Eico ST70 Rebuild

Part 1

The first ST70 I owned was in the early 80's and it was my first tube amp. After having a Bryston 2B with a Hafler 101 preamp and then getting a matching Hafler 100w/ch power amp , the stock ST70 had a sound that blew me away compared to the other components. At that time I paid $35 for the ST70 with all original 7591 EICO's,a mullard 5AR4 and mullard 12ax7's and 7247's. This led to a friend finding me the last 2 NOS Output transformers from a HF-50 Eico. From there we built 2 mono blocks on one chassis. This amp had power, detail etc....wow. I'll post the schematic later, but if you own an HF50 change the input tube to a 12ax7, it will sound much better.Well just recently I bought another ST70 from Ebay. When I finally started on the unit it looked stock except someone had replaced the can filter caps and the 12ax7 's were replaced with Sovtek 12ax7's. The first thing I did was to replace the frayed power cord. Then I checked and this unit did have the ground attached to the black terminal on the output transformers but were terminating on the 4 ohm tap. CHECK THIS...Many were built like this, the schematic is wrong.
Thanks to my friend Vern who had this part of the schematic pasted on his website.I have attached some pix showing my change to this and then scrapping the terrible speaker screw terminals and making up some small boards and mounting banana jacks on them. I'll change these when the unit is fully complete to 2 or 3 piece gold jacks so you have a choice of banana or spade plugs on your speaker cables. Below shows the inside the amp with the transformer fix and the back of the amp with the banana plugs. NOTE-the channel balance resistor etc. has been removed.
The back of the amp will have a cover where the centre channel speaker used to be. I have removed all the lettering at the back that had anything to do with the centre channel speaker. As seen in the above pix remove all the wiring to the balance check switches on the front along with the big 16 ohm resistor. This balance check does nothing but mess up the sound. In doing this you have to install the feedback components somewhere. I used 2 small terminal strips .The 16 ohm tap goes there and then replace the wires back to the amp for the feedback connection. I tested the resistors and caps and found they were off more than 10% so I replaced all the components. While installing new connectors on the tranformer taps, strip back 1/4" so you have clean unoxidized wires for soldering. I used the 8 ohm tap so I just taped the end of the 4 ohm tap. You could install another banana jack and then you could have the option of using either the 4 or the 8 ohm tap. The next step I did was to replace the paper cap for the bias filter and the resistors. These are old and can cause major trouble if they short out. For a few dollars it's worth replacing. At this point I decided to plug in the tubes, fire the unit up and check voltages on the power supply. All were very close to the manual. I then balanced the output tubes and set the bias to .38v. I hooked up the speakers and a cd player and had a listen. No hummmm...That's always a good start. And the sound was quite acceptable. After a few minutes the one channel starting cutting in and out and by tapping the chassis It kicked back in again. I checked the balance control and it worked towards one channel but did next to nothing on the other channel. Also the blend control and loudness control did next to nothing. This was a good start ...The schematic below shows the removal of the balance check and the feedback network and the proper transformer taps. Common "C" goes to GROUND!!!
The image at the right is what the ST70 should like, quite similar to the HF50.

Part 2

The next step in the rebuilding of the ST70 was to remove the blend control , the high/ low filters and then properly rewire so the loudness control really does something.

As many know the loudness control was another design flaw from EICO that many have stumbled upon.

Below is another schematic showing to locate the output from the selector control that goes to the tape monitor switch.

An easy way is to use an ohmeter and hook one lead to the input of say the aux and then choose that input from the selector switch. There will be a lead off the selector switch and this should now go directly to the input of the balance control.

Do this for both channels and so the tape monitor switch and blend control have been disabled. You will most likely have to install new longer leads to the balance control.

While I did this on mine I found 2 or 3 bad solder connections and a few that were entirely missed being soldered. This was probably the cause of the one channel cutting in and out.

You will require 2 -68pf (use silver mica 5%) 2- .02uf and 2- 22k resistors to make the loudness control actually work.

The above schematic shows the path from the input to the balance control, the new components to make the loudness control work and the bypass of the blend switch and filters.

After I rewired the amp as above the EICO ST70 now had both channels working solid, the balance control really worked , and the loudness control finally does something.

What a difference from the stock amp that didn't image at all , half the controls did nothing and was putting out about 30% less power due to the connection of black lead of the transformer to the 4 ohm speaker terminal.

Total cost about $4 in parts, but maybe 8 hours of your time!!! I normally remove the tone controls in my preamps but have left this one as is for now.

Part 3 (Power Supply Rebuild)

One of the components of the power supply of the ST70 is the 5AR4 tube which is a full wave dual diode rectifier.

The output DC from the tube has still some of the AC component in the DC. This would be very visible on a scope.

To further clean and remove more of the AC component or so called ripple , a series of capacitors and resistors filter out the DC so the AC component gets minimized enough so it will not be heard through your speakers as hum.

The first stage of capacitance in the ST70 requires a 40uF and a 20uF @500v electrolytic capacitor. What I found to replace this was a can type cap that had 2 sections of 50uF each @500v. (See picture to the right)

It is very important if using a tube rectifier that the first capacitor after the rectifier does not exceed 50uF. After this stage and the 1.8k 5W ,you can increase the capacitance if you wish.

On this unit I did used a 50uF/50uF @500v. The next can capacitor with 4 capacitors in one can be very difficult unit to replace. This has 40/20/20/20 uF @400V.

Therefore what I did was to install 4- 47uF radial type electrolytic caps under the chassis.

The can caps on this ST70 appeared to have been previously replaced, but I'm not sure the condition so I just replaced them both.

I have left the other old one in place just for looks, it beats having a hole sitting in the middle of the chassis.

There are also 2 other electrolytic caps under the chassis that need replaced as well. These are a 16uF/350v and a 30uF @400v.

Also replace the resistors between each section of caps. They may test ok, but remember they are 40+ years old and run hot. Their resistance may change considerably when hot. When you have replaced all the power supply caps and resistors, you should now have a very stable and clean supply for your ST70 that should be trouble free for years.

Part 4

With the power supply now completely rebuilt the next step is to remove those old black coupling caps which are .1 and .25 uF . You can't beat the 715P sprague caps Which are a great coupling cap for the money. I have tried MIT PPMFX caps and I noticed very little difference in the sound and the MIT's cost about 3 times as much.

Also replace all the balance resistors around both balance pots. These again may have drifted over the years. These components are the 100k and 150k resistors.

While I was at it I also replaced the 33k and 28.75k resistors from the plates of the 6SN7 tube. To get the 28.75k I used a 75k and 47k in parallel, this will get you around the 29k range.

Use 1 watt's for these. There are also a few more electrolytic caps that need replaced. These are 2-25uF at the cathodes of the 12ax7 driver tube which feeds the 6SN7's.

With all the above replacements this amplifier sounds absolutely great.

At this time I am going to remove the terrible selector control, Install a 6X2 switch as rewire all the input leads back to this selector.

On the existing selector switch there are 4 components which are the RIAA network required for the Phono input.

What I will do is wire these directly on the 12ax7 tubes. These components are a 400pf in parallel with a 200k resistor in series with a 4 meg resistor in parallel with a 1200pf cap.

The phono input now will be wired directly into the 12ax7 and then output will then go to the selector switch. EICO had a lot of jumping back and forth wiring which can really mess the sound up.

Always try and keep the signal lead lengths down to a minimum.

Below I have pasted in more pix showing the rest of the power supply capsand the coupling caps.

Also I have inserted a few pages from the original ST70 manual on troubleshooting and some parts lists.

Loudness Control Fix

New Coupling Caps and Power Supply Caps

The manual can be found by joining the Yahoo Eico group.

Part 5

The input selector has now been replaced with a 6 position switch. Also all the rca input jacks have been replaced. I created an outline of the original holes using the jack mounting brackets and then created a new bracket. Email me below if you are interested in a bracket . The spacing of the jacks is now greater to allow for those large rca cables that are becoming a norm with new audio equipment. This still leaves 5 sets of jacks which will still give you plenty of inputs. In this particular amplifier I have wired 1 set for the phono, 3 sets for line inputs (CD, TUNER, AUX ETC) and the fifth can be a line out for recording or it can be used as a fourth line input. I have also gone back and replaced the speaker terminals with gold plated banana plugs.
This EICO ST70 now is complete and sounds many times better than when it was originally assembled. The design flaws have been corrected and the new components make this amp sound superb. I would definitely recommend doing a rebuild on one. There are still many units out there and the price is still reasonable until people start realizing how good and large the iron is on these units. If you would like any information or have comments please email me on the link below. This rebuild was documented for personal satisfaction only, and to help others share my information.

Pix of New Selector Switch, output terminals and new input jacks

If you don't like the look of the ST70 or just want a power amp , see the pix below of an ST70 that I made into a power amp.

Any comments or additional information: Dan Nicotera