Mr. E.Z. Tube is definitely not audiophile gear - it is a platform for learning about tubes (bias, output/input capacitors, etc) without making a huge upfront investment in 'iron'. There are tons of tutorials on working with tubes, this is not one of them. This is a platform that helped me understand what the tutorials were talking about through hands-on experience.
Compatible with a variety of popular tubes (including tubes currently in production).
IR remote control interface (power, volume, 4 auxiliary outputs).
Cheap and easy SMPS (no 'iron'!).
I think it sounds great (but I have awful hearing).
Standard warnings apply: Mr. E.Z. Tube will not hesitate to kill you given the chance. Mr. E.Z. Tube can be run from batteries or an isolated DC wall-wart, but the same precautions apply as when working with a transformer and mains current!
Read on for more.
Step 1: Eliminating the 'iron'
Power transformer - The vast majority of tube amps use a transformer connected to mains power (AC) and a rectifier circuit to produce clean high voltage DC for the tube. This heavy chunk of metal and wire is expensive and dangerous. The rectifier diodes and capacitors alone are a whole project. Mr. E.Z. Tube uses a 240 volt switch mode power supply I originally designed for nixie tubes. The SMPS is simple, with the option of battery operation!
For a detailed overview of the SMPS operation see my SMPS instructable for nixie tubes:
The Mr. E.Z. Tube SMPS has a few enhancements to eliminate switching noise:
1.A larger inductor (1.2 amp continuous rating).
2.A soft recovery rectifier diode (BYV 26C) was used.
3.Two large output capacitors and a choke were added to the HV output.
4.Updated firmware with high voltage 'enable' switch.
With these enhancements there is VERY LITTLE switching noise - it is only noticeable with no audio playing and power amplifier volume at 100%.
Step 2: The Tube
There are a ton of tutorials on correctly biasing vacuum tubes. I followed the suggestions here:
My output coupling capacitors are a bit large (1uF/250V), I plan to try the recommended 0.1uF caps soon. Thats the beauty of the Mr. E.Z. Tube development board - swap parts and hear the difference for yourself, no hype.
Step 3: The Interface:
Volume is adjusted with a DS1807 I2C audio potentiometer. I have not received this chip, so this functionality has yet to be implemented in the beta control firmware.
The PIC is programmed with a staggered startup and shutdown routine - a FET is used to power the tube heater/filament several seconds before the high voltage supply is enabled. This is supposedly good for tubes. The heater is run from 5v, also for better life (be sure and put a heat sink on the 7805). Heater control gives Mr. E.Z. Tube a true remote controlled 'power off'.
4 leds on the front panel indicate status. LEDs show, from left to right: system power, filament/heater power, high voltage supply, low voltage supply (for future use with a gainclone power amp).
Four auxiliary outputs are brought to header pins on the left side of the PCB. These can be used for anything, but I intend to use them as follows:
AUX1: low voltage SMPS enable for a future gainclone power amplifier (next month's instructable).
AUX2: button to trigger remote control code programming (already set).
AUX3: digital/analog source select for a (very) future SPDIF DAC.
FIRMWARE NOTES: this is a beta firmware version. I will release an update here when I receive the DS1807. The IR decoding routine isn't the best. It samples according to the RC5 protocol - but I don't know if thats what my remote uses. Codes are stored in the PIC EEPROM so you can see what they look like by reading them out with a PIC programmer. If you need a cheap easy programmer try my enhanced JDM2, also available here at instructables:
My Happauge WinTV remote works well, but I can see in the EEPROM that I'm not getting the bit stream correctly. None the less, by identifying the 1's and 0's, and noting the errors (with $FF), I get pretty reliable results. This will be fixed when I get a remote that I KNOW uses RC5 (I have no idea what the Happauge remote puts out...).
Step 4: Files
Circuit and PCB: Cadsoft Eagle format - made with the freeware version.
SMPS & Interface firmware - written with the freeware version of MikroBasic.
OO.org documents - this instructable in the .odt format (no more MS Office exports).
Interface firmware is still in beta. A final version will be released when I recieve the DS1807 potentiometer chip.
The file was missing for about five minutes. Its there now!