MK1,SOB,MK2 Constant current Diodes

[tomasana]

Hi, any realy serious amptech around?
I've serviced lots of Boogies in the past 25 years. Some of them had constant current diodes (1N5303, TCR 5303, TCR5305...) instead of the kathode resistor. Can anybody explain, what happens exactly in a pre-amp tube with this kathode diodes added? Why aren't they used in later models any more? A TCR 5303 is a 1.6 mA constant current diode, but as far as i know the 12AX7 tube delievers only 1.2 mA. So what's the reason for these strange parts? SOBs use a TCR 5305 (which is 2.0 mA) at the coupled kathodes of the driver tube 12AT7. What happens here?
Thanks for your help, Thomas

 

[Mr. Bacon]

Hi, good question!
I have a 60W. SOB with power limitter myself.
I use a 12AT7 as PI tube.
The diode seems to put the kathode on a +50 volt level!?
So the tube works on a -50 v. grid curve!?
(the books normaly mention approx. -2 volts)
On some schematics they even mention 12AX7 for V3.
So what is the right one? Do you have the right schematics?
I just plugged in a 12AT7 as the amp came without tubes.
The amp sound good but........ good to know for sure.
Cheers,
Bas

 

[kaluablue]

I've been looking for an answer to that question aswell... Perhaps it's just a solution for the 'Fetron' tube replacement that could be used on those old amps?
Current limiting is much more important (live saving) for solid state units than for tubes, I think.
Just a thought though, I'm no amp tech in any way ;-)

Boogiebabies mentioned replacing those TCRs with 1k5 resistors is an official mod by Mesa:
http://www.forum.grailtone.com/viewtopic.php?p=110981&highlight=#110981

 

[Mr. Bacon]

I knew I did something wrong there.
I mentioned -50 volts, but that is the powertube bias voltage.
It is -7 volts on limit=0 to -15 volt on full limmit.
So the grid works on -7 to -15 volt curves.