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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)

Chan-1 schem illus

20 - 20kHz phono response in dB

Power amplifier modifications


Reader Comments

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.

Posted by georgeh May 22, 2022 - 10:53 pm
Hi Steve, after completing the modifications to my ST-70 I powered the unit up and proceeded to make the DC balance and bias adjustments. For your knowledge I changed both pairs of bias and balance potentiometers to 50K 10 turn Bourne 3590P-4 units for finer adjustment. I don't know if this is of consequence but for the resistor networks on the potentiometers I used metal film resistors as I had them available. To my dissatisfaction, I could not achieve a null voltage as measured at J19, J21 and J18, J20 for either pair of output tubes. I came close but reached the maximum clockwise travel of my potentiometers, and could not further adjust. I then proceeded to try the bias adjustment, and could not lower the target voltage as measured at J19 and J18 to chassis ground below 0.5V with the potentiometer adjustments in the full clockwise direction. In my attempt to perform some troubleshooting I measured the negative supply voltage at the diode anode at -109V. The voltage at the wiper terminal of both bias adjust potentiometers measured -18V with the potentiometers in the full clockwise position. I then disconnected the wire from the wiper lead connection of the bias adjust potentiometers leading to wiper lead of the balance adjust potentiometers. When I then measured the voltage at the wiper lead of the bias adjust potentiometers it measured -22V in the full CCW position with the voltage increasing as the potentiometer was moved in the CW direction. With the information I provided I hope you can make some suggestions to correct this problem. Thank you for your help and I look forward to our reply.

Posted by Steve L. April 24, 2022 - 07:42 pm
George H. sent an email message with the following questions, answered here:

Do you recommend replacing the two conductor power cord with a grounded three conductor cord and if so what would be the best way to install it.
--- I do try to follow modern safety practices where possible. However, the problem with introducing the mains protective ground (call this MPG) is that it's likely to cause a ground loop unless you can be sure that it is the only connection from audio ground to MPG in the entire system. It can be tricky to ascertain that and to maintain it when other equipment and connections are added. In modern equipment, the MPG may be connected to the metal enclosure but audio grounds are kept insulated from it. Most vintage hifi equipment like the ST-70 though, has audio grounds intermingled with the enclosure. My solution is to avoid using the MPG.

With your modifications, will the "speaker phase" and "balance" switches remain fully functional. Regardless, do you think it is a good idea to keep the "speaker phase" switch functional and if not can you recommend a good way to remove it from the circuit.
--- No but, let's recognize that those functions are only functional if you use Eico's rather odd technique of connecting the Channel-2 speaker through the third rear screw-terminal strip and then connect a jumper from the Impedance Tap terminal to the selected tap on the Channel-2 terminal strip. I don't know anyone who does that. Hence, the phase and balance switches don't get used for their original functions. The phase switch will have no effect. The balance switch can be left in its off (down) position. The common way to connect the speakers is simply, directly to the Channel-1 and Channel-2 screw-terminal strips. By the way, the balance function didn't take into account speaker unbalance, drift of certain part values and unbalance in whatever source was used, so it wasn't very useful anyway.

The speaker phase switch could in principle, be useful if it were implemented before the power amp. But placed after the power amp as it is, it could only work for the chosen impedance tap. That's only applicable if you're using the third screw-terminal board as it was intended.

Fortunately, you can just ignore the third screw-terminal strip, the balance switch and phase switch and use the Channel-1 and Channel-2 screw-terminal strips to connect the speakers. No mods needed regarding that issue. The mods package does include rewiring the transformer output connections to make the "C" or common terminals go to ground instead of the odd grounding of the 4-ohm taps as Eico did. With the connections Eico had, the two speakers could not have a common ground and the input and output could not have a common ground. That caused problems for speaker switches, headphones and test equipment. When redone with the mods package, speaker switches, headphones and test equipment work fine with the amp. But the phase and balance switches have no function.

From what I have read, it is my understanding that the loudness switch was incorrectly wired from the factory. Do you happen to have a wiring diagram or images that show the correct wiring for that circuit?
--- I wouldn't call it incorrect. It's true that it lowers the level of the audio in addition to applying the bass-empha­siz­ing loudness contour. I think Eico did that intentionally because the original intent of the loudness switch was to correct for the loss of ear sensitivity to low frequencies at low levels. It was valid only at low levels and this function would give you both the lowered level and bass correction in one step. If you just wanted to boost the bass, you could use the bass control.

However, modern usage is that some people just treat the loudness switch as an extra bass boost. Thus they don't expect the level to drop a bunch with that and don't understand why it doesn't work the way most other hifi amps and receivers do. Incidentally, I find that I tend to play the amp with the volume pretty low in its range, even when it seems fairly loud. I guess the ST-70 has some extra gain. Operating at the low volume setting worries me at times that the balance might be a little off down there. Having the loudness switch on would be a way to avoid the ragged low end of the control. That might have been part of Eico's thinking. E.g. "Wanna listen to it low? Just turn on the loudness switch and avoid the low end of the volume control." I wouldn't want the bass boost though and have thought about modifying the switch for flat attenuation.

Posted by Steve L. April 24, 2022 - 07:30 pm
Hi George H., I don't think there will be a problem with using 15K instead of 16K for R53A and R59A. R53A/C61A are used to improve high frequency stability. R59A/C59A were included to match the other side of the phase inverter.

Posted by George H. April 24, 2022 - 07:29 pm
Hi Stephen, could you please provide a brief explanation for the implementation of the R53A/C61A and R59A/C59A RC networks in the power amp section of the EICO ST-70 and would it be possible to substitute the 16K resistor with a 15K with no adverse effects.

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