ST70 power supply question

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ST70 power supply question

Postby franciscano » Mon Sep 12, 2016 1:58 am

I would like to build a st70 clone from scratch.

Ok, i need to clarify a few things about the power supply. I'm planning to run the amp with kt88 tubes.

So, i'm going to order the PT locally, it will be a toroidal. So i need to confirm the power ratings:

2 x 6.3v heaters at 5A each with center tap
1 x 720VCT (360-0-360) at 300 ma with 55V bias tap - should i raise the current rating for this winding to say 400ma?
1 x 5v at 4A - no center tap

Thank you.
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Re: ST70 power supply question

Postby Geek » Mon Sep 12, 2016 4:37 am

Hi,

So, what you're basically doing is a stereo MK-III amplifier.

Cool thing about this is you can use the plethora of ST-70 drivers for it.
(my test bed is a giant stereo MK-III and I swap KT88 or EL34 as I need to test driver boards)

Here's what you need to do:

Take a look at the MK-III power transformer specs and double them for current:
http://www.triodeel.com/pdf/p782new.pdf

If you're sticking with tube rectifier, parallel 2 x GZ34.

Don't forget to double the current capability of the choke.

Use MK-III output iron, or equivalent.

If you don't mind a BIG chassis, it may be more economical to do a dual-monoblock on one chassis.... big power iron and choke is crazy $$$ $)

Don't skimp on a large amp design. A smaller amp done well with sound far better than a mediocre large amp (been there :$ )

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Re: ST70 power supply question

Postby franciscano » Mon Sep 12, 2016 5:54 am

Hi, i actually don't know what is the difference between the mk-iii and st70 circuit.

I want to build the whole amp in one wooden box. Under the hood, i was thinking of building two monoblock.

I have to look onto the prices for one pt with one 360-0-55-360 300ma, 1- 6.3V 5A, 1- 5v 3A. Vs one with all the secondaries.

For the output transformer i want to use Edcor cxpp60-ms-4.2k.

I would also chose a smaller amp, but i have some 86db sens. speakers. Can you recommend one? :D
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Re: ST70 power supply question

Postby fp2000 » Wed Nov 16, 2016 2:23 pm

please check out what bob latino did with his st 120 stereo amp. I think that is what you should be aiming for.
if you just want an st70 with KT88 tubes the regular PT with 300mA will work.
the st120 uses higher B+, and it pulls more current so the use a beefier PT with 400mA.
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