DIY Audio Projects Forum

Hey guys!
New to the forum, looks great.

Im building a stereo tube preamp, I have purchased the already built tube preamp, tube rectiified power supply, choke and power transformer. All were designed for eachother. This is my first attempt at building a tube relate piece of equipment and have spent months on the net and reading books to give me enough knowledge so that I dont kill myself trying to build this preamp. Im up to connecting the power transformer to the power supply but Im pretty unsure on how to do it and I have spent soo much time trying to find out how to do it on the interent but cannot find anything. Here are some pics/schematics of the preamp, power supply and tranny:



*the schematic for the power supply should be attatched to this post as a pdf, theres a possibility that there may be slight differences between the schematic and my actual power supply as there are slight changes made by the company to suit the preamp you are using (im using the marantz m7 upgraded design, made by the same company).

I understand that 6.3v from the transformer goes to the power supply to supply heat for the 6x4 tube, the 15v from the transformer goes to the two terminals labelled L. AC (on the right) which ends up coming out of the terminals on the right labelled as DC1, DC2 and DC3 (each puts out 12v) which then goes to pin 4 and 5 of the tubes (two 12ax7 and two 12au7) on the preamp for the heating, and the 280v from the transformer goes to the two terminals on the power supply labelled H. AC which is the B+ for the preamp. *Note: One of these 12v dc outputs connects to the two 12ax7's in parallel and the other two connect to the two 12au7 (one for each 12au7(not in parallel)). What Im stuck on is what connects to what. For the 15v that goes to the filaments for the tubes on the preamp board, does the 15v pin from the transformer connect to the L. oin on the power supply board or the AC pin on the power supply board? And does the 0v from the transformer go to the L. pin on he board or the AC pin on the board? Im guessing for the 6x4 tube heater on the power supply board, the 6.3v pin on the transformer connects to the pin on the power supply board labelled as 6.3v and the 0v pin on the transformer connects to the pin on the power supply board that isnt labelled at all. (please use the photos I took of the power supply above for reference so you know what Im talking about.

This is for a major project at school (year 12 Major Design Project) so your help will mean sooooo much to me.

never mind, its almost completely wired up, just one thing though, ive measured the voltage for the heaters for the two 12au7's and two 12ax7's and they are each putting out 25vdc which is way too much for the tubes. There is three heater outputs, one for the two 12ax7's in parallel and the other two for the two 12au7's. What am I supposed to do?

Hi Barrington and welcome!

Did you measure the heater output with the filaments connected??
The heater output should should sink to 12,6 VDC with a connected load (filament).

Unloaded - the heater supply voltage is measured right from the capacitors and surely way to high...

The 15VAC from the transformer connects to the right side of the 2 boards (Heater supply board) at "L. AC".
Yep - this ONE board is a TWO boards design. Where the board on the left side is the "High Voltage supply".
The one board is designed to cut it in the middle. So you can mount the HV and the heater supply on several places.

Yep - the 6,3VAC connects to the "6,3" pins on the left board (HV board) for the rectifier tube.
"H. AC" are the "High voltage" pins, coming from the transformer.
The transformers "0" should be connected to "E" or "GND" - but - that's not clearly visible on the HV board.
In each case, the "0" or GND MUST connect to the "-" side of the capacitors!

Yep!

The left over 12VAC tansformer output with 0,3A / 0,6A is an "Auxillary" winding that connects to a input relay or other stuff...

Please check the "HV GND" connection until you're sure where it connects...

Have fun and succes with your project .

_________________
Cheers,
Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.

Thank you so much for the help!
Yea I measured 25v without any filaments connected. So you are saying that the 25v is ok because once the filaments are connected it will drop down to the required 12.6 v?

Hi again .

Yes, the heater supply voltage should(!) drop down to 12,6VDC with a connected load (filament).
Usually, LM type voltage regulators can NOT regulate the fixed / adjusted voltage, if there isn't any current flow through a load..
This regulators are connected to heater supply GND. If they can't read a current flow (output voltage and adjust voltage)
they can't adjust the output - and they pass by the full rectified voltage to the tank caps.
The tank caps add some more volts and so you'll get the higher "unloaded" (and unstable) voltage...
The output voltage will drop down in a second, once a load is connected. Then the regulator does the job.
You could test it with a simple resistor (about 1kOhm) or a simple 12V halogen bulb to simulate the filaments.

Have fun!

_________________
Cheers,
Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.

Ok sweet! thanks so much. Just to double check, are my tubes non polarised? How will I be wiring them up? Will I have pin 9 and 5 together or just leave pin 9?

Yes and no...

The heaters from the 12AT7, 12AU7, 12AX7 type tubes (ECC81, 82 and 83) are centre tapped at PIN 9.
For use with 12,6 Volts DC the heater pins are only 4 and 5 (non polarity).

Pin 9 is for use with 6,3 Volts. (Then, 4 and 5 are connected together and 9 to "minus" or AC).

Please refer to this datasheet: http://www.jj-electronic.com/pdf/ECC%2082.pdf
You'll find all the connections here.

The tube connections are allways counted clockwise, viewed onto the bottom from the socket.
f = filament
a = anode / plate
g = grid / control grid
k = cathode

Have fun .

_________________
Cheers,
Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.

So I am meant to have 4 and 5 connected to positive and nothing connected to negative for 12.6v?

I think i wrote a bit too much...

No, for 12,6 Volts you connect the "+" from the heater supply to PIN 4
and the "-" from the heater supply to PIN 5.
The heaters are not polarised. There is no special "+" or "-" for the heaters.
You could also connect "+" to 5 and "-" to 4.
But - for a reason, it's a good idea to hook up the "+" to 4 and "-" to 5.

This PIN 9 is only used, if you would power up the filaments with 6,3Volts.

_________________
Cheers,
Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.

ok well I initially had it wired like that (pos to 5, neg to 4) but that only powered one of the filaments on the 12ax7's, and I had it wired the same on the 12au7's but again only one filament glowed, but it was barely glowing, you could only just see a tiny bit of red when the lights were turned off. So thats why I re wired so it was pos to 4 and 5, and neg to 9. With it wired like that for the 12au7's and 12ax7's, the two filaments lit up in each tube nicely. The tubes looked overall lit up properly.