My fellow Amp-Freaks,

herewith I'd like to document another amp build for all those who might be interested. The project will be based on the Fender Narrow Panel 5E5A Tweed Pro, which was built from 1956 to 1960. This model is a favorite among West Coast jump blues players. It's typical tone is generated by a mighty 15 inch speaker and it produces about 30 Watts. This is plenty of power to fill any stage but at the same time it can be pushed into overdrive at a reasonable volume level.

This page will be updated as the build comes along.

Basically I will rely on the original schematic from 1956, because there is simply nothing wrong with it. Leo Fender's team has designed wonderful amps, which are still the standard which every amp on the planet is measured against. I don't think that this design needs to be improved. Some things are a matter of personal taste though so I might stray from the original layout a bit to tune it to my liking. Of course everything will be hand wired point to point.

Two small but important modifications from the original design will be incorporated for sure:

•  The amp will be grounded. I am old fashioned but not tired of living.

•  For easier and more accurate biasing after tube changes I will install a bias pot.

For the components I will only use the best stuff that is available today. Nevertheless this whole project will not cost me more than an off the shelf Fender amp, which is produced with PCB boards and some cheap components and it will be about half the price of a custom shop Fender or boutique amp, which would probably be the same amp only with a different price tag. Nothing compares to a homemade amp ... it's your own creation and you know exactly what's inside.

Of course I will use my experience from previous builds. At the same time I will take the opportunity and try some new components and building approaches to find out which of the rumors and theories that I keep hearing and reading about are myth or fact. This time I will use a different brand of capacitors for example. Instead of using a ground buss or using the back of the pots as ground I will use the old fashioned brass plate. I think research is better than blind faith...

Most of the parts were supplied by Marsh Amps. Everything I need is available there and Mike and Denise will walk that extra mile for you. Marsh is highly recommended ...

Before we start, heres the obligatory safety advice :

IF YOU DO NOT KNOW HOW TO SAFELY WORK ON A TUBE AMPLIFIER AND DO NOT KNOW ABOUT THE LETHAL VOLTAGES INSIDE WHICH ARE STILL PRESENT AFTER THE AMP IS SWITCHED OF, PLEASE LEAVE THE BUILD TO A QUALIFIED PROFESSIONAL. SAFETY ALWAYS COMES FIRST!

parts used for the project

the build

results

 



- parts used for the project

cabinet

The cabinet is a lacquered tweed cab by Mojo. This is my fourth Mojo cab. Workmanship is great, measurements are accurate and not to forget ... they look great!



I like to put my amp on the ground on stage and tilt it backwards. Improvised constructions made of chairs and beer crates are ugly and instable. Therefore I will install tilt back legs. They are not vintage correct because they were first introduced on some of the bigger blackface models but who cares.

 

speaker

One of the most important components for a good tone is without a doubt the right choice of speaker. I have two to choose from and a final test will tell, which one will stay in the amp. One is an original Chicago Jensen C15N from 1961, which I purchased a while ago and is lying around my house since and the other one is a new Weber 15A150.



The original Jensen has some serious mojo for sure but it's a ceramic. That's not necessarily a bad thing but for that classic tweed tone an alnico might be the ticket. I will try it nevertheless ... who knows.

In my humble opinion the great late Ted Weber produced some of the best Jensen style speakers since the originals from the Chicago factory are not produced anymore. I have tried a bunch of other brands as well but a Weber is a Weber. I think the sound smoother and more balanced than the current production Audiovox/Jensens and don't have that buzz in the higher frequencies when they break up. But it's all a matter of personal taste, I don't want to put down any products here and it's always a matter of finding the best speaker/amp combo. I have a feeling that the Weber might be the winner in the end.

 

capacitors

Filter caps are TADs. They are equally nice, smaller and cheaper than the Spraque Atoms so that's what I am going to use.

For the coupling and tone caps I will use either Orange Drops or Mallory 150s. I got me a set of both because they don't cost a fortune. On my previous builds I have used Orange Drops exclusively with great results. Some say that the Mallorys have a warmer tone though. I will probably start with the Mallorys to find out if I like them.

 

resistors

I am using old fashioned carbon comps here. They are said to help shaping the tone by compressing the signal a bit. Although I performed some swap and compare tests, I was not able to determine if there is really a tonal difference between carbon comps and other resistors. If there is one then it's marginal. At the same time they are prone to producing unwanted noise. Sometimes an amp will run dead quiet with carbon comps only, sometimes it's necessary to replace some of the preamp resistors with metal film types. The signal in this section of the amplifier is not strong enough to produce any compression here anyway so the sound is not affected by this.

UPDATE: After some considerations I will change my plan and start with metal film resistors throughout the whole build. This avoids heat inducted value drift that can sometimes be pretty dramatic as well as hiss and crackling. Should I notice a negative effect on tone (which I seriously doubt) I will replace some of them in cruical places with carbon comps to see if this changes anything.

 

trannies

Tried and true Heyboers are used here. Mercury Magnetics might be a bit more rugged but more expensive as well. If you are like me and don't kick your amp down a staircase twice a day you will probably be happy with the Heyboers for a lifetime.

 

tubes

The rectifier is a 5U4GB.

In the preamp I am using low gain 12AY7s like the originals in the 50s. In the first stage I might as well try a 12AU7 or 12AT7 but 12AY7 is usually fine. Many of today's builders and even Fender itself use 12AX7s in the preamp to achieve a more modern, higher gain sound. I prefer lower gain tho and leave more distortion to the power tubes. That has a bluesier character to me. The phase inverter is a standard 12AX7.

In the power section I will try a pair of 6L6 as well as a pair of Tung Sol 5881. The 5881s are rated at 23 Watts instead of 30 Watts and break up a little sooner with a really nice harmonic distortion. In most cases they are my favorite power tubes for 6L6 type amps.

 

miscellaneous parts

Pots are by Alpha of course. For jacks and switches Switchcraft is the obvious choice.

A vintage cloth covered power cord is used for a cool look.

An old fashioned cardboard eyelet circuit board is used. In case it starts warping due to humidity I will make a plastic one but I never had problems with this type and many of those are still working perfectly in 50+ years old amps.

All the wiring is done with cloth covered pushback wire.

 

So let's turn this pile of parts into an amplifier ...


- the build

power cord

Let's start with something easy. The ends are covered in solder lead and the cloth covering is protected with shrinking hose. Then the plug is assembled and that's it. OK, I agree ... that wasn't very interesting.

 

Circuit Board

First I attach the caps and resistors to the cardboard by feeding the legs through the eyelets and bending them over. I will save the soldering for later when the board is fully stuffed. The only exception are the cathode resistors which need to be attached to the corresponding cap legs. Their own legs aren't long enough to reach all the way from one eyelet to the other. Some like to solder each finished eyelet immediately which is of course absolutely fine as well. A uniform orientation of the resisturs (value facing the pots, tolerance facing the tubes) cleans up the board a bit. I also like to position the caps in a way that the printed value is facing up so it can be easily read as well.
   

Day 2: As mentioned above, I changed my plan and use metal film resistors throughout. They have the advantage of having longer legs, so I can also connect the cathode resistors directly to the eyelets, which makes it easier to replace them later if necessary and improves serviceability. I use 1 watt versions only. With the exception of the power supply dropping resistors, 1/4 watters would be sufficient, but the 1 watters have stronger and more sturdy legs.

Next, the protruding ends on the back are cut off and the connections that go behind the board are installed. These wires should not run directly next to each other and should cross in a steep angle in order not to interfer with each other. The connection in the first pic between the 56k bias regulationg resistor and the 100µF cap will be removed again, because that's where the Bias Pot be connected to. Then the external connections will be attached to the board. The expected distance to the corresponding connection as well as the routing should be taken into consideration when cutting them to length.
Now is the time to finally solder everything together. After this is done, the ends of the resistor's and cap's legs which are still sticking out a bit are once more cut off. Otherwise they might pierce through another wire and cause a short.

Chassis and Transformers

Up until now everything was pretty easy. Now as we dig into the chassis, space is getting limited.
This chassis has all the tranny mounting holes predrilled. The correct placement should be checked but in this case they were correct.
The holes for mounting the eyelet board are also drilled ... don't forget the second board that goes underneath. They should be put in a place where they don't interfere with the trannies later. The corresponding holed in the chassis are also punched, drilled and smoothed out.
The holes where the tranny wires go through need to be enlarged to make the rubber grommets fit. The power tranny, output tranny and choke are now bolted to the chassis. IMHO it's better to put the bolts into the holes from the inside so the nuts and threads don't stick into the chassis. Wires ate twisted together and fed through the grommets.

I slipped a piece of shrinking hose over the wires, where they pass the mounting bolts in order to avoid damage to the isolation, caused by the threads. The first couple of wires can now be seen inside the chassis.
 

Chassis components

All ground connections are soldered to the brass plate and then everything is stuffed into the chassis and secured with three screws. Switches, fuse holder and the pilot light are also mounted in their dedicated holes. The securing lugs are cut off the pots, because they are not needed. The brass plate is held in place by the bots and input jacks. The jacks are positioned in a way that those lugs which are connected to each other later are facing each other. This makes wiring easier. The ground connections are only visible in the back of the chassis and will not get in the way of other wires. So far I like using the brass plate. The noval sockets for preamp tubes and phase inverter are mounted as well as the speaker jacks and octal sockets for the power tubes and the rectifyer.

Bias potentiometer

A little improvisation is necessary to mount the bias pot because in the original design there is none. Therefore I use an old PCI-slot cover from an old PC. It is cut to length, bent and drilled. Isn't that a nice mounting bracket?

filament wires

I like to route the filament wires along the corner of the chassis. This way it is easier to reach the rest of the wires and components later. It's important to twist them tightly. This takes quite a bit of fumbling in a tight space but the patience pays off in the end.

board connections

The remaining wires coming off the board are now soldered. I start with the preamp tubes. Routing the wires close to the chassis improved shielding and ensures good access to all the components later. Running the wires at a right angle has no functional reason. It just looks neat.

For the ground connections on the lower side of the chassis I use a bare wire which is soldered to a cable terminal. That wire gets attached to a power tranny bolt. When wiring up the power tranny, the correct wire, according to the tranny's schematic has to be used. In this case it is the yellow 8 Ohms tap. The grees 4 Ohms tap remains unused. Therefore it is isolated off.

The unused connections on the power tranny are isolated off, twisted together and stored away in the corner of the chassis. The power cord's ground connection gets a cable terminator and is fastened to a fixed ground connection at the chassis. The remaining power cord and power tranny connections are routed and soldered.

The last thing to do is to finish the tone and volume pots and to connect the input jacks. Internal wiring is now finished.

speaker cable

The phone plug connector is soldered together and isolated with shrinking hose. The wires are trwisted together and at the other end two cable terminals are installed. I like to leave the wires a bit longer in order to be able to connect the speaker to another amp for testing purposes.

final assembly

Now the chassis is mounted to the cab. Therefore the holes for the bolts are marked and drilled. IMHO a weak point in the traditional way of mounting the chassis with two bolts is that the heavy transformers put too much stress on the mounting bolts, especially on the left side. To support the weight I fabricate a mounting bracket, which is bolted to the side of the chassis and secured with two screws at the top of the cab. They are not easily accessible but in a tweed amp the chassis hardly ever needs to be removed anyway.

The power cord is secured to the side of the cab. The back panel is shielded with metal foil.

All connections are once again checked against the schematic. For the first power up, only the rectifyer is installed to check voltages across the board. WIth no additional tubes installed the voltages will be a bit higher but it's easy to spot a major wiring error if the readings are way off. If that is correct, all tubes are installed, voltages are checked again and bias is set. Because there is an additional pot wired in series with the bias regulating resistor, as expected the value of this resistor needed to be lowered to get into the desired range. FInally the tilt back legs are installed. Believe it or not, this thing is ready to roar!!!


And that'S how it looks :

fine t uning

We have a mighty fine amp here. SInce I can't leave well enough alone, we'll get into the fine tuning. First I try a couple of different tubes. For the power tubes I have EH 6L6GC, SED "winged C" and Tung Sol 5881 to choose from. The differences are rather dramatic. Even when biased pretty hot, the EHs sound rather harsh and have an unpleasant bite in the upper frequencies. That'S not quite what I'm looking for. The SEDs are a whole lot better. They sound very balanced, loud and defined. I do like them. The Tung Sols are true 5881s which means that they have a maximum power rating of 23 Watts as opposed to the 30 Watts of a 6L6. It's no surprise that they break up a little sooner. The sound stands out among the rest. They are smooth, balanced, defined, very musical and everything you could ask for in a power tube. They will definitely stay in the amp.

In the second gain stage and for the phase inverter I use JJ 12AX7s. They are sturdy, solid and sound good. No need to change these. In the first stage a 12AY7 turned out to be the perfect choice. The 12AU7 sounded too thin and the 12AT7 and 12AX7 had too much gain for my taste. The only 12AY7 I have at the moment is an EH but they are fine tubes. I might try other brands in the future tho.

   

Let's move on to the speaker. S mentioned before I have a Weber 15A150 and an original '62 Jensen C15N. Both turn out to be marvelous speakers and it hurts me to put one of them back into the box. Bot are very efficient and produce a lot of volume ... more than I expected. The Weber, with it's Alnico magnet compresses a bit earlier and sounds fatter. The Jensen ceramic sounds a bit clearer and more airy. Bass frequencies are a tiny little bit more defined and more treble if present, without ever getting obnoxious. Both have a fat punch and smoothly drift off into breakup without ever sounding harsh. To me that's where they really shine and that's why they are well worth the extra bucks over the Jensen reissues.
For now the '62 Jensen will stay in the amp.

   

I never really liked the bright channel on tweed amps. The bright cap which lets some higher frequencies past the volume pot, gradually loses it's effect, the more you turn up the volume and sounds strange to me. I can't quite put it into words but even experimenting with different values did not make me happy. Therefore the whole thing must go. To avoid having two channels with the same sound I turn the bright channel into a normal channel by removing the cap and turn the normal channel into a "dark" channel. This is achieved by using a .1µF coupling cap in this channel instead of the .022µF. The sound gets a bit rounder and darker while the other channel keeps it's bite. The difference is quite subtle but at least I have to useable sounds. I might try a couple of other things to increase this effect but I need to get some parts first.

   

OK, that's it. Now I need to find out, how it behaves in a live situation. Some changes and comparisons are already planned and some might arise over time. I will try to document these as well.

 



- results

My first impression is very good. I already own a couple of tweed amps but this one is quite different. Even the Super, which is almost identical apart from the speakers is a different beast. The 15" speaker has less crunch and breaks up smoother. Treble is smooth and airy, bass frequencies are simply BIG.

What really surprises me is the volume. It seems quite a bit louder than the Super and even louder than the 5E8A Twin which is supposed to push out roughly 10 Watts more. I'm curious if this will be practicable in a live situation or if I have to do something about it.

The use of the brass plate works great. I have als tried builds with a ground buss or grounding through the pot housing. If done correctly all approaches are fine. Again it's a matter of taste.

The Metalfilm resistors sound just as good as the carbon comps to me. Instead of reading boutique amp ads I have studied some scientiffic articles on the topic. The theory is that it is necessary to have DC voltage of at least 100 volts across the resisstor as well as a strong amplified signal going through it. That is only the case at the plate resistors of the preamp tubes in the second gain stage and the phase inverter. So I changed all plate resistors to carbon comps and did not notice an audible difference. To be fair I have to mention that I did not notice additional hiss but since I have had problems with carbon comps in the past which developed over time, the metal film resistors are back in. Maybe in an amp with more gain stages where there is a stronger signal the difference is more noticeable.

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