My first post......I have a chance to get an ST-70 and wondering what I should look for? This is a working unit with only enough upgrades to keep it working. Are there some basic checks with DVM that I can try (O/P transformer windings, voltages etc.). Are there any switch positions in particular that I should check and listen for hiss and hum? I will check play in switches and pots etc. but for those of you who have experience I would appreciate your advice. Thanks in advance............
I don't think any more than you mentioned. It should be recapped so you don't endanger the transformers as you probably know. Good to check resistors and clean pots, tube sockets (re tension pins as needed) ,& switches.
There is a loudness switch mod out there and maybe preamp tube sub mod out there. Google amp to find out more. AK and this form may have more info. AA too. Check in Eico yahoo forms. You will have to join to get info.
Should be a nice sounding amp when redone!
Randy
There is a loudness switch mod out there and maybe preamp tube sub mod out there. Google amp to find out more. AK and this form may have more info. AA too. Check in Eico yahoo forms. You will have to join to get info.
Should be a nice sounding amp when redone!
Randy
Be sure that the output transformers are the same. I would look up the original EICO part number to verify the originals weren't stripped off & sold separately. They are very good and get put into service on other amp projects. Also see what generation 7591 output tube is in it, some are better than other. Beyond that, it is a very nice integrated amp. With a few mods it can be even better.
6L6s will work with the O/P trafos, but require a larger drive voltage than is provided by the OEM circuitry. The 7591 is as easy to drive as EL84s and 6V6s.
This PDF about modifications may be of use. Trying to find a URL for a complete schematic is frustrating. However, the power section and PSU are here. "Classic" Mullard style topology is used.
I dislike the AC heating used in the low level mag. I/P section. The Eico ST70 is yet another vintage design that (IMO) needs reworking of its source selection capabilities, to accommodate current realities. Having a tape head I/P makes no sense in today's environment and places switching in circuitry handling mV. signals. If I were working on an EICO ST70, the OEM source selector switch would be replaced by an inexpensive Lorlin part from Mouser that would yield up to 6 possible signal sources. The low level mag. I/P would be hard wired for phono as 1 of the possible sources and the rest as "line" level.
PECs, which are encapsulated combinations of resistors and capacitors, were employed by EICO and frequently are a source of consternation. Fisher used 'em too.
Here is how I did a few of them including my own with 6P3Se tubes.
These are 6L6gc equivalent that is superb only its a bit easier to drive than the 6L6gc.. As Eli had mentioned,the circuit can lack drive so you have to do some resistor changes on the 6sn7s to make them swing a bit more voltage.
The hf50s,60s,hf87 and 89,all have the same basic circuit however,they have no problem driving the KT66 or the 6L6gcs because they running a higher B+ on the 6sn7s. I was able to raise the B+ on the 6sn7s of the ST-70 by about 19vdc and with other complementary changes,it came close to the drive of the units mentioned.
You can see the way i rebuilt this one and I used a film caps for the power supply..I can give you the many parts and resistor I had used..I have paper in oil caps as well as a few teflons and silver micas.
These are 6L6gc equivalent that is superb only its a bit easier to drive than the 6L6gc.. As Eli had mentioned,the circuit can lack drive so you have to do some resistor changes on the 6sn7s to make them swing a bit more voltage.
The hf50s,60s,hf87 and 89,all have the same basic circuit however,they have no problem driving the KT66 or the 6L6gcs because they running a higher B+ on the 6sn7s. I was able to raise the B+ on the 6sn7s of the ST-70 by about 19vdc and with other complementary changes,it came close to the drive of the units mentioned.
You can see the way i rebuilt this one and I used a film caps for the power supply..I can give you the many parts and resistor I had used..I have paper in oil caps as well as a few teflons and silver micas.
Thanks for all the tips and advice. I went ahead with the purchase and it's now sitting in my garage. I haven't had any time to have a look except that the balance control knob does not line up with the others. I have a feeling that it has been replaced with a non OEM type but I will have a closer look later today. The fellow did say that the volume control was replaced last year with an OEM but maybe he was a little confused. He said he had a tech do it. Could anyone tell me what the Eico part # would be for the balance pot and the volume pot just in case both need to be replaced. Here is a pic. of the Vol. and Bal. pots. Thanks again.
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Would appreciate any details on your upgrade components that you wish to share. If it is ok to post here or maybe send me a pm. Thanks.
A couple things...
1. I have a complete Eico ST-70 factory schematic - drop me a note if you need a copy.
2. Keep the 7591As as the output tubes. The EH tubes are EXCELLENT!!
3. If you are going to do any mods start with the power supply. Improving the power supply improves virtually every area of the amp's performance.
Then look at the audio path caps - the cathode bypass electrolytics need to be replaced and increased in value as do the coupling caps.
There's a LOT you can do to improve the ST-70, the power amp section is nearly identical to the HF-89 which is a real nice amp - it uses EL34s instead of 7591As.
You can find most schematics and build manuals at the Yahoo EICO users group. I need to restore my dad's ST-40 one of these days.
HIFI Engine has the complete EICO ST70 manual.
JMO, a stock EICO ST70 is in need of some serious modernization.
JMO, a stock EICO ST70 is in need of some serious modernization.
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