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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. November 25, 2020 - 06:44 pm
Hi Loomis, Sorry you're in quarantine--hope all goes well, otherwise. Answering your questions:
#1 - (Higher voltage caps okay?) - Yes, higher voltage ratings will be fine. Sorry that was left off of that line.
#2 - (Use 56K instead of bridging?) - Yes, that will be fine. The only thing is, the method of using fixed resistors instead of a pot means that the setting won't be optimum. As mentioned in the text, the pots in my unit ended up about 420K and 290K. The best setting is highly dependent on the tube. I included the fixed 360K alternative as a rough shot at it for folks who can't do the adjustment. With an extreme tube, results might not be satisfactory.
#3a - (remove all wires and/or switches etc going to the center speaker, the speaker phase switch, and the balance check switch?) That should be okay, as long as the circuit is left with the same connections it would have with those functions unused.
#3b - (What about the hi and lo filter switches?) Again, that should be okay, as long as the circuit is left with the same connections it would have with those functions unused.
#4 - (Will the loudness circuit correction mods interfere?) I would have to see the mod schematic to answer definitively but I did a brief search and what I saw at one link didn't seem to indicate a problem.

Thank you for your interest and I will be happy to help if you have any other questions.

Posted by loomis November 25, 2020 - 02:23 pm
Hello. Thank you so much for creating this detailed package of modifications.

I have a few easy questions that I'm hoping you'll be able to answer, so that I can order parts and perform these mods during my quarantine.

1) Usually you comment that it's ok to use higher voltage rated parts. But you don't say that in the parts list next to C7-C10, the 0.1uf 400v film caps in the phono section. Can't I simply upgrade these, and the rest of the coupling caps in this unit, to Panasonic or Orange Drop 630v polypropylene film caps instead of 400v? I wasn't clear if there was some reason to specifically need no higher than 400v there, of if you simply forgot to type "higher voltage ok" next to the parts there.

2) If using fixed resistors, instead of bridging R27 and R28, can't I just remove R27 and R28 and put in something like 56K resistors, instead of bridging the existing resistors with 360K's?

3) After doing these mods, can I then remove all wires and/or switches etc going to the center speaker, the speaker phase switch, and the balance check switch? What about the hi and lo filter switches?

4) Will doing the loudness circuit correction mods interfere in any way with any of your modifications?

Thanks again for all your help!

Posted by Mark W. May 03, 2020 - 03:13 pm
Hi Steve, Really interesting Hot Rod st70 article well written too. I have decided that going the way of mono blocks is better fro me due to my low power draw Klipsch Cornwell II and Forte' I spkrs. I'm after sound rather than power. Yes even if power is lost to the achievement of better quiet space and definition of actual sound not fb. I use a local tech who has decades of exp. & very sharp so I'm sure he can get done whatever is doable. I'd be grateful to get your thoughts on mono blocking my matching ST70s. mlw

Posted by Steve L. April 30, 2020 - 03:21 pm
Hi Mark, I hate to admit it, but there may be some truth to your suspicion. It will make a difference of 3dB, which is quite significant but could be accused of not being "big." Often, lots of effort is expended to get improvements which are actually very subtle. Does anyone really notice the difference between 15kHz and 20kHz frequency response? Between 1% and 0.1% distortion? Between 80dB and 100dB signal-to-noise ratio? I doubt it.

But people may still insist on such things and want to know that their equipment goes above and beyond good-enough. Some may even claim to hear a difference. I imagine it's mostly a psychological difference though. Yet, since music is a subjective experience, one could argue that a psychological difference matters. For example, would you still feel the same about a song if you found out it was generated by a computer simulation?

The answer for you may depend in part on your speakers. If they are very efficient, such an upgrade would be less effective than if they're "space heaters," as my friend with Klipsch Cornwalls refers to my classic AR9s :) With some rewiring, you could actually do an experiment with your existing ST-70s, to use each as a monoblock to see how it sounds. Post a reply if you want to know more about that.

By the way, there is an article on this website which describes modifications to an ST-70 which almost double the power and would leave your second unit intact:

Posted by mark whalen April 30, 2020 - 02:27 pm
I own two functional st70s in original configurations in need of going through.

I'm told that it will be possible using parts from each to create one 70 WPC unit. Apparently this hasn't been tried so from that P.O.V. the prospect intrigues me, but my suspicion is that the extra 35 watts wouldn't make a big difference in the sound. Any thoughts?

Posted by Steve L. November 12, 2019 - 06:43 am
Hi Peter, Thank you for your posting and kind remarks. First, your mention of "shiny blank piece of steel" brings to mind the fact that many ST-70s today suffer from corrosion and rust problems. I trust that there will be appropriate plating or coating to preserve the chassis.

- "...could a 100k pot be used...without the 68k resistor" -- Yes, you are correct that the nominal value is expected to be roughly 57K and thus replacing R27 and R28 with 100K pots should work well.

- "One option for improving performance of the output stage is to regulate the screen to maintain a constant difference between the plate and screen voltage through Zener diodes...Is it worth doing either of these for better performance?" -- There is little doubt that reducing screen voltage droop will increase the output power but this would also increase screen power dissipation. The 7591 tubes work pretty hard in this amp and the dirty little secret of these and similar power tubes is that screen dissipation may be more limiting than plate dissipation. It's also insidious because a glowing screen is partly hidden and tends to be obscured by cathode glow. The tube makers weasel-worded the screen power spec by setting it at only 3.3W but adding a footnote permitting up to 6W "during the periods of maximum input of speech and music signals". No mention of how long those periods are allowed to last. (How long can I test it at 6W?) A datasheet example somewhat like the ST-70 shows almost 5W screen power at max output. I would have to see screen power test data before endorsing a mod which increases screen power in this amp.

- "In addition to your modification to balance the 6SN7 driver/phase inverter, another modification I have regulate the cathode current to 6 - 8 ma versus grounding through a 18k resistor...does it substantially impact performance?" -- Increasing the impedance of the cathode current source does make the second output of the phase inverter closer to the same level as the first output, given that the plate loads are the same. Of course, Eico's circuit and my version adjust the plate loads to take the cathode resistor effect into account. But having the AC balance adjustment corrects any unbalance from the phase inverter or power stage, so adding a cathode current source seems superfluous and isn't an adequate alternative to having the adjustment.

- "Removing the rectifier tube and going solid state seems like it could be beneficial as it takes the filament load from the transformer...Unfortunately the resulting higher voltage may require additional dropping resistors...Any thoughts?" -- Yes, it would be nice to reduce the load on the power transformer. One problem with the solid state rectifier would be the fact that initially, the high voltage supply chain comes up without a load, making its initial voltage far higher than it would be if the delay provided by the GZ34 weren't there. I guess you could use (much) higher voltage caps in the supply chain or provide a delay relay but is all that really worth it? The GZ34 burns 9.5W of heater power. If the transformer is 85% efficient, the transformer dissipation saved will be about 1.4W. I doubt this will make a big difference in temperature.

- "Does DC for the filaments of the preamp tubes really produce change that is audible?" Yes, particularly if done for the line stages as well. Hum in middle-class tube preamps was one of those plagues which I've had to live with all my life. I've often dreamed of doing a conversion (starting when I was a teen!) but somehow never got around to actually doing it. Not only does it remove the tube-induced hum, but it also removes the 6.3VAC snaking through the sensitive phono circuits (shudder). However, you would need to be careful that the current pulses charging the filter caps are not comingled with signal grounds and that the DC is free of higher frequency line harmonics. A possible issue is that, if you're using the existing 6.3V windings as the source for the DC heater supply and have a filter cap at the rectifier, the spikes of current will impose higher frequency harmonics on the 6.3VAC. The concern is that this could get into the circuits with AC-powered heaters. You can't use a choke input filter in this case because the resulting DC voltage would be too low. I don't know how much of a problem (if any) the harmonics on the 6.3VAC would be with the cap at the rectifier, though.

Peter, thank you for the excellent questions and I hope your ST-70 project goes well. Will be happy to help if there are questions or problems. Please keep me posted on how it goes.

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