That is the newer version of the output transformer. Connect the feedback to the 8 ohm terminal.
No specific changes, but verify whether all the output transformer lead connections are correct,
for both channels.
for both channels.
Good afternoon! I have another question. The power transformer (PA-774) indicates a secondary of 330VAC-0-330VAC, my transformer has a secondary of 250VAC-0-250VAC, as the manufacturer said this is the correct voltage. If it were set to 330, which is wrong, it would give 470VDC. The EL84 valve works with 300V rectified. Is this correct?
Here is the data sheet for the PA-774 power transformer. The secondary is 660 VAC center tapped.
The primary is 120 AC.
The primary is 120 AC.
Exactly. The one I bought has a secondary of 250VAC-0-250VAC, because the manufacturer of my transformer said that this is the correct voltage. With the secondary of 660VAC CT (330V-0-330V) it is wrong, because after rectification it would give 470VDC, and the EL84 valve would not be able to handle this voltage.
Is the correct secondary 250 or 330V?
Is the correct secondary 250 or 330V?
ST35 has 385V after the rectifier, the plated app 375Volt
This according to the manual.
A transformer giving 275Volt ( mean ). will give 388volt after recification as top value
A transformer giving 250V will after rectifier give 353Volt after rectification, which seems too little for a st-35.
This according to the manual.
A transformer giving 275Volt ( mean ). will give 388volt after recification as top value
A transformer giving 250V will after rectifier give 353Volt after rectification, which seems too little for a st-35.
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You also have to take into account the DC voltage drop of the 50 ohm power supply resistor (10 VDC)
and the 95 ohm cathode bias resistor (14 VDC), so those together lose about 25 VDC from the B+ value.
So in effect the B+ is actually 355 VDC when using the original Dyna power transformer at 120 VAC line voltage,
and the DC voltage to ground just after the rectifier is 380 VDC according to the original Dyna spec.
and the 95 ohm cathode bias resistor (14 VDC), so those together lose about 25 VDC from the B+ value.
So in effect the B+ is actually 355 VDC when using the original Dyna power transformer at 120 VAC line voltage,
and the DC voltage to ground just after the rectifier is 380 VDC according to the original Dyna spec.
The issue is the 380v print. Does that make any sense without a further explanation? Calculate peak voltage to the first input cap. Where are the losses?
The only explanation I can think of is that the PT has a very high internal resistance that gets loaded beyond a normal value to create a larger V_drop than one would expect.
I would add that that might make sense when you look at the rectifier current rating. It's kind of low for SS rectifier into 60uF input cap and so Dynaco may have designed the PT with an extra high internal resistance to protect the rectifiers on a cold or hot restart.
The only explanation I can think of is that the PT has a very high internal resistance that gets loaded beyond a normal value to create a larger V_drop than one would expect.
I would add that that might make sense when you look at the rectifier current rating. It's kind of low for SS rectifier into 60uF input cap and so Dynaco may have designed the PT with an extra high internal resistance to protect the rectifiers on a cold or hot restart.
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The Dyna transformer may be rated at no load, as many were back then.
Someone would have to measure the AC secondary voltage while under normal loading to find out.
Someone would have to measure the AC secondary voltage while under normal loading to find out.
I understand and agree with your point of view. However, it is very difficult for someone to have such an open device to measure.
I don't know what to do, but I will continue the assembly with my swallow and see what results. Again I thank you all for your help.
I don't know what to do, but I will continue the assembly with my swallow and see what results. Again I thank you all for your help.
You and your transformer supplier are correct that a typical EL84 amplifier often uses a 250-0-250 Vac power transformer. But the ST35 runs the EL84 at higher plate voltages and does so with proven results. The original Dyna PA-774 was rated 330-0-330 VAC at 180 ma, which was slight underrated in current IMO.
You are right again, a 330 -0-330 secondary calculates to a B+ of ~460 V, without taking into account the losses, reductions in resistors and droop under load. In reality with the PA-774 the actual B+ is only 385 V under load (380 V on the plate and 14 V on the cathode, so 366 V across the EL84).
Do you know what the current capacity is of your 250-0-250 VAC transformer? If it is robust with large current capacity and can supply the required current without much droop you could perhaps use it, but:
I think your best path is to try to return your transformer and ask the supplier for one that will supply 385 VDC after rectification at a load of 180 - 200 ma.
If the supplier cannot honor that specification in that way I would suggest ordering a slightly higher current transformer (the ST35 circuit benefits from a more robust power transformer) that provides something like 320-0-320 Vac @250ma, or 330-0-330 Vac @ 200 ma. Good luck!.
Mr. François, I thank you for your considerations. Tomorrow I will take some tests on the amplifier and pass your recommendations to the supplier.
Hugs!
Hugs!