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DIY 6T9 Tube / Valve Amplifier Project

 Mark Houston  Australian Flag   To email Mark, type out the email address.

6T9 TUBE / VALVE AMP

If you are just getting into do-it-yourself (DIY) audio and have wondered about building a valve amplifier but been put off by the seeming complexity and high voltage, this 6T9 valve based amp may just be the answer. The high voltage concern is a real one and is no less important in this project than any other. But what does soften the build complexity is the entire amplifier is based on a single printed circuit board (PCB). The valve used in this project is the 6T9 compactron which is a triode and pentode in the one envelope. The 6T9 valves I used are new old stock (NOS) General Electric which are cheap (about $7) and available from several online sources.

The schematic of the amplifier and power supply sections are shown in Figures 1 and 2. (Please note that this circuit is © Spare Time Gizmos and permission to host the schematics on this site has been provided by Spare Time Gizmos.

6T9 Single-Ended Tube Amp Schematic
Figure 1: 6T9 Single-Ended Tube Amp Schematic


6T9 Valve Amp Power Supply Schematic
Figure 2: 6T9 Tube Amp Power Supply Schematic

The PCB for this amplifier can be purchased from Spare Time Gizmos for $16US. In a single afternoon you can have the whole board assembled and dropped into a chassis of your choice.

PCB for 6T9 Tube Amp Kit
Photograph 1: PCB for 6T9 Tube Amp Project


PARTS SELECTION AND MODIFICATIONS

The value and type of component is laid-out in the parts list. But what I like to do is to put my own twist on each amp I build. If you are not an experienced valve amp building follow the parts specification to the letter. I increased the input and coupling capacitors to 0.22 uF and used polypropylene types. These components are physically larger than what is meant for the PCB so some manipulation is required. I also like to use snubber capacitors across any possible noisy components such as 0.01 uF poly caps. across the diodes and 1 uF capacitors across the filaments of the valves. Snubbers on the filtering caps is also a good idea and I used 0.1 uF polies. I added an extra 200 uF of capacitance to the power supply and found that this helped the bass response. I used a Hammond 369JX power transformer and Edcor XSE15-8-5K for the output transformers (OT). A smaller OT can be used but expect the bass and treble to suffer. The Edcor OTs are very reasonably priced and also have a ultra-linear (UL) tap so you can run the amp in true UL mode.


TUBE AMPLIFIER CONSTRUCTION

I mounted the tube sockets on the back of the board so I could expose the cute little hot bottles outside the chassis. A drilling template is provided with the boards. Instead of an expensive chassis I used a $7 cooking pan. Drilling thin metal can be more problematic than drilling a heavy chassis, so be patient with drilling. I mounted the power and OTs on rubber grommets that were sliced in half. This was done to reduce the potential for transformer vibrations reaching the tubes which can be microphonic. The thin chassis is mounted on thick lacquered pine and silicon feet to also help absorb vibration.

An Alps blue velvet 50k stereo potentiometer with an audio taper was used for signal attenuation. The front of the amp was finished off with a 4 oz solid brass knob. The brass knob and attenuator are expensive but what I saved on the chassis allows me to spend in other areas and still make this an inexpensive amp to build. Total cost for the amp was about $375AU (Hammond and other imported parts are expensive in Australia) but one can easily build this amp for about $250US in North America using good iron.

Single-Ended 6T9 Tube Amp Kit
Photograph 2: Finished 6T9 Tube Amplifier


6T9 TUBE AMP MEASUREMENTS

With the Edcor OTs the -3 dB points are at 30Hz and 20kHz. The shots below show the square wave response at 100Hz, 1kHz and 10kHz.

100Hz Square Wave Response of 6T9 Tube Amp
Photograph 3: 100Hz Square Wave Response of 6T9 Tube Amp

1kHz Square Wave Response of 6T9 Tube Amp
Photograph 4: 1kHz Square Wave Response of 6T9 Tube Amp

10kHz Square Wave Response of 6T9 Tube Amp
Photograph 5: 10kHz Square Wave Response of 6T9 Tube Amp

While the waves are not outstanding, there is no sign of ringing, overshoot or instability. The tilting at the top of the waves is caused by low frequency roll off, which is not entirely unexpected for a small and simple single-ended (SE) tube amp. The maximum rated power output is about 4W with only a Watt or two of good output. So depending on your room, efficient speakers are likely required.


6T9 TUBE AMP SOUND

The big shock which will hit you when you play this amp, is the depth and breadth of the soundstage. With a pair of Fostex FE167E bookshelf speakers 3.5 m apart the soundstage extends past my listening position 4.5m back. The width of the sounds stage extends a couple of meters either side of the speakers. This occurred through other speakers also. Classical music through this amp is excellent. Some big orchestral heavy going tracks get a little muddied but string quartets etc. come though balanced and detailed. Voice and piano come though natural and earthy. Drums and cymbals have a "club atmosphere" like sound.

I would recommend this little amp to new and experienced builders alike but teamed with some high efficiency speakers to really enjoy what it is capable of. The image below shows the 6T9 tube amp with a NAD C542 cd player, DIY silver interconnect cables and Fostex FE167E bass reflex speakers. This makes for a great and musical combo.

6T9 Valve Amp with Fostex FE167E Bookshelf Speakers
Photograph 6: 6T9 Valve Amp with Fostex FE167E Loudspeakers

For additional information and discussion about this project see the DIY AUdio Project Forum thread: 6T9 Valve Amplifiers.

 UPDATE  4 September 2008: I have completed a second 6T9 Tube Amplifier which operates in Ultra-Linear mode as opposed to Pentode mode. For more information, see the Ultra-Linear 6T9 DIY Tube Amplifier Project page.