Hi!

Having restored a pair of tube gears - Fisher 400 and Pioneer SM-83, now I am thinking about Sansui 1000 or 1000a. The Sansui 1000 uses 25E5/PL36 output tubes, 1000A - 7591.

I would like to re-wire output stage and replace these with EL34 or KT66/88. 25E5/PL36 use 400K grid leak resistors - very high value indeed. Probably they had to be reduced (and coupling caps increased). What other changes do I need to make? There is a negative feedback loop, which is different for 1000 and 1000a. Any other changes required if I replace tubes?

I also wondering about audio transformers - will they match output impedance of EL34/KT66/88?

Schematic of power amp of both models attached.

If my idea is not so good please do not hesitate to tell it.

Thanks in advance for any suggestion.

Hi Linux,

The first thing that comes to me is the large increase in filament current required by the EL34 compared to 7591. Almost twice. I doubt the filament winding has adequate reserve. I recently finished a restoration of a 1000a. It runs quite hot around the transformers due to the close proximity of the output tubes to the transformers. I wouldn't want to add to the heat load, even if it the extra current was available. Some folks have installed muffin fans in theirs to control the heat.

Regards,
John

For the 1000, you'd need to add a filament transformer for sure... PL36 are not that common, but they ARE cheap... buy a few and be happy.

Is there room for new 7591EH tubes? If so they are pretty darned good.

Hi Linux,

The first thing that comes to me is the large increase in filament current required by the EL34 compared to 7591. Almost twice. I doubt the filament winding has adequate reserve. I recently finished a restoration of a 1000a. It runs quite hot around the transformers due to the close proximity of the output tubes to the transformers. I wouldn't want to add to the heat load, even if it the extra current was available.
John

Hi, John,

Where you have bought 1000a? Do you have by chance second in decent condition?