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Say Doug; how would you go about.. [message #31801] Sun, 05 March 2006 12:19 Go to next message
Manualblock is currently offline  Manualblock
Messages: 4973
Registered: May 2009
Illuminati (13th Degree)
converting the complicated CCS loaded linestage to a smple resistively loaded 12b4? That sounds about as good? I know it has been done; just wondering how it would work.

Look here for 12B4 line stage info! [message #31804 is a reply to message #31801] Mon, 06 March 2006 04:35 Go to previous messageGo to next message
Norris Wilson is currently offline  Norris Wilson
Messages: 361
Registered: May 2009
Grand Master
FYI
Norris

Re: Look here for 12B4 line stage info! [message #31805 is a reply to message #31804] Mon, 06 March 2006 06:28 Go to previous messageGo to next message
Manualblock is currently offline  Manualblock
Messages: 4973
Registered: May 2009
Illuminati (13th Degree)
Thanks Norris; I really appreciate that.

Re: Look here for 12B4 line stage info! [message #31807 is a reply to message #31804] Thu, 09 March 2006 14:45 Go to previous messageGo to next message
PakProtector is currently offline  PakProtector
Messages: 935
Registered: May 2009
Illuminati (2nd Degree)
Hey-Hey!!!,
I took some part in that discussion. I just resurrected my first pre as a resistive plate loaded 12B4. With B+ of only 170V, I ran a 10k plate load at 10 mA with 4 red LED's for biasing the cathode. Piddling 1 uF coupling caps too.

It sounded really good, but it was a bit more prone to hum than with the CCS. I had to beef up the PS capacitance( 120 uF/330V salvaged photo flash caps from disposable cameras ).
cheers,
Douglas

Re: Look here for 12B4 line stage info! [message #31808 is a reply to message #31807] Thu, 09 March 2006 18:38 Go to previous messageGo to next message
Norris Wilson is currently offline  Norris Wilson
Messages: 361
Registered: May 2009
Grand Master
Hi Douglas,

With the change to smaller output capacitors and the use of disposable camera flash capacitors. It looks as if you have developed a bang for the buck preamplifier.

What type of power supply filter scheme are you using, CLCLRC, or?

How did the overall sound character change, when you changed to the 10k plate load at 10mA and 4 red LED's at the cathode?
This must have reduced the overall gain somewhat?

Now, if I can figure out how to get a cheap power transformer to obtain the 170V without a big series resistor to drop some Voltage.

This preamplifier could be built from mostly junk box parts.

It would be nice to have a great preamplifier with a tiny investment.

As always, thanks for your input.

Norris

Re: Look here for 12B4 line stage info! [message #31809 is a reply to message #31808] Fri, 10 March 2006 04:02 Go to previous message
PakProtector is currently offline  PakProtector
Messages: 935
Registered: May 2009
Illuminati (2nd Degree)
Hey-Hey!!!,
The PS is a CRCRC with a shared first C then branched. Power Iron is a 261G6 which is ~$30. 125-0-125 at 130 mA, plus a 2A 6.3 winding.

I never built this with 12B4 and CCS, so I don't know what the gain *MIGHT* have been. It has a 50k volume pot with a 75k shunt resistor so it is gain-reduced anyway.

Don't worry too much about the B+, with 10k plate loads, there is no dissipation issue until it's over 400 anyway. 4 or 5( or maybe 6 ) LED's will be fine too. Just make sure your plate loads are up to the task. Remember, the output voltage is going to be less than 10 p-p, so we don't need a lot of cathode bias....back to less LED's no?
cheers,
Douglas

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