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Postby audiog » Mon Jul 21, 2008 6:24 am

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Postby Shannon Parks » Wed Jul 23, 2008 6:41 am

So the power transformer will have no center tap, correct?
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Postby audiog » Fri Jul 25, 2008 4:01 am

Hello Shannon,
Yes, no center tap.

I would use for example 360-0Vac and not 360-0-360Vac.

I plan to use or a SS graets bridge, with a Vac lower and a delay with a resistor in series for 20-30 sec. or a Hybrid graetz bridge (2xuf4007+2xGZ34) with a Vac higher and without delay of course.
If I build a power transformer with
0-6,3Vac 10A
0-5Vac 5A
0-Vlow-Vhigh / ?A for HT
I could experiment both solutions, I did so with my ST35 from scratch.
So I have to decide the value of Vlow and Vhigh on transformers ad the current.
Thank's and Best Regards.
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Postby kheper » Sun Jul 27, 2008 9:21 am

audiog wrote:Hello Shannon,
Yes, no center tap.

I would use for example 360-0Vac and not 360-0-360Vac.


Usually, the non-center tapped transformers are expressed as 'Vac between the windings', not as 'Vac-0'.

For a capacitor input, full-wave bridge rectifier, a 360 Vac non-center tapped transformer will yield 507 Vdc unloaded.

The stock MKIII B+ voltage in the Dynaco schematic was 490V.
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Postby Shannon Parks » Mon Jul 28, 2008 6:34 am

Giuseppe, go ahead and download Duncan's Power Supply Designer II from the Duncan Amps website. Great tool to tweak exactly what you are doing.
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Postby audiog » Tue Jul 29, 2008 1:37 am

Hello Shannon,

Yes I will use PSUDII for simulation.
It will be of some help to know the Rprimary and Rsecondary (HV) of P782 power transformer and the Vout without load. If someone know this figures, please replay the post.

Examinating the datasheet of KT88, I think that for a "Class AB PP UL fixed bias KT88" with Va=490V and I(a+g2)=70mA for each tube in idle state, it will be necessary consider about Imax=150mA for each tube at max power, for first stage Ik=1.1mA costant because it work in class A, for phase divider Io=6.2mA fixed by corrent source. So for one channel I need 308mAdc, the rated rms current in the secondary could be 218mAac with FWCT rectification and 435mA with graetz bridge rectification at max power.

My idea is to design a new power supply for PP kt88 and investigate about various configurations: FWCT rectification with GZ34 and SS diode,
hybrid graetz bridge rectification, ss graetz bridge rectification.
Another possibility is to use a HV stabilized power supply for first and second stage.

I for the time being am waiting for to receive the ST70 board from Shannon, where I will mount the components for a stereo MKIII.

Meantime I design a first power supply with a 400Vac secondary, with a tap at 360Vac, since the st70 board have only one supply line for both channel. I suppose for best performance a rated secondary current of 700mAac would be enough.

Ciao.
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Postby kheper » Tue Jul 29, 2008 7:44 am

audiog wrote:Hello Shannon,

Yes I will use PSUDII for simulation.
It will be of some help to know the Rprimary and Rsecondary (HV) of P782 power transformer and the Vout without load. If someone know this figures, please replay the post.


The P782 has a 117 Vac primary and a 400-0-400 Vac secondary. This is used with full wave rectifier tube to achieve B+ voltages at around 490Vdc, loaded.

Examinating the datasheet of KT88, I think that for a "Class AB PP UL fixed bias KT88" with Va=490V and I(a+g2)=70mA for each tube in idle state, it will be necessary consider about Imax=150mA for each tube at max power, for first stage Ik=1.1mA costant because it work in class A, for phase divider Io=6.2mA fixed by corrent source. So for one channel I need 308mAdc, the rated rms current in the secondary could be 218mAac with FWCT rectification and 435mA with graetz bridge rectification at max power.

My idea is to design a new power supply for PP kt88 and investigate about various configurations: FWCT rectification with GZ34 and SS diode,
hybrid graetz bridge rectification, ss graetz bridge rectification.
Another possibility is to use a HV stabilized power supply for first and second stage.

I for the time being am waiting for to receive the ST70 board from Shannon, where I will mount the components for a stereo MKIII.


A stereo MKIII will need a substantial power supply. One P782 will not provide adequate amperage.

Meantime I design a first power supply with a 400Vac secondary, with a tap at 360Vac, since the st70 board have only one supply line for both channel. I suppose for best performance a rated secondary current of 700mAac would be enough.

Ciao.


700ma should do it.

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0033508569

This transformer has (2) 360Vac coils at 500ma each. You could parallel these HV coils. With a capacitor input, full wave bridge rectifier, you should get 507Vdc with about 620ma current capacity.
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Postby Blair » Wed Aug 06, 2008 3:58 pm

I did this with my Poseidon boards before it dawned on me that the ST-70 board was more along the line of what I needed Yellow_Light_Colorz_PDT_02

Anyway, it worked well. The tranny is ugly, and it is a bit of a growler, but not audible at all from a few feet.

Image

I used a 400-0-400 tranny though. The AN-360 would be much better for this application. I'm running 580v B+.

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