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DIY 6T9 Push-Pull Tube Amplifier Project

 Steve Geary  USA Flag   To email Steve, type out the email address.

6T9 Push-Pull Tube Amplifier

This vacuum tube amplifier project was inspired by the Spare Time Gizmo's 6T9 tube amp (see Mark's 6T9 Tube Amp and Ultra-Linear 6T9 Tube Amp). This got the ball rolling of ideas in my head and soon it was evolving from one idea to another. I decided to make the amplifier with a push-pull output stage using circuit ideas from my EICO HF-81 and my RCA Receiving tube manual.

DIY 6T9 Push-Pull Tube Amp
Photograph 1: DIY 6T9 Push-Pull Tube Amplifier

The 6T9 compactron tubes draw 940mA each on their heaters for a total of 3.76A and the transformer I used is only rated for three amperes on the 6.3V winding. Since I am not using the 5V 2A winding, I hope that allows some leeway on the 6.3V winding. I have seen smaller power transformers powering more tubes, so I'm feeling some confidence it will be ok.

Tube Amplifier Power and Audio Output Transformers
Photograph 2: Tube Amplifier Power Transformers

Initially I started with 6EU7 tubes in the preamp stage but later switched to 12AX7A tubes as they are easier to find. I only have two 6EU7 tubes and I feel I should save them for another project. The 12AX7A heaters are powered by a separate transformer. I also chose to rectify and filter the heater supply to help further reduce the chance of hum in the preamp stages. Here is the 6T9 Push-Pull Tube Amplifier Schematic I came up with.

Construction - 6T9 Push-Pull Tube Amp

6T9 Tube Amplifier Construction
Photograph 3: 6T9 Tube Amplifier Construction

After building the amplifier and making the mistake of reversing the control grids and plates on the 6T9s (OOOOPS! I'm so glad I brought it up slowly on a Variac!) The amplifier came to life! Fortunately, the 6T9s survived the assault on their grids.

I did have some excessive hum on the left channel which I tracked down to a defective 6T9 tube. There was some residual hum left after that. I needed to rework the power supply a little to improve filtering as well as put a shield across the front panel. I would get hum as I reached for the Volume or Treble controls. I also made sure I grounded everything to a common point on the chassis. I also played around a bit with the cathode resistor in the output stages to get the bias voltage where it should be.

Also, a note about the pots. I'm using some old Allen Bradley dual pots that I picked up at Mendelsons Electronics Surplus in Dayton, Ohio. They have a short shaft with screwdriver adjustment. I decided to drill into the end of the shaft, then tap the hole for a 4-40 screw. Then I used a 1/4 diameter spacer and bolted it on. It works very well for making these pots useful!

6T9 Tube Amp - Point-to-Point Wiring
Photograph 4: 6T9 Tube Amp - Point-to-Point Wiring

The combination wood and brass chassis came about when I found a small sheet of brass shoved under a cabinet and long forgotten about at work. It took a bit of work to clean it up and polish it to that shine! The wood is Poplar purchased at Lowes, stained with Minwax Red Mahogany stain and finished with Amber Shellac.

Sound - 6T9 Push-Pull Tube Amp

I'm very impressed with the performance of this amp. Plenty of gain and nice rich sound. The amp does have a rather bright upper midrange, maybe a little too bright, but it is not distorted and has crisp highs otherwise.

DIY 6T9 Compactron Tube Amp
Photograph 5: Finished 6T9 Push-Pull Tube Amplifier

The speakers I have it connected to in the photos are Olson from either the late 60's or early 70's. They have a single 8 inch speaker inside. All I know about these speakers is the information shown on the rear label (acoustic suspension, 15W, 8ohms, ceramic magnet, Olson Electronics Inc., Akron, Ohio, product of Japan). They sound wonderful with this amp! I'm very impressed with the deep bass they provide.

While I was taking pictures of it playing on my workbench, it was playing "Brandenburg Concerto No. 3-I. Allegro" by Wendy Carlos from her album "Switched on Bach" Amazing music from a Moog Synthesizer!

I plan to use it with my computer to replace the cheap powered speakers it has. I need to make room for it first!