Comments for page: Eico ST-70 Mods

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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 donít 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 Ė 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 donít 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

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:
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 donít 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

Posted by ted eilers November 23, 2013 - 07:58 am

Posted by Steve L. February 02, 2012 - 09:34 am
Hi Jon, Thank you for the postings and kind words. So glad that you found the hum issue. The DC power supply for the phono preamp is a great idea.

Ordinarily, when people talk of switching a tube amp's heaters to DC, the key problem is that the power transformer isn't sized to power a DC supply. In this case though, powering just the phono preamp on DC is such a small load (relative to the overall amp), that it isn't an issue.

Posted by Jon Y. February 02, 2012 - 12:45 am
I added a voltage doubler and fed DC to the phono tubes and the 12AX7 driver. I also added a choke (for the Dyna ST-70) in line with the B+.

I also discovered the reason for the hum. I had connected the speaker returns to the chassis ground point where the power supply transformer grounds were connected. Bad idea.

Now the amp is so quiet I can't hear any hum or noise.

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