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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. [Please note that the line amp and power amp mods must go together.] 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 DAK August 03, 2018 - 01:43 am
Ok, i have other chokes. My reasoning for the selection was to try and match the original value for setting the G2 voltage. I have tried a G2 supply choke with zeners, and that actually worked pretty good in a single ended amp. I will try to get the G2 to around 370v as Steve recommended. I have lots of chokes and Zeners so that seems like a simple course to take. Best regards, Dak

Posted by Dave July 28, 2018 - 09:48 pm
Hi Dak -- That's still nearly 1700Ω; feeding the screen circuit. Understand that between quiescent and full power conditions, the pair of screen grids in each channel will increase their current draw by nearly 320%. Ohm's Law will tell you what that will do to the static voltage drop across a 1700Ω; resistor. Of course, the screen grids won't be able to increase their current draw by that much because the increased voltage drop from the resistor won't allow for it. When that happens, power output drops, and distortion increases.

You might try using the 470Ω; resistor (or a lower resistance choke) in series with an appropriate Zener(s) device, so that then, only 470Ω; is subject to the increased drop with increased current flow.

If you go the EFB route, check on the AK Fisher Forum where one AKer applied it to his 500C receiver. For this exercise, you only need to use the EFB Screen Grid Regulator from that project, although the ST-70 would benefit greatly from using both the EFB Screen Grid and Control Grid Regulators.

Dave

Posted by DAK July 28, 2018 - 02:39 pm
Hi Dave, and many thanks for the most helpful suggestions. I will be implementing them over the next week.
In my current amp for the G2 supply, i have ditched the 1.8K 12watt resistor and added a 470 5 watt resistor + 20uf cap and a 1.2K DCR choke. IIRC, my G2v is about 360. Should i replace this setup with the EFB circuit? regards, Dak

Posted by Dave July 26, 2018 - 11:47 pm
Hi Dak -- Understand about the 7591 concerns. Against even the best American versions, the 6L6 family of tubes are simply far more durable pieces, which history has rather undeniably shown.

The revised ST-70 power amplifier circuit will work just fine as a platform to base your build on, but besides the basic socket rewiring, you will need to address three significant areas:

1. Since the 6L6 tube family has about half the Gm of 7591 tubes, you will need to revise the bias supply and adjustment circuits to provide about double the negative control grid voltage that the stock design provides. The bias supply is easily capable of the necessary voltage, but you will need to adjust the dropping resistors associated with the bias controls to achieve the correct range. Also make sure that the filter caps connected to the wipers of the bias controls are appropriately voltage rated as well.

2. You will want to drop the screen grid voltage to about 375 vdc under quiescent conditions. This doesn't appear to be much lower than the voltage that Eico specifies, but I've found that their 390 vdc specification at that point is typically about 25 volts low anyway. You will also want to stabilize this voltage as much as possible. Simply raising the value of the screen grid dropping resistor would not be a good way to go, as increased screen grid current at elevated power output levels will cause significant voltage drop, reducing power output, and increasing distortion. Zeners can help, or even use of an EFB™ Screen Grid Voltage regulator, which would be an ideal solution. At this screen grid voltage, quiescent cathode current should be 40 mA per tube, and the amplifier should easily develop 35 low distortion watts in each channel individually, or 30 watts with both channels driven -- assuming the screen grid concerns are properly dealt with. With 375 screen grid volts, the control grids will typically require about -35 vdc in this scenario.

3. AC Drive: 6L6 tubes will require more AC drive from the phase inverter stage (about double the stock amount). The existing design should be able to provide the necessary drive, but it will be near it's maximum capability with these tubes. Make sure the driver tube is strong, and ideally, consider changing the inverter plate resistors to 43K and 51K (at 2W each), and the tail resistor to 22K at 2W. You would still use the AC Balance control with these values, but this change will ensure plenty of low distortion drive to the new output stage.

Any further details are beyond the scope of this forum, so you'll need to work them out with some of your own experimentation. Ultimately however, done properly, the 6L6 conversion can provide every bit as good of performance results as the 7591 tubes did, and will do so without the availability concerns of the latter tube.

Good luck with your project!

Dave

Posted by DAK July 26, 2018 - 03:05 pm
Hi Steve and Dave, i hope all is well with you folks. I am going to convert the ST70 from 7591 tubes to 6P3S-E , i believe that is a Russian 5881, mostly to save on the cost of using a 7591. I know they make new 7591 but the reliability is much less than vintage so i am going to convert. I am going to start with the schematic posted from the comment on January 22, 2017 - 06:10 am. Of course there is changes needed to be made to the octal socket hook ups but, i thought at first, i would just follow your revised schematic as shown and see if that works. Do you have any recommendations or any suggestions to add to my plans. Thank you for your comments, best regards, Dak

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 = V²/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.


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