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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. March 17, 2016 - 03:04 pm
Hi Kevin, The power amp mods did not significantly change the DC conditions. Hence, the expected DC voltages are the same as Eico listed.One might carp about the fact that the DC cathode resistance for V3 increased 4.5%. That would increase the DC cathode voltage slightly but considerably less than 4.5%. That would be well within the range of error, so Eico's value of 0.70VDC at idle stands. (0.55VDC at 30W/chan, both chan driven)

Posted by Kevin March 17, 2016 - 11:29 am
Hi Steve,

Thanks for the speedy answer. May I have the Vdc estimate on V5 (pin 1,3) and 6SN7 (pin 2,5,3,6,1,4) as well please? I want to compare them with the original circuit voltage chart.

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
Giovanni


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