HTTP/1.0 200 OK Content-type: text/html

Can't connect to database, trying again....HTTP/1.0 200 OK Content-type: text/html

Can't connect to database, trying again.... Tubes Asylum

Tubes Asylum

Questions about tubes and gear that glows. FAQ

Return to Tubes Asylum


Message Sort: Post Order or Asylum Reverse Threaded

To Doc.B: C4S active load.

206.24.138.67

Posted on March 23, 1999 at 18:35:12
Tom S.


 
Greetings Doc,

In a back issue of Valve, you describe the active load. Can the active load be substituted in for most plate loading resistors?

I am going to crack open my Jolida once again this weekend to remove the negative feedback. While I'm poking around in there, I am going to find out the values of plate loading resistors used as well as the voltage across them and current through them. Would you be able to use these measured values to choose the appropriate component values for a C4S active load? (DC resistance, current through resistor, voltage at trannie and voltage at the plate)

Thanks for your time and have fun,
Tom S.

 

Hide full thread outline!
    ...
Where are my manners?, posted on March 24, 1999 at 06:14:03
Tom S.


 
As one of the forum moderators on this site, I must welcome your insight and extensive practical knowledge to the Asylum.
I have built your foreplay kit and I was very satisfied with it until it was made obsolete by an integrated tube amplifier. I am sure, that down the road, it will be put to use again. I wish I could show you a picture of the completed product because it looks real classy. I went to the hardware store and found some polished brass corners @4 for $2. People who visit and don't know anything about audio make comments like "Looks like that antique was exquisitely taken care of.">grin<

Tom S.

 

Re: To Doc.B: C4S active load., posted on March 24, 1999 at 09:46:47
Doc B.


 
The load can go pretty much anywhere with the following constraints:

It can run from a few piddling mA ( like in the Anticipation Upgrade) to (theoretically) as hot as 100 mA ( tho I haven't gotten it to do this right yet) with a big heat sink on the MJE 350

It can see up to 300V at the top (where the B+ comes in) of the load, no higher.

So you are talking about a situation where the loaded tube can run about 250 Vp max, which leaves you 50V of compliance (even 30V is enough compliance for the load to work) Naturally you would run correspondingly lower Vp if you need more than 50V of peak output from the tube.


 

Thanks, Doc!, posted on March 24, 1999 at 10:58:01
Tom S.


 
Looks like it can be done unless jolida drives the piss out of the 12AX7's and 12AT7's. I'll find out this weekend.

Thanks and have fun,
Tom S.

 

Damn!, posted on March 28, 1999 at 13:57:56
Tom S.


 
Jolida runs their 12AT7's and AX7's off the same DC supply. They use a voltage divider to supply DC voltages to each of the tubes. The AX7 plate sees about 260VDC while the 66K loading resistor sees 440V. 180V/66K=2.7mA. It looks like the C4S would be able to handle the current if the voltage wasn't so high on the plate loading resistor.:-(

Each AT7 plate sees about 360VDC while its plate load resistor sees 440 volts as well. Since both see more than 300V at the top, it looks like I won't be able to use the C4S. Maybe for the foreplay preamp in the future.....

Thanks again Doc B.,
Tom S.

 

Re: Damn!, posted on April 1, 1999 at 08:55:13
Doc B.


 
Hey, don't give up that easy!
Give the circuit just a little more analysis-

If the plate of the 12AX7 is sitting at 260V, you have 40V of compliance above that, that is still within the 300V Vmax of the C4S. I don't imagine the 12AT7 needs anywhere near the 40V peak of grid drive that would be available...

How about installing a 52K 1W or better WW resistor (use a convenient value close to this, err on the high side) in place of the 66K. This will drop your B+ to 300V at that node when 2.7 mA is drawn. Put a 10 mfd or bigger bypass cap to ground at this node, and attach the top of the C4S there. R1 should be .95/.0027=352 ohms 1/4W, R2 should be about 300K 1W, because with only 2.7 mA going thru the load, you only want about 1 mA of bias current thru the LEDs.

The 12AX7 curves say this is not that great an op point, at a cathode bias of 1.25V you are in the space charge region, but we'll stay with it. Your cathode resistor should be around 1.25/.0027= 463 ohms. Eliminate the cathode bypass cap across the cathode resistor if there is one.

I would have to see the schematic to go any further than this, but there may well be ways to work the C4S into the driver stage too.



 

Re: Damn!, posted on April 1, 1999 at 16:12:32
Doc B.


 
Oops. That 52K 1W resistor ahould be a 38K 1W resistor. The additional bias current of the C4S will create a 3.7mA draw, not 2.7mA.

 

Page processed in 4.025 seconds.