tube voltage regulator error amp

[nhuwar]

What kind of mu do you need for an error amp in a tube voltage regulator. Is anything over 100 good?

Thanks,
Nick

 

[SY]

Pentodes are ideal here. The higher the amplification, the better. Take a look at some of the regulators that Tek used to use.

 

[nhuwar]

Is a triode with a mu of 100 good, my problem is the voltage around 1 to 1.5 kv.

I have found a few triodes that will do this but no pentodes so far.

How do you compare the amp factor of a pentode to a triode, if it's even listed for the pentode.
All the ones I have looked at to are very low like a mu of 6 to 10 for pentodes.


I will take a look at some old Tek schematics but if memory serves they did some funky stuff with the high voltage or nothing at all.

Thanks

Nick

 

[nhuwar]

SY I looked at a few TEK tube scope schematic for reference as you said.

Out of the 2 I looked at all the error amp tubes were 6au6's and it's mu is 36.


The question I have is can I use a triode for this application if it has a high mu say 100 or 150?




Thanks,


Nick

 

[Miles Prower]

How do you compare the amp factor of a pentode to a triode, if it's even listed for the pentode.

u= gm X rp for all devices

Example: 6CB6:

rp= 280K
gm= 8.0mA/V

u= (280E3)(8E-3)= 2240

They don't list it for pents or solid state since it's not very useful for anything but triodes since only triodes exhibit significant Ip change with varying Vpk. Calculations for devices other than triodes use gm. (You can also use gm with triodes as well, if you so desired.)

All the ones I have looked at to are very low like a mu of 6 to 10 for pentodes.

That would be for trioded operation (screen in parallel with plate). For most pentodes, u should be several hundred to several thousand.

 

[HollowState]

The question I have is can I use a triode for this application if it has a high mu say 100 or 150?

Yes you can. The old Kepco tube supplies did it all the time. Actually they used a dual triode (12AX7) in differential mode with one side as the reference and the other as the error amp.

Victor

 

[SY]

nhuwar, a 6AU6 will give a gain MUCH higher than 36, since they use it as a pentode, not a triode. At 4-5mA/V transconductance, a 100k plate load resistor will give a gain of 400-500.

 

[peranders]

A tube super regulator can be found here Mr. Bob Pease has dig up something from the stone age.

 

[Geek]

Hi,

6CB6A's are great for this. Can be had really cheap too.

Here's a decent performing reg using a high-mu triode/pentode:
http://geek.scorpiorising.ca/GeeK_ZonE/index.php?topic=3665.0

Steve Bench has great examples too.

Cheers!

 

[cerrem]

Keep in mind the stability of the closed loop system.... You need to calculate the compensation values to maintain a safe and stable loop.... You can plot your POLES and ZEROS on a BODE plot if you prefer graphical representation....
The objective is stay above 60 degrees of Phase Margin at your cross-over frequency... and better than 15 degrees of Gain Margin..

Chris

 

[nhuwar]

I knew pentodes had high mu but I was questioning myself since I didn't see much of anything when I looked at data sheets, so thanks for clearing that up.


I know my options are very limited do to the high b+ and I haven't found many pentide that work at these high voltages.

Originally I was looking at using a 6293 since it can handle a b+ of 3kv but was thrown off do to the low mu listed, now I know why.

I will calculate the mu for this tube and maybe I will be able to use it.



Thanks for the help Guy's


Nick

 

[HollowState]

Have you considered the 7239 or 8068?

 

[nhuwar]

I don't know the specs on them so no, But I am open to suggestions though.

I did look at the 8068 before but hadn't been able to find a data sheet on it.


Nick

 

[ray_moth]

A 6AU6 should do fine here. There's no need for the pentode error amplifier to be exposed to the full regulated voltage. Normally, there would be a reference tube or two (or a zener string) between the pentode's cathode and ground. It is that reference voltage that is compared with the sampled voltage at the control grid. If the reference voltage is e.g. 500v, that effectively subtracts 500v from the plate-cathode and screen-cathode voltages as seen by the pentode.

 

[nhuwar]

Well I stated 1 to 1.2 kv, but it could be higher then that. My input voltage could be close to 2kv. I want to make the circuit as stout as possible and I want a few hundred extra volts on the back side of the regulator for heavy current draws.

It's still a work in progress so it's all on the chalk board.



Nick