The original Dynaco instructions called for the quad cap lug D (#2) to connect to the rectifier output. Why was this later changed to the "square" lug (#4)?
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ST-70 Quad Cap Wiring - Why Two Different Ways?
maltboy1- Posts : 5
Join date : 2024-09-10
maltboy1- Posts : 5
Join date : 2024-09-10
Correction - square lug is #3, not #4
jimmyb- Posts : 2
Join date : 2024-10-03
I see nobody has responded to this yet. I'm new at this and not qualified to answer, but I have just gone through the exact same question this week. There are detailed directions on this on this forum that clear this up, so I suggest you look for that. But, I interpret that by "later changed" to different orientation you are referring to instructions for the upgraded quad cap with increased capacitances. I think the key is that the wise folks instruct that no more than 60 mf should be connected to the rectifier. The 80 mf section should be connected to the output transformers. So in replacing the original (30-20-20-20) cap with the upgraded 80-40-30-20 cap (what I just got from Tubes4HiFi), the 40 mf section gets connected where the old 30 mf section was, and the 80 mf section is connected where one of the 20 mf was. Thus you install the upgraded one rotated by one section compared to the original.
maltboy1- Posts : 5
Join date : 2024-09-10
jimmyb wrote:I see nobody has responded to this yet. I'm new at this and not qualified to answer, but I have just gone through the exact same question this week. There are detailed directions on this on this forum that clear this up, so I suggest you look for that. But, I interpret that by "later changed" to different orientation you are referring to instructions for the upgraded quad cap with increased capacitances. I think the key is that the wise folks instruct that no more than 60 mf should be connected to the rectifier. The 80 mf section should be connected to the output transformers. So in replacing the original (30-20-20-20) cap with the upgraded 80-40-30-20 cap (what I just got from Tubes4HiFi), the 40 mf section gets connected where the old 30 mf section was, and the 80 mf section is connected where one of the 20 mf was. Thus you install the upgraded one rotated by one section compared to the original.
I'm guessing it was intended for a different application (MKIII?), but it can serve as an upgrade for the original ST-70 quad cap if it is rotated 90 degrees.
Bob Latino- Admin
- Posts : 3273
Join date : 2008-11-26
Location : Massachusetts
maltboy1 wrote:The original Dynaco instructions called for the quad cap lug D (#2) to connect to the rectifier output. Why was this later changed to the "square" lug (#4)?
The reason for the difference in the two quad cap arrangements is that the top photo is the quad cap arrangement for a STOCK Dynaco ST-70 and the lower photo is for the (Dynaco similar) VTA ST-70 and VTA ST-120 amps. The 6800 resistor and the 22,000 resistors are used to drop the DC voltage to various places on the ORIGINAL Dynaco driver board that used 7199 driver tubes. A VTA ST-70 and VTA ST-120 use 12AU7 driver tubes. This resistor should be a 2200 ohm 2 watt resistor on a VTA ST-70. On a VTA ST-120 this resistor should be a 4700 ohm 2 watt resistor. The VTA board itself should receive about 375 - 400 VDC.
The VTA ST-70 driver circuit is a much better driver circuit than the original Dynaco ST-70 driver circuit and used THREE (dual triode) 12AU7 driver tubes. The center tube is the voltage amplifier for BOTH channels and the two outer 12AU7's take care of the splitter/inverter part of the circuit - one tube for each channel. The original Dynaco circuit used just TWO 7199's to save the cost of one tube. A 7199 tube back then cost about $1.50 (believe it or not !). Back then in the 1960's you could buy a Dynaco ST-70 with all tubes for $99.95 USD. Sometimes they would have a sale and you could get a complete kit for $89.95 USD !
Bob
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