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The Eico ST-70 is a 35 Watt per channel vacuum tube amplifier from 1962,
which is great in many respects. This project fixes a few flaws which prevent
it from reaching its full potential. Here are some before and after results:

Table of ST-70a output power versus frequency
Table of ST-70a line-level signal-to-noise ratio

You can choose which mods you would like to use. The separate mods are:
Phono preamp, driver balance adjust and line amp/power amp. Get the
illustrated step-by-step instructions in the download:

ST-70A mods package icon

-Eico ST-70A mods article (1MB)

Chan1
Chan-1 schem illus

20 - 20kHz phono response in dB

Power amplifier modifications

 

Reader Comments


Posted by Steve L. February 03, 2016 - 05:05 pm
Hi Giovanni, The approach you describe sounds wise. That's an excellent idea, if you decide to use the unneeded switches to select either the lower sensitivity direct bypass or the regular modified configuration. It does require a DPDT for each channel so the two available switches will do it. Hope it goes well.

Posted by Giovanni February 03, 2016 - 11:59 am
Dear Steve,
well it seems that things are not so simple as I thought! Here you can see how green I am, haha. Thank you so much dear Steve, your help is really precious here. First, I don’t have the 7247 tube on my amplifier. In its place there was already a 12AX7 tube. When I saw that, before to put my hands on it, and having found your Mods article, I even thought that somebody had already implemented your modifications. But indeed the amplifier is all original, the only odd thing was the 12AX in place of the 7247 tube.
Now, due to your competent suggestions about how to skip the tone controls stage, I think that I will do the following. I will finish the mods of the article, will do the calibrations, do some oscilloscope checks and then listen and see how it works. If everything is ok, then most probably I will keep it as it is. If I will have some noise, then I think that I could use the speaker phase switch and balance switch, that I have now disconnected, for disconnecting the V3 and connecting V5-2 directly to the level pot, as for your first suggestion. That way I will have the “high sensitivity/tone control” and “low sensitivity/no tone controls” options. It may be interesting, and I will keep you updated.
Thank you very much again dear Steve.
Kind regards
Giovanni

Posted by Steve L. February 03, 2016 - 10:59 am
Hi Giovanni, Your analysis of the AC balance pot power requirements is spot on!

Regarding bypassing the tone controls, I'm not sure I understand your proposal correctly. Did you mean to connect the junction of C15 and R41 [call this point-A], to V3B pin-7 [abbreviated V3-7]? If so, the main concern I have is that V3B is a high-gain stage which is constrained by negative feedback [NFB] through the tone controls. Point-A is low impedance and that will eliminate much of the NFB at V3-7, resulting in too much gain and noise though that stage.

My approach to doing the bypass is to disconnect V5-2 from the existing circuit and to connect it to the wiper of Level Control, R33. [Call this point-W.] Disconnect other components from the wiper. The only issue with this approach is a reduction in sensitivity, which could be a good thing, depending on your needs. (The lower sensitivity minimizes noise and distortion.) Line input sensitivity for 35W out would be about 1.07V instead of the original spec of 0.48V. You may want to leave V3 in-place to avoid changing the load on the B+ and heater systems but it's not a big deal anyway.

If you need to maintain the original sensitivity, then we can employ V3A as follows: I've minimized changes by keeping R37 and R43 at the original values of 33K and 1.5K and by keeping the tube as a 7247. No cap is added across R43. As you probably know, the 7247 is a combination of one 12AU7 stage and one 12AX7 stage. Here, the 12AU7 stage is at V3A. Since we're not using V3B and 7247 tubes are less available, you could substitute an actual 12AU7 for V3 if you wish.

Even with the minimal-changes-scenario we DO need to insert R33a, defined by the standard mods package. To get the original sensitivity, make R33a = 220K and R41 = 820K. Disconnect point-A from S6 and C23. Then disconnect V5-2 from the existing circuit and connect it to point-A. A rough sketch of the circuit is here: http://www.tronola.com/misc/Gain_stage_for_tone_bypass.gif (The DC return path for V5-2 is through R41, R33a and R33 so R51 is not needed.)

I will be happy to help if you have any questions or problems. Best wishes with your project!

Posted by Giovanni February 01, 2016 - 06:35 pm
Dear Steve, I really appreciated your answer, thank you SO much. Thanks for the explanation about the 4 ohm tap grounding, now I understood that. I fully agree with you about the central speaker. I don’t need it and was thinking to remove the related wiring.
Thank you for your recommendation about the AC balance, I will implement it. I had a bit of perplexities because the cermet potentiometers that I bought for the AC balance are rated 0,5W with the maximum current on the slider of 100 mA, so I was afraid that they were not suitable. But I have seen on the wiring diagram that the voltage drop across the 33k resistor on the plate of the 6SN7 is 360 – 265 = 95V, which means a plate current of 95/33,000 = 3 mA. Since we have two times this current on the pot slider, we have 6mA and the power on each half of the pot is about 0,045W, let say about 0,1W on the whole potentiometer. So no problem at all if I am not wrong.
My ST70 is a factory one, these gears are rare here in Italy. I bought it for almost nothing because the seller told me that the sound was extremely low. I did not switched it on yet, I just tested the tubes which are all perfect and the transformers, so I changed all the capacitors and the carbon resistors because their values have increased of about 20 percent. Then I will try to clean the contacts of the rotary inputs selector, but most probably I will replace it with a modern one. Phono and Line inputs are enough for me. I usually do bypass completely the tone controls, I don’t like them. For doing that, I will connect directly C15 (0.1 mF) to the pin 7 of the 12AX7 (ex 7247). Do you agree on that?
When I will switch on the amp, I will keep you posted with the results.
Best regards
Giovanni

Posted by Steve L. February 01, 2016 - 11:08 am
Hi Giovanni, That's an interesting question about grounding the 4Ω tap. I wouldn't call it a "mistake by Eico." The main reason that Eico grounded that tap was to support the center-speaker feature. It's also involved with the Balance Check feature. With the 4Ω tap grounded, the signal on the C-tap is opposite phase from the signals on the 8Ω and 16Ω-taps. Thus, a center speaker connected between C-tap on Channel-2 and (for example) the 16Ω-tap on Channel-1 will properly sum the two channels for the center.

I think few people use the center channel feature. Many would not like the reduction in stereo effect. To use it not only requires dealing with the confusing extra terminal board connections, but also requires adding a 25-watt rheostat on the back of the center channel speaker. (This is according to the Eico instructions.) Most folks who would want a center channel would have a separate power amp for that and would control the level that way.

The balance check feature is of limited value because it balances the level of the signals driving the speakers but does NOT account for any imbalance in the speakers themselves or the effect of the room environment. Hence, I don't think many use this either.

Even back in the day, I remember that we found the convoluted connections on the back of the ST-70 to be a frustrating mess. What a relief it was to realize that if you ignore the third terminal board (and fancy features) and just use the two individual channel terminal boards, it works just like a normal power amp! However without mods, you still can't generally connect the output speaker commons or even have common output and input grounds. Those are things which are often needed for headphone connections, speaker switches and bench testing.

Note that rewiring the C-taps to ground requires that the negative feedback be adjusted, since the loop gain will be affected. In developing the mods for the article, feedback had to be re-optimized anyway so including the common ground mod was easy.

In conclusion, there was no reason for Eico to ground the 4Ω-taps other than to support the questionable features. To address the Audio Research case, I would need much more information about that amplifier. In general, there isn't any advantage to grounding the 4Ω-tap, so you don't find that in most high-end amps.

Regarding the AC balance mod, I highly recommend installing it. The adjustment is a much more effective way of balancing tubes than the crude matching that tube sellers are able to do. (I would still recommend buying matched tubes though.) Also, it can compensate for unbalance that arises as the tubes age differently. If drilling the chassis is a big problem, you could simply put the pots underneath the chassis. While that would require removing the bottom to make adjustments, you should rarely need to do that. By the way, I might have overdone the cautions about drilling steel. A good titanium or cobalt coated drill bit shouldn't have any problem with that. In fact, in the article about doubling the power of the ST-70, the experimenter who contributed the design is quoted as having drilled the holes in the steel chassis with a 1-inch (uncoated) spade bit, intended for wood! What a man! :)

Posted by Giovanni January 31, 2016 - 06:38 pm
Dear Steve,
I am an old guy from Italy, one of my hobbies is to bring vintage stuffs to new life. It seems that they are so happy and sings better than before, when they were young. But I am not that expert. I stumbled on a nice Eico ST70 and am restoring it. I did all the mods that have been suggested here except the AC balance, which I am wondering if it will be worth to install or not considering the necessity of drilling the plate.
In doing some search about the AC balance, I found the following description from Audioresearch:
http://www.audioresearch.com/ContentsFiles%5CD51D75D75AD76_BiasAdjust.pdf
There I have seen that they too has the 4 ohm tap of the output transformer connected to ground, instead of the common tap. Due to that, I suspect that the grounding on the ST70 is not a mistake by Eico but a deliberate choice for some reason that I don’t understand. What do you think? Is it better to move the ground to where it apparently sounds logic or it will be better to leave it where it is, i-.e. the 4 ohm tap?
Thank you so much in advance
Giovanni


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