Input impedance

[Shoog]

You need to think parafeed to use the interstage as a phase splitter. I encountered bass resonance with a 1uf parafeed cap, but 4uf gave a flat response. I use the DIYaudio CCS by Pinkmouse in my amp which consists of a CCS loaded parallelled 5687 driving a pair of 6AS7's. This is a difficult valve to drive and I needed a full 40mA to get it to work satisfactorily. It looks like the 813 has similar input capacitance, but I think the issues may be a bit more complex than the simple figures might suggest.
Try it with a beefy parafeed driver through your interstage and onto your 813's. If the top end is disappointing then look to add the powerdrive.

Shoog

 

[nhuwar]

Parafeed I though parafeed was for single ended this is push pull. I'm thinking it would be fairly tricky to implament and could cause some anode load problems am I correct in thinking this our not?

Thanks
Nick

 

[David Davenport]

Hi Nick,

An example of a parafeed differential amplifier is here:
http://www.raleighaudio.com/chapter_4.htm

Dave

 

[Shoog]

If your going to use the interstage to do your phase splitting, you will be going from a SE driver to PP outputs. In that case you put your parafeed cap before the primary of the interstage.
If thats not what you intend to do then I'm a bit lost.

Shoog

 

[nhuwar]

the other idea I was toying with is the 6sn7's in srpp. And it would be easy to config it in parafeed this way. I don't know how well it would work though.

Nick