I am starting a Dynaco ST-35 clone build and have questions regarding which output transformer to choose.
The Edcor’s have 16 ohm taps so would drop right in and give me the proper feedback. But are overkill in wattage and Price.
Musical Power Supplies’s OPTs are less cost and smaller but have to choose between 4-8 or 8-16 ohm, I’m leaning towards the 4-8. Can I pull the feedback off the 8 ohm if I change the feedback circuit? How do I calculate that?
Thanks!
The Edcor’s have 16 ohm taps so would drop right in and give me the proper feedback. But are overkill in wattage and Price.
Musical Power Supplies’s OPTs are less cost and smaller but have to choose between 4-8 or 8-16 ohm, I’m leaning towards the 4-8. Can I pull the feedback off the 8 ohm if I change the feedback circuit? How do I calculate that?
Thanks!
Are you interested in a Z565 clone that looks like an original Z565 or one that also performs likethe original Z565 ?
Regards,
Regards,
I did not look up the schematic. But I bet the negative feedback from the 16 Ohm tap is a resistor that has a capacitor across it.
If the phase and frequency response of the substitute transformer are the same as the original Dynaco transformer, then change the negative feedback resistor to be 70% of the original resistance value; and change the parallel cap, if any, to 1.4 times the original capacitance value.
That provides the same amount of negative feedback from the 8 Ohm tap, as the original amount of negative feedback from the 16 Ohm tap.
Likewise, to use the 4 Ohm tap as the negative feedback point, reduce the resistor to 1/4 of the 16 Ohm tap resistor, and increase the capacitance to 2 times the capacitance of the original 16 Ohm tap feedback capacitor.
These are just a good starting point, but may work just fine without any further adjustment.
If the phase and frequency response of the substitute transformer are the same as the original Dynaco transformer, then change the negative feedback resistor to be 70% of the original resistance value; and change the parallel cap, if any, to 1.4 times the original capacitance value.
That provides the same amount of negative feedback from the 8 Ohm tap, as the original amount of negative feedback from the 16 Ohm tap.
Likewise, to use the 4 Ohm tap as the negative feedback point, reduce the resistor to 1/4 of the 16 Ohm tap resistor, and increase the capacitance to 2 times the capacitance of the original 16 Ohm tap feedback capacitor.
These are just a good starting point, but may work just fine without any further adjustment.
You considered Edcor transformers “But (they) are overkill in wattage and Price.“ Not sure which transformers you considered, but I beg to differ. Their EL84 output transformers are not overkill or overly expensive for what you get in today’s market.
If you want to build a Dynaco ST35 clone you must use original or clone Dyna z565 output transformers, and they are expensive, else you can build of course anything that resembles the topology with whatever transformers you can afford, but it will not be a “real Dyna ST35 clone”. Don’t misunderstand, you can certainly build a nice sounding amplifier this way, but it will not be a Dyna ST35 clone.
With that said, I urge you to check the Z565 clone among Musical Power Supply offerings of output transformers, rather that the smaller and cheaper ones. The model is OT19PP-DYN35-4-8 USA at ~$95 each. They also have the option for the 8-16 Ohm as alternative. [No affiliation, but I own a pair).
If you want to build a Dynaco ST35 clone you must use original or clone Dyna z565 output transformers, and they are expensive, else you can build of course anything that resembles the topology with whatever transformers you can afford, but it will not be a “real Dyna ST35 clone”. Don’t misunderstand, you can certainly build a nice sounding amplifier this way, but it will not be a Dyna ST35 clone.
With that said, I urge you to check the Z565 clone among Musical Power Supply offerings of output transformers, rather that the smaller and cheaper ones. The model is OT19PP-DYN35-4-8 USA at ~$95 each. They also have the option for the 8-16 Ohm as alternative. [No affiliation, but I own a pair).
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@Elemental
Please note a small typo in what @6A3sUMMER wrote:
For 8 Ohm tap vs 16 Ohm tap it is correct to multiply by 0.707 the resistor and by 1.41 the capacitor of the feedback circuit.
For 4 Ohm tap vs 16 Ohm tap you need to to multiply by 0.5 the resistor and by 2 the capacitor of the feedback circuit.
Please note a small typo in what @6A3sUMMER wrote:
For 8 Ohm tap vs 16 Ohm tap it is correct to multiply by 0.707 the resistor and by 1.41 the capacitor of the feedback circuit.
For 4 Ohm tap vs 16 Ohm tap you need to to multiply by 0.5 the resistor and by 2 the capacitor of the feedback circuit.
zintolo,
Yes, where I said "1/4", I was incorrect. I should have said "1/2" there.
1 V on 16 ohm tap has 0.7V on 8 ohm tap. There is less feedback voltage on the 8 ohm tap. So we need less voltage drop across the feedback resistor. A resistor that is 0.7 times the resistance of the 16 Ohm taps resistance, is the correct starting point for the feedback resistor that now goes to the 8 oHm tap.
Example, a resistor that is 10k from the 16 Ohm tap is replaced by a 7k resistor going to the 8 ohm tap. Correct?
We need less high frequency voltage drop for a capacitor that is on the 8 Ohm tap, versus the cap that is on the 16 Ohm tap.
Example, a capacitor on the 16 Ohm tap that is 100 pF, will require a 140pf cap on the 8 Ohm tap.
Yes, where I said "1/4", I was incorrect. I should have said "1/2" there.
1 V on 16 ohm tap has 0.7V on 8 ohm tap. There is less feedback voltage on the 8 ohm tap. So we need less voltage drop across the feedback resistor. A resistor that is 0.7 times the resistance of the 16 Ohm taps resistance, is the correct starting point for the feedback resistor that now goes to the 8 oHm tap.
Example, a resistor that is 10k from the 16 Ohm tap is replaced by a 7k resistor going to the 8 ohm tap. Correct?
We need less high frequency voltage drop for a capacitor that is on the 8 Ohm tap, versus the cap that is on the 16 Ohm tap.
Example, a capacitor on the 16 Ohm tap that is 100 pF, will require a 140pf cap on the 8 Ohm tap.
@6A3sUMMER
I know you know it better than me, that's why I called it typo.
You can find the schematic here: http://diyaudioprojects.com/Schematics/Dynaco-ST35-Tube-Amp-Schematic.htm
Basically 27k||27pF on 16 Ohm tap will be equivalent to 19k||38pF on 8 Ohm tap (18k and 39p will be fine) and will be equivalent to 13,5k||54pF on 4 Ohm tap (13,3k and 56p will be fine).
I know you know it better than me, that's why I called it typo.
You can find the schematic here: http://diyaudioprojects.com/Schematics/Dynaco-ST35-Tube-Amp-Schematic.htm
Basically 27k||27pF on 16 Ohm tap will be equivalent to 19k||38pF on 8 Ohm tap (18k and 39p will be fine) and will be equivalent to 13,5k||54pF on 4 Ohm tap (13,3k and 56p will be fine).
I just built my first tube amplifier, an ST-35, using pcbs available on eBay.
I wanted to treat this as a bit of an experiment in OPT due to the cost to get good ones delivered to Australia. I used the M1120 line transformers and it seems to be working well. Distortion less than 0.5% up to 10 watts.
I have an HP DSA and the ability to run some more tests, I would happily take advice on what other tests I could perform. Others may be interested in the results.
Cheers
I wanted to treat this as a bit of an experiment in OPT due to the cost to get good ones delivered to Australia. I used the M1120 line transformers and it seems to be working well. Distortion less than 0.5% up to 10 watts.
I have an HP DSA and the ability to run some more tests, I would happily take advice on what other tests I could perform. Others may be interested in the results.
Cheers