ago.

Now I would like to proceed this change, and knowing the existing soft start kit for DC couple 2A3.

My question is, do I need the soft start circuit since the plate voltage for driver tube is getting from C4S, from the Cathode of 2A3?

How can it be possible to damage the 2A3 because of high voltage on grid during start up?   The voltage of 2A3 grid is power from the 2A3 cathode thru C4S, it seems like the 2A3 grid will always be lower in voltage than 2A3 Cathode.

Thanks for your long term support and help!!!

William

 

Title: Re: DC coupled 2A3 AfterGlow with PGP 8.1, soft start needed?
Post by: Paul Joppa on March 26, 2018, 02:49:01 PM

You are correct, for the Afterglow you do not need the soft start.

In the later Paramount, we switched to taking the driver power from a shunt regulator, at a higher voltage. Some boutique tubes exhibited a spark at start-up, and the soft start was implemented to cure that problem. But the earlier design, taking power from the 2A3 cathode, does not have that problem.

Title: Re: DC coupled 2A3 AfterGlow with PGP 8.1, soft start needed?
Post by: tongwi on March 26, 2018, 05:27:57 PM

Thank you Paul!!!

William