As a matter of course I would replace the two bias fiter capacitors also.
Remember that the bias voltage has to be negative with respect to ground.
When powering up an ST-70 with new tubes or if its in unknown condition you should first make sure that the maximum amount of negative bias voltage is applied to each pair of tubes.
With no tubes installed, power up the amp and check the voltage at pin 5 of each of the EL34 sockets. Connect the black DMM lead to the chassis and place the red probe in the pin 5 tube socket of each tube. The voltage should read as a NEGATIVE. Adjust the bias pot for the side you are checking until that voltage is at its MAXIMUM NEGATIVE value...around -55 volts or so. The right side pot adjusts the right side power tubes and the left side pot the left power tubes. That way the tubes will conduct the least amount of current when all the tubes are installed for the first time. Then comes the process of adjusting each pot ping-ponging from side to side until the correct bias current is reached. Most of us run only 40 to 45 ma per tube (80-90 ma per pair) rather than the 50 ma per tube (100 ma per pair) as recommended by Dynaco.
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I'm printing this and putting it in my notes as a reference. Thank you very much!
And if you have the original phenolic Dynaco 7199 driver board, those big black cylindrical paper caps are also another serious liability, the "black cats" and "bumble bees" weren't meant to last for 40-50 years. The outer cases can crack, and the caps will go leaky if not just plain short out, shich can kill the EL34s, at worst, or just really mess up the bias settings at the minimum.
generally these phenolic boards turn into "crispy critters' after a few decades...
This is something I've been studying on ever since I purchased these amps last year. These are the 3rd and 4th st 70's i've owned. The first one was all stock. Something went wrong on the board about 6 months after I first powered it up and ruined more than I could afford to fix at the time. I was very happy with the way it made my old Klipch LaScalas sound...VERY happy! So happy that I went on to purchase the second one which had been modified. I don't know what board was in it, but I know it had 3 input tubes and giant yellow Wonder Caps on it. I remember it sounded phenomenal on those speakers. That amp was built by a fella named Robert Cunningham in Chamblee GA (I was sort of hoping he might be around here somewhere). After a marriage and then subsequent divorce, I had to sell the entire setup and start over. That system is what has spurred me on into the world of DIYTUBE and ultimately led me to purchase the 3rd and 4th st 70.
I have done quite a bit of reading about some of the different options that are available today. I've read about Curcio, Van Alstine, Triode Electronics, the DIY Tube board, VTA and Mapletree. I'm leaning toward VTA and have recently contacted them. Believe me, the stock stuff in these amps won't be around for long and this is where I'll be needing help with things I know I'll run into in the process. I should have all of the parts together in a few weeks...
Hmmm... Flowery Branch, GA is just about 60-70 miles south of here.
I hear that name on WDUN 550AM, which comes in OK around here.
Well, we were really more of a twigg than a branch when I moved here 6 years ago, but urban sprawl has arrived. And yes, I get WDUN pretty strong up here. I switch to it when I lose 640 WGST on my way home from ATL. You must be pretty close.
WA4SWJ, your handle would lead me to believe I should listen out for you sometime. What bands do you frequent?