Croft Super Micro...

[HVfanatic]

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

 

[JonSnell Electronic]

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.

 

[PRR]

> 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.

 

[kevinkr]

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

 

[HVfanatic]

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.

 

[kevinkr]

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.