If you want to go that route, there's this:
The page explains what they're changed and updated (a lot); it's been well reviewed.
There are a couple of older reviews out there for this amp--its been around for a long time. Here is one worth a read (date unknown) Jeff Dorgay: http://www.tonepublications.com/review/ava-ultravalve-vacuum-tube-amplifier/
There is also a very good Audiocircle ’circle’ dedicated to AVA--Frank moderates it and he has lots of adherents. For the price it is certainly a very solid way to get into tube amps; given the product’s rep I would suspect it has reasonable resale value (AVA’s design hasn’t changed much as I understand it over the years).
Full disclosure--I own a Bob Latino VTA 120, sort of a competing product. Latino also makes a more direct competing product based on the 70.
There are several others out there doing the same thing, i.e., producing modern versions of the Dynaco line but with improved power supplies, better transformers, improved driver circuits, higher tolerance parts such as caps, coils and resistors, SS chassis, either point-to-point wiring or mil-spec PCB boards, etc.
Most importantly all of them use highly available modern tubes.....
Couple things to consider before pulling the trigger:
1. Make sure you get what you’re looking for in terms of tube rolling, can the AVA run any other output tube besides EL34s? In my case it was important to me to be able to run the different output tubes (KT88s and KT120s, not just EL34). I honestly don’t know if the AVA allows for this (might, just not sure).
2. Does the circuit allow for switching between triode/pentode operation?
I only have a laymans understanding of the topic (triode, pentode and ultraliner mode configurations).
As I understand it you can configure the output circuit of your amp runing pentode tubes at least three ways.
In “triode mode”, the output circuit is configured to bypasses certain elements of the pentode tube, in effect runs the pentode tubes (five elements) as if they were triode tubes (only three elements), resulting in the tube producing less power then it is actually rated for, and resulting in (claimed) a more lush tube-like sound.
In pentode mode, I believe the circuit is wired to take full advantage of the pentode tube elements, producing more power output, and yielding a different sonic profile.
They may be other differences between the two circuits beyond bypassing elements in the tube, in particular how feedback is used in each of these configurations to control or create distortion—someone more technically knowledgeable will have to pipe in to explain feedback and or correct my comments.
There is also ultralinear mode, which is another variation of the output circuit, supposedly yielding yet another sonic signature (I believe ultralinear mode can be utilized with either triode or pentode tubes, as the changes are implemented with respect to how the output transformer is configured in the output circuit....)
At a practical level, my amp has a switch that allows me to run in either mode.
Not all tube amps under discussion in this thread have this feature.
hope that helps.
The brilliant Dave Gillespie is the first person to take a full set of published, objective, measurements of the ST-70 and correlate them with the design strengths and compromises. Interesting reading:
Interesting to the point I can understand about 10% of the content :) thanks for sharing this.
One thing he does point out that I know to be an issue, even with the modern versions of this amp—the tube rectifier. Per his article, “A much bigger compromise -- in my opinion -- is the use of a single rectifier tube to support a 70 watt amplifier.”
I swapped out my tube rectifier earlier this year for a Weber WZ68 (SS device), which has twice the rated current output as the single 5AR4 rectifier tube that came with my VTA 120—IMHO it resulted in a big sonic improvement when the volume is turned up.
It is important to realize that the current draw of the VTA 120 is much greater than the ST70 as it uses four 6550/KT88 tubes as opposed to EL34s. The analog in the Dynaco line would be the MK3 mono amp which supplies the B+ voltage for only two 6550/KT88 from a single 5AR4.
I would never use a tube rectifier in the VTA 120, however in the 70, though it makes less power, it is a marvelous contributor to the overall sound and I prefer it to solid-state rectification of the B+. Just an opinion.