Thursday, June 26, 2014

'Fender Blues Junior III Modifications and other stories'.

This week I had on my workbench a limited edition Fender Blues Junior III. This example came in a 'Chocolate Tweed' cabinet, a burgundy metal jewel-light and a Jensen P12Q Alnico speaker. The owner
had done a lot of research on how to make it sound better and brought me the amp along with the BillM- Audio and Fromel Electronics upgrade kits. This post is about these modifications, but while writing a story about this little spider came to mind...

The SGSystems SG212 Amplifier, or; 'why I'm so nervous around Tube-Amps'.

I haven't worked on a lot of tube-amps yet because I'm not really comfortable with the high voltages if I'm not a 100% sure what I'm doing. A few years ago I acquired an SGSystems SG212 and trying bias it to use different tubes the 660V operating voltage scared the hell out of me. 
It didn't help that I found a spider that I thought looked a lot like a black widow lurking inside the amp, and while getting a jar trying to catch the little critter I lost sight of it, not making me any less nervous. You really don't want to have a scare-reflex and accidentally pull a 660V amp with red-hot glass tubes onto your lap. I eventually found the spider and learned it was the so-called 'false widow', which can have a painful bite but is only lethal  in combination with a running tube-amp, so I found it a nice new home in the yard. 

1974 SGSystems SG212 amp. Yes, that's a the legendary Meastro PS1A phase shifter built-in.
I bought this amp for 50 Euro's from the man who changed my 'okay' Fane DEA100 into a marvelous Plexi Super-lead monster that I suspect may one day evoke the 2nd coming of Hendrix.
It was in a broken state but the 3 colored knobs grabbed my attention, suspecting it might be a Maestro 
PS-1A Phase-Shifter built in. And it was! Now the PS-1A is actually the first ever Phase-Shifter built, 
the genesis of phasers if you will, and it should be no surprise to you that the phaser is one of my favourite effects. It was designed by Tom Oberheim, build by Maestro/Gibson and can be heard on the organ parts 
of Led Zeppelin's 'No Quarter'.

Now the SGSystems amp is quite rare and it was hard finding info on the schematics and re-biasing to use 6L6 instead of 8417 tubes. The 8417 output-tubes are no longer made and go between a 200 and 340 euro's for a matched pair on eBay. Compare that to the 26 euro's that get you a platinum matched pair of JJ 6L6's...
When trying to bias the amp for the 6L6's the tubes where running way too hot, resulting in a bright red glow, a funny smell, an increase in room-temperature and the sound went into a worrying tremolo stutter effect. 
I managed to hear for about minute how great the phaser sounded, but decided to shut off the amp and haven't touched it since. I'm still deciding if I'm going to take out the phaser and build it into a pedal, or if I'm going to restore the amp when I have more experience in biasing and tube-amp circuits.

Modifying the Blues Junior

Luckily the Fender Blues Junior III is a very common amp, and information on modifications, biasing and schematics are easy to find. Also the upgrade kits came with full instructions so I wasn't too scared to take on this project. 

The BillM Audio kit included a off/standby/on switch to replace the stock on/off switch, a bigger TO20B output transformer and a presence control mod.
The Fromel Electronics kit included power-filter and tone capacitor upgrades, a different bias resistor and
a panel mounted input jack to replace the board-mounted input-jack (which is used in almost every current- production amp, and is always the first thing that breaks .)
The reverb tank's RCA plugs where cheap and they didn't connect tightly enough, causing the reverb to drop out of the signal so I hardwired those connections to fix this.

After reading up on safely discharging the power capacitors I found them already discharged to around 7 volts due to the drain resistors that are in this amp, so it was time to remove the board from the chassis to de-solder the stock parts.

The Tweed Deluxe and Eyelet-/Printed Circuit-Board comparison.

I get why modern electronics construction moved on to printed circuit-boards; the old Point- to-Point/ Turret/ Eyelet method has to be done by hand, while PCB's can be made in great number by machines, keeping the cost down so you can have an affordable amp. 
Still, and especially with regards to tube-amps, I don't like working on them. To illustrate why let's have a look at two outwardly very similar looking amps I've been working on this week, one being the 'PCB' equipped Blues Junior, the other a Tweed Deluxe that uses an 'Eyelet-Board';

The Tweed Deluxe 'Eyelet-Board'. This amp was designed in 1954...

The Blues Junior's 'Printed-Circuit-Board'
The Tweed Deluxe had problems with the switched-connections on the 4 input jacks, so I replaced those with high-quality Switchscraft jacks. When I got it it had a cheap tone-pot which caused problems because it didn't ground well to the chassis so I replaced it with a better one similar to the other pots and put in better tone capacitors along with a new grid-capacitor, a new Jewel-light assembly and a new bypass switch, which I had borrowed to build that B-15 Flipster FET pre-amp.
To get back to why I don't like working on PCB's; trying to take out the board of the Blues Junior to even get access to the solder-connections took me as much time as as it took me to do all the work on the Deluxe mentioned above. All the screws, jacks, pots and cables needed to be disassembled to get the board free and lift it out of the enclosure, and even that was tight work.

Installing the mods

After removing the parts that where going to replaced I put in the new quality F+T filter-caps. The main 47uF filter-cap is doubled in value to 100uF which along with the bigger output transformer helps to give the amp a bigger sound and better bass response.

The new F+T filter-caps in the power section.

The old (right) and new (left) output-transformer.

The new output-transformer in place.

From L to R; 'Orange Drops', Silver Mica and switched-pot
of the presence control.
Next are the replacement tone-caps along with the added presence control. The presence control has a switched-pot that allows you to take it out of the circuit completely

The caps are two 'Orange Drops' and a Silver Mica that are used often in quality Fender (type) amps.

These mods helped a lot in getting rid of the somewhat thin and spikey stock sound. It now has clear and pronounced mids and the presence control dialed out only that specific annoying treble frequency that Fenders sometimes have, without really affecting the overall treble setting. It gave the amp very 
flexible control over the high frequencies.

After installing the new Off/Bypass/On-switch, hard-wiring the connections to the reverb-tank and installing the chassis mounted input-jack it was time for testing. .

Inside the tank.
I used the existing input on the reverb-tank to feed through
the wires, using heat-shrink tubes to make it a tight fit.

I played some 'Wind Cries Mary', a little Tori Amos (I somehow suddenly remembered how to play the piano to guitar conversion of 'Winter' that I had puzzled out more than 10 years ago.), and a little Pulp Fiction to test the reverb. I couldn't turn up the amp high enough to compare its sweet-spot and breakup point with the Tweed Deluxe, but with the master at 5 and volume at 2 it already was very loud, clear with that undeniable fender character. It had the sparkling blackface cleans but with those Tweed Deluxe's mids added which will make the amp stand out very well in a band setting. The overall impression of the amp after the modifications is that it now delivers the great sound the beautiful exterior promises it has.

With all the mods in place.

The finishing touch

The last thing needed now was putting it back together and making a 'Liquitone Modified' name-plate to put on the amp. Perhaps I've been watching too much Chip Foose's 'Overhauling', because the dull grey metal of the tube-grill annoyed me so much I couldn't resist polishing it to match the shiny name-plate.

I didn't make any audio recordings of the amp, but here is a good video review with most of the mods that gives you an impression of its sound;

And a stock vs. modified demo;

1 comment:

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