Tube Guitar Amp Project

digitalvoodoo · 17th December 2006, 12:14 PM

Though I’d post some details of a tube guitar amp I’ve been working on for a while. I’ve been tinkering away at it for over a year now, but I’ve resolved to make an effort and finish it off before Christmas.

It’s a clone of circa 1966 Marshall JTM-45. The JTM-45 was the first amplifier from Marshall (essentially a copy of the Fender 5F6A Bassman) and my favorite sounding guitar amp.

Schematic:

Click to view full-sized image!
Hosted by UGBox Image Store

This is the earliest construction picture I can find. The first thing I did was solder the components to the turret board. Big and chunky! Much nicer than working with fiddly little pcb’s

In this pic, I’ve cut the holes for the tubes in the chassis, but not for anything else.

Click to view full-sized image!
Hosted by UGBox Image Store

The output transformer is from Marstran in the US. The black transformer is a power transformer I sourced locally, however I decided to replace it, as I decided its current ratings were a bit low for my liking.

Here’s a close up of the turret board:

Click to view full-sized image!
Hosted by UGBox Image Store

digitalvoodoo · 17th December 2006, 12:35 PM

Progress Pics:

Here’s the top of the amp with all the hardware mounted. From right to left is the power transformer, output transformer and choke.

Click to view full-sized image!
Hosted by UGBox Image Store

Starting the internal wiring (this is hard work!).

Click to view full-sized image!
Hosted by UGBox Image Store

From the front.

Click to view full-sized image!
Hosted by UGBox Image Store

The Glass!

Click to view full-sized image!
Hosted by UGBox Image Store

Dr feelgood · 17th December 2006, 2:28 PM

very nice, Love to see these amps being built. Will you be recreating a cabinet like the original or will you customise your own?

I have been thinking of doing one of the fender's but I need to find a local place to source transformers at reasonable prices

.

][nfector · 17th December 2006, 4:26 PM

nice job. What does this put out, like 15watts?

wabbit · 17th December 2006, 5:01 PM

Quote:

Originally Posted by Dr feelgood

I have been thinking of doing one of the fender's but I need to find a local place to source transformers at reasonable prices

.

WES in Sydney have Fender replacement output transformers (5k ohm, 25 Watt) for only $34.50. Part number NSC318. Also Marshal 50 and 100 watt power and output transformers (much more expensive).
www.wescomponents.com
Cheers
Rob

digitalvoodoo · 17th December 2006, 5:34 PM

Quote:

Originally Posted by ][nfector

nice job. What does this put out, like 15watts?

I’m using KT-66 tubes, which will get run with a plate voltage of around 450V. This should put it in the 30-40 Watt range.

I ended up getting all the transformers from the US. Even with the shipping, it ended up about the same or cheaper than sourcing them locally - particularly given I ordered a bunch of other stuff at the same time from Metroamp.

Output:
http://marstran.com/

Power and Choke:
http://www.metroamp.com/

digitalvoodoo · 21st December 2006, 11:06 PM

Bust of activity tonight…

Finished wiring up the turret board:

Click to view full-sized image!
Hosted by UGBox Image Store

Installed the board and completed all the wiring:

Click to view full-sized image!
Hosted by UGBox Image Store

I really hope I haven’t screwed anything up (like wiring the tube sockets backwards) – cause it’s going to be a pita to undo that wiring.

I’m too tired to fire it up and check voltages at the moment. I’d like to double check everything when I’m a bit more alert. Here’s hoping the next post has glowing tubes and sound clips (that don’t consist of oscillation).

Quote:

Originally Posted by Dr feelgood

very nice, Love to see these amps being built. Will you be recreating a cabinet like the original or will you customise your own?

I haven't really decided on the cabinet yet. I probably won't do an exact copy, but I'll probably stick close the traditional Marshall look.

Dr feelgood · 22nd December 2006, 3:09 PM

looking good,

Quote:

I’d like to double check everything when I’m a bit more alert.

Very good idea, good luck.

You have almost inspired me to get a little more active in sourcing the gear to make my amp.

digitalvoodoo · 29th December 2006, 12:39 PM

Update:

First check, no tubes installed - all AC voltages were OK. Heaters were 3.38V and 316V on the AC side of the rectifier.

Installed the rectifier valve and the rectified voltage was reading 429 Volts DC. All good and within spec so far.

Installed the pre-amp tubes, everything lit up nicely with when AC was applied.

Switched the standby switch (to apply DC to the circuit) and immediately blew the HT fuse (Centre tap to ground). I couldn’t spot any shorts in my wiring, so I guessed something was arcing (possibly between the pins on the tube sockets). I bent the pins on the sockets further apart, replaced the fuse, held my breath and success!

Checked everything against a voltage chart I’d found for the amp and everything was within 15% (still no output tubes at this stage).

Moment of truth. Installed the output tubes, attached a load to the output transformer in the form an old speaker from a practice amp. Switched on the power, everything lit up, let it heat up for a few minutes. Switched in the DC with the standby switch…

…and it sounded like a foghorn bellowing from the speaker. DOH!

Got some troubleshooting to do. Oh well, I like a challenge. Pity I’m on leave from work at the moment, no easy access to a CRO. Hopefully it’s something simple.

Here's some pics of the finished exterior

Click to view full-sized image!
Hosted by UGBox Image Store

wabbit · 29th December 2006, 1:20 PM

Just a thought - try transposing the output transformer primary leads.

digitalvoodoo · 29th December 2006, 3:00 PM

Cheers wabbit. Yeah, that was my first guess as well.

wabbit · 29th December 2006, 3:26 PM

Hey - did it work?

digitalvoodoo · 29th December 2006, 5:28 PM

Bingo! Wrong phase. Measured the resistance from the primaries to the centre tap, indicated the leads were the reverse of what I expected. Swapped them and it’s all good.

Set the bias, plugged in a guitar and it’s sweet, sweet tube guitar bliss.

I’ll get it all set up with a proper speaker cabinet and post some sound clips.

That was a good get wabbit! You got some experience with tube gear? This has certainly been a big learning experience for me.

wabbit · 29th December 2006, 5:49 PM

Quote:

Originally Posted by digitalvoodoo

That was a good get wabbit! You got some experience with tube gear?

Yeah, you could say that.
I got into electronics in the late 1950's. Transistors were becoming available but beyond my means as a schoolie so tubes were the go. Lots of WW2 surplus gear around at relatively low cost and old radios virtually free.
I haven't done anything with tubes for nearly thirty years now but I guess it's like riding a bike (which I never learned how to do).
I'm tempted to get back into it now that some cheaper components are becoming available. Maybe a stereo amp with PP 6L6's.
Cheers
Rob

digitalvoodoo · 29th December 2006, 6:57 PM

My supervisor at work is an old school engineer from the Soviet Bloc. He thinks I’m crazy making tube stuff. He can’t understand why anyone in their right mind would want to use tubes over transistors.

He laments that that they were stuck using “tube junk” while the rest of the world moved onto transistors (which were really expensive and hard to get in Eastern Europe at the time).

17th December 2006, 12:14 PM	#1
digitalvoodoo Member Join Date: May 2003 Location: Adelaide Posts: 247	Tube Guitar Amp Project Though I’d post some details of a tube guitar amp I’ve been working on for a while. I’ve been tinkering away at it for over a year now, but I’ve resolved to make an effort and finish it off before Christmas. It’s a clone of circa 1966 Marshall JTM-45. The JTM-45 was the first amplifier from Marshall (essentially a copy of the Fender 5F6A Bassman) and my favorite sounding guitar amp. Schematic: Click to view full-sized image! Hosted by UGBox Image Store This is the earliest construction picture I can find. The first thing I did was solder the components to the turret board. Big and chunky! Much nicer than working with fiddly little pcb’s In this pic, I’ve cut the holes for the tubes in the chassis, but not for anything else. Click to view full-sized image! Hosted by UGBox Image Store The output transformer is from Marstran in the US. The black transformer is a power transformer I sourced locally, however I decided to replace it, as I decided its current ratings were a bit low for my liking. Here’s a close up of the turret board: Click to view full-sized image! Hosted by UGBox Image Store

17th December 2006, 12:35 PM	#2
digitalvoodoo Member Join Date: May 2003 Location: Adelaide Posts: 247	Progress Pics: Here’s the top of the amp with all the hardware mounted. From right to left is the power transformer, output transformer and choke. Click to view full-sized image! Hosted by UGBox Image Store Starting the internal wiring (this is hard work!). Click to view full-sized image! Hosted by UGBox Image Store From the front. Click to view full-sized image! Hosted by UGBox Image Store The Glass! Click to view full-sized image! Hosted by UGBox Image Store Last edited by digitalvoodoo; 17th December 2006 at 1:57 PM.

17th December 2006, 2:28 PM	#3
Dr feelgood Member Join Date: May 2005 Location: Mornington peninsula Posts: 1,739	very nice, Love to see these amps being built. Will you be recreating a cabinet like the original or will you customise your own? I have been thinking of doing one of the fender's but I need to find a local place to source transformers at reasonable prices . __________________ There's always someone out there who will make me feel wiser and less arogant than I may actually be. I'd Rather Be A BASS Player Club Member #04 "It is better to sleep on what you plan to do than to be kept awake by what you've done." Rowland Croucher - Living Without Regrets

29th December 2006, 12:39 PM	#9
digitalvoodoo Member Join Date: May 2003 Location: Adelaide Posts: 247	Update: First check, no tubes installed - all AC voltages were OK. Heaters were 3.38V and 316V on the AC side of the rectifier. Installed the rectifier valve and the rectified voltage was reading 429 Volts DC. All good and within spec so far. Installed the pre-amp tubes, everything lit up nicely with when AC was applied. Switched the standby switch (to apply DC to the circuit) and immediately blew the HT fuse (Centre tap to ground). I couldn’t spot any shorts in my wiring, so I guessed something was arcing (possibly between the pins on the tube sockets). I bent the pins on the sockets further apart, replaced the fuse, held my breath and success! Checked everything against a voltage chart I’d found for the amp and everything was within 15% (still no output tubes at this stage). Moment of truth. Installed the output tubes, attached a load to the output transformer in the form an old speaker from a practice amp. Switched on the power, everything lit up, let it heat up for a few minutes. Switched in the DC with the standby switch… …and it sounded like a foghorn bellowing from the speaker. DOH! Got some troubleshooting to do. Oh well, I like a challenge. Pity I’m on leave from work at the moment, no easy access to a CRO. Hopefully it’s something simple. Here's some pics of the finished exterior Click to view full-sized image! Hosted by UGBox Image Store Click to view full-sized image! Hosted by UGBox Image Store

29th December 2006, 1:20 PM	#10
wabbit Member Join Date: Sep 2003 Posts: 408	Just a thought - try transposing the output transformer primary leads. __________________ I started out with nothing and I still have most of it.

17th December 2006, 4:26 PM	#4
][nfector Member Join Date: Jan 2004 Posts: 167	nice job. What does this put out, like 15watts?

29th December 2006, 3:00 PM	#11
digitalvoodoo Member Join Date: May 2003 Location: Adelaide Posts: 247	Cheers wabbit. Yeah, that was my first guess as well.

29th December 2006, 3:26 PM	#12
wabbit Member Join Date: Sep 2003 Posts: 408	Hey - did it work? __________________ I started out with nothing and I still have most of it.

29th December 2006, 5:28 PM	#13
digitalvoodoo Member Join Date: May 2003 Location: Adelaide Posts: 247	Bingo! Wrong phase. Measured the resistance from the primaries to the centre tap, indicated the leads were the reverse of what I expected. Swapped them and it’s all good. Set the bias, plugged in a guitar and it’s sweet, sweet tube guitar bliss. I’ll get it all set up with a proper speaker cabinet and post some sound clips. That was a good get wabbit! You got some experience with tube gear? This has certainly been a big learning experience for me.

29th December 2006, 6:57 PM	#15
digitalvoodoo Member Join Date: May 2003 Location: Adelaide Posts: 247	My supervisor at work is an old school engineer from the Soviet Bloc. He thinks I’m crazy making tube stuff. He can’t understand why anyone in their right mind would want to use tubes over transistors. He laments that that they were stuck using “tube junk” while the rest of the world moved onto transistors (which were really expensive and hard to get in Eastern Europe at the time).