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The Eico ST-70 is a 35 watt per channel vacuum tube amplifier from 1962, which is great in many respects. This project fixes a few flaws which prevent it from reaching its full potential. Here are some before and after results:
Table of ST-70a output power versus frequency
Table of ST-70a line-level signal-to-noise ratio

You can choose which mods you would like to use. The separate mods are:
Phono preamp, driver balance adjust and line amp/power amp. [Please note that the line amp and power amp mods must go together.] Get the illustrated, step-by-step instructions in the download:

ST-70A mods package icon

-Eico ST-70A mods article (1MB)
Chan1
Chan-1 schem illus

20 - 20kHz phono response in dB

Power amplifier modifications

 

Reader Comments

Posted by Steve L. December 25, 2022 - 08:29 am
Hi FarmerBoy, Congratulations on fixing that ground issue! Regarding your question about whether one could do all the mods with only a VOM for test equipment, I sadly have to admit that I can't see a way to do that at the moment. I guess I would also be concerned that these mods would be too challenging for someone without basic lab equipment (e.g. a scope and an audio generator). The worry is what if a problem occurs, you wouldn't have the resources to investigate it. But there are plenty of other things you can do to grow in your electronics skills and I would encourage you to do all you can. I started out as a kid building test equipment kits and eventually earned degrees in electrical engineering, having lots of fun along the way! Best Wishes, Steve

Posted by FarmerBoy December 24, 2022 - 01:58 pm
Thank you for the reply and knowledge! It was a ground issue! The common screw was painted black and I assumed that was correct. An easy but not so easy fix, i really appreciate your help!I have not done any mods. The only reason why is I have no scope or signal generator to calibrate the final mod. Is there a way to complete all modification with out the extra equipment to set the pots? I know it might not be top performance but is there another method to get things set up to sound OK and do no damage with out the extra test equipment. I do have a volt/ohm meter .

Posted by Steve L. December 23, 2022 - 09:25 am
Hi FarmerBoy, Normally, the main thing which would cause sound on both channels after pulling one input connection would be the Mode switch set to Mono 1 or Mono 1+2. It should be set to Stereo. Also, make sure the Balance √ switch is in the down (off) position and the Spkr Phase switch is in the up (N) position. One thing which may cause confusion with the ST-70 is the strange wiring Eico used for the output connections. This has to do with their grounding the 4-ohm taps. If you implement the ST-70A mods, the strangeness is eliminated and the output Common connections go to ground. I mention this because an improper or missing speaker ground connection could lead to hearing something on both speakers when only one amplifier channel is playing.

The problem is, I don't know whether you have implemented the ST-70A Mods in your "overhaul" so I can't advise in detail. Not that I'm advocating implementing the ST-70A Mods at this point. If you haven't attempted those, it would be best to get the stock design working properly before doing those. Since this is getting "into the weeds" for some readers, I would invite you to contact me at the email address given on the About page if you have further questions. You can find a button for the About page near the top of the Home page.

Posted by FarmerBoy December 22, 2022 - 04:38 pm
Hello I just finished overhauling an St70. It had alot of hum. I noticed someone moved the ac switch to the volume pot and rerouted the ac wires along the other pots. So I added a toggle switch and moved the ac wires . Now no hum.

So my problem is that I noticed no 3d sound which led me to pull an input . To my surprise i still had sound coming out left and right speakers. I tryed every adjustment and came to the conclusion that someone rewind the new volume pot wrong. I tried chasing the wires the best I could but could not find the problem. I'm kinda lost on the wiring diagram but followed the signal path the best I could .making sure everything corresponded with the appropriate location of the double pot left or right channel and everything seems right but it obviously it is not.

Any help or guidance is greatly appreciated!

Posted by Steve L. June 28, 2022 - 07:50 am
To wrap up the thread started by Georgeh 5/22/22 and initially answered 5/23/22: It turned out that indeed, the middle tab of the pot had been taken as the wiper, whereas the Bourns 3590 wiper tab is farthest from the shaft, at the back of the control body.

Posted by Steve L. May 23, 2022 - 07:59 pm
Hi George H., Sorry you're having a problem with that. First let me verify that I understand what you saw. I have:
  • Couldn't balance the cathode currents for either channel, though they were close.
  • DID have -109V at CR1-Anode (good).
  • Cathode voltages wouldn't go below 0.5V, even with bias pots full CW. (This suggests CW is more negative grid voltage.)
  • The wipers of the bias pots were at -18V for the max CW position.
  • With the bias pot's load removed, the wiper reads -22V full CCW, "increasing in the CW direction."
Did I get all that right? The -18V full CW reading should have been far more negative.
Calculations show that with bias and balance pots in the center, we should have -15.2 and -48.8V at the ends of R102, with the wiper in the center. Now, due to the loading, the wiper doesn't exactly go between those voltages. Instead, -16.9 and -37.5V are expected at the wiper when it's at the ends. This takes loading into account. With the load removed from the bias pot, we should expect -23.8 and -59.9V at the ends of (say) R102. You saw -22V on the full CCW wiper so that reasonably agrees with the -23.8V figure (8% delta). Did you get a reading for full CW?

Now, I have a couple questions: First, are both channels acting about the same? Second, with the pots centered and connected normally, are you seeing -15.2 and -48.8V at the ends of (say) R102? Caveat: I'm a little concerned that the cathode current could go too high at the low bias voltage end and suggest that you watch the voltage at one of the affected cathodes to make sure it doesn't go much over 420mV. (Implies 42mA.)

Oh, and just to confirm: You know that the wiper is NOT the middle tab of the pot terminals, right? The wiper is farthest from the mounting bushing.


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