Introduction: Mr. E.Z. Tube Development Board
Goal/purpose: Mr. E.Z. Tube is a cheap vacuum tube audio platform without the 'iron': no power transformer, no output transformer(s). A tube amplifier will typically have several heavy, expensive transformers: output transformers that protect speakers from high voltages in the tube, and power transformers that provide high voltage by rectifying mains (AC) power. Mr. E.Z. Tube eliminates both, but still uses popular high voltage tubes (EU:ECC81/2/3/4/5/8, American:12AT7/12AU7/etc.).
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'
Output transformer - to eliminate the output transformer we stick with a line amplifier design. Line amplifiers, unlike power amps, can be capacitor coupled. We still get to experience the joy of tube sound, but without the burden of driving speakers directly with tubes. In the picture below you can see a Mr. E.Z. Tube connected between a PC line-out and cheap PC speaker amplifier. My next instructable will cover the construction of a 'gainclone' op-amp power amplifier to pair with the Mr. E.Z. Tube.
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
Mr. E.Z. Tube will take any tube with a pinout similar to the ECC81/ECC82 (12AT7/12AU7). These tubes are quite popular and common. Tubes are available as 'new old stock' and new tubes manufactured in China and eastern Europe. I have only tested an ECC82, but I look forward to trying other types.
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:
What's an amplifier without a remote control? Mr. E.Z. Tube has an IR remote interface powered by a PIC 16F628A. The pic decodes IR remote commands in the RC5 format. Codes are programmed by shorting the jumper connected to RB0 on the pic. The front lights will prompt for codes (with a blink) in the following order: power, volume up, volume down, mute, source-select/auxillery one.
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
This ZIP archive contains all the files for the current version of Mr. E.Z. Tube, including the PCB:
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!