Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 1blueheron on February 16, 2021, 02:22:04 pm
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An old college buddy gave me an Eico ST70 integrated amp to keep if I want. I am guessing it needs a recap and resistors have likely drifted. Has anyone restored one of these? Would be interested in hearing some thoughts and opinions on the process and maybe what to expect. This would be my first "hi-fi" tube project, all others having been Guitar amps.
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I am restoring one right now. Is it DOA? The power supply is the place to start. Those two cans and the bias supply. There are a couple of good articles on the web. Did you find them? Jim
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Wow a real monster, great project.
Some pictures here:
https://www.canuckaudiomart.com/details/649487701-eico-st70-integrated-amplifier-clean-amp-working/images/2105314/
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An old college buddy gave me an Eico ST70 integrated amp to keep if I want. I am guessing it needs a recap and resistors have likely drifted. Has anyone restored one of these? Would be interested in hearing some thoughts and opinions on the process and maybe what to expect. This would be my first "hi-fi" tube project, all others having been Guitar amps.
the choke is another weak point. it filters the entire amp and is grossly underrated. there are 3rd party PCB for improving the filter string. scrap the 5AR4/5V4 rectifier and go solid state.
--pete
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The last I heard it working was back in about 1994/95. It was in our bachelor days when all we did was work, go to the used record stores, and play with stereo's and guns. I found it at a flea market and didn;t have any cash with me. I think he paid $40 for it. We cleaned it up and he used it in his home setup for a few years on and off.
We both met the girls of our dreams a few years later and the stereo hobby took a back seat for both of us as we got married and with our wives went our separate ways. After that it went through several moves and eventually ended up in storage in WPB FL. He dropped it off at my house during a visit a couple years ago and it has lived under my bed since waiting to be restored. I was somewhat hesitant to plug it in after he dropped it off. Since then I have learned a ton from the good folks here on this website and am gathering enough courage to attack this project and attempt to unleash some of its potential. It has some fond memories and sentimental value so I don't want to blow it up.
I have found a couple websites where they have been discussed and some mods been performed. Some of them got pretty outrageous plating the chassis, adding tubes, and other such things. Not looking to go that far. I want to start simple. We messed with it a bit back in college and if I recall, we had found telefunken preamp tubes for it. Not sure if they are still in it or not. I just looked on tube depot and it looks like they are now selling for 500.00 ea. I'm stunned. Guess I need to drag it out and blow out the dust bunnies.
We have no power at the house right now. Lost it last Sat morning and have been trying to live on generator power. Camping out at work right now where I have power and internet while the next ice storm approaches. Spring can't come soon enough right now. Will dig it out and post up some pictures when the power comes back on. 'Til then I will do some searching the interweb on PCB. Seeing lots of stuff about the Dynaco ST70 mixed in. I thought them distinctly separate beast? Will keep digging.
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Ok, i missed that this is an EICO ST70 - Dynaco ST70 locked in brain when i read the ST70 - apologies. they are much different beasts. when you said it had/has telefunken preamp tubes, then it clicked, as 7199 (dynaco ST70) were RCA exclusive at that time.
replace filter caps, coupling caps to power tubes, check bias, check tubes, call it good and jam.
they are a well regarded integrated.
--pete
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There is no choke in this amp. Looks for fried stuff underneath carefully. Ask your buddy when and how it failed. Check the fuse for continuity ---should be 2 1/2 amp fast blo (312 3AG 1/2 amp). Check the tubes if you can ...if not use a light bulb current limiter if you have one and check it with speakers and source input. Mine produced some low level audio ugly audio. If the fuse fails---that's why it is there. Get to work. If you have audio it might not that tough. PM me..this is a guitar amp forum and this thread might be moved. PM me for more info...Jim
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I have one that I recapped a few years ago. These amps have very nice output transformers. The early ones had undersized power transformers (not enough filament current) and then Eico changed to a more robust one that is a bit taller than the original. You might start your work by ohming out the various PT secondaries before you spend money on parts.
A known failure point for several Eicos is the on/off switch …. on a ganged treble pot..... in the lower corner. A difficult part to replace, and it is in a vulnerable spot. There are workarounds if this is broken on yours, and if it is not, take care as you move the amp around.
I thought it was a very straightforward recap; I used like-for-like on all the coupling caps, and power supply stuff. I used JJ cans , a 2x50uF and a 4X20uF. Bias supply caps are positive-to-ground, so watch for that. This amp has individual bias and balance for each channel, and it is accessible from the top so that is very helpful. If yours still has the original Westinghouse power tubes try to use them if at all possible. The original rectifier was a Mullard - still the best choice if yours is working.
There are lots of mods out there, as you saw. The Tronola hot rod was of no interest to me, as this amp is already too powerful for my needs, but the Loudness mod is one that most people do. The Balance Check is pretty weird, and the Phase Reverse means that the speaker connections look strange (but you probably already know that).
I did the phono mod from the Tronola site but (for me anyway) I think it has too much gain, and I could have done without it.
Perhaps the oddest thing was that the OT reference to ground is the 4 ohm tap. Many people change this to use the Com to ground, but doing so requires different feedback values. I want to think the Eico engineers knew what they were doing here, but it is largely misunderstood and frequently changed. In retrospect I would have left mine stock.
Please ask questions as you get into it. Lots of seasoned tube guys on this site that are great about troubleshooting.
Dick
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Nice project! I like this site https://diyaudioprojects.com/Technical/ (https://diyaudioprojects.com/Technical/) but you have likely already found it. On their Forum they argue about signal caps even more than folks do here. :icon_biggrin:
I have rebuilt a few Fisher and Grommes mono-blocks and stereo power amps. Love the sound. I have kept all of mine stock except for fixing a recurring power rail problem on a big Fisher SA-300. I recall my wife saying she thought something was wrong with the stereo; me waving her off; and she pointing out that there was smoke curling up from the amp. At that point it was hard for me to argue that she was wrong.