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Thread: Mod to prevent cathode follower arcing on startup

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    Mod to prevent cathode follower arcing on startup

    Hi All,
    Getting back to finish my work in progress JTM45 circuit build. Ive read, and also heard first hand from friends, that with the standard JTM45 circuit, the cathode follower has a tendency to arc. Some comments on various blogs say its due to the tube not yet conducting, but HV is placed on the tube, and the cathode follower has no plate resistor, so its plate is at higher voltage, the plate to heater voltage is too high and it arcs. Also, read some anecdotes that vintage tubes were less likely to arc, or at least self destruct due to construction.
    Merlin's book has two suggestions, one is a neon bulb, the other is dc (elevated?) heaters with a diode. Has anyone had experience with either solution, or a different one? Id hate to roast the tube before I ever got sound out of the thing. Hopefully, there is a good reliable solution that doesn't make the amp character too different.

    Thanks!
    Sparky.


    EDIT: I think what I wrote above is wrong. The cathode follower has DC on the grid. its grid to cathode, or grid to heater V. (Sorry)

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    Last edited by mikepukmel; 02-25-2020 at 03:37 AM.
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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    Don't forget the joker g1's Avatar
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    If you have a tube rectifier it may not be an issue.
    nickb has done a lot of analysis on this but I can't find the thread right now. Helmholtz referenced it recently here: https://music-electronics-forum.com/...l=1#post549650

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    I think this is Nick's initial post on the matter. I didn't look for follow ups.

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

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    Cool, thanks i thought I read that a while ago, but couldn't find it. Geez Im still working on problems I had 2 years ago.

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    I don't buy it. Plate to heater voltage? Lots of space between them. Usually we worry about heater to cathode voltage. And that only comes along AFTER the tube warms up. WHen the tube is not conducting there is no voltage on the cathode. Cathode makes its own voltage, so to speak. Elevate the eaters if it concerns you.

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    I read and re-read NickB's thread, and this looks like a different issue. What I read about the JTM45's were arcing when the standby is flipped off. But NickB's post is about a very specific Marshall amp where the B+ is always "on" on some tubes, and it kills the tube as it is heating. He even mentions putting the neon bulb in, and the tube in this spot still failed.

    So, for the arc'ing problem, is the neon bulb good enough, without having to elevate the heaters?

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    Ooo, is this a strong argument *for* a standby switch? I remember reading lots of posts, some argued that the standby switch is just not needed, then some comments about 'cathode stripping'. Is NickB's investigation indication that it does occur?

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    Quote Originally Posted by Enzo View Post
    I don't buy it. Plate to heater voltage? Lots of space between them. Usually we worry about heater to cathode voltage. And that only comes along AFTER the tube warms up. WHen the tube is not conducting there is no voltage on the cathode. Cathode makes its own voltage, so to speak. Elevate the eaters if it concerns you.
    OK thanks Enzo, but what causes the arcing?

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    Lifetime Member Enzo's Avatar
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    I could be wrong, but I would think between heater and cathode.

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    Quote Originally Posted by mikepukmel View Post
    OK thanks Enzo, but what causes the arcing?
    Isolation between grid and cathode is supposed to be passed through. When cold we have whole HT voltage on the grid whilst cathode stay at 0 potential. ( cold heaters meant no current flow, no voltages dropped over resistors in circuit ).Say max 500v potential difference may determine an arcing between grid and cathode of an 12ax? Never happen to me. Heater elevation will not protect at startup the junction between grid and cathode but heaters to cathode instead.( this is other junction which have its isolation limits stated in data sheet). The limit between grid and cathode is never mentioned cause the grid it is not designed to conduct current. Just construction aspects of build of specific tube can give you an ideea if the grid is disposed far away enough from cathode as the distance in between to not be passed through by an electric arc at a certain potential difference.

    Keep the meter on previous stage plate and cold power on: you.ll record max ht voltage at beginning, say 450v then slowly stabilize at plate working voltage , say 150v. Do the same thing with the meter in CF cathode. Get 0V at beginning and stabilise slowly at say 152V. The grid of cf will stay at the same voltage to previous plate stage when dc coupled. So the diference between grid and cathode will be 2V when conduct but 450v when cold.It depends by internal tube build if the isolation between grid an cathode will not be passed through at this diference when startup cold tube.

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    Last edited by catalin gramada; 02-25-2020 at 10:46 AM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    Quote Originally Posted by Enzo View Post
    I don't buy it. Plate to heater voltage? Lots of space between them. Usually we worry about heater to cathode voltage. And that only comes along AFTER the tube warms up. WHen the tube is not conducting there is no voltage on the cathode. Cathode makes its own voltage, so to speak. Elevate the eaters if it concerns you.
    I think it's CF grid to cathode voltage that's the problem. The CF grid is directly coupled to the plate of the driver stage. At switch on from cold when neither tube is conducting the CF grid will be at B+ and its cathode at ground.

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    Quote Originally Posted by Enzo View Post
    I could be wrong, but I would think between heater and cathode.
    Yes, sorry Enzo, the first thing I wrote was wrong. There's a post someplace that talks about the distance between the plate, grid, cathode, heater. I didn't find the image anymore, but it said grid-cathode is close and cathode-header is very close.

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    Quote Originally Posted by catalin gramada View Post
    Isolation between grid and cathode is supposed to be passed through. When cold we have whole HT voltage on the grid whilst cathode stay at 0 potential. ( cold heaters meant no current flow, no voltages dropped over resistors in circuit ).Say max 500v potential difference may determine an arcing between grid and cathode of an 12ax? Never happen to me. Heater elevation will not protect at startup the junction between grid and cathode but heaters to cathode instead.( this is other junction which have its isolation limits stated in data sheet). The limit between grid and cathode is never mentioned cause the grid it is not designed to conduct current. Just construction aspects of build of specific tube can give you an ideea if the grid is disposed far away enough from cathode as the distance in between to not be passed through by an electric arc at a certain potential difference.

    Keep the meter on previous stage plate and cold power on: you.ll record max ht voltage at beginning, say 450v then slowly stabilize at plate working voltage , say 150v. Do the same thing with the meter in CF cathode. Get 0V at beginning and stabilise slowly at say 152V. The grid of cf will stay at the same voltage to previous plate stage when dc coupled. So the diference between grid and cathode will be 2V when conduct but 450v when cold.It depends by internal tube build if the isolation between grid an cathode will not be passed through at this diference when startup cold tube.
    Thanks, Catalin. So, this arcing really is a warmup issue. Once the CF is warmed up, and conducting, when the standby is turned off, even if the HV is higher than normal by 20-30v, it might not be enough to cause an arc. Id love to build a test circuit, and put increasing HT on the grid, and see what happens when the standby is turned off, and at what point (if any) an arc would occur, with a warmed up, conducting tube.

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    Quote Originally Posted by mikepukmel View Post
    Thanks, Catalin. So, this arcing really is a warmup issue. Once the CF is warmed up, and conducting, when the standby is turned off, even if the HV is higher than normal by 20-30v, it might not be enough to cause an arc. Id love to build a test circuit, and put increasing HT on the grid, and see what happens when the standby is turned off, and at what point (if any) an arc would occur, with a warmed up, conducting tube.
    Be sure to take pictures!

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    Quote Originally Posted by eschertron View Post
    Be sure to take pictures!
    Oh yea. will have the camera rolling!

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    Quote Originally Posted by mikepukmel View Post
    Thanks, Catalin. So, this arcing really is a warmup issue. Once the CF is warmed up, and conducting, when the standby is turned off, even if the HV is higher than normal by 20-30v, it might not be enough to cause an arc. Id love to build a test circuit, and put increasing HT on the grid, and see what happens when the standby is turned off, and at what point (if any) an arc would occur, with a warmed up, conducting tube.
    Someone consider a hundred volts as a safe limit. For this reason a diode or neon bulb will work. As time the voltage drop below 90v it will be blocked so in fact is out of circuit.

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    the cathode follower has a tendency to arc.
    As you speak of arcing: Has anybody really seen an arc?
    I think arcing at a "lowish" voltage of 450V or so would mean poor vacuum. Which tube brands are concerned?

    I re-read Nick's thread on DCCF failures and found no indication of arcing between any electrodes. (Hope Nick will chime in).

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    I have no expertise to add, but I will say that there about a million JTM45s out there that aren't having any problems with the stock circuit. You don't need to fix a problem that doesn't really exist.

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