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Current sourced heaters in a CF

Salas

diyAudio Chief Moderator
Paid Member
2002-10-08 11:31 am
Athens-Greece
current sourrced heaters in a CF

Hello

Can I use a current source heater PSU (LM317 R=1.25/I type) for a cathode follower and still bias it with voltage so to keep the cathode no more than 100V DC positive in respect to the heater?
I want to use 0.3A current source for 12AU7.
Do you prefer the sound of a current sourced heater?
 

Salas

diyAudio Chief Moderator
Paid Member
2002-10-08 11:31 am
Athens-Greece
got the parts already

I have got the parts already (he he) but I was mainly interested in floating CT with voltage to avoid exceeding over 100VDC difference between heater and cathode. So I asked if they do this and how on a current sourced heater. I will listen to it anyway and let you know p10.

PS did you see that?:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=57210

you are right to be a proponent
 
The constant current source for filament supply is mainly used for Direct Heated Tubes like the 300B and 2A3. The reason is that the filament forming part of the cathode circuit which would affect the sound.

However, the indirectly heated tubes like the 12AX7 having the filament separated from the cathode, I don't see what CCS could do anything at all.

If you want to float the filament voltage, I think you can connect the -ve side of the CCS supply to the B+'s divider.

Just my 2 cents.

Johnny
 

Salas

diyAudio Chief Moderator
Paid Member
2002-10-08 11:31 am
Athens-Greece
is this ok?

I have prepared this. Its ready. Is it ok to apply it?
 

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SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
Re: is this ok?

salas said:
I have prepared this. Its ready. Is it ok to apply it?

The one glaring problem is that you're using a CCS to feed a parallel pair of heaters. A CCS is best used on heaters in series- in a sense, it forces matching between tubes (or, in your case, between sections of tubes).

You probably do not need that reverse-voltage diode since this is not a HV application.
 

rdf

Member
2004-06-21 8:04 am
big smoke
When driving indirect heaters with a current source is there any benefit to bypassing the coils with capacitors? A small film cap driven by a very high impedance sounds like a cheap and cheerful way to minimize the potential of mains-bourne junk entering the circuit via the heater. Another consideration is that a solid state current source will only act as one to a certain frequency (well below RF?) Anyone played with this combination?
 

EC8010

diyAudio Moderator Emeritus
2003-01-18 7:57 am
Near London. UK
It would indeed be an excellent filter, except. Except that most CCSs for this application are not only not very high impedance, but they're distinctly iffy above a few hundred Hz. I've been doing some preliminary investigations on using power FETs or bipolar audio transistors (such as BD131) in a cascode.
 

rdf

Member
2004-06-21 8:04 am
big smoke
EC8010 said:
...not only not very high impedance, but they're distinctly iffy above a few hundred Hz.

Thanks for the head's up! Heater CCS's were in consideration for my first self design but I hadn't considered the possibility of such poor behaviour from typical implementations. When you say 'most', you mean the the usual IC voltage regulator restrapped as a current source?