Eico ST-70 vintage tube integrated amp restoration/upgrade

Fongolio

Today I made a straight across swap for a 100% working Eico ST-70 for a spare Carver TFM-25 I was no longer using. I probably lost a bit on this swap but I really wanted one of the old Eico or Dynaco tube pieces for a restoration project so I think I did alright to get one in fully functioning condition.

This thread will journal the restoration and upgrade process. I'm posting pictures of the unit as it is now and will post more as work progresses. I'm totally stoked about this project and especially happy to get it right before Christmas. At the same time, I also made a trade for an old Thorens TD-160 turntable which is pretty much a basket case. I will also be journaling its' restoration in another thread.

So here are the initial pics of the unit as I received it:

Fongolio

Now with the top off:

Fongolio

As you can see the whole unit is dusty and dirty but it's all there. All the tubes appear to be original Eico's and the original Eico transformers are quite highly regarded. Here are a couple of pics of the back connection board which I plan to modify quite a bit. Binding posts for speakers, gold rca inputs etc.

Fongolio

Now a few pics of the underside with the bottom cover off:

Fongolio

The underside really makes me want find schematics for this NOW. I've joined the Yahoo Eico group and they have manuals and schematics for this unit. It's going to be a lot of work but I know it will be worth it. I'm hoping it will eventually be the center of my two channel rig and power my SDA-1C's to sonic nirvana. Well, at least sound really nice.

michaeljhsda2

I would have made that trade. Nice acquisition!

mhardy6647

Are you just going to do a rehab or are you going to bypass the tone controls, loudness, etc.?

My interest is more than academic... I found an ST-40 on the scrap metal pile at our town dump earlier this year in surprisingly good shape and I also recently was given an ST-70 (in unknown working order) a couple of weeks ago. They're not in the class of the earlier "brown and brass" EICOs, but the iron's still decent and there's definitely potential (no pun intended) in these amps.

Fongolio

Cool finds!! I'm going to be doing a kind of combination of rehab and upgrade. The tone controls will remain but improved, the loudness will be made much more effective (even though I doubt I will ever use it), the third center channel (yes this unit has three channels) will be totally disabled. The mod I'm doing is identical to the one Stephen H. Lafferty has done and documented in a paper he published online and is posted in the Yahoo Eico group. Stephen says "The upgrade fixes problems with line-level noise, phono
preamp bass response, increases low frequency and high frequency power output, reduces distortion and improves power amplifier stability into difficult loads." I can send you a copy of the full description of the mod procedure if you like.

mhardy6647

Thanks; PM me a link if possible, otherwise I can PM you my e-mail address.
Do keep us posted. I am pretty optimistic that both of my carcasses may still be viable.
Good luck and have fun!

Fongolio

PM me your email address and I'll send it to you. I also have a pdf of the manual for the ST-70 if interested.

Fongolio

Ok, spent several hours last night and 4 or so more tonight cleaning the unit. The aluminum bottom that holds the tubes and transformers had a thick coat of dust grime and other unidentified stains on it. There's a screen that covers the tubes and transformers so 50 years of filtering has left it's mark. This was serious hard work with sandpaper, miniature wire brushes and aluminum polish. It's not quite as nice as I'd like it to be but short of a total dismantle and rebuild I'll have to be happy. Anyway, it's light years better than before. I even plished up the aluminum can capacitors and the aluminum tube cozy's (or whatever they're called. I'm new to tubes cut me some slack) Check out the before and after shots. Next up, start collecting the parts for the various mods.

Fongolio

Here is the pic. Go to the previous pics to see before. Thanks.

[Deleted User]

Nice job, that looks great! What kind of aluminum polish did you use?
Regards, Ken

mhardy6647

Fongolio wrote: »

PM me your email address and I'll send it to you. I also have a pdf of the manual for the ST-70 if interested.

Thank you; some serious sweat equity you have in that beastie already!
It looks great to me; like new!

Fongolio

Kenneth Swauger wrote: »

Nice job, that looks great! What kind of aluminum polish did you use?
Regards, Ken

Thanks. Metal Polish Cream by Blue Magic.

[Deleted User]

Beauty! Can you go over your process? How did you get rid of the pitting so well?
Thanks for the polish info, vintage audio restoration is very rewarding. Nothing beats bringing one of those great devices back to life. Very cool!

Fongolio

First I used a wire toothbrush style tool to remove most of the caked on scum which actually worked quite well and gave it an initial shine but I wasn't happy with the way it was still stained so I went deeper with 320 grit sandpaper. Then I used a synthetic kitchen scrubby type thing (the green scouring pads) with the bluemagic metal polish which of course turned everything black until I buffed it out with a cotton rag. My shoulder is still sore from all the scrubbing but the result was worth it.

Kelvin

[Deleted User]

Great info, thanks! The guys on the Ampex group all use the green scrubby pad and soap. Must work, your results look great.

TroyD

Barkeepers Helper....that stuff is freaking amazing.

BDT

geoff727

Thread subscribed to. I'm anxious to see what you do with this.

Geoff

Fongolio

First bench power up. I don't have a variac so I very cautiously powered up the unit and did the hum adjust, balance adjust and bias adjust according to the manual. The manual is very detailed on exactly to do this which I'm very greatful for. There is a little bit of excess hum but I was expecting this considering how old some of those electrolytic caps are. The channel two balance floats around a little and I'm not sure what that indicates. Bias adjusted to the recommended 0.38 volts perfectly on both channels. All tubes are functioning but how well only a tube tester will be able to tell. I have my little Polk R20 satellites hooked up with my Kenwood KD2020 turntable and it sounds luscious. No repairs or mods at all and this little unit has excellent imaging and soundstaging. Even on the bench you can hear the 3 dimensional imaging. The R20's don't have a lot of bottom end but they still sound very good with this amp. Most pleased. I'm tempted to through it into the main system just to see how it would sound with the SDA's. I'll eat dinner and contemplate that move. See pic for chestnuts roasting on an open fire. It almost brings a tear to my eye it's so lovely glowing away like that.

Oh I almost forgot, transformers are barely warm to the touch with the volume playing at about 1/3. These Eico's are well respected for their iron and I see why now.

mhardy6647

You should build yourself a dim-bulb tester (assuming you don't have one?)!
http://www.audiokarma.org/forums/showthread.php?t=41523&highlight=poor+man%27s+variac

The output transformers shouldn't get warm (although they may absorb some heat from the output tubes). EICO is actually (and unfortunately) notorious for using smaller-than-prudent power transformers (e.g., in the HF-12 and HF-81) so if your power transformer's staying cool, count yourself quite fortunate.

As long as the can cap containing the first filter is cold, the electrolytics in that can are probably OK (at least for now). If it gets warm (especially if it gets even a bit warm quickly after power-up), it's (sadly) on the fast lane to self-destruction (and it may decide to take other stuff with it).

Fongolio

mhardy6647 wrote: »

You should build yourself a dim-bulb tester (assuming you don't have one?)!
http://www.audiokarma.org/forums/showthread.php?t=41523&highlight=poor+man%27s+variac

The output transformers shouldn't get warm (although they may absorb some heat from the output tubes). EICO is actually (and unfortunately) notorious for using smaller-than-prudent power transformers (e.g., in the HF-12 and HF-81) so if your power transformer's staying cool, count yourself quite fortunate.

As long as the can cap containing the first filter is cold, the electrolytics in that can are probably OK (at least for now). If it gets warm (especially if it gets even a bit warm quickly after power-up), it's (sadly) on the fast lane to self-destruction (and it may decide to take other stuff with it).

Thanks for the info. I'm still learning a lot of this stuff. The big can caps both are not hot and are only warm from being near all the warm tubes. I was checking out the dim bulb tester and I will definetly build myself one. A very cheap but effective way to power up old pieces.

On a different note, I went ahead and put the Eico into the main system with my two turntables, music server dac, and SDA 1C's. In a word, UNREAL. This amp is the same age as I am, from what I can tell has never been worked on, obviously has something not quite right (the faint hum heard through speakers) and it still kicks serious a$$. I live in an apartment and can't crank it often so low volume detail is what I'm after and this amp serves it up in spades! I also could not believe how much good tight bottom end it has, but the real standout is the pin point imaging. With the Carver TFM-45 the imaging was not nearly as precise. The other surprise was how well the SDA effect works with this unit. I was playing Robbie Robertson's first solo LP (which is recorded extremly well) and more than once heard effects at 90 degrees to where my head was facing. Superb!! The mods I want to do are only going to make this great sounding amp even better. It's a huge bonus as well that it plays so nicely with my SDA's. Fongolio is happy!!!

mhardy6647

welcome to the club! :-)

Fongolio

I purchased some new old stock original Svetlana 12AX7's today on ebay. These are alledgedly the original iron curtain Svetlana's and not the newer ones. They get fairly reasonable reviews. I only need two for the mod (they are to replace the 2 12DW7's) but bought 4 for $38 dollars. Not bad I think. In reading the trouble shooting guide in the manual it says the hum could be coming from a bad V1 or V2 which are original Eico Mullard 12AX7's so it will be handy to have some tubes to roll in and see if it helps.

mhardy6647

probably P/S caps, but could also be related to bad coupling caps or poor lead dress. The latter is one drawback of kit equipment... you get the skill of the builder (for better or worse) :-P I am assuming (guessing) that yours would've been a kit and not factory assembled(?).



and its little brother the ST-40...

Fongolio

Mine was for sure a kit. I can see markings for placement of parts on the aluminum panel all over the place. I was thinking PS caps as well. I think I'll just replace all the electrolytics just to be safe. What's your opinion on the ceramics? Should they go too. I think they are ceramics. I'm talking about the disk style. And how can I tell if the leads are dressed correctly? Thanks for all the help now and that which you may give me in the future.
Kelvin

mhardy6647

ahh... the issue of ceramic capacitors :-) Well, the EICO HF-81 notoriously used ceramic coupling caps to the EL84s. Some folks ascribe much of the HF-81's magic to those; I am not one of them! Ceramic capacitors are not terribly linear; almost anything that carries the signal would be an improvement to ceramics in the signal path. Looks like you don't have ceramic coupling caps (at least to the 7591s)... "we" should check the schematic.

FTGV

mhardy6647 wrote: »

Ceramic capacitors are not terribly linear; almost anything that carries the signal would be an improvement to ceramics in the signal path.

Exactly,they are very well suited to power supply decoupling but not for coupling or feedback loop usage.

Fongolio

In place of the ceramics what is recommended? And on a completely different note, I have my SDA's connected to the 8 ohm taps but the SDA's are rated 6 ohms. Would connecting them to the 4 ohm tap make much of a difference. I appologize for all the newbie questions but...well....I'm new. Thanks.

mhardy6647

I'd say any decent film-type capacitors; for my money the venerable Sprague (SBE) "Orange Drops" (either 715 or 716 series) or the cheap but perfectly decent Illinois "lemon drop" capacitors sound very good and won't break the bank. They're also not physically huge ('specially the Illinoises) which is a plus in retrofitting most anythine (and even though the EICOs are pretty spacious below decks).