I'm building a ST70 clone with Hammond 6.6k OPT's with the original board to start. I wanted to upgrade it towards a more optimum amp. The 6.6k gives a little less power but a little larger class A operation. I've been reading the posts and will try to make a universal board for experimenting with 6SN7's for input and 6H6n for the differential phase splitter.
Bias is of concern, from the posts the 6SN7 likes a little current, is 4.2ma adequate or is it starved, and 6ma through the 6h6n differential?? I can put a bipolar current limiting scheme on the diff pair if there is a sonic reason for doing so. I've added different biasing ideas for the 6SN7 , and input bandwidth limiting filter 7 to 44k should I ever want to stifle the source a little. The transformer is stout so I can increase the B+ to either circuits up to about 450V. KT88's have been chosen for outputs, but I have a set of EL34's as well. Just starting the wood work, I'm finding myself challenged in this area. Hopefully the PCB for the power supply will go easier.
Please review and advise.....
Bias is of concern, from the posts the 6SN7 likes a little current, is 4.2ma adequate or is it starved, and 6ma through the 6h6n differential?? I can put a bipolar current limiting scheme on the diff pair if there is a sonic reason for doing so. I've added different biasing ideas for the 6SN7 , and input bandwidth limiting filter 7 to 44k should I ever want to stifle the source a little. The transformer is stout so I can increase the B+ to either circuits up to about 450V. KT88's have been chosen for outputs, but I have a set of EL34's as well. Just starting the wood work, I'm finding myself challenged in this area. Hopefully the PCB for the power supply will go easier.
Please review and advise.....