A single-ended tube amplifier

This is a recent project: 2A3/300B tube speaker amp with headphone out.

Classic topology: 3-stage, cap-coupled with fixed bias.
Input tube: 6SN7, Drivers: 6V6GT, Output: 300B or 2A3
Separate power supply chassis from signal chassis.
Resistor load for the 6SN7 (with Grid Chokes on the grids to the drivers).
Plate choke load on the drivers.
7 separate power supply circuits: 2 HT supplies (one for input/drivers and another for the output stage), 2 negative bias supplies (one for the drivers and another for the output tubes), 3 DC filament/heater supplies.
Rod Coleman DHT filament regulators for the output tubes.
HT supplies use small first cap input and LCLC filtering.
LT supplies use RCLCRCRC filtering.
Tribute Amorphous C-Core OPTs (3.5K primary, 8ohm secondary).
Custom Electra-Print power transformers.
Hybrid rectification using MUR4100 diodes and tube rectifiers (GZ37 for input/drivers and 5AR4 for outputs).
Lundahl filament chokes throughout.
Electra-Print, Bartolucci or Tribute chokes for HT filtering.
K&K Audio timer relay kits for the negative bias supplies to the output tubes.
The amp is convertible between 2A3s and 300Bs by switching the secondaries of the power transformer (with a high voltage relay) and switching resistors in the filament supplies.

Here is the link to my Flickr album: https://flic.kr/s/aHskagaFHE

 

Beautiful work. The ability to use either 2a3 or 300b is pretty slick. Is there sonically any compromise in having the ability to switch back and forth? Or is it simply a question of cost/complexity of the build?

 

I should have mentioned that this project is a re-build of an earlier version of this circuit.

So I bought the OPTs and choke direct from Pieter years ago when he still sold to end users.

Very envious of the commercial builders like EC and DNA who can still buy his stuff.

In the manner I did the switching, there shouldn't be a compromise sound wise. The 2A3s are currently running at an operating point of 300V anode with ~50mA current. Which for a 3.5K primary is pretty much where you want to be. The 300Bs are running at ~370V anode and 60mA current.

It is just a matter getting the operating points for both tubes just about right. I am still testing, so it may take a little while for me to come up with acceptable operating conditions. (Note that I can raise or lower B+ by increasing or decreasing the value of the first cap in the power supply.)

The complexity of switching the HT supply is made easier by the use of an Omron HV relay. Just a matter of getting the wiring right, and you are good to go.

The other thing that you have to be careful of is the filament supply. I wanted to make sure that I got exact voltages for both tubes, and allow the filaments time to heat up. In this case the Coleman filament regulators are just perfect. And to make the tube filaments happy, I use the timer relays in the negative bias supply to almost shut the tube down upon startup by forcing full negative voltage on the grid. After 90 seconds the relay closes the circuit, and current is allowed to flow to normal operating levels.

So yeah, overall not a simple or cheap build.

 

Thanks!

Yep, I stopped thinking about the costs involved a long time ago. YOLO is my mantra when it comes things like this.

Also, I hate metal work, the biggest pain of DIY.

The P2P wiring comes from looking at a lot of Japanese tube audio magazines (Stereo Sound-Tube Kingdom). And making sure I did a neat job in terms of layout.

There are many builders who frown upon this method of wiring. And try to wire everything with just component leads. (Decware comes to mind: http://www.decware.com/newsite/TABOO.htm) I personally just prefer the Japanese way of wiring.

And I should note that the amp is as quiet as my balanced headphone amp.

 

That is a really impressive piece of work! I like the Adafruit ruler you got back there.

Do you have a schematic available?

Edit: Nevermind, I just checked the Flickr album.

 

That's just beautiful. Turrets and terminal strips make for a much cleaner layout and wire runs. Probably more secure solder joints/components and easier to repair/mod in the long run.

 

Adafruit makes some nice tools and sources others from Japan, I think. Very useful.

Note that the schematic is still a work in progress. I recently bought a 'Scope and Signal generator to test various DIY projects. (Just need to figure out how to use them.)

So, some of the component values may change to tweak the operating conditions.

 

What are you using to make the schematic?

 

I use a Vector Drawing program on my Mac: EazyDraw.

It's not as full featured as Adobe Illustrator, but for schematic drawing and quick drawings it serves my needs.