• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Eico ST-70 Supermods

andresfont

Member
2011-06-25 1:51 am
Hi guys. Sorry for my bad English I am Venezuelan ..... Here present my latest project, I call it "supermod Eico st-70." A brief review: I bought the Eico ST70 on eBay, after the restore, the sound I thought it was great! Then he performed "tronola mods", the sound improved considerably, but I wanted more power and then I was thinking "as having more power" and I got the crazy idea of putting 4 tubes per channel (PPP).

Well get to work...... first calculate the ratio of the OPT , these were the approximate results:
40:1 for 4Ω,
29:1 for 8Ω and
20:1 16Ω

after making the "design on paper" and working on the amp, here are the results of the "supermodificacion" : Output power Apply 1kHz, to a FM1 input and set the Selector to it. Measure the output across the load 8Ω. 22.6Vrms ± 64WPC GREAT SOUND!!!!!!! THE BASS AND TREBLE SOUNDS INCREDIBLE! Eico ST-70? I put two GZ34 and a transformer for the filament of the four new 7195 I'm planning to use solid state rectifiers to increase the B + because it is now 400V

Desmostracion: http://www.youtube.com/watch?v=7ninqmkRA4
 

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I am also curious the measurement. If I remember correctly, the PT is rated at 170VA in the HT section, and the OT's are rated at 35W. I will love to see how far these PT and OT's can be pushed without destroying mine :) Also, the 5V section is only rated for 2A, I hope you are are driving 2 x GZ34 filament from the stock PT.
 

andresfont

Member
2011-06-25 1:51 am
Hi Fred.
Well, you must remember that these are nominal values, ​​these transformers supply much more than that. believe me that initially I connect the four 7195 to PT and work well " you can calculate the current filament with four additional 7195 ....." (everyone said it was impossible that I could not break the laws of physics), after I put an additional transformer for the filament, to avoid overheating of PT. also placed at the beginning a single GZ32 (see video) but had 390V B +, putting two GZ34 voltage is raised to 400, but yesterday probe using SS rectifier and voltage increases to 440V (surely the power output must be increased too) of course I have to use a delay circuit for the SS, I'm thinking about it:bomb:
 

andresfont

Member
2011-06-25 1:51 am
Hi Steve
I'm going to test at low frequency and then I'll tell you. but certainly should be better than before because they feel better the bass with the music.:hphones:

Andres
Hi Steve
Today I was testing and I put SS rectifiers the results at different frequencies:
20Khz=22.3Vac
1Khz=23.8Vac(8Ohm)
300Hz=23Vac
30Hz=21Vac
20Hz=20.6Vac
The improvement is evident with the SS rectifier!:p
Note: these results are no visual distortion on the oscilloscope, I have no distortion meter.....
 
Hi Andres,

Thank you for the additional measurements. They are interesting and yet puzzling. In my measurements on the Eico ST-70A, at 20Hz, it achieved only 54% of the power that it had at 1kHz. However, my measurements were taken at 1% distortion. As I recall, "clipping" was somewhat nebulous at 20Hz, so a visual interpretation of maximum might be quite different.

Since the output transformer is being operated at much higher power in your case, one would expect that low frequency saturation effects would have more of a limiting effect than they do at lower power. Yet, we don't see that here. I wish that you had a distortion analyzer. There is software which can do distortion measurement using your sound card. It should do a pretty good job up to at least 5kHz or so. One example: TrueRTA http://www.trueaudio.com/

Anyway, you have shown something very interesting.

Steve L.
 
Hi Fred
well, you must remember that these are nominal values ​​these transformers supply much more than that. believe me that initially connect the four 7195 to PT and work well you can calculate the current filament with four additional 7195 .... . (everyone said it was impossible that I could not break the laws of physics ), after I put an additional transformer for the filament, to avoid overheating of PT. also at first I put a single GZ32 (see video) but B + had 390V with two GZ34 voltage is raised to 400, but yesterday probe using SS rectifiers and the voltage increases to 440V, {surely the power output must be increased too ) of course I have to use a delay circuit for SS, I'm thinking about it:bomb:
Hmm, I was saying the 5V section is rated at 2A, each GZ34 draws 1.9A, and you have 2. That will be 3.8A. I suppose it is more like overclocking a processor - it is more on how fast it goes, not how long it will last :xfingers:
 
Don't get me wrong, Andres. Many members here love to push way beyond the specs, I am very interested in how much the ST-70 can be pushed.

Attached is the Sam's sheet. From here, you can see the specs of the PT. The HT is only rated at 170ma, and 8 x 7951 at 38ma bias = 304ma which is almost 80% above the specs!!
 

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Hi Fred
I do not know how they(Sam) obtained the data from PT, I just want you to see the picture with the value of B +.Do you think that this value of B + can be had with 80 % about the specs?
Andres
Sams provides many the specs and the schematics of many equipments especially in the era. Those specs are usually published by the manufacturer. They are the best thing next to the original manufacturer manual of the vintage gear!!

There seems to be some disconnection here. I said 80% above the specs, not about the specs. I did assume each 7591 was drawing 38ma per Eico instructions. 38ma x 8 = 304ma total when idling. The PT specs is 170ma. Therefore, you are drawing 134ma (304 - 170) above the PT's specs which is approximately 80% of 170ma. This may present potential hazard as the magnetic wire might not be spec for such level of current either.

If you read the Sams from my attachment (you can download all 6 pages via the Eico group files section) on the specs of the OT, you will see the primary impedance is 6500 which is somewhat high for PPP of 4x7591 as well.
 
Hi Fred
Sorry my english is bad, i want say "above" not "about". well I checked the amp for several days using it for several hours and so far has not had any problems. I am clear that the PT is over the limit, however now I have a bigger PT, I have not put because until now has not failed the original. On the other hand before of the mod I calculate the ratio of OPT, please if you see the attached schematic you will notice the taps of impedance values ​​in the secondary of the OPT is different.........
 
Andres,

No worry, English is not my native language either, and there are plenty of members like us here.

It is indeed good to see that you have fun and good result. I just wanted to do the
due diligent of a fellow hobbyist to warn the risk of pushing the (vintage) irons way beyond what they were designed for. As long as you are aware of that, I am happy for you. Safety is always first, and fun is only secondary :)