Risk of DC coupled input with tube pre-amp ?

[tingtong5]

Hi all.

I recently build a simple cathode follower (B+ = 200V) to replace the solid state pre-amp (I did not need the extra gain) in my integrated stereo amplifier. The reson I choose a tube (6H30P-DR) is because in my opinion it sounds better. This is debatable of course, but please not in this thread ;-)

There is a 1M resistor from grid to ground and thus the input does not have a DC component. I still use a capacitor on the input though cause I am afraid to blow up my expensive sources when something goes wrong (as in high voltage DC at the input).

My question is, what is the risk involved with DC coupling the input here? Or can I safely omit the input capacitor ?

Regards,

Ronald

[mastodon]

The risk is not so much blowing up the input equipment, as putting a dc voltage (coming from the input equipment) on the cathode follower's grid, resulting in bias problems in the tube. If you're absolutely 100% sure that there is never going to be a dc voltage on the input, you could remove the input cap. But I'd leave it in place personally.

[SY]

Close to zero danger.

DC outputs are a different story.

[tingtong5]

Originally Posted by mastodon

The risk is not so much blowing up the input equipment, as putting a dc voltage (coming from the input equipment) on the cathode follower's grid, resulting in bias problems in the tube. If you're absolutely 100% sure that there is never going to be a dc voltage on the input, you could remove the input cap. But I'd leave it in place personally.

Normally there should not be any dc at the input, but even if it does occur, there is a cap directly after my cathode follower, AC coupling it (since there is DC on the output) to solid state power amplifier circuit.

So I suppose I it will be safe to ommit the input cap then? No chance of getting DC at the input when the tube gets defective?

Regards,

Ronald

[tingtong5]

Originally Posted by SY

Close to zero danger.

DC outputs are a different story.

Yeah, I know, fried speakers and such ;-)

[SY]

Originally Posted by tingtong5

So I suppose I it will be safe to ommit the input cap then? No chance of getting DC at the input when the tube gets defective?

Low enough probability that it's not something I'd worry about at all. I've never seen a tube go bad in such a way that plate voltage was sent to the grid. I have once seen a grid to cathode short, but in this case, you won't have more than a volt or two.

Keep in mind that most sources will have an output cap of their own.

[mastodon]

Originally Posted by tingtong5

No chance of getting DC at the input when the tube gets defective?

Not unless some freak mechanical failure occurs in which the grid shorts to the plate of the tube. Which is incredibly unlikely under any other circumstances than an earthquake or a plane crash.

[tingtong5]

Originally Posted by SY

Low enough probability that it's not something I'd worry about at all. I've never seen a tube go bad in such a way that plate voltage was sent to the grid. I have once seen a grid to cathode short, but in this case, you won't have more than a volt or two.

Keep in mind that most sources will have an output cap of their own.

I still got a high voltage on the cathode ;-) My main source (logitech transporter) has a DC coupled output ( since I built a new analog output stage in it), the other is a surround receiver which probably has output caps.

But if I understand correctly, risk is near zero and I can safely ommit the input caps, which I do prefer because of the small gain in transparancy of the sound.

Ronald.

[SY]

If you have a high voltage on the cathode, how can you have a 1M resistor to ground at the grid? That makes no sense. Perhaps you should post a schematic.

[revintage]

With the grid at ground level a 6H30 should have ca 6V to ground. Unfortunately this leads to higher distortion as the cathoderesistor will have quite a small value. Better to use a negative voltage and a CCS.