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
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
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
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.
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.
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.
If that's what he's done, agree 100%. But it's unclear what he actually is doing- if the input cap is to allow a self-bias arrangement and the 1M he's talking about doesn't actually go to ground, but rather to a tap on the cathode load, getting rid of the input cap could be a real problem.
Schematic will clear up the contradictions.
Schematic will clear up the contradictions.
I guess i am kind of a low probability magnet. I cannot say that i had something like a plate-grid short, but after changing a couple of 1969 tubes for another 1977 couple, half a minute after applying B+, i heard a loud pop from the speakers. My dac was dead.
Could that be a cathode-grid short?
Attachments
It can definitely happen with an oscillation. Using a high transconductance tube with 100% feedback and no grid stoppers is a very effective way to make a self-destructing power oscillator.
If the pots in the source of the MOSFETs are turned close to zero, you will also get sparks and flame. Try 180-200R fixed resistors- that should give you 15mA more or less. But definitely see if the MOSFETs have survived the ordeal- use a 10-20k 5W (or a pair of 33k 2W in parallel) resistor in place of the tube and measure the voltage across it.
If the pots in the source of the MOSFETs are turned close to zero, you will also get sparks and flame. Try 180-200R fixed resistors- that should give you 15mA more or less. But definitely see if the MOSFETs have survived the ordeal- use a 10-20k 5W (or a pair of 33k 2W in parallel) resistor in place of the tube and measure the voltage across it.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.