Comments for page: Eico ST-70 Mods

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Posted by Steve L. March 17, 2016 - 09:54 am
Hi Kevin, Thank you for your kind comments. Since I don't have direct measurements of the AC voltages you're interested in, I'll have to rely on Eico's readings, adjusted by theory. They give values for 35W output (at 1kHz) from a single channel at a time. The fact that we can hang our hats on is that, to develop the same output as the original circuit, the internal levels must remain pretty much the same at midband frequencies.

However, at the grid and cathode of V5, things have changed. Still, we need about the same drive at the plate, so the AC voltage from grid to cathode must remain the same. Eico has that difference voltage as (0.24 - 0.20) = 0.04Vrms. Since the grid voltage is 650mV, the cathode goes to 610mV. Notice that the percentage drop from grid to cathode is reduced by the same factor that the negative feedback is increased. Including those numbers with the ones from the Eico table results in the answers to your question. The 1kHz AC voltages for 35W output (16.7Vrms into 8Ω;) in the ST70 power amp should be:
V5-1 2.60Vrms
V5-2 0.65
V5-3 0.61
V7-1 2.60
V7-2 18.00
V7-3 1.24
V7-4 0.00
V7-5 18.00
V7-6 1.24
Let us know how it turns out!

Posted by Kevin March 17, 2016 - 02:12 am
Hello Steve,

Thank you so much for the Eico ST-70A mods project. I have been saving the transformers for years and just recently got around to build the power amplifier section per your modifications.

Since the PA input sensitively is now lowered to 650mV from 200mV per your article, please tell me the approximate voltage measurement on V5 plate (pin#1) and cathode (pin#3), also the 6SN7 plate (pin#2 & #5), grid (pin#1 & #4) and cathode (pin#3 & #6) should be? Thank you!

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

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: (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

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