by Bob Latino Wed Dec 12, 2012 3:48 pm
If you place the two amps side by side - yes you can see some but not all of the differences between the original Dynaco ST-70 and the VTA ST-70. The design concept behind the VTA ST-70 was to make the amp LOOK LIKE an original Dynaco ST-70 yet take advantages of 50+ years of advances in electronics and have the VTA ST-70 SOUND LIKE a modern tube amp. Efforts were also made to make the amp more reliable than the original amp. Some things were not changed like the chassis layout. The size and shape of the chassis and the parts layout of the VTA ST-70 is identical to that of the original Dynaco ST-70. An original or aftermarket Dynaco tube cage will fit this chassis perfectly. The silk screening of the Dynaco markings on the front and rear face of the amp are also the same as on the original amp. The OUTPUT transformers are the same size and shape and more importantly, are wound the same way (interleaved/layer) as the original Dynaco A-470 output transformers. The assembly manual for the VTA ST-70 is written in detail for the beginner just like the original Dynaco ST-70 assembly manual.
The many improvements over the original amp include ..
A. Chassis - The original ST-70 chassis was made of 18 gauge nickel plated regular steel. Eventually most of these chassis will develop rust spots. The chassis on the VTA ST-70 is made of 16 gauge (.015 inch thicker metal) polished stainless steel and will never rust. All the screws on the VTA amp are also made of stainless steel.
B. Driver board and driver circuit - The driver circuit on the original amp with the two 7199 tubes was done to "save a tube". In light of circuits available today, the driver circuit can easily be improved upon. The original driver board was made of an inexpensive phenolic material with open solder traces on the bottom of the board. This phenolic material was not very heat resistant and the board would many times turn black from heat under the 7199 tube sockets. The open solder traces sometimes lifted from the board. The carbon composition resistors found on the board would, after many years, stray from their original values. The VTA board is made of epoxy/fiberglass and has no open solder traces. It has plated through holes so that parts may be placed on either the top or bottom of the board. It has an "on board" power supply using Nichicon low ESR caps and an on board bias system that allows individual biasing of each output tube. The new board also uses 1% metal film resistors that will not stray from their original value over the years.
C. Power transformer - The original ST-70 used an undersized PA-060 power transformer with a 1 1/2 inch stack that usually runs pretty warm. The upgraded power transformer on the VTA ST-70 has a 2 1/8 inch stack, runs cooler, and has a much better power transfer capability.
D. Choke - The original ST-70 C-354 choke was rated at 200 milliamps and would run very hot. Sometimes a "brown goo" would melt out of the choke and deposit like candle wax on the bottom cover below. The C-24X choke in the VTA ST-70 is rated at 240 milliamps and runs cooler.
E. Quad cap - The 30, 20, 20, 20 uF quad cap on the original ST-70 has been replaced with an 80, 40, 30, 20 uF cap on the VTA ST-70. The total capacitance on the original amp was 90 uF. On the VTA ST-70 the total capacitance is 464 uF.
F. RCA input and output binding posts - The original ST-70 had aluminum RCA input jacks narrowly spaced at 3/8". The VTA ST-70 has gold plated input jacks spaced at 9/16" and will accept modern thicker interconnects without the interconnects touching. The output binding posts on the original amp were simple screw terminals. The VTA ST-70 has modern 3 post gold plated binding posts which may be set up for 4 and 8 ohms, 4 and 16 ohms or 8 and 16 ohms.
G. Wire - The original amp kit was supplied with 22 gauge solid core PVC hook up wire. The VTA ST-70 amp kit is supplied with 20 gauge (one size thicker wire) tin coated solid core irradiated PVC wire. The tin coating makes for easier soldering and the irradiated PVC insulation is a better insulator than the insulation on the wire in the original Dynaco kit.
H. Triode/ultralinear operation - The original ST-70 ran in ultralinear mode only. The VTA ST-70 runs in either ultralinear mode or triode mode. The change for each channel can be made at the flick of a switch while the amp is running.
I. Other optional items - The original amp had no options like the VTA stepped attenuator to allow you to use the amp directly with high level signal sources or VTA TDR (Time Delay Relay) which is a good addition if you use solid state rectifiers.
Bob
Last edited by Bob Latino on Thu Nov 29, 2018 7:25 pm; edited 1 time in total
