[Joeztech]

Steve;

Yes the ST-70 is known for somewhat loose bass due to power supply limitations and too much gain on the low end. The power supply upgrade (similar to one offered by Joe Curcio) plus the lower limit of around 14 to 16Hz and high end limit of 20kHz while the amplifier itself has a wider frequency range helps. The highs are quite good if the preamp is and the tubes are in good condition. I did notice that the restored PAS-2 has a better high end frequency response than the PAS-3X for whatever reasons. Yes I did keep the tone controls. I am old enough that my hearing at high frequencies is poor and I compensate by advancing the treble as needed. I have a Hafler multi-frequency equalizer that I could use and need to try soon.

I am using my AR-2Ax speakers that I bought in 1964. I have had to repair them by re-gluing the woofer surrounds and replacing the tweeters with ones suggested by the Classic Speaker Pages site. This brought them back as good as new. I also have a set of home constructed acoustic suspension speakers using some 10in. woofers from some RS Optimus-6 speakers, new mid ranges and some AR silk dome tweeters. I used 3-way cross-over networks from Parts express with L-pads to tweak the midrange and tweeter levels. They actually sound quite good - far better than they did with the original RS tweeter only plus woofer design. The home constructed speakers require more drive level than the AR speakers do. Right now I have them both in my study where I can switch between A and B speakers on a given recording and even change to 4 ohm impedance and use A and B at the same time. I just started the evaluation of the two speaker types and need more listening time. I have an audio signal generator that I can use to run some frequency sweeps to see if either speakers have noticeable peaks. Of course the room gets involved too as does speaker placement and listening position.

It is possible you might like the VTA board offered by Tubes4HiFi, which is a long-tailed pair design. Some others offer similar designs but not necessarily with the individual grid bias like the VTA unit. I like the individual bias setting for each output tube because you can compensate for slightly different tube conduction to balance the output better and not have to use matched pair tubes. Periodic bias checks can correct for changes in tube current as the tubes age.

I plan to install four bias adjustment controls, one for each tube and install 10 ohm resistors at each tube's cathode allowing direct reading of each tube's current. Those cathode voltages can be brought out to pins on the octal sockets at the front of the amplifier for easy access with a meter. I removed the B+ feed and heater feed to those two front sockets as I will never be using them with PAM preamps. These new bias controls will be installed and used with the existing OEM driver board equipped with 6GH8As and some minor resistance changes in the bias control network to keep the adjustment range right after adding the four controls instead of the original two controls.

I am having fun along the way. Hope that you achieve your goals and have fun with your own restoration and/or modifications. Check out the DIY Tube forum too.

Joe