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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. January 22, 2017 - 06:10 am
Hi Joe,
I truly appreciate your kind comments. For the benefit of readers, I'm posting below, your excellent question and the answer I sent, slightly edited:
Question: Hi Steve. Your work with electronics is very inspiring. I unfortunately do not have any near your training and knowledge of valve circuits. I do love to learn though.

I have a question if you have a quick second. I own an Eico ST70 that was scratch built using Stancor iron from a parts bin. It is configured as just the amp section of the 70. I was wondering if I were to take the feedback circuit and remove it from the 16 ohm tap and solder it to the 8 or 4 ohm tap, if I would have to change any of the component values. My speakers are a very stable 4 ohm impedance so I use the 4 ohm tap. I would like to refine the Sonics of the amp as best as I can. I can do the work but certainly not the engineering. I've been an avid parts swapper for years.

Thanks for your time and have a great day.
Regards, Joe

Answer: Thank you, Joe. Yes, if you change the tap to which the feedback (FB) is connected, you would need to change the values of the FB components. To advise on what those changes should be, I would have to verify the details of your configuration. From your description, I visualize it as the Eico schematic here.

The critical issue is whether you followed Eico's grounding of the 4-ohm tap. I will assume so for the moment. In that case, you would connect your speaker between tap-C and tap-4. I'm not sure why you want to move the FB tap but I will assume it is to minimize whatever distortion or other maladies might come from differences between taps. Unfortunately, you cannot use tap-4 because that is the amp's ground, so it would provide no FB at all. Tap-C would have the "4-ohm signal" but its phase is inverted, so it cannot be used.

Tap-8 could be used but I would advise against that because the voltage between Tap-4 (gnd) and Tap-8 is just 41% of the voltage between Tap-C and Tap-4. That is, between Tap-4 and Tap-8, the nominal impedance is only about 0.7ohms, so it certainly isn't closer to what is seen by your 4-ohm speakers operating between Tap-C and Tap-4.

You might well ask how this can be and why the impedances don't add directly. The clearest way to look at it is in terms of voltage, because those values DO add and subtract the way one would expect, across taps. Here is a list of the impedance taps and voltages for the transformer as it would classically be used, with Tap-C grounded, plus a column with Tap-4 grounded instead.

In the illustration, voltage is scaled with the 4-ohm tap set arbitrarily to 1.0. Notice that impedance goes with the square of the voltage. If we calculate the power available at each tap, they will all be the same, with P = Vē/R.

The bottom line is that, with Tap-4 grounded as Eico does, Tap-16 is the only one that makes sense to use for FB. In the Tronola article, "Simple Mods to the Eico ST-70...", we changed the ground to what Eico calls Tap-C and took FB from Tap-8. If you wanted to change ground to Tap-C and take FB from Tap-8 or Tap-4, that could work but would need different FB component values. Perhaps the best approach would be to implement the power amp mods given in that article, using Tap-8 for FB. It would also reduce input sensitivity by roughly 10dB (which is a good thing for hum, noise and distortion). The dynamic balance parts of the mods (R61a, 61, 63) would be optional. [I'm citing only the left channel parts.] The essential power amp mods are highlighted here.

I will be happy to help if you have any questions.

Posted by Joe A January 20, 2017 - 05:25 pm
Steve,

Regarding a PM that I sent you, I want to semi publicly thank you for an incredibly detailed response that you sent back to me. A response that makes me have a little hope that there are still decent people walking around on this giant floating rock. Thank you for your time, patience and audio advice. You're a good man and keep up the great work.

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!


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