Eico ST 70

[xbart]

Picked up this Eico ST 70 two weeks ago.
Starting to clean it up and come up with a game plan.
The outside is pretty nice, inside a lot of dust. Undersides look OK, with no obvious damage.

Being that it looks pretty nice, I'm thinking that I'll do a basic recap and a fix a few known flaws and give it a listen.
I have a lot of the needed caps on hand and I have re-stuff the old metal twist lock cans in the past.
The original can for the first couple stages of power supply filtering is a 40/20uF 500V and I don't have any 500V caps that will fit inside the old can. So looking at my options, I could use a snap-in 50/50uF 500V from the likes of JJ, F&T or a twist lock from CE Mfg. The CE twist lock caps are 3 times the cost of the snap-in caps, any reason to stay away from one of the snap-in caps other that looks, is there a quality difference?
I also notice that there is a -10 to +30% tolerance on the 50/50uF cap. If it much higher than 50uF, it might be a issue for the rectifier.

Eico ST70 owners, I would like to hear your thoughts on this amp.

 

[Retrovert]

Excellent score. The ST-70 is an awesome amp; better than my ST-40 which is excellent. I have the matching ST-97 tuner which is equally angular.

You might want to read Lafferty's excellent articles on Tronola about redoing the ST-70:

tronola.com/ST-70A_mods_package.pdf
tronola.com/html/st-70_mods.html
tronola.com/html/st-70_hotrod.html
tronola.com/html/st-70_hotrod_page-2.html​

EICO HiFi Looks like some German Bauhaus or Brutalist dressed in black went all jackboot on an amplifier, paring away anything that might be considered to be decadent bourgeois styling, or superfluous to function. Great output iron on those amps; with some efficient speakers you can liquify your brain with the volume at ten o'clock.

 

[xbart]

Excellent score. The ST-70 is an awesome amp; better than my ST-40 which is excellent. I have the matching ST-97 tuner which is equally angular.

You might want to read Lafferty's excellent articles on Tronola about redoing the ST-70:

tronola.com/ST-70A_mods_package.pdf
tronola.com/html/st-70_mods.html
tronola.com/html/st-70_hotrod.html
tronola.com/html/st-70_hotrod_page-2.html​

EICO HiFi Looks like some German Bauhaus or Brutalist dressed in black went all jackboot on an amplifier, paring away anything that might be considered to be decadent bourgeois styling, or superfluous to function. Great output iron on those amps; with some efficient speakers you can liquify your brain with the volume at ten o'clock.

Thanks for the links, I'll look them over when I get some free time.
I would like to get it back to near stock before I decide what other modifications I might do to it.

 

[radioactive]

The loudness mod is one mod that should be done.my st 40 has it and is otherwise stock.

 

[xbart]

The loudness mod is one mod that should be done.my st 40 has it and is otherwise stock.

OK, I did the same fix on my HF 85. The other issue that I'm aware of is the ground coming off the 4 Ohm OPT tap, instead of the common tap.

 

[Retrovert]

The loudness mod is one mod that should be done.my st 40 has it and is otherwise stock.

EICO legend has it the engineers balked at adding a loudness control and deliberately botched the implementation.

Makes about as much sense as the loudness circuit itself.

This has PECs (talk about awful technology) which will need to be replaced. Sam made a replacement board for these, I think. Check over in BT.

 

[xbart]

EICO legend has it the engineers balked at adding a loudness control and deliberately botched the implementation.

Makes about as much sense as the loudness circuit itself.

This has PECs (talk about awful technology) which will need to be replaced. Sam made a replacement board for these, I think. Check over in BT.

I actually bought some from Sam and put them in my HF85, they worked great.
I bought several extras, so I'm set, if I get into it that deep.

 

[radioactive]

EICO legend has it the engineers balked at adding a loudness control and deliberately botched the implementation.

Makes about as much sense as the loudness circuit itself.
I heard the same story or read about it on the Internet but don't know how true it is.

This has PECs (talk about awful technology) which will need to be replaced. Sam made a replacement board for these, I think. Check over in BT.

 

[Retrovert]

I heard the same story or read about it on the Internet but don't know how true it is.

It's one of those stories that should be verified or debunked. So I did some digging and halfway down in my search I discovered dcgillespie had already done the heavy lifting:

audiokarma.org/forums/index.php?threads/eico-st-40-poor-low-end-response.342209/#post-4271329
Most loudness switches produce only a slight change in basic sound level, while adding to the low frequencies, with some also adding to the high frequencies as well. This is what most folks are accustomed to, and that then allows you to turn the volume down to a background music level, yet still enjoy the boost provided.

In Eico's case, they decided to design theirs to operate so that turning the loudness switch on automatically provided a level reduction, while adding a low frequency boost function all at the same time. The advantage of this approach was to give much more flexibility to the level control under low volume conditions when the loudness switch was activated. Most folks however, logically reacted to that by saying what's up with that? Very understandable given that nobody else's loudness switch worked that way!

For the record, I also think that Eico's approach was lame as well. It was one of those ideas that sounded good from a feature standpoint, and even has merit from an engineering standpoint. But the reality of the design is that nobody liked it, and it leaves the resulting sound sounding very muddled.

If you change out the existing loudness circuit for the ones offered, the loudness switch will react much more "normally", leaving the basic sound level largely unchanged, while adding both a low and high frequency boost. May Eico's loudness circuit rest in peace!

 

[whoaru99]

Picked up this Eico ST 70 two weeks ago.
Starting to clean it up and come up with a game plan.
The outside is pretty nice, inside a lot of dust. Undersides look OK, with no obvious damage.

Being that it looks pretty nice, I'm thinking that I'll do a basic recap and a fix a few known flaws and give it a listen.
I have a lot of the needed caps on hand and I have re-stuff the old metal twist lock cans in the past.
The original can for the first couple stages of power supply filtering is a 40/20uF 500V and I don't have any 500V caps that will fit inside the old can. So looking at my options, I could use a snap-in 50/50uF 500V from the likes of JJ, F&T or a twist lock from CE Mfg. The CE twist lock caps are 3 times the cost of the snap-in caps, any reason to stay away from one of the snap-in caps other that looks, is there a quality difference?
I also notice that there is a -10 to +30% tolerance on the 50/50uF cap. If it much higher than 50uF, it might be a issue for the rectifier.

In one of my previous rebuilds where I was not concerned about retaining original appearance I used the F&T can caps. Best as I can tell they're good caps. Going by the data sheets they ought to be a step up from JJs.

As far as the tolerance, the originals probably had a wider range than that. FWIW, new Nichicon caps I've measured recently all measured within spec, but on the minus side. Around 4-5% under.



(Btw, I did nix that eye burner pilot light)

 

[xbart]

As I was going through this, replacing all the electrolytic and bypass capacitors, it became obvious that time had taken a big toll on this amp. Corrosion had weakened a lot of the wiring, I had wires breaking just by bending them. The solder tabs on the sockets weren't much better, and several broke quite easily. Visually the amp looks good and there is no sign of corrosion.

So I need a new game plan. Looks like I'm going to have to strip this and start from scratch,
using the iron and chassis. BuzzK's ST 70 build http://www.audiokarma.org/forums/showthread.php?t=496518 looks like a good possibility.

 

[xbart]

After several hours and a yard of wick!
The nine pin sockets are all bad and will need to be replaced.
Started painting the transformers and cleaning up some of the reusable components.

 

[Retrovert]

Wow! That looks like an EICO kit from the factory. Nice work!

If you haven't already concluded this, I suggest replacing all of the resistors with 1% metal film. Carbon composite have no magical mojo. What they have is noise and distortion, the gifts that keep on giving.

 

[xbart]

Wow! That looks like an EICO kit from the factory. Nice work!

If you haven't already concluded this, I suggest replacing all of the resistors with 1% metal film. Carbon composite have no magical mojo. What they have is noise and distortion, the gifts that keep on giving.

Yes, the carbon comp resistors will get replaced.
I have used KOA Speer SPR2 series carbon film resistors in my last couple builds and really like the results.
They are a little more expensive then metal film, but still reasonable.

 

[Retrovert]

Yes, the carbon comp resistors will get replaced.
I have used KOA Speer SPR2 series carbon film resistors in my last couple builds and really like the results.
They are a little more expensive then metal film, but still reasonable.

About twenty percent, which is pretty negligible on a ten cent item.

Carbon film has a lot more noise than metal film. It has problems at higher voltages and frequencies which don't apply here. The accuracy of metal film for the same price is 1% vs. 5%.

Carbon film has no mojo and just retains some of the debunked mystique of carbon composite. So there's no reason to use it.

 

[xbart]

About twenty percent, which is pretty negligible on a ten cent item.

Carbon film has a lot more noise than metal film. It has problems at higher voltages and frequencies which don't apply here. The accuracy of metal film for the same price is 1% vs. 5%.

Carbon film has no mojo and just retains some of the debunked mystique of carbon composite. So there's no reason to use it.

Are there any metal film resistors that you can recommend that are physically bigger and have thicker gauge leads? I don't care for the fact that the metal film tend to be very small and have short thin leads.

 

[Retrovert]

Lead gauge depends on wattage.

Why do you want thicker leads?

 

[primosounds]

Are there any metal film resistors that you can recommend that are physically bigger and have thicker gauge leads? I don't care for the fact that the metal film tend to be very small and have short thin leads.

you can get the mil spec ones that have the prefix RLR
there are numbers after that relate to wattage and value. I know for 1/2 and 3/4 and 1 watt sizes they use a thicker lead wire.

 

[primosounds]

Hey Retro, i like to use thicker leads on resistors especially because i do point to point wiring and the heavy gauge leads are better at holding their shape and stay in place better than the thinner lead out resistors. It is primarily that for me. cheers.

 

[xbart]

Hey Retro, i like to use thicker leads on resistors especially because i do point to point wiring and the heavy gauge leads are better at holding their shape and stay in place better than the thinner lead out resistors. It is primarily that for me. cheers.

I agree, and for myself the larger resistors make for a nicer looking job.
The physically larger resistors along with thicker leads have longer leads and when rebuilding a vintage piece the shorter leads of most 1/2 watt metal film will often not have the needed length.