40 Watt Tube, Guitar Amp

Jon Tirone

(also known as John Fisher)

 

This was a very rewarding project for me. Although I have some experience with electronics, I have never done anything with tubes before. So researching this project and building it taught me a lot.

Most of the materials that I used were from junk.

Parts for tube amps are either very hard to get or very expensive if you can find them new, so I started collecting junk from friends and junk men.

I came across a very old hi fi amp that I was able to get the output transformer and the power transformer from. The date written on the amp was 1954. (older than me) The output transformer and the power transformer are the hardest things to come by so I was happy about that.

I must note that when you use parts as old as these, your are very likely to run into bad parts, especially old capacitors that are leaky or shorted, resistors that the values have drifted drastically and tubes that are no good. When you are as inexperienced as I was, this can be frustrating as things fail or burn up and you can't figure out why. Because these generally use high voltages, they are hard to find.

I began to search a lot on the web for info and asked a lot of people a lot of questions and I am very thankful for everyones help which was very valuable.

 

What I originally planed to make was a simple straightforward 40 watt amp but as I began to research it, I wound up adding much more

 

 

Below is a list of the features:

 

  Celestian 12" speaker

  1 rectifier tube (5U4G), 8 preamp tubes ( GE 12AX7), 2 power tubes (6L6 GT)

  2 switchable channels with Marshal style preamps, Each channel also has a switch to reconfigure the preamp for another style of preamp.

  Channel 1 has volume and gain controls with a 3 band EQ with a switch for 2 separate high and low gain preamps.

  Channel 2 has a volume and gain control with a 3 band EQ , a frequency sweep control (kind of like a parametric EQ) and a switch for 2 more separate high and low gain preamps. This particular channel gets quite a bit of variety of sound.

  Each channel has a master volume which enables it to be clean or with overdrive.

  The switching is done with relays and has a jack in the back for a foot switch.

  There is a red and green LED indicator on the faceplate to determine which preamp is on.

  All tube effects loop with ajustable send and return volumes on the front pannel.

  All tube spring reverb unit for both channels

  Fixed bias and cathode bias switch on back.

  Global negative feedback switch on the back

  Standby and on and off switch on the front

  Line out jack in the back

  External speaker jack in the back

 

 

 

I basically made the amp out of whatever junk I could find around. I found some scraps of wood in the yard. I also found some very short piled carpet laying around to cover the outside. The metal chassis is homemade from sheet metal and the faceplate is a piece of aluminum. The front grill is a piece of wire mesh that I got at the hardware store.

 

 

Face Plate:

The faceplate was made of a piece of aluminum U channel from a doorframe that was laying around.

I used Letraset to make the names and indications of the face plate. For the lines I used some pinstripe for cars that comes in a roll that I got in an auto supply store. After I finished putting on the face plate markings I then sprayed it with a can of clear coat to protect the Letraset markings.

As you can see, because I used what was at hand, the amp has a very non -traditional look to it, but if you were to see it, it really doesn't look so bad. Most people that don't know about amps don't even realize that it's homemade.

Schematics

I used several different ideas from different schematic to make this amp. The preamps are basically Marshal design and the power amp circuit is based on a Fender Blackface amp. There are some modifications. For example: I installed a Fixed bias to cathode bias switch. There is also a switch on the global feedback resistor to switch from a 820 ohm resistor to a 100 K resistor that gives the option to practically eliminate global feedback which gives the amp a lot more volume and a more bluesy sound.

Rather than draw out the whole schematic below are different schematics that used I to make the amp. There are a few modifications to these circuits.

For any further questions you might have, please write me.

Here are a couple of MP3 sound files of my amp you can download to let you know how it sounds. They were recorded with the amp EQ fairly flat with a little of the spring reverb from the amp itself. I did not do any other processing or add any other effects to the sound. The amp has quite a variety of tones you can get and these are only a few.

- Clean sound

- Overdrive sound

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