henry4841 Posted November 27, 2020 Share Posted November 27, 2020 I just now posted a picture of my latest purchase, an Eico ST-70 on Pictures of your Tube Amplifier. The amplifier was bought on Ebay for what I call the price of the iron. And I do mean heavy iron. The owner says it plays fine as is but I will not turn it n until I replace the electrolytic caps just to be safe. All the original tubes except possible the rectifier tube which is branded RCA with a Britain recognition. Could possible be the original as well. Many of these amplifiers were sold both as kits and as a retail item. Many you find have had the preamp part removed but that was not what I wanted. I waited till I found one that was not molested. Unlike the Dynaco ST-70 these can still be found at a decent price. As far as appearance it looks surprisingly good. Some of the center caps are missing on the knobs but there is a punch you can buy to make them again. I paid $300 for this amplifier which as I said is basically what new transformers would cost to build a new amplifier. Shipping was not cheap because this thing is really heavy, a good thing. There is online modifications one can do to improve certain parts of the amplifier as well as info on hot-rodding this amplifier. That is not my intention though. I want to keep it as original as possible. And yes it came with the cage. For those that missed the picture. 1 1 Quote Link to comment Share on other sites More sharing options...
cincymat Posted November 27, 2020 Share Posted November 27, 2020 Very nice Henry. Are you going to document your rebuild in this thread? It would be nice to be able to follow along. cincy Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted November 27, 2020 Share Posted November 27, 2020 Eico has always been one of the best values in tube amps, even when they were new. Restored, they typically can hold their own against almost any modern costly unit. Like Lafayette tube amps, they are always worth picking up at the right price. The “guy down the road”, a professional musician, said he wants to be buried with the HF-89 which I restored for him. It drives his CWs to perfection, and he says it is the best sounding amp he has ever paired with them. Maynard 1 Quote Link to comment Share on other sites More sharing options...
mike stehr Posted November 27, 2020 Share Posted November 27, 2020 I bought a Eico ST70 at ST. Vincent DePaul in the as-is area for 3 bucks...back in 2007-08... All the original tubes, all testing strong. The clerk, (jail work release) had no clue what it was. Unlike Henry, who wants to rehabilitate and restore his ST70, I promptly ripped mine apart wanting to build a power amp out of the unit. Getting over my head, I ended up with an oscillating mess. Gave up, and it's been on the shelf ever since. I still want to build a power amplifier out of the unit, but maybe using 6L6 instead of 7591. Quote Link to comment Share on other sites More sharing options...
henry4841 Posted November 28, 2020 Author Share Posted November 28, 2020 Before I bought mine I did a search on this forum about the Eico and found the thread where you bought this amp years ago Mike. I would like to keep the same appearance in my restoration including the can capacitors. From research I have found that many of the new can capacitors are nothing more than empty cans filled with modern capacitors and not made like the old ones. I would like to gut the old can and install new ones in them but it may not be possible. We'll see down the road. I may just have to buy new cans. I have a number of projects to do before I tackle this one but at least I have found a good example to restore one day. It is supposed to work and sound just fine the way it is now. Still I am reluctant to try it as it is now. One thing I like is it is a full preamplifier with a phono stage along with being a power amplifier. The phono stage is where some modern mods need to be done from what I have read. Increasing the capacitance of one capacitor and installing a pot in the RIAA circuit to compensate for the change of the cap along with replacing the mica's with film caps are some of the changes others have advised to do. I have not thought about documenting the restoration with pictures as Cincymat suggested but if there is enough interest I may do so. I am going to have to pony up some more money, probably as much as I paid for the amp, to complete the restoration but I knew this before I considered this project. I will be lucky if the Eico labeled tubes made by Mullard are still good. I can expect the front end tubes to still test good but I will assume the output tubes are now weak and need replacing. Quote Link to comment Share on other sites More sharing options...
henry4841 Posted December 11, 2020 Author Share Posted December 11, 2020 Making some progress on this restoration project. I replaced the old electrolytic caps but not without a problem. I installed one cap with wrong polarity and had a dead short. It was in the bias circuit where the positive goes to ground instead of the negative. Easy to over look, at least for me, if one is not careful. It took me hours over a couple of days to find the problem where a good technician would probably find in an hour or so. But it does make it hard when one has changed many caps finding the one that was installed wrong. I am old and late in the game of audio gear repairs having dealt with other types of electronic products in the past. I took on restoration projects of old equipment, mostly SS, for the challenge of finding problems. The SS receivers I look for on Ebay are the ones that are sold for parts and not working. My Eico ST-70 was assembled as a kit sometime in the past. They were sold in 1962-63 for the price of $99 as a kit. Does not sound like much now but at the time it was a good chunk of change. Right now I am not sure that it is working in stereo. Might be that I just do not have the switches in the right spots or it never did work as stereo. I have only checked for sound on both speakers and cut it off being satisfied for having it working. One is never sure with a kit build from the past. Some switches are going to have to be taken apart to be cleaned. Just spraying D-oxit did not work on them. Common problem on a piece that is close to 60 years old. The old yellow knobs are soaking in hydrogen peroxide and it appears to be working turning them white again. I am missing some of the metal inserts but I have found one source for them. At least I now have a working recapped amplifier. More to come. Quote Link to comment Share on other sites More sharing options...
Marvel Posted December 11, 2020 Share Posted December 11, 2020 10 hours ago, henry4841 said: One is never sure with a kit build from the past. TRUTH! I got a Dynaco ST70 from a friend who had built it. I got it in 1975, paid him $50. It sounded great, and I used it for years. In early 2005 I decided to try the Curcio driver board in it. While working on it, I found some miswired components. One spot, it still sounded ok. I currently have it apart toput in a VTA board, and may adjust to put in 6L6 outputs instead of EL34. Quote Link to comment Share on other sites More sharing options...
cimchazz1 Posted December 21, 2020 Share Posted December 21, 2020 Hello Henry, I also recently purchased an ST70 that looks to be all original. I got it with no tubes. Ordered all vintage RCA and Sylvania and mMullard tubes, so faer, except for the new Tung Sol 7591matched quad. I am now in the process of checking all the resistor values. I will probably buy the CE can caps, and maybe the cap resto kit on Ebay? Anyway, I also have the metal insert knobs missing 3 inserts. From what I have found out, the metal insert knobs are shown in the 1964 Eico catalog and it is called the "classic series". I was wondering where you found the metal inserts for the knobs? I need to get some. Also need the top cover. Are you following anyones published resto instructions? I will probably be restoring what I have, themn will start on the simple mods, like the loudness switch. Thanks, Chuck S Quote Link to comment Share on other sites More sharing options...
mike stehr Posted June 26, 2021 Share Posted June 26, 2021 On 12/11/2020 at 4:34 AM, henry4841 said: My Eico ST-70 was assembled as a kit sometime in the past. They were sold in 1962-63 for the price of $99 as a kit. Mine is a kit as well. The price for a 5AR4/GZ34 rectifier in 1961-62 was 4.50 to 5.00 bucks. At least the tubes came with the kit... I have my Eico ST70 currently on the bench, and will follow the Tronola mods. Quote Link to comment Share on other sites More sharing options...
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