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Need work done on a dynaco power amp
Need work done on a dynaco power amp
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Old 6th August 2007, 06:31 AM   #1
ag2217 is offline ag2217  United States
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Default Need work done on a dynaco power amp

I have an old stereo 35 model of power amp but it won't even light up and I was wondering if anyone knows someone whon works on these things.... thanks
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Old 6th August 2007, 11:35 PM   #2
Tubehead is offline Tubehead  Sweden
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Location: Stockholm

I dont know if I can be of any help but I do own one which I pseudo triode connected back in those days when I didnt know what I was doing. Sometimes I take a look at it intending to make it original or at least close to. However there are always other projects with higher priority. I do have the original schematic.

Some advice to start with:

1. Check the power tranny (and of course the fuse!). If there is a break in primary or secondary winding youre in trouble. The ST35 was run very hot and was intended for 110V/220V, not todays 115V/230V.

2. The power supply electrolytic cap is probably not good due to aging and too much heat. If I recall correctly, this is a 4 section unit with A+, B+, C+ and one (1!) cathode bypass cap for all 4 output tubes! If you want an original look for the amp, I would dig out the innards of that cap and try to replace it with 3 modern lytics caps inside the original can. Not sure this can be done though, due to the restricted space inside the can. There is very little space in this amp overall.

3. Voltage HAS to be reduced due to raised mains voltage. Increase first resistor in power supply. A little more than is necessary to compensate for mains voltage increase seems advisable as these amps were really hard on the EL84 output tubes.
Todays EL84 might not be as sturdy as old tubes. Even if you can get NOS tubes you probably dont want to run them too hard.

4. Check heater voltage, this has probably gone up too for the same reason as above. In my opinion 6V is fine. Not 6.3V or higher.
Emission will fall quickly to a level wich is less than with 6V. This is true for all tubes. Theres an article somewhere on TubeCad Journal about this. Insert small resistor in series with heater, probably 3W and 0.1 ohm or something like that, you will have to try different values to get it right. This is for all tubes.

5. Replace the one(!) cathode resistor with 4 resistors, one for each tube. You will have to cram them in where you can find the space in the tiny chassis. They should be around four times the resistance of the original one (Ugh!). Maybe a little bit more to reduce current further. Dont forget to bypass these with a small ie physical size (diameter approx 10mm to fit inside the extremely low chassie) lytic cap. These could be about 1/4th of the capacitance of the original cap. Preferably a bit more. These have to be low ESR type.

6. Replace the crappy RCAs with new gold plated ones to your liking. Same with output terminal blocks, I disconnected and isolated the 16 ohm outputs and installed some standard gold plated binding posts connected to the 8 ohm output taps.

7. Replace the crappy coupling caps, there are 6 of them. Use high quality caps like Auricaps, MIT, Mundorf etc etc. Be sure to check that there is space enough to fit them. Dont change the values.
They should be at least 400V rating.

8. Remove the input cap, it only does harm. Any source you connect it to should have zero bias on its output anyway.

9. You may want to/have to replace at least some of the resistors due to aging problems. These are mostly carbon (mass?) types and may have changed value too much. Metal film will do nicely. Be sure to check power rating though.

This list is not written in any particular order, Im just writing out of memory as Im on vacation and dont have a schematic or amp around to look at.

That said, I think that this is a really nice little amp and it deserves to be saved. The output transformers are probably good iron even if there are better ones around today, but perhaps not by much.

Good luck
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