90v neon bulb vs resistor and 1N4007 (to protect cathode follower tube)

[mikepukmel]

Reading Merlin's page on arc protection for cathode follower (e.g. marshall tone stack driver). what is better, i.e. more reliable and transparent to the audio: diode + resistor or bulb?

[Chuck H]

For the last amp I built with a cathode follower I elevated the heater voltage to mitigate the cathode to heater differential. I've had no failures. nickb has a thread on the matter and believes it's the current causing failures. Which I believe is what the solutions you propose account for. But I'll wait for more replies and see where this goes.

[Zozobra]

I too usually elevate the heaters as a matter of course. This is a pretty neat way to do it using the screen supply:

[the fatch]

Reading Merlin's page on arc protection for cathode follower (e.g. marshall tone stack driver). what is better, i.e. more reliable and transparent to the audio: diode + resistor or bulb?

The diode is reverse biased during normal operation, so the resistor and diode are out of circuit, so might as well not be there as far as audio is concerned. The capacitance of the diode is reduced to a negligible 1pF or so by the 'reverse Miller Effect'.

[Chuck H]

I'm linking the post because it directly addresses the problem in question.

https://music-electronics-forum.com/...thode+follower

And I don't have Merlin's book, but fatch is right that the reverse diode would make that circuit basically invisible unless an overcurrent issue activates it. My concern would be that, since these tube failures as cathode followers has become more common that this overcurrent condition is "normal" for the circuit and therefor part of the tone. So eliminating it may alter the tone negatively in some way.?. I don't know how the neon bulb circuit is arranged.

[uneumann]

Not sure if this has come up already, but nickb points convincingly to current as the failure culprit. I'm looking at that 47k cathode resistor vs a more traditional 100k value. The 100k has been used for yrs (since 5f6a days) with relatively few issues. The 47K can allow over 8ma of cathode current during overdrive if B+ is over 400v. That's over the max spec for a 12ax7. I know specs are ignored at times, but the 100k resistor has a history of operating safely. Does the 47k sound noticeably different or better? Is it worth the risk?

[Helmholtz]

I interprete Nicks findings as a (reversible) cathode degeneration effect caused by cathode currents being drawn before the electron cloud surrounding and protecting the cathode has established. Obviously for some tubes initial cathode currents considerably lower than 8mA suffice to produce this effect. Cathode warm-up time of the specific tube surely also plays a role.

I found this:
http://www.tubebooks.org/Books/Atwoo...um%20Tubes.pdf
and I think pages 18..36 give may some further clue.

P.S., I noticed that the pdf above mentions negative ions hitting the cathode. This must be a misprint, as the stripped atoms/molecules being accelerated towards the cathode always are charged positive.

[R.G.]

Well one way to protect a cathode follower in the face of high cathode to heater voltages is to use a MOSFET follower instead. You can leave the tube there if it looks pretty.

[nickb]

The 100k has been used for yrs (since 5f6a days) with relatively few issues. The 47K can allow over 8ma of cathode current during overdrive if B+ is over 400v. That's over the max spec for a 12ax7. I know specs are ignored at times, but the 100k resistor has a history of operating safely. Does the 47k sound noticeably different or better? Is it worth the risk?

The problem with the JVM205, which so happens to have a 47K, is that full 450V HV supply is present from cold. The lack of space charge during warm up coupled with the high current being the root cause of the failure, IMHO. In many (most?) of the other cases there is a standby switch so you are never in that situation. I don't think the 47K vs 100K is the primary issue, but it's not helping either.

If you the replace CF with a MOSFET, you don't get the second harmonic warming of the signal, which I presume is the primary sonic purpose of the tube CF.

My solution was pragmatic. Use a Chinese tube in that slot and stop worrying. JJ may be OK but I didn't have any to test.

[Helmholtz]

The problem with the JVM205, which so happens to have a 47K, is that full 450V HV supply is present from cold. The lack of space charge during warm up coupled with the high current being the root cause of the failure, IMHO. In many (most?) of the other cases there is a standby switch so you are never in that situation. I don't think the 47K vs 100K is the primary issue, but it's not helping either.

If you the replace CF with a MOSFET, you don't get the second harmonic warming of the signal, which I presume is the primary sonic purpose of the tube CF.

My solution was pragmatic. Use a Chinese tube in that slot and stop worrying. JJ may be OK but I didn't have any to test.

How about delaying the HV ramp-up? Don't know if NTCs can be slow enough.
The NTC's time-constant should be similar to the cathode heat-up time.

[mikepukmel]

The problem with the JVM205, which so happens to have a 47K, is that full 450V HV supply is present from cold. The lack of space charge during warm up coupled with the high current being the root cause of the failure, IMHO. In many (most?) of the other cases there is a standby switch so you are never in that situation. I don't think the 47K vs 100K is the primary issue, but it's not helping either.

If you the replace CF with a MOSFET, you don't get the second harmonic warming of the signal, which I presume is the primary sonic purpose of the tube CF.

My solution was pragmatic. Use a Chinese tube in that slot and stop worrying. JJ may be OK but I didn't have any to test.

Sorry for sounding like I don't understand what's going on completely, but I don't understand what's going on completely. I had thought there were two issues: 1) current, and 2) startup condition that caused arcing. I thought (sounds like wrong) that using a chinese tube would satisfy problem 1, but ... its likely to arc due to the design, no matter what tube you put in. I found a couple of awesome discussions on the cathode follower problems, got lost in some of the discussion, picked up some of it.

For the JTM45 type thing Im building now, I put the standby switch after the first two filter caps, which i thought would help since the spike that occurs after flipping the standby off would be less since the two big caps right after the tube rectifier would not be 'empty'. Its this startup condition that is part of the problem, right?

[mikepukmel]

I interprete Nicks findings as a (reversible) cathode degeneration effect caused by cathode currents being drawn before the electron cloud surrounding and protecting the cathode has established. Obviously for some tubes initial cathode currents considerably lower than 8mA suffice to produce this effect. Cathode warm-up time of the specific tube surely also plays a role.

I found this:
http://www.tubebooks.org/Books/Atwoo...um%20Tubes.pdf
and I think pages 18..36 give may some further clue.

P.S., I noticed that the pdf above mentions negative ions hitting the cathode. This must be a misprint, as the stripped atoms/molecules being accelerated towards the cathode always are charged positive.

Thanks for the book link, reading ...

[nickb]

Sorry for sounding like I don't understand what's going on completely, but I don't understand what's going on completely. I had thought there were two issues: 1) current, and 2) startup condition that caused arcing. I thought (sounds like wrong) that using a chinese tube would satisfy problem 1, but ... its likely to arc due to the design, no matter what tube you put in. I found a couple of awesome discussions on the cathode follower problems, got lost in some of the discussion, picked up some of it.

For the JTM45 type thing Im building now, I put the standby switch after the first two filter caps, which i thought would help since the spike that occurs after flipping the standby off would be less since the two big caps right after the tube rectifier would not be 'empty'. Its this startup condition that is part of the problem, right?

If you have a conventional standy switch in the HV then you won't have the current issue or the arcing one, as long as you use it.

[mikepukmel]

Cool, thanks Nick. Then, in case of occasional user error (standby off, but user flips the power on), the neon bulb would help prevent/reduce the arcing problem? Is this the right bulb to use:


https://www.mouser.com/ProductDetail...VjWACq6akdA%3d

[mikepukmel]

I hate to kick a dead horse, there have been so many great threads and comments already about this topic, but has any of you guys put a time delay HV relay, in series with the standby switch, that closes, say 25 seconds after power on? This way, even if the user forgot to turn standy switch "ON" (not conducting), when the amp was turned on, it wouldn't put HV into the circuit for a while.

Are there cheap, reliable relays that can handle the voltage?