50/50 Deep Mod

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Triaxstasy

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Since I did it to mine and there´s not too much info floating around, here´s a detailed instruction to the 50/50 Deep Mod:

The blue text is copied from the original MB tech document for the modification:

before the mod:
Before-1.jpg

assembled and wired 6-20-1990 :D the board says "Mesa 50/50B , 1989"

MODIFICATION INSTRUCTIONS FOR FIFTY/FIFTY

When the Fifty/Fifty was first introduced, it was designated as the companion poweramp for our Studio Preamp. Now, with the advent of the TriAxis Programmable Preamplifier, we are offering a single mod which greatly improves the compatibility between the TriAxis and the Fifty/Fifty by adding warmth and fullness plus a little additional power.
Although the procedure is simple and requires less than one hour, you must REFER THIS SERVICE TO A QUALIFIED TECHNICIAN.

1. Bolt the filter choke to the side of the chassis near the input jacks. It is not necessary to drill holes. Just run the pair of 8-32 screws through the cooling slots and mount the filter choke horizontally. NOTE: To ensure proper grounding, “metal-to-metal” contact must be made. Scratch a little paint off the inside of the chassis where the hardware mounting the choke will make contact, and use the star lock washers between the chassis and choke.
The best location is in the third row of slots up from the bottom, farthest slot toward the rear. Thread the nuts on the bolts and – before tightening – slight the choke toward the front of the amp as far as it will go on that rear slot.

2. Solder the two orange choke leads across the 1000 ohm, 7 watts sand block resistor, R501, one lead to each end.
(I simply unsoldered R501 and put the choke leads in its place)
1a.jpg


3. Replace the cathode resistors and capacitors at V1, the 12AX7 tube nearest the input jacks. These four parts are lined up next to each other just to the right of V1, as seen looking from the front towards the inside rear of the chassis. R202 and R203 each become 2.7k and C5 and C6 become 15.6µF/35v. Be certain to align the “+” ends with the “+” sign on the circuit board.
original:
2.jpg

modified:
2a.jpg


4. Look at the 8ohm speaker jacks and locate the red and orange wires. Unsolder these wires and add a .003µF disc capacitor between each wire and the jack where it was originally attached. Glue the capacitor bodies to the back of the jacks.
5.jpg


EXTRA MOD INFORMATION

If you have one of the early Fifty/Fifty´s and it has a noticeable buzz in the background, here is a very simple cure:

1. On the V1 tube, remove pins 2 and 7 (counter-clockwise from the gap). These can be difficult to unsolder so it´s ok to clip off pins 2 & 7 just flush with the PC-board. Be sure to leave enough of the pin material so you can bend and solder to them. Then unsolder the grey and white wires which come from the Level Controls to the circuit board just below the cathode parts. Bend up the remaining pieces of the pins from the socket and solder the wires to them directly. The white lead goes to pin 7, the grey goes to pin 2. Lift C18 .1/400v cap for easier access.
4a.jpg

white=black
grey=red



Required parts:
- 1 Filter choke (I used a 125C1A from TAD @ 25€)
- 2 .003µF disc caps (I used Orange Drop 716P)
- 2 2.7k resistors
- 2 15.6µF/35V caps


edit: Since I had the complete board out of the chassis I went for "major overhaul" and replaced all filter caps ((4x 220µF/350V; 2x 50µF/100V; 1x 33µF/500V), all Orange Drop (there were 715Ps in which I replaced with 716Ps), a couple of resistors and all old ceramic tube sockets with new high-quality Belton-Micalex ones.
Mine was originally a 110V US Version so I got in contact with a small electronics workshop here in Germany who does custom power and output trannies and had them wire a customized 220V PT to the specs of the original (for 80€). It also has a higher quality WAGO connector.
I also unsoldered all original wiring and rewired the whole amp with new 18gauge high-quality silicon wire and shielded wires for input and Volume.
All 4 pots where replaced with new Bourns pots.
The small blue/white trimpot is a modification to the Boogie-typical fixed-bias....guess what?!? It´s now adjustable :) (Thanks @ Crane !!!!)
The original fixed bias was set at about 21mA...ooooh, icy cold :nono: It sounds waaayyy better at about 35mA....

The final result is now the quietest tube power amp I have ever heard....and I simply love it!
After all modding:
Boardnew.jpg


Allwiredup.jpg


Micalex6L6.jpg


BoogieassembledBias.jpg


pilotlight.jpg


Foto023.jpg


ISAFRig-1.jpg
 
I know this thread is old, but did you add a 6.3V lamp to your amp instead of the neon lamp? Can you show me how you connected this to the circuit? Thanks
 
Hi Brion,

I used a 125V,75W pilot light assembly (http://www.banzaimusic.com/Pilot-Light-Assembly-round.html).
You can then use any 120V T3 1/4 (BA9s) light bulb in there.
The pilot light is wired according to the schematic.....so same way as the original one....
5050pilotlight.jpg
 
Thanks for posting pictures of your work. This thread is about the only place I've found such detailed information on this mod. I'm considering having mine done. At the risk of asking a stupid question, are those 2.7k resistors .5W?
 
Hi! The schematics say 0.5w but I used 1w metaloxide resistors. The two caps I used in there are High quality Panasonic caps...
 
Thanks a lot for posting this mod. Where were you able to get 15.6uF/35v caps? I have searched the internet and haven't been able to find them.

p.s. wonderful job on the the power amp!
 
I just verified it with my old parts order list.... I used Panasonic FC 15µF/50V caps
http://www.banzaimusic.com/Panasonic-FC-15uF-50V.html
 
Hello, why pût à choke 100 ohm in place of the 1k ohm résistor ?
I found the tda choke at banzaimusic, but it's a Transformer T-CHK-125C1A it's it ?
http://www.banzaimusic.com/Transformer-T-CHK-125C1A.html

The résistor are 2w ?

Do you now the "AT deep mod" ?
http://www.rig-talk.com/forum/viewtopic.php?f=26&t=33688
It's very simple, work it on the fifty ?

Thanks, merci
 
Thanks for the step-by-step, very useful and clear. I've just some (dumb) questions:

1- on Banzai (and other parts websites) the TAD125C1A isn't listed into the chokes but into the trannies... O__o maybe you can spot a light on that and make me understand :)

2- when you solder the 2 choke's leads across R501 you unsolder the block resistor but you don't state you put it back again after but I can see it in the "here it's done" picture, is it right?

3- did you also swapped the fuse holder?

4- any before/after soundclip? :D

Thanks again for sharing!

Keep rockin'
 
Hi,

- at TAD the 125CA1 is lister under chokes...and that´s what it is ;-) http://www.tubeampdoctor.com/product_info.php?products_id=138&url_node_name=Transformators_misc_transformers_and_inductors&language=en

- I went with the original instructions and soldered it "across" the original resistor...which i swapped for a new one as well

- The fuse holder is also a new one

- I don´t have any before/after soundclips, sorry



Rock On!
 
Hello, why put à choke 100 ohm in place of the 1k ohm résistor ?
THE AMP WOULD NOT EXPLODE ?
 
Ioanq said:
Hello, why put à choke 100 ohm in place of the 1k ohm résistor ?
THE AMP WOULD NOT EXPLODE ?

Dunno....
Mine hasn´t exploded yet, although it sounds bombastic 8)
 

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