protecting a VR tube

[jarthel]

I have built a shunt regulator using CCS + a VR150. the CCS is supplying 82mA. 68mA goes to the headphone amp and the rest to the VR tube.
My concern is that if the amp somehow stops working, it'll probably take the VR tube with it since 82mA will be passing through the VR tube which is possibly double the max rating of the tube.

Is there any way I can protect the VR tube?

Thank you.

[Sheldon]

Could use something like a 60ma slow blo fuse on the VR tube? But you'd have to have your preamp filaments warm before power is applied to the B+, or the fuse would go on power up. You'd also want to make sure your pre would handle the extra voltage, should the VR tube go.

Sheldon

[dsavitsk]

Why is the amp going to "stop working"? And, why bother protecting a $5 VR tube?

[d2134]

I have built a shunt regulator using CCS + a VR150.

This is odd. Can you post the schematic, please?

[Sheldon]

I don't know his schematic, but the concept is not odd. All the supplies using a VR tube have a current limiting (or regulating) device before the VR tube. The simplest is a series resistor. But a CCS can be used instead.

Sheldon

[d2134]

The simplest is a series resistor. But a CCS can be used instead.

This is correct!
But in this case the CCS is supplying 82mA and is not protecting the VR tube, like a regular resistor. This is odd to me. It is like using a too small resistor.

[DHT112A]

[jarthel]

Originally posted by dsavitsk
Why is the amp going to "stop working"? And, why bother protecting a $5 VR tube?

electronic/electrical devices breaks down all the time. it the amp breaks down, it'll stop working.

as for the comments on the price of a tube, these tubes are not abundant locally. I believe it's different situation in the US (I looked at Ebay). So getting a replacement may take several days if I need to buy it overseas.

[jarthel]

Originally posted by d2134


This is correct!
But in this case the CCS is supplying 82mA and is not protecting the VR tube, like a regular resistor. This is odd to me. It is like using a too small resistor.

how would a resistor protect the VR tube? The resistor provides a constant current.

if the amp stop working for whatever reason, the current must go somewhere and I believe the current sinks to ground via the VR tube.

maybe my understanding is wrong?

[Sheldon]

Originally posted by d2134


This is correct!
But in this case the CCS is supplying 82mA and is not protecting the VR tube, like a regular resistor. This is odd to me. It is like using a too small resistor.

The problem is not in the CCS here. Under operating conditions, he is only passing 14ma through the VR tube. That's on the low end of what I've seen recommended here. The consensus (if there is one) seems to be that you want to run the VR tube around half of maximum with about an equal amount of current to the amp.

The problem in this situation, is that the amp draws 68mA. So the current source (resistor or CCS) has to deliver that, plus some for the VR tube.

Frankly, I'd make sure the amp is safe if the VR tube fails, and not worry about the VR. Make sure the voltage after the CCS cannot go too high for the amp.

The other option, assuming that both channels are driven with one CCS and VR at present, is to add a second VR and CCS and power the channels separately. Then you can get the current down near the limit of the VR if the amp is not drawing current. Voltage consideration for the amp safety still applies.

Sheldon