NOS Astron mustard caps in 5f6a Bassman

[bepone]

My wall voltage isn’t that stable today so forgive a bit of variation. 



Cathode is normally at 185-190v ramps up to 230v briefly during power up before settling back down.
Grid gets full 450-475v then down to 190 after everything is warm, unless you add a neon from grid to cathode, then it stays under 240v. 
Grid to cathode voltage surged to 475 until tubes heat up. With a neon, Grid to cathode was 75v until heat up.

Neon is a good idea, though I once noticed some weird distortion when using a smaller neon. Didn't have time to investigate that one instance, but I have stuck with larger versions since noticing that and haven't had issues.

all correct.. soldering directly to pins, diode from grid to cathode in cathode follower to protect the tube,
not series diode+resistor like in book from blencowe, because that resistor is preventing diode to do desired protecting action.

[maxkracht]

Forgot. No filament, no cathode voltage. Unless you have neon of course.

[martin manning]

[Without standby] Grid gets full 450-475v then down to 190 after everything is warm, unless you add a neon from grid to cathode, then it stays under 240v. 
[Without neon] Grid to cathode voltage surged to 475 until tubes heat up. With a neon, Grid to cathode was 75v until heat up.

Neon is a good idea, though I once noticed some weird distortion when using a smaller neon. Didn't have time to investigate that one instance, but I have stuck with larger versions since noticing that and haven't had issues.

If standby is on standby at power on, and the heaters are allowed to come up to temperature before switching to play, it's all good. Vk ramps up from zero to its steady state level when standby is switched to play with no overshoot, and Vg tracks Vk closely.

If standby is eliminated, or simply not used, there could be some damage due to the ~450V Vg-k, and that's why Blencowe recommends adding g-to-k arc protection... he is also an advocate of deleting the standby function altogether.

For arc protection Blencowe has shown both a diode alone and a diode in series with a 10k resistor from g to k. A Si diode alone can become forward biased at high signal levels and bypass some current, so the resistor is added to reduce (but not totally eliminate) that effect.

A diode with higher Vf would also work (an LED, or back-to-back Zeners), and neither of those options has the high-signal bypass problem.

A neon bulb limits Vg-k to the neon strike voltage, ~75V. That should be fine, and it will not conduct under high signal conditions. A nice thing about going that route is that Leo could have done that in the '50's ;^)

[maxkracht]

For arc protection Blencowe has shown both a diode alone and a diode in series with a 10k resistor from g to k. A Si diode alone can become forward biased at high signal levels and bypass some current, so the resistor is added to reduce (but not totally eliminate) that effect.

A diode with higher Vf would also work (an LED, or back-to-back Zeners), and neither of those options has the high-signal bypass problem.

Interesting, thank you for the info! Is there anything one should look out for when selecting an LED? It might be fun to poke a 3mm LED into the tube socket hole to give a little light show/rudimentary fault indicator, assuming it wouldn't be damaged with heat/prolonged use.

[martin manning]

I don't think it matters much. Most will tolerate 5V reverse bias, and Vf varies with the color: white and blue are ~4V, red ~2V. You could leave your neon in place and the LED should limit Vg-k to its Vf, and illuminate until the tube starts to conduct. It won't light if you warm up on standby. IDK how much heat they will stand.

[maxkracht]

I once LED biased a tremolo LFO with the LED in the tube socket. Tube was pointing up and did not have a shield. I believe it is still working fine after several years, but not sure if I would attempt with a tube pointing down. I think most cheap 3mm LEDs are 85c rated, so might not last too long above a tube.