Triode Electronics of Chicago offers a new power transformer for the ST-70 that has increased current for the rectifier circuit. This allow you to run dual 5AR4s in parallel rather than one- thus fixing the biggest single weakness of the original amp (which had distortion low enough that it rivals the classic Marantz 8B), which is that it eats rectifier tubes.
You can reduce distortion in the amp by replacing the 7199 driver tube with a 6GH8A, by using an inexpensive adapter for the conversion available on eBay. 6GH8s are a lot cheaper and easier to find than 7199s!
Finally, a set of ODAM V-Caps work out very nicely as the main coupling caps in the ST-70- better sounding and better performance. Of course the stock filter caps should be replaced, along with the selenium rectifier used in the bias circuit. The filter cap to use is a 4-section part made by JJ, available from CE Distribution. Its small enough it can be mounted beneath the chassis, allowing the 2nd 5AR4 to be located where the old filter can was.
Out of spec resistors should be replaced. I don't like the speaker terminals but without chopping up the chassis there's not much for it.
You can reduce distortion in the amp by replacing the 7199 driver tube with a 6GH8A, by using an inexpensive adapter for the conversion available on eBay. 6GH8s are a lot cheaper and easier to find than 7199s!
Finally, a set of ODAM V-Caps work out very nicely as the main coupling caps in the ST-70- better sounding and better performance. Of course the stock filter caps should be replaced, along with the selenium rectifier used in the bias circuit. The filter cap to use is a 4-section part made by JJ, available from CE Distribution. Its small enough it can be mounted beneath the chassis, allowing the 2nd 5AR4 to be located where the old filter can was.
Out of spec resistors should be replaced. I don't like the speaker terminals but without chopping up the chassis there's not much for it.