Re: [neonixie-l] Multiplexing nixies in a tube preamplifier

Michel van der Meij Fri, 04 Jan 2013 14:48:54 -0800

Hi Tim,

Not that I want to influence your design but it sounds a bit odd to have the nixies go from 0 to 63 in a valve (pre)amplifier. It is really related to something digital that IMHO doesn't really match a design of a valve amplifier. I think it is nicer if it would go from 0 to 99. If you use a few latches before the 74141 that would solve all your limited I/O problems. Another thing that comes to mind is using an IN-13 bar graph tube to indicate the position of the volume button.

Michel

on Jan 05, 2013, Tim <[email protected]> wrote:

Hi,

Thank you for your reply David. My reason for multiplexing was due to not having enough spare pins on the PIC, however having givin this considerable thought I have the IO pins that were being used to switch the MSD and LSD anode transistors and a third IO pin which was available to control the enable pin for the tayloredge SMPS, so by using these three pins allows me to to have 0 - 7 on the MSD which is great as I wanted 64 steps to indicate the volume control potentiometer position.

Sadly there is a but to this (is there not always) I need to be able to blank the digits which normally involves using all 4 bits of the 74141. Now as I do not need 7 - 9 I was thinking I could shift all the digits along one position, so 0 in the nixie is connected to 1 on the 74141 1 to 2 and so on. This results in being able to blank the nixie by sending 000 to ABC and having D permanently tied to ground. This digit shift being easy to work around in the firmware.

I can position the SMPS in such a way that it will not interfere with the audio signal path so I would like to try and stick with it for the nixie HT supply as the valve HT supply has a delayed start to give chance for the soft started heaters to warm up. I want the volume indication to come on at power up with a possible count down on it to show the remaining warm up time. Additionally to this I am trying to keep the analogue and digital power rails separate from one another.

This is where my questions begin: Can I leave the cathodes 7 - 9
floating or do they need to be tied to something? Will floating digits ghost? I would also like to
keep my enable signal to disable the SMPS when the amplifier is in standby so can I use the now unused 0 output on the 74141 as a logic
signal to drive the enable pin on the SMPS? The only trouble with this is when blanking the nixie the SMPS will be disabled unless I use extra logic to look at the LSD bits? (I guess it is quite environmentally friendly to turn the SMPS off when the digits are blank but if I was worried about this then I guess I would not be building a power hungry inefficient valve amplifier!)

Regards,
Tim

On Wednesday, 2 January 2013 22:25:13 UTC, nixiebunny wrote:
On 1/2/13 3:01 PM, Tim wrote:

> I am in the process of building an audio preamp using valves and I am

> using nixie tubes to indicate the volume control position. I am using

> two nixie tubes to indicate the volume and they are being multiplexed

> via a PIC and 74141.

>

> My question to you fine folk is how should I provide power to the nixie

> tubes. I have two options available to me:

>

>

> Regards,

> Tim

Tim,

If you can afford the extra four pins on the PIC and another 74141, then

you can run the tubes direct (non-multiplexed) and not worry about it.

If you will be multiplexing them, then the power supply will not conduct

the noise to any noticeable extent. You will need a high anode resistor

value, so you can insert a simple two-stage RC low-pass filter (10K

series, 0.1uf polyester shunt) between the power supply and the anode

resistors to eliminate noise from the anode power.

You will want to be careful about mounting the display unit away from

the input stage of the amplifier, to prevent radiated noise from getting

into the input stage where it will be amplified.

--

David Forbes, Tucson AZ

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Re: [neonixie-l] Multiplexing nixies in a tube preamplifier

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