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Dynaco st-70 rebuild questions.

It strikes me that some information collected in one place could be useful here. If I am offending by posting this, please forgive me, I only intend to help.

The 55 volt output is used to obtain 32 volts on the "grid" (pin 5) of the output tubes. A change of the bias resistors will correct the voltage for the 60 volt output.

Since you have no transformers you should be aware that the EL34 is capable of more then 35 watts output in push pull. Dynaco used a power supply that did not respond well to very rapid demands in current draw. (That kept the price down) To compensate they biased the outputs to run class A. That kept the current draw in the output tubes nearly constant. You have the chance to build a more robust power supply that could respond to current changes quickly and therefore change the bias of the outputs to operate in class AB, getting more power out of them.

As to the output transformers. They are very different from power transformers. Power trannys operate at only one frequency, 60 hz. Output transformers must operate over the entire range of human hearing, considered to be 20 Hz to 20,000 Hz. The winding for that is very different from a power transformer. It is important to actually use an audio output transformer in this application. As mentioned earlier there are a number or options out there. The Dynaclone's are suposed to be made to the original specs of the Dynaco Transformers. If true (I have no reason to doubt it I just never used one) then they are excellent for this purpose. Also the Edcor output transformers are very well thought of here on this board as well.

You will need to set up a power supply filter (can cap and choke in the original ST70) Those circuit pulls usually don't include those parts. A resistor could be substituted for the choke and work almost as well (sorry I don't know a value for that) but you will need caps capable of very high voltages. You may need to use caps paired up and wired in series along with high value bypass resistors across each of them to guarantee the voltage across each is kept within limits. (Example, a pair of 80 microFarad caps in series, and a 300K to 500K ohm resistor wired across the leads of each cap. Use photo strobe caps rated at 450 vDC. The network will be rated at 900 vDC and 40 microFarads).

Good luck with this and please let us know what you did and how it worked.

Shelly_D
 
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In summary -
Choke transformer - no idea if mine has it, but I can sub in a resistor and high voltage caps as yet to be determined.
Output transformers - stay with a stock clone, edcor one is nice. Stock replica in any good make.

Power transformer - bias of 60 is fine if adjusted with resistors, and a bigger power supply tranny will get the amp to run better. Bigger = more voltage or higher current ?
Whatever it may be, if I have a range of trannies to work with, I'd pull apart medical equipment and find my tranny.
Thanks much, I will be back with more Q's I am sure.
Cool.
Srinath.
 
You're going to need around 450-500 VDC for the ST-70 to run correctly. If you pull apart medical equipment, I doubt you'll find a stock PT that will do this. Your best bet is to find a 2:1 step up transformer and use a voltage doubler circuit. You'd then need to get some low voltage transformers to provide heater current to the tubes. All of this will take space that I'm not sure the ST-70 chassis will have.
 
No chassis problem here, I am stuffing it in a dead R-500 yamaha chassis, much more room ...
I'll start looking, you mean a 450 0 -450 ? yeow that much ?
Cool.
Srinath.
 
A 450-0-450 transformer will produce about 600 volts DC with tube rectification, due to way that a capacitor-input filter works. (peak of a sine wave is 1.414 * the RMS value listed).
 
srinath said:
No chassis problem here, I am stuffing it in a dead R-500 yamaha chassis, much more room ...
I'll start looking, you mean a 450 0 -450 ? yeow that much ?
Cool.
Srinath.
Click to expand...

Since there's no chassis, tubes, or transformers, you're basically free to build whatever you want. Is there a reason you want to recreate the ST-70? Basically from scratch?

You'll have a real hard time staying under your $100 budget. Just the output and power transformers will set you back at least $200 if you buy new. And then you'll need tubes and all types of little things that add up.
 
srinath said:
No chassis problem here, I am stuffing it in a dead R-500 yamaha chassis, much more room ...
I'll start looking, you mean a 450 0 -450 ? yeow that much ?
Cool.
Srinath.
Click to expand...

If you use tube rectification you will need a a 360 - 0 - 360 Volt center tap transformer to duplicate the voltage output of the PA-60 power transformer in the Stereo 70. If you go with a 4 diode bridge solid state rectifier then yo would need 4 diodes and a transformer of about 290 to 300 volts or so to get about the same output voltage, and it need not be a center tap. The reason for less voltage is that the tube rectifier drops some voltage through it. The reason you don't need to have a center tap is that the nature of the solid state diodes (not needing a B+ to operate) can be arranged in a 4 diode, full wave bridge that eliminates the need for a center tap to get a fully rectified output.
 
I am rebuilding the dynaco cos well, I got it cheap.
I prolly will scavenge for parts and take my time with it.
I got other tube amps, also got cheap, a Precsion electronics one, a couple of consoles etc. No hurry at all.
Mods especially in the parts I dont have are great, and if I have a wide range of transformers I could fit, that means I'd have a easier time finding it cheaper.
So that 360V 60 bias is useable, I will check if somehtig similar is out there cheap.

Power supply tube or SS wont make a difference in the sound ?

Now 300v non center tap is just about what I would have in that wall wart I got, its a 36V 500VA monster. Running it backwards would just about give me 330V. I could look for a transformer that is closer to 40V and run it backwards.
I should find something easy and cheap if going to ss power.
Then resistor for the choke, and that leaves the A470's for outputs.
I should come across a dead st-70 with these parts intact but some else dead ... I hope.
Cool.
Srinath.
 
srinath said:
... Now 300v non center tap is just about what I would have in that wall wart I got, its a 36V 500VA monster. Running it backwards would just about give me 330V. I could look for a transformer that is closer to 40V and run it backwards.
...
Click to expand...


With that much output are you sure it is not a switching power supply? If so, it will not have a usable transformer in it.

Shelly_D
 
Switching ? OK its got a brick sized transformer, 2 ceramic caps and that is about it. I should get a pic of it, and put up the pic of the other tube amp I got.
Cool.
Srinath.
 
With no circuit board and no IC's or transistors in it then it is not a switching supply. The transformer may be usable for your purposes.
 
Better power and same output transformers ?

Essentially I stay with the output transformers.
Then I get better power transformer and put the right resistors to get 32V @ pin 5 of the output tubes.
Now what would be a better power transformer ? anyone help me here.
I believe I have a set of 470's located.

Also the 100 buck project limit didn't include tubes, of which I have almost a 100 out of the 5 amps I have.

Cool.
Srinath.

shelly_d said:
It strikes me that some information collected in one place could be useful here. If I am offending by posting this, please forgive me, I only intend to help.

The 55 volt output is used to obtain 32 volts on the "grid" (pin 5) of the output tubes. A change of the bias resistors will correct the voltage for the 60 volt output.

Since you have no transformers you should be aware that the EL34 is capable of more then 35 watts output in push pull. Dynaco used a power supply that did not respond well to very rapid demands in current draw. (That kept the price down) To compensate they biased the outputs to run class A. That kept the current draw in the output tubes nearly constant. You have the chance to build a more robust power supply that could respond to current changes quickly and therefore change the bias of the outputs to operate in class AB, getting more power out of them.

As to the output transformers. They are very different from power transformers. Power trannys operate at only one frequency, 60 hz. Output transformers must operate over the entire range of human hearing, considered to be 20 Hz to 20,000 Hz. The winding for that is very different from a power transformer. It is important to actually use an audio output transformer in this application. As mentioned earlier there are a number or options out there. The Dynaclone's are suposed to be made to the original specs of the Dynaco Transformers. If true (I have no reason to doubt it I just never used one) then they are excellent for this purpose. Also the Edcor output transformers are very well thought of here on this board as well.

You will need to set up a power supply filter (can cap and choke in the original ST70) Those circuit pulls usually don't include those parts. A resistor could be substituted for the choke and work almost as well (sorry I don't know a value for that) but you will need caps capable of very high voltages. You may need to use caps paired up and wired in series along with high value bypass resistors across each of them to guarantee the voltage across each is kept within limits. (Example, a pair of 80 microFarad caps in series, and a 300K to 500K ohm resistor wired across the leads of each cap. Use photo strobe caps rated at 450 vDC. The network will be rated at 900 vDC and 40 microFarads).

Good luck with this and please let us know what you did and how it worked.

Shelly_D
Click to expand...
 
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