I have a Croft Super Micro.
Its voltage was total out of range, so I saw that the neon it used as reference burnt at around 120Volts instead of around 90. I replaced the neone with a new one (The old looked blackened in a way...) and the voltage after the regulator tube was back to the desired 250V
Today I listened to music, and sudenly there was a rythmic popping, the same as the NE2 got blinking....
I asked Mrs Google, and I got:
https://www.southampton.ac.uk/~apm3/diyaudio/Croft_SM_III_circuit.gif
Wow. No Neon2, but a 90C1.
Reference voltage is the same, but: 90C1 has a current of around 1....40mA, but NE2 should only get 0,6mA(!!!)
Can the linked schematic work with the NE2???
Or:
How can I change the regulator schematic, that it works with the NE2??? It seems to me, that the regulator tube has to cope with the current from the 82K resistor (more than 1 mA...) and with the current from the triode in the regulator tube...
So I expect it to be around 3mA...
Can a NE2 handle this current?
Any experiences with a croft and this regulator setup? Please advise.
Regs, Dirk
Its voltage was total out of range, so I saw that the neon it used as reference burnt at around 120Volts instead of around 90. I replaced the neone with a new one (The old looked blackened in a way...) and the voltage after the regulator tube was back to the desired 250V
Today I listened to music, and sudenly there was a rythmic popping, the same as the NE2 got blinking....
I asked Mrs Google, and I got:
https://www.southampton.ac.uk/~apm3/diyaudio/Croft_SM_III_circuit.gif
Wow. No Neon2, but a 90C1.
Reference voltage is the same, but: 90C1 has a current of around 1....40mA, but NE2 should only get 0,6mA(!!!)
Can the linked schematic work with the NE2???
Or:
How can I change the regulator schematic, that it works with the NE2??? It seems to me, that the regulator tube has to cope with the current from the 82K resistor (more than 1 mA...) and with the current from the triode in the regulator tube...
So I expect it to be around 3mA...
Can a NE2 handle this current?
Any experiences with a croft and this regulator setup? Please advise.
Regs, Dirk
Don't need experience with the amp but the design dictates that more than a small neon is required in this schematic.
There were, years ago, neons with an SES cap. They could handle a fair current.
There were, years ago, neons with an SES cap. They could handle a fair current.
> with the current from the triode
Current in triode is like 1/3rd mA. Work it out. 350V on top of 470K, a little under 200V at plate end.
NE-2 can stand 2mA "or so". Life drops as current is increased. Which may be where you are at now. 0.6mA is a happier value for long life.
Can you find the right part? 90V gas-regs are available in several shapes/sizes, but many drop-out below 5mA.
ALL gas tubes are either very-old or super-cheap. The old ones have a pinch of radioactive to help them start-- by now this is half-depleted and starting is erratic. But not as erratic as the very-very-very cheap new neons still on the market.
Are you allergic to Silicon? 45V Zeners are standard parts. Less hiss than gas, and IMO more reliable.
Current in triode is like 1/3rd mA. Work it out. 350V on top of 470K, a little under 200V at plate end.
NE-2 can stand 2mA "or so". Life drops as current is increased. Which may be where you are at now. 0.6mA is a happier value for long life.
Can you find the right part? 90V gas-regs are available in several shapes/sizes, but many drop-out below 5mA.
ALL gas tubes are either very-old or super-cheap. The old ones have a pinch of radioactive to help them start-- by now this is half-depleted and starting is erratic. But not as erratic as the very-very-very cheap new neons still on the market.
Are you allergic to Silicon? 45V Zeners are standard parts. Less hiss than gas, and IMO more reliable.
I'd look for the original 90C1 - you may find some on eBay from time to time.
3mA is too much current for an NE-2 which otherwise makes a decent if not brilliant reference.
A 91V 5% 1W zener would work well too and might be a bit quieter. (You can add some capacitance across the zener as well) Select one from a small number of samples that gets you close to 90V at 3.3mA
3mA is too much current for an NE-2 which otherwise makes a decent if not brilliant reference.
A 91V 5% 1W zener would work well too and might be a bit quieter. (You can add some capacitance across the zener as well) Select one from a small number of samples that gets you close to 90V at 3.3mA
Hi, thanks for your commments.
I have 10x 500mW 9,1Volt Zener. I'll put them on a little PCB with a cap across. They have a design goal of 5mA, that will be sufficient. And noise like superior. So I'll get a good reliable upgrade.
I have 10x 500mW 9,1Volt Zener. I'll put them on a little PCB with a cap across. They have a design goal of 5mA, that will be sufficient. And noise like superior. So I'll get a good reliable upgrade.
Yes that should work well and should be somewhat quieter than a 91V zener, just check their characteristics and make sure the current is sufficient to get them well beyond the knee in their zener characteristic.
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