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

[mikepukmel]

Chuck, you asked about the configuration of the bulb, this is the source:

http://www.valvewizard.co.uk/dccf.html

[mikepukmel]

Related question, in a JTM45 type "thing", what is the reason that the cathode follower is DC coupled? Is there any reason not to put a cap before the grid on that tube?

http://www.marstran.com/JTM45.gif

[Chuck H]

Related question, in a JTM45 type "thing", what is the reason that the cathode follower is DC coupled? Is there any reason not to put a cap before the grid on that tube?

http://www.marstran.com/JTM45.gif

If you added a cap you'd also need to add another resistor to bias the cathode follower grid. And the cap and resistor also form a HP filter, add phase shifts and increase the time constant of the circuit. I doubt it would be very audible at all in reality though. Point is, I don't see an advantage to adding all this.

[nickb]

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

Seems like the smart thing to do. Yes, that one will work.

[nickb]

Related question, in a JTM45 type "thing", what is the reason that the cathode follower is DC coupled? Is there any reason not to put a cap before the grid on that tube?

http://www.marstran.com/JTM45.gif

I think it will fundamentally alter the asymmetric distortion and so have a significant sonic effect. Merlin has the scoop, and here.

[SoulFetish]

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

That bulb seems fine. The traditional part number for your standard neon used in any number of applications is NE-2, but this bulb looks to be an appropriate analog at a quick glance (if not direct replacement). Some important things to note are that datasheets will give you a range of initial breakthrough voltages at which the neon will begin to conduct, and will usually differ at AC and DC voltages. (I'm guessing that this is due to AC voltages being measured in RMS, while the peak voltages of the AC waveform will cause the neon to flash over at a lower "measured" RMS voltage. But I digress....) You just want to check the datasheet for the particular type of tube you using in this application to insure that the turn on voltage of the neon doesn't exceed any of the max inter-electrode voltage ratings. But I use neons to protect the grids in my EL84s from dropping to -150V at turn on and it works like a charm.

[mikepukmel]

If you added a cap you'd also need to add another resistor to bias the cathode follower grid. And the cap and resistor also form a HP filter, add phase shifts and increase the time constant of the circuit. I doubt it would be very audible at all in reality though. Point is, I don't see an advantage to adding all this.

Oh boy, thanks, all way over my engineering pay grade.

[mikepukmel]

I think it will fundamentally alter the asymmetric distortion and so have a significant sonic effect. Merlin has the scoop, and here.

Thanks Nick, reading...

[mikepukmel]

That bulb seems fine. The traditional part number for your standard neon used in any number of applications is NE-2, but this bulb looks to be an appropriate analog at a quick glance (if not direct replacement). Some important things to note are that datasheets will give you a range of initial breakthrough voltages at which the neon will begin to conduct, and will usually differ at AC and DC voltages. (I'm guessing that this is due to AC voltages being measured in RMS, while the peak voltages of the AC waveform will cause the neon to flash over at a lower "measured" RMS voltage. But I digress....) You just want to check the datasheet for the particular type of tube you using in this application to insure that the turn on voltage of the neon doesn't exceed any of the max inter-electrode voltage ratings. But I use neons to protect the grids in my EL84s from dropping to -150V at turn on and it works like a charm.

Thanks for the info, and photo, and wow, beautiful work, almost too beautiful to close up and use as an amp! Mind me asking: where do you get your turret lugs, and the little discs you have your neon lamps sitting on?

[SoulFetish]

Thanks for the info, and photo, and wow, beautiful work, almost too beautiful to close up and use as an amp! Mind me asking: where do you get your turret lugs, and the little discs you have your neon lamps sitting on?

Those turret lugs, I bought from https://www.surplussales.com. Ill try and track the actual link down. The discs are just repurposed little rubber feet you can get at Home Depot or whatever.
(But, as an aside, those tube socket are complete shit and were replaced.

[Chuck H]

But, as an aside, those tube socket are complete shit and were replaced.

As pretty as your work is that's a real slog. It's one thing to build like that. It's another to tear down and rebuild like that! And I have no doubt you did too.

[mikepukmel]

Those turret lugs, I bought from https://www.surplussales.com. Ill try and track the actual link down. The discs are just repurposed little rubber feet you can get at Home Depot or whatever.
(But, as an aside, those tube socket are complete shit and were replaced.

Good deal, thanks. I think these are some of what they have:

http://www.surplussales.com/Hardware/Standoffs/dap.html

wow what a great place.

[SoulFetish]

Good deal, thanks. I think these are some of what they have:

http://www.surplussales.com/Hardware/Standoffs/dap.html

wow what a great place.

That’s it. Yeah, they’ Great.
I tapped my chassis to take 2-56 threaded inserts. But be carefull when using them like i did right at a grid input. They can act has a very good conduit for transferring mechanical vibrations and microphonic noise. I was able to damp that by making a gasket layer using a piece of duel wall heatshrink. (See the red layer under the standoff)

[nsubulysses]

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

[ATTACH=CONFIG]50921[/ATTACH]

Just curious why the 100K resistor to ground in parallel with the cap? is it just to drop the voltage a bit? Or is it just the resistor that is in the same position as the 39K resistor in the image I posted?

Elevated Heater Supply and Bleeder Network.pdf

[frus]

Beautiful work SoulFetish!
That green screen resistor doesn't seem to be well soldered though (in the photo at least)

A question: why would EL84 grids go to -150 V if not protected by neon bulbs?