Slow Blow Main Transformer

Hi everybody !!

I'm interested by building the Slow Blow

i already have a G9 !!! sound beautiful
it's for sure my main vocal preamp.
Great thanks Jakob !! :thumb:

I read here, that the Slow Blow have a more clean sound
and i think it's what i looking for my MK-012 stereo pair.


I have read a few times the schematics, but i'm unable
to find a place to buy the main transformer with 3 secondary !

:roll:

is there another way to build a PSU with the same voltage ?
or an adress to buy this strange xformer ?

thanks very much

Pidjay,

One option is to use two or three separate xformers. Another is to use the G9 style powersupply. I'm going to do one using the second option soon. I have two G9's and I am going to convert one to a slowblow, at least temporarily to see if I like it. I might even make it switchable from G9 to slowblow, using the same infrastructure for both pres and swaping out the gain stages. IIRC, Fotis ran one off his G9 and was very happy, so while the G9 supply might not be exactly what Ollie intended to run a slowblow, it seems to work fairly well at least. Note that if you are using the G9 supply, you will want to use a high wattage trimmer off the B+ to bias up the heaters about 50V. You might also think about reworking the HT section to put out a higher B+ for more headroom.

-Chris

So a G9 PSU with the HT part of the Slow Blow
should work fine ?


thanks very much

Yes. You may not even need to rework the whole HT supply either. Try the 245V that Jakobs design gives before redoing the whole thing. You might be fine with the reduced headroom. If not, then adjust R34-R36 to get a higher output voltage. Just be sure that you are still within the Vin-Vout and current limits in the TL783 spec. If that dosn't work, just pop out R33 and tap in the slowblow supply. Slowblow specs 230VAC input to the rectifier, Jakob, 220VAC. Close enough.

If you already have a G9, it's easier to try what you have first. You can also skip the 50V heater bias, but your tubes won't last as long. The heater bias isn't depicted in the slowblow PS supply schematic that I have anyway.

-Chris

:roll:

what is 50v heater bias ? :roll:

i don't understand that...

I didn't understand myself until recently. Check this thread. Rafafredd does a good job explaining it and shows a schematic where he uses one.

here

It's basically a voltage divider off the B+ which floats the heaters up, so instead of the heater lines being 12V and 0V, they are 62V and 50V (or whatever you want, were deltaV is still 12V). The heaters still only "see" the 12V differential and draw the same amount of current, but they are closer to the cathode voltage inside the tube. This way, the tube is also happy.

-Chris

I found Amplimo, (www.amplimo.nl) a dutch transformer company has really nice toroids for tube psu's.

For example the 3N604 has 2 115 primaries and the following secondaries
250V 200mA
6.3V 2.2A
30V 100mA

You can use a simple voltage doubler and a regulator to get phantom from the 30V winding.

There are more nice trannies to choose from. Go to www.amplimo.nl, click High end audio section -> scroll down and click the button Power transformers for valve amplifiers

They are 51 euro's ex.VAT though.

You can use separate trafos but you will need more space. Slowblow isn't a compact pre. It needs big caps , chockes etc. If you don't find something i can send you whatever you want. My cousin has a small but quality industry. He made toroid power trafos and chockes. My trafo is 250V/300mA-50V/100mA-15V/4A =150W toroid and cost me 45Euros.
Having made some quick tests, i can say for sure that without TL783 for regulation, it sounds better. Keep in mind that better sound come with HV about 400V!!! . The first tests that i do was on 250V from G9 psu and it sounded fantastic. Then, i powered from G9 reservoir caps (330V) and i heard a nice difference. I am planning to go up to 380-400V but the last week i was very busy to try it.

Cheers
fotis

for sure i'm interested !

let me know how to buy this trafo
and it's ok for me ! thanks very much !

HS : is the VU really necessary ? :roll:


it's possible to obtain 400v with the 250v/300mA
of the trafo ??

Ok you can pm for more info.
The meters are not needed ( if you don't want to see them :grin: )
From 250V you will have about 350V. That is what i have , too.
Cheers
fotis