What a great find the Dynaco Tube Audio Forum has been! I am totally stoked about all the great information I have discovered regarding my Dynaco components, which include a ST-70 amp, PAS 3 preamp, and FM-3 tuner, all of which I assembled back in the mid sixties.
One of the immediate benefits I have gained from the forum was learning about the selenium rectifier/diode swap out on the ST-70 amp, which I just completed yesterday. Unfortunately it was during that job that I discovered a previously unknown fact regarding my amp. I erroneously thought I still had a “stock” amp! However, when I opened the unit to do the swap, I discovered that several components have been modified.
Some time back I had “repair” work done on the ST-70. The technician was asked to check out a hum in the amp and to repair as necessary. The work tag indicates the replacement of several resistors and capacitors and the installation of new tubes. However, I had always considered the unit to simply have been brought back to original factory specs with the work performed. I was then, and remain today, pleased with the results of his work and have experienced many trouble free hours of enjoyment, but I would like to return my unit to stock conditions.
I found the following modifications:
1) The original installation called for the connection of 1000 ohm resistors (brown-black-red) between pins 5 and 6 of V2, V3, V6, and V7, respectively. In each case, they have now been replaced by 680 ohm resistors (blue-gray-brown-silver).
2) There are now 60UF-500VDC capacitors running from the quad filter capacitor to the ground lugs on the V6 and V7 sockets. Quad Pin 1 connects to V7 and pin 2 to V6.
3) On the PCB, the 4 0.1UF/400VDC and the 2 0.05UF/200VDC capacitors have been replaced by SIX 0.1 UF/400VDC capacitors.
4) Replacement tubes for my original Mullard “Dynaco” EL34/6CA7 fx2s are Ruby EL34Gs and the 7199s are now Sovtek. I understand about this part----Mullards are rare and expensive.
My questions for anyone who can help are as follows:
1). Do these mods make sense? I do not understand enough to know why the tech made the changes I have noted.
2). With the exception of the tubes, can this ST-70 be brought back to “Stock”? If so, can you give me some directions on how to go about it? Is it as simple as removing the modified parts and replacing them with original spec components?
3). Since I want to continue to operate the unit until I decide what direction to go, given the current modified configuration, is the bias setting still 1.56VDC?