Driver Board for the ST-70 Using Common Mode Feedback PI

[Geek]

Hi Joe,

Sent you payment for an octal board. Is there a schematic that I could download so that I can get parts ordered?

Boards will go out tomorrow :-)

BoM is a few posts above.

Schematic is here:

http://geek.scorpiorising.ca/contrib/Ge ... de_v2a.png

Replace the 5751 in the schematic with a 6SL7 and the 6GC7 with 6SN7 (two are needed).

I'll try and get the docs for this up before Christmas.

Cheers!

[WA4SWJ]

Gregg,

PM sent.

[tomlang]

I did a Mouser BOM here (I hope this link works)...

https://www.mouser.com/ProjectManager/P ... 19c5492151

[burnedfingers]

Your link worked great! Just finished ordering the parts using it.

Thanks!!!!

[Quad]

A trivial nit - the board has very nice left/right symmetry,
except for the silk screen legends for R12/R13 and R14/R15.

[Geek]

A trivial nit - the board has very nice left/right symmetry,
except for the silk screen legends for R12/R13 and R14/R15.

When proofreading the file before it goes to the mfr., the silkscreen is practically invisible.

It's really hard to do anyway on tight component boards like this while still keeping it visible without a magnifying glass, LOL!

Anyways fellows, here's the schematic and board view you will really need to complete this.




Don't forget the heatsinks on Q1.

The power rails are meant to be versatile. I'm working with guys locally who insist on a seperate power supply for each rail.

For the standard ST-70 PS, tie the left and right 6SL7 "B+" together and feed that from the end of the power supply filter (the "pre" supply) and do the same with the left and right 6SN7 "B+" and feed them from the "pahse inv." supply.

Same with the bias... they're seperate to allow for tube balancing (Triode PSU, my PSU board, etc.), but for a standard ST-70 bias, just tie the two "Bias" inputs together on left, same for right and feed them as a stock ST-70.


Cheers!

[tomlang]

Regarding the Q1 heatsink, the one you spec'd is about 1.5" long, actually for two transistors. This size needed for heat rejection? I assume so as it is so close to the tubes. One simply bolts it onto Q1 and let it stand tall?

Finally, the components shown on the rear of the board mount on the underside, correct?

Great job, Geek, as it seems you have thought of everything!

[Geek]

Hi Tom,

The heatsink size was chosen because 1) Digikey did not carry singles of that style and 2) when I originally designed it point-to-point, the CCS sinks went under the chassis, where poor ventillation demanded a much larger heatsink.

Yes, rear components mount underside.

There is also a sequence to add the parts. Docs will have that, but it's not hard to figure out.

The output caps, C2 and C3 are real easy to mess up the installation. The silkscreen will point to the exact holes for each. Double check the traces if you are unsure.

Cheers!

[Geek]

One thing I did not incluse in the BoM was the power supply divider resistors

As mentioned in the schematics, they should be chosen for ~250V on the pre tube (6SL7 for the octal board) and 400V on the PI (6SN7 for the octal board).

The standard ST-70 divider uses 6.8K followed by 22K. This needs to be changed, depending on your B+.

My stock PA-060 + SS GZ-34 replacement gives me 450-460V. For this I used: 2.2K/2W and 68K/2W.

For a 420-430V B+ as specified on some schematics (what I had with a 5U4 rectifier), use 1.2K - 1.5K/2W for the first resistor.

Tested tolerances for performance are:

PI: 390-415V
Pre: 240-270V

Sorry for the omission.

Cheers!

[burnedfingers]

You have a reference to "top EL34" do I assume that you mean V6& V3? as the top EL34's?

[Geek]

Hi,

Top EL34 as in the one with the Blu/Wht anode lead. Get 'em backwards and you have a power oscillator

I just drew this for the docs. I'll post it now to clarify:

Either right or left channel:


Cheers!

[Geek]

DOCS!

http://www.classicvalve.ca/docs/DynaMutt_octal.pdf

Please let me know if I left something out, or if something is "clear as mud".

Cheers!

[burnedfingers]

Received my board the 26th. This is one fine board and I can't wait to get started on it.

Thanks
Joe

[burnedfingers]

I see C6 on the board....do I need it?

[Geek]

I see C6 on the board....do I need it?

Nope!

It's part of a future implementation, not used on this version.


Note to folks:
My email has konked tonight. Tom, I got your email and replied. My emergency backup email is classicvalve <at> gmail <dot> com

Cheers!

** edit **

Email issues seem to have been fixed.