Transformers

The most expensive part of the amp build is the transformers.

Amps all have a power transformer and an output transformer. The GA-40 also has a choke. A choke is used to filter the audio signal.

From the information we obtained relating to the valve specifictaions and form the schematic, we can start to build up a picture of the type of transformers we need to use.

Finding transformers online is difficult her ein the UK for some reason. The best source I have found is Bluebell Audio who sell transformers made by the Canadian manufacturer Hammond.

The power transformer needs to have three secondary windings:

1. high voltage 300-0-300VAC. This winding has to power all of the plates of all of the valves. So you need to figure out how much plate current in total will be drawn by the valves. That is why I collected the plate current figures for each valve in hte previous post. I make the total about 113 mA. So we need the high voltage winding to produce at leats that much current.

2. Heater 6.3V CT

3. Heater 5V CT

Looking at the Hammond website, I found that the Hammond 372fx provides all three of these secondary windings and the high voltage winding is a 150 mA. It is £57.

The output transformer needs to be capable of coping with 14-16 watts and provide a matcning inductance of about 8000 ohms. Philip at Bluebell advised me to get the Hammond 1650E which gives 15 watts and 8000 ohm inductance. It costs £49.00.

The choke is a 3H from the schematic and the rectifier outputs 125 mA. So the choke needs to be a 3 Henry choke @ 125 mA. The closest I have found is 3.5H @ 150 mA in the Hammond 157Q for £10.25.

A total of £106.25 which is pretty reasonable I think.

It's all about valves

With the schematic in hand, we can see that this amp has seven (count 'em) valves or "tubes" as our American cousins call them.

If you want to learn about the different types of valve this is a great site.

The pre-amp valves are two 5879s. The phase inverter is as 12AX7. There are two power valves which are 6V6GTs, a rectifier which is a 5Y3GT. Last but not least is a 6SQ7 valve which is used in the tremelo circuit.

Now we need to get hold of the specifications/data sheets for these valves. The best resource I have found is at the Tube Data Sheet Locator (TDSL).

From this we learn:

5879: This is a pentode valve that uses a 9 pin noval B9A socket. It requires a heater circuit of 6.3v. It has a plate current of 5.5 mA.

12AX7: This a triode valve that uses a 9 pin noval B9A socket. It requires a heater circuit of 6.3v. It has a plate current of 1.2 mA

6V6GT: This is a tetrode vale that uses an 8 pin octal socket. It requires a heater circuit of 6.3v. It has a plate current of 47 mA. Because it is the output valve we need to note its output inductance which is 8000 ohms with a two valve output stage (this is relevant to the selection of the output transformer).

5y3GT: This rectifier uses an 8 pin octal socket. It requires a heater circuit of 5v. It outputs 125 mA of DC.

6SQ7: THis valve uses an 8 pin octal socket. it requires a heater circuit of 6.3v. It has a plate current of 1.1 mA.

I sourced two NOS 5879 from ebay for £10.
I got the 6V6GTs, the 5y3GT and the 12AX7 new from Hot Rox
The 6SQ7 was harder to find but I got one from watford valves.

From all of this we can see that we will need:

3 Noval B9A sockets

4 Octal Sockets