Modifications: Eico ST-70 Vacuum Tube Amplifier


	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:


	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:


	-Eico ST-70A mods article (2MB)




	20 - 20kHz phono response in dB

	Power amplifier modifications

Reader Comments

Posted by Steve L.

July 23, 2023 - 02:37 pm

Hi Jack (TriodeLuvr), Thank you for letting me know about this interesting discussion at Audiokarma.org. As this has been going on for a while, it will take me some time to study the proposed alternative (making V3,4 12AU7) before I can comment in detail. One preliminary impression is that it involves the issue of changing the line input sensitivity. The ST-70a mods package seeks to keep that and the phono input sensitivity as it was originally. The goal was to fix the hiss problem and take advantage of the PA gain reduction to reduce distortion and increase power (at 1% distortion).

I guess the 12AU7 line stage solution ends up reducing sensitivity and uses a higher line input level. I see a comment that "350mv for full output is too sensitive." Of course, that depends somewhat on personal preference and I might not disagree within the context of vintage line levels. But today, line levels are typically much lower so it's harder to argue against Eico's choice. Also, since the phono input sensitivity would be similarly affected, would it be adequate with reduced gain?

Back in the day, 775mV was a common line level for consumer gear but these days, 316mV is mentioned as common. Folks want to connect MP3 players and smartphones as audio sources and headphone outputs seem to have taken the place of formal line outputs. The full scale headphone output from my somewhat-high-end laptop is only 50mV! So it seems that increasing line input sensitivity level far above the original 350mV could cause problems for some.

Posted by TriodeLuvr

July 23, 2023 - 08:43 am

A recent thread at AudioKarma discusses high noise levels (hiss) in the Eico ST-70. This was present both before and after the user completed the mods shown at this site. The solution requires replacing the 7247 in each channel with a 12AU7. I did the groundwork in SPICE, and the owner of the unit performed the new mods. The gist of the discussion begins here:

https://audiokarma.org/forums/index.php?threads/eico-st-70-noise-floor-still-high-after-restore-and-mods.1034313/post-16505009

Feel free to contact me through AK or to contribute to the discussion if you're interested.

Jack

Posted by Steve L.

June 06, 2023 - 02:52 pm

Partly in response to FarmerBoy's and others' comments, I did some experiments using ARTA demo software, a PC and a $20 audio adapter module to substitute for the audio generator and distortion analyzer needed in the mods. I'm pleased to report that the experiments were successful. This is covered in the Appendix which is added to the new Rev.7 of "ST-70A Modifications Package". I will be happy to help if you have any questions, best posted in these Reader Comments, which are live and monitored. Note that if you have a desktop PC with a decent audio input/output card, not even the audio adapter module would be needed. I would still encourage reading the Appendix because it addresses using a built-in audio input/output card too.

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.

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