[BUILD] STA LEVEL PCB OFFICIAL BUILD & HELP THREAD

[bernbrue]

Hi Donnie,
Any progress on your build? Would love to hear your comment especially in comparison with your drip sta level.
Bernd

 

[Purup]

Hi Bernd and anyone else who can help,

I have found some nice Jørn Schou input transformers that I plan to use in my build. They are 5K:250K giving a ratio of 1:7 - close to the 1:8 of the Sowter trafo.
Now the question is how to make the input pad. I could make one with different amount of attenuation to compensate for the changed ratio of the input transformer, but how would you suggest to change the input and output impedance of the H attenuater in order to match the transformer and maybe give a higher input impedance for the compressor? Could it be 5K for both input and output impedance of the H attenuator?
Also guessing I need to increase the size of the input pot. But to which value?

Hope you can help.

Mikkel

 

[bernbrue]

Hi,
I suggest to build it according to the values given by the schematic. Then test and modify it if necessary. I have to admit that I'm not good at determining impedance. Maybe an expert can chime in.
Cheers
Bernd

 

[Purup]

Hi Bernd,

I decided to go with impedance matching between the pad and input transformer and modified it into a 20dB pad with 5k input and 5k output impedane. Will see how it works.

Another question for you:
I do not see R43S (200K ohm) in the original schematic. What is the reasoning to put this resistor in parallel with the input pot?

Cheers,

Mikkel

 

[bernbrue]

Hi Mikkel,
There is a later revision of the schematic (1958). You'll find the 200K there.  Concerning impedance matching you can always change the pad later.  Report back when you fire it up.
Bernd

 

[bernbrue]

Added some observations when using different input tubes (6BA6 combo or single 6386 tube). See 3rd posting.
Bernd

 

[gareth33]

Greetings all

Thanks to Bernbrue and Zayance for this project, this'll be my first project with so much iron and tubes  ;D Looking forward to it...!

I haven't found much info in this thread regarding the meter. The one on Don Audios page looks great but is over £150!! I looked on ebay for Simpson Model 27 meters and there are several in the States for around $30, are these a good match? Such as:

http://www.ebay.co.uk/itm/Simpson-Model-27-DC-Volts-Analog-Panel-Meter-0-500V-/181785254004?pt=LH_DefaultDomain_0&hash=item2a533ed874

Would also be interested to hear subjective opinions on the sound of finished unit or audio demos... !

I'm likely going to use a 6386 valve. Does anyone have any good sources for these? I found one in the UK but am keen to keep the price down on the overall project so any advice is appreciated.

Thanks, Gareth

 

[bernbrue]

Hi Gareth,
Original meter is 200 micro ampere (DC) . You can try other values, e.g. 100 micro ampere or 1mA. They should work as well. Sound wise there are some demos on YouTube. The 6386 tube is very nice. A good pair of 6BA6 performs extremely well, too. So it's your choice. Being very careful while building saves a lot of time afterwards.
Bernd

 

[gareth33]

Boards arrived. Look great, thanks very much!

£419 for all 3 transformers from Sowter. Not too bad I guess. How much difference sonically would you be looking at between the Edcor's, Hammonds, Sowters etc?

I'm also interested to hear what other people are doing regarding meters?

Can't wait to start this, looks like a lot of fun, this is my first tube compressor project and I'm looking forward to learning.

Gareth

 

[bernbrue]

Sonical differences are marginal. I've built another vari mu with Sowter and the same circuit with lundahls. I can't hear any difference. In the end it's your choice how much money you want to invest. Sowter transformers are very good but in my opinion a bit overpriced . You can also use a Sowter for input and an Edcor or Hammond for output.
This one looks good. It'll work fine.
http://www.ebay.de/itm/Vintage-Ernest-TurnerMoving-Coil-Meter-1mA-FSD-GWO-/221841837891?hash=item33a6cdcf43

This one is used in the original and has even the same specs.
http://www.ebay.de/itm/DC-MicroAmmeter-Simpson-Model-27-0-500DC-UA-3-1-2-Meter-New-in-Box-USA-/161787425817?hash=item25ab483419

Best
Bernd

 

[gareth33]

Hi Bernd

Thanks for the information. If I'm going to invest in one Sowter, would you say Input is the best position for it? Then go for Hammond power transformer and Edcor on the output. I'm looking for good sonics, but am on somewhat of a budget, if I get the money together I'll go all Sowter.

Appreciate the heads up on the meters too...

Gareth

 

[bernbrue]

Hi
I would take the input transformer from Sowter , Edcor or Hammond output transformer and the don audio power transformer.
Bernd

 

[bernbrue]

Hi,
Are there any finished units out there? We are of course interested in some feedback, questions, suggestions or just some pics.
Best
Bernd

 

[djfatum]

It's a great project
very professionally produced PCBs
I intercede some pictures of my building  8)

Heater regulator mount on the heatsink
Use insulating pads

[size=12pt]regulators working with high voltage must be isolated from the heatsink for the safety remember it's high voltage !!!!!![/size]

 

[bernbrue]

that's what we like to see, Andrej! Keep us up to date.
Best
Bernd

 

[prescott]

Will LL1676 work on the input?

http://www.lundahl.se/wp-content/uploads/datasheets/1676.pdf

Thanks!

 

[bernbrue]

Hi,
Wire it 1:4 and it should work. Leave out the input pad.
Best
Bernd

 

[prescott]

Thanks!

Can I use 6k4p tubes in place of 6ba6?
I have about 50pcs...

 

[bernbrue]

Yes, we can!  ;D

http://www.radiomuseum.org/tubes/tube_6k4p.html

Best
Bernd

 

[prescott]

In the PSU BOM there is a line: for mono 1000uf for stereo 2200uf.
Why is that? Is the PSU capable of running a stereo unit?

Peter