Topic review

Hey Bob,

The damn thing (my ST-70) had been sitting on the floor in my closet for about 10 years. I had put it there when I switched over to a home theater system with a multichannel receiver. I did bring it out on Sunday and fire it up and it still works OK but it has this low hum. I saw on one of your other posts here about the metal can capacitor being the source of hum in a lot of old ST-70's. I'm going to replace the can with the 80, 40, 30, 20 capacitor when I get the chance.

RockyAM

We don't really care that you downloaded the manuals and put them on a CD for your personal use.

The PAS-4 preamp was created by Panor, a Japanese company who bought the rights to the Dynaco name and produced the PAS-4 in a plant in Taiwan. Some made it over here but they never sold in large volumes in the USA like the PAS-2 and PAS-3. Very rarely does one show up on Ebay or Audiogon and when they do they go for $500-$600. The PAS-4 is a much better sounding preamp than the PAS-2 or PAS-3.

As for starting up your old ST-70 the best thing to do is use a Variac to bring the voltage up slowly over a period of an hour so the quad cap has a chance to reform gradually. Most Dynaco users, however, don't have a Variac so here is what I would do in this order ..

1. Pull the rectifier tube and start the amp up without a rectifier. Let the amp stay on for 5 minutes or so and check that all the other tubes light up OK. If the amp was working OK when you put it away there should be no problem here. This just tells us that the power transformer is working OK. Turn the amp OFF.

2. Open the amp up, set your meter for its highest resistance scale. Place the black probe on the chassis and the red probe on each of the 4 sections of the quad cap and measure the resistance on all four sections of the quad cap. What SHOULD happen is that you should get a RISING RESISTANCE. The resistance will gradually go up as each section of the quad cap charges from the battery on your multimeter. If the resistance on each section is over 1 MEGohm (1,000,000 ohms) you are probably OK. If the resistance peaks out at 5000 to 10,000 ohms the cap is probably bad and should be replaced.

3. Assuming the resistance on all four sections of the quad cap is sufficiently high then plug in the rectifier and turn the amp on and KEEP YOUR EYES ON THE RECTIFIER TUBE AND YOUR FINGER ON THE POWER SWITCH. The key time is about 10 to 15 seconds after you turn the amp on. The indirectly heated 5AR4/GZ34 rectifier will have heated up sufficiently to start applying high voltage from pin 8 across to the quad cap. If the quad cap is weak or has gone bad it will not accept the high voltage and the rectifier will "arc over" and you will see sparks inside the rectifier. The second you see anything abnormal inside the rectifier > you shut the amp off! (you still have your finger on the switch - right ?)

The quad cap is the weakest link on an old ST-70. Replace it with the CE Distribution or the Dynakitparts 80, 40, 30, 20 quad cap, Both are very well made and should last along time. This cap should be oriented properly > All you do is place the 80 section towards the back of the amp and all the other sections will be in the proper place. If you have a Mark III instead of a ST-70 use the one from Dynakitparts because it has a slightly higher voltage rating.

Let us know how you made out this weekend when you "fire her up" ?

Bob

Bob and Roy,

I have followed Dynaco tube stuff over the years since the '60's and found a few manuals online in the past few years. I have also seen some guys selling individual manuals on Ebay for maybe $6 to $10 or so. There are also some guys selling CD's of all the manuals but even they didn't have the PAM-1 manual or the PAS-4 manual like you guys do. I actually had one of those skinny PAM-1 preamps many years ago in a mono system. Kinda tells you how old I am ? LOL. I had never even heard of the PAS-4 preamp? Where the hell did that come from ? I downloaded all the manuals anyways from Roy's web site and put them on a CD. I hope you don't mind ? I think it is really nice that you and Roy offer these free for guys like us who still have some old Dynaco tube stuff and like to tinker with our amps. I still have an old ST-70 in the bottom of my closet in the spare room that I haven't played in at least 10 years. I think I will pull it out this weekend and see if it still works. Should I use any precautions when I start it up since it hasn't been used in so long ?

RockyAM