3W Tube Power Amp
In loving memory of my grandmother. She passed away a day before I finished this amp. We’ll always remember you, Nana 🙁
I wanted to have a small but nice sounding tube power amp that I can use with my preamps and pedals. Having a separate power amp is very flexible, since you can combine it with any preamp you want. Also, I wanted it to be able to be a standalone clean amp without a need for additional pedals to drive it.
- LED cathode bias for output tubes. As long as the LEDs are lit, they provide a constant voltage at the cathode, so it should sound more like fixed bias, without the added complexity of the bias circuit. For more powerful amps that use higher bias voltage and draw more current this wouldn’t be practical, but here it was sufficient to use two arrays of few LEDs to get the desired cathode voltage and current handing. I used a standard loadline calc to determine the bias point of -10.5V for the grid (or +10.5V at the cathode). Quiescent plate current is about 11mA per tube, plus few mA for screen. That’s enough to keep the LEDs glowing, but I used two arrays of LEDs to make sure they can take the higher current at higher signal levels. Two arrays should be able to handle 50mA without problems. As far as getting to 10.5V, I hoped to achieve that with three white LEDs, but the ones I got had slightly lower Vf, so I added one red diode in each array for total Vf of 10.6V. The two LED arrays should be matched, so the current is distributed evenly across them.
- A choke is applied at the beginning of the power supply, so it filters even the current for the power tubes.
- Two inputs, one goes directly to the 6021 gain stage, and one that uses a LND150 gain stage to boost low level guitar signal. That way I can plug a hot pedal or preamp straight to the 6021 gain stage, and plug guitar in the boosted input to be able to drive the output to its full potential. In front of the LND150 there are two 12V zeners that are supposed to protect the LND150 which has max gate-to-source voltage +-20V.
- Apart from one Sanyo solid electrolytic serving as cathode bypass capacitor from the preamp, no electrolytic capacitors are used, even in the power supply. Poly caps have lower ESR and will not drift over time like electrolytics. Also, they are not *that* expensive. I got five of Vishay 10uF 250V caps for about $10 and added few of 6.8uF 250V I already had. The biggest problem with film caps in the power supply is their size. For bigger amps that require 450V or higher voltage rating and more capacitance, capacitors get much bigger, but it’s not a problem here.
- Cathodyne phase inverter using one spare 6021 triode.
Schematic and Layout
Click here to download layout in PDF format.
- Custom aluminum chassis, 26x17x9cm big (or small), powder coated in cream.
- Custom wound toroid power transformer with 135V 110mA and 6.3V 0.7A secondaries. You could use two standard transformers back-to-back to achieve the same thing. For example 230:6.3V (~2A) and 230:9V (1A) wired backwards, or 120V:6.3V (~2A) and 120V:18V (1A) for US mains.
- Custom wound ridiculously overspec’d output transformer with 16K primary and 8ohm secondary. Hammond 125B or 125C should work fine too.
- Hammond 194A choke rated 4H at 50mA.
- NOS RCA 6AK6 power tubes.
- NOS Raytheon 6021 submini tube.
- A few 10uF 250V poly film Vishay capacitors and some 6.8uF 250V poly film (don’t remember the brand) in the power supply.
- ERO poly film coupling caps.
- Mostly Xicon resistors.
- NKK toggle switch with LED on the tip of the lever. Great quality and saves space for a power indicator light. I power the LED off of heaters, using one rectifier diode in series with the switch and one 510ohm resitor to drop the voltage.
- Cliff UK isolated jacks.
- CTS volume pot.
The circuit is layed out on an 1/8″ thick eyelet board for durability and flexibility. Since it’s a new circuit, not verified over and over again, it’s important to be able to easily replace or add components if needed. All tubes are mounted directly to the board, but using traditional chassis mounted sockets. That way I get compact layout without drawbacks of PCB mount sockets. LND150 and 6021 are installed using gold plated pins from IC socket pin strip. That way I can change them if needed without desoldering. Most of the filter caps and twisted heater leads are installed below the board. By using all three dimensions, we’re saving some space and ensuring that heater leads are as far as possible from signal leads. Also, I tried to have heater runs go behind the filter caps which act as a shield, since one side is grounded. I used hot glue to mount the filter caps to the board and make sure they stay put.
Click on an image to see more details.
Click on a thumbnail to play the video on YouTube.