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

Getting the Schematic

The first step then is to get hold of the schematic.

Happily, these days all it takes is a google search. I found a reworked schematic (above) and the original as a pdf that can be downloaded from here. The original is particularly useful because it has a voltage chart.

About the GA-40

So what is all the fuss about?



Well the GA-40 was an amp sold by Gibson in the 50s as part of a matched set with its flagship solidbody electric guitar, the Les Paul. 4,810 of them were made between 1954-59.

Surprisingly, given the full on rock world that awaited the Les Paul, the GA-40 was not a high power amp. It is not a 40 watt amp as the name might suggest, rather it uses a pair of lower-powered 6V6 output tubes and has a conservative 14 to 16-watt rating.

The GA-40 has an unusual "pentode" preamp section - meaning it has five functional elements other than the three of the standard 12AX7 dual-triode—called a 5879. The 5879 sounds nothing like the familiar 12AX7, and has a higher gain and a fatter, thicker overall tone. This higher gain doesn’t necessarily mean that this tube itself distorts more easily, but rather that it pushes a firm, bold signal onto the next stage, whihc is easier to push to distortion if you want to.

It has two channels, eahc of which are voiced differently: Channel 1 (designated Instrument/Microphone) is hotter than Channel Two (designated Instrument). You can patch them together (by plugging into Input 1 of Ch1 and running a jumper cable from Input 2 to Input 1 of Ch2, or vice versa) to blend the two voices together all the time.

Channel 2, the Instrument channel, has its own little treat in store in the form of what is regarded as one of the most delectable tremolo circuits available. Powered by a single 6SQ7 preamp tube, the GA-40’s tremolo is deep, thick and lush, and provides anything from a throbbing, swampy wobble to a machine-gun staccato.

Building a Gibson GA-40 clone

Hi. This blog is going to set out the story of my attempt to build a clone of a Gibson GA-40.

To date I have built two amps myself. The first was from a kit supplied by Barry at ampmaker. A lovely 5w single ended amp. Barry's kit and instructions are great. I loved the whole experience. It took about a day to build and at the end of the process I had a very nice sounding amp. I can't recommend amp maker highly enough.

My second amp build was a Marshall JTM45 clone. There are a number of JTM45 kits out there but most base din the US. With the exchange rate, customs duty and postage costs it was just too expensive to order a kit from the US to be sent to me here in London U.K.

Thankfully, George at Metro Amps has placed the instructions for his kit online and available to download for free. I can't thank you enough George. So I sourced the parts from within the UK mostly (although I did get some parts for biasing from Metro Amp because I was feeling guilty!).

I sourced a JTM45 chassis from Valve Power - pre-punch for £20 - thanks Steve!

I sourced the turret board making equipment and materials from Barry at amp maker along with the necessary hardware and wiring

I then spent quite a bit of time trying to find the right transformers online. It is really hard to get hold of tranformers in the UK I think. Eventually I was lucky enough to find Bluebell Audio based in Dundee. Philip there offers what appears to be the entire range of tranformers by the Canadian manufacturer Hammond.

Philip was unbelievably helpful. I ordered by potentienometers form him and he spotted that I had ordered the wrong type of pot for a JTM45 for example. He also sent me the bolts to be able to attach the transformers to the chassis for nothing. What more can you ask for in terms of service?? Philip is a star in my book.

I also got Chris Uff to make a cab for me. You can contact him through the amp maker site. Chris's cabs are just super quality. I can't recommend him highly enough.

After many months of being too busy to be able to do any work on the project I finally got two days and blitzed it. It came together really well. The test measurements were all great. I switched on and hey presto a really fantastic working JTM45.

Well, the amp bug has really got me hooked now. So I have been thinking about what amp to do next. Then I was chatting to a friend who is mad about Ry Cooder and he mentioned that Ry sometimes uses a really old Gibson amp. I was curious to find out more and googled only to learn about the GA-40 for the first time.