DYNA SCA 35 - voltage doubts

[havols]

Hi AK members
I decided to power on my dyna SCA 35 tube amp.
It looks original (maybe the power transformer has been re-routed) in the past, by another person.
So I take out all the tubes ( no one tube on amp)
I take some voltage readings on it, with 3 different wall VAC voltages, with a buck transformer.

112 VAC on the Wall:
heaters secondaries: 5.5 VDC
B+: 380 VDC

118 VAC on the wall
heaters secondaries: 5.6 VDC
B+: 410 VDC

127 VAC on the wall
heaters secondaries: 6.2 VDC
B+: 450 VDC

If the B+ are ok (within scheme), the heaters are too low.
If the heaters are fine (6.2 VDC), the B+ are too high

What can i do with this amp?
I would appreciate your help
Thanks you all

 

[BinaryMike]

B+ voltages will be greater than normal if the tubes are missing. It's best to have the tubes in place and amplifier line voltage at 117VAC for internal voltage measurements, but watch carefully for red-plating or excessive power tube cathode voltage. If the power tubes are of good quality and correctly biased, then B+ and heater voltages should both be pretty close to spec at 117VAC on the line. If they're not, then check your voltmeter before proceeding.

 

[gadget73]

All the voltages will be high with no tubes, but the B+ will be off by the largest amount.

 

[havols]

Thanks for replies
I installed all tubes (good knows)

With 118VAC on the wall (measured with my digital multimeter - VDC mode)
5.1 VAC on heaters
332 VDC on B+

With 132 VAC on the wall (actual voltages)
5.8 VAC on heaters
372 VDC on B+

It seem that the power tap was rewound in the past (a few years ago by someone)
All wires of primary and secondary are different than scheme.
Thanks

 

[BinaryMike]

You should be measuring AC voltage across the heater wires, not relative to ground. DC heater voltage is derived from the power tube cathodes and is an indication of total power tube cathode current. It's better get that measurement directly across the cathode resistor, R42. If I recall correctly, it should be near 13.5VDC (not the 16VDC quoted in the SCA-35 manual) when the output stages are healthy.

 

[havols]

Sorry by my mistake about VDC in heaters.
All voltages on heaters are in VAC. The B+ in VDC.

I take new voltages.

With 132VAC on the wall:
13.3 VDC across R42
5.8VAC on 2 heater secondaries
368VDC on B+ (measured after 2 diodes)

I suspect that this power tap was rewounded in the past, with different specs than original. Could be?
I attached some pictures

Regards

 

[century tek]

Does it work? What seems to be the problem? How do you know the power transformer has been replaced or rewound?

I would connect it to some speakers and check it out.

 

[havols]

Yes, it plays sounds normal
The power transformer was painted, an all the wires have different colors than original.

What seems to be the problem?

The voltages are different than manual. heater secondaries mainly.
Thanks

 

[century tek]

Then I wouldn't worry about it. Enjoy it!

 

[BinaryMike]

It appear that the AC line input voltage should go a bit higher than 132VAC to achieve normal operating voltages, but this is worrying. Why on earth would anyone wind a power transformer for something like 140~150VAC on the primary?

 

[havols]

Then I wouldn't worry about it. Enjoy it

Thanks Century. I'm a bit worried about the low voltage for the heaters.

 

[century tek]

If the power transformer has actually been replaced, then there is really nothing you can do about it except that lower voltage is always better than higher voltage anytime providing that the lower voltage doesn't go below the manufacturer's spec.

 

[havols]

It appear that the AC line input voltage should go a bit higher than 132VAC to achieve normal operating voltages, but this is worrying. Why on earth would anyone wind a power transformer for something like 140~150VAC on the primary?

Every time i listened this DYNA, with 117 VAC on the wall, i thought the sound was cold, normal, as if it were not from tubes.
What could I do now? Replace the power transformer for one with original voltages?
Regards

 

[century tek]

Look at auction sites for a junk SCA-35 with a good power transformer?

Or, there are companies that will rewind to spec.

 

[havols]

lower voltage doesn't go below the manufacturer's spec.

Is safe for the tubes use it with 131˜133 VAC?

Yes, i can find one companie to do it. Expensive, but OK
Maybe i will use this Dyna sporadically

 

[Dandy]

You can buy a new SCA35 or ST35 power transformer from companies that supply kits of these amps. Maybe cheaper than a rewind? e.g. 70 USD here: https://www.dynakitparts.com/shop/pa-774-power-xfmr/

 

[havols]

Tanks Dandy.
I will see. A new has more quality too, than rewind here.
Regards

 

[havols]

Thank you all for your help.
As soon as possible I will buy a new power transformer
Greetings

 

[havols]

Hi AK members.
Someone here have the complete specs of Power Transformer by Dynaco SCA-35?
Primary and Secondaries.
Greetings

 

[BinaryMike]

Specs for Triode Electronics' replacement appear on this page: http://triodeelectronics.com/pa7dynst35sc.html