Tank,
Let me address your last question first: Both Triode Electronics and Dynakitparts.com offer power transformers that have 230V primary windings. They come with appropriate wiring diagrams. It's simply the addition of a tap that allows you to run the same secondary windings at a greater primary voltage and still retain all the original secondary specs....which is what the amp runs on; the B+, the bias, and the heater windings.
As to your questions 1 and 2, here's my take on it: The solid state rectifier will deliver more voltage due to it's reduced voltage drop. So, the increase in voltage amounts to a bit more potential power from the amp. Then you have the question of Power Supply Filters.....or what I guess in your case are the separate caps installed somewhere along the line to replace the multi-cap. Generally speaking, when the original power supply multi-cap is replaced, it's done with something either 'stock', or with an upgrade. If 'stock'...then you have some voltage limitations where the use of a solid state rectifier may create some problems.....like filter cap failure. However, if the Power Supply (PS) filter section was upgraded....to a voltage level of 550 volts or more....then the use of a Solid State (SS) rectifier may be possible without any problems resulting from the increase in voltage.
One of the weak points in the Dynaco ST-70 design was that the PS filter section ran right on the edge...as did the Power Transformer (PT) itself. You'll find much discussion about replacing original PTs with upgraded replacements. This is due mostly to the fact that the current draw on the original based on it's specifications would cause that thing to run pretty hot. That heat has been the cause of a good many failures along the way. Thus when you 'upgrade' to your 230 volt version, you'll be likely getting one that will also run cooler....which is a 'plus' for the entire unit.
As to the SS rectifier vs the Tube rectifier....in the case of the 5AR4, the difference in voltage isn't quite as bad as with others. The voltage drop on a 5AR4 is about 17 volts.....vs the SS 'diodes' which will drop only about 1.5 to 2. So, your PS filter section does need to be stout enough to take this slight increase....though many have run the original 525 can cap with SS rectifiers and not suffered greatly. I fear that, and at minimum, unless you've upgraded to the newer can caps that run 550 volts with a surge of 600.....I think you're 'at risk' running a SS rectifier. HOWEVER, if you've got individual caps in your unit, you may find that it's been set up such that the voltage level may be upwards of 700...which is what the boards are usually good for. This makes running a SS rectifier no problem at all. But, it may be better than that.......
In the case of most filter cap replacements in the ST-70, there is generally either the motive of either 'repair'....using an original to replace a failed original, or 'upgrade'. When upgrading, not only is the issue of voltage capability addressed, but so is 'capacity' as well. Many of these filter boards, as well as some of the newer multi-caps, will add a greater 'uf' capability.....which increases the voltage reserve in the filter section. With this comes the ability to reproduce transient passages in your music with a greater potential for 'less' distortion due to lack of voltage...particularly if played loud. This is commonly referred to as 'more head room'.
So, from my perspective, I prefer the combination of enhanced filter section with the solid state rectifier. To me, it just gives the amp more dynamic capability....better 'bass' response...more head room, and greater power overall. I do NOT find any detriment in doing this. In other words, I don't hear any 'white noise' of 'transistor hiss' or any of the other drawbacks often mentioned in these discussions. But, that's just me. I will mention that there IS one drawback that I do acknowledge...which is that of having the full voltage potential of the power supply hit the tubes as soon as the 'on' switch is flipped. If you're concerned about 'cathode stripping' from that, then you might want to stick with a tube rectifier.....or install a 'standby' switch, or some kind of 'soft-start' mechanism. I personally don't worry about it. Fisher for one built nearly all their units with solid state rectification....including the venerable 400, 500, and 800 series of receivers....as well as their integrated amps. To my knowledge, I've never heard of complaints about that design as related to tube failure from cathode stripping due to SS rectifiers. IMHO!!
But, this is the 'fun' of an ST-70. You get to play around with it, experiment on it, learn from it....and in the end, form you own opinions. Then you can get on AK here and assert your point of view with authority!!......lol.
Good luck............ Tom D.
