Refurbishing a Chandler Tube Driver Pedal

[electrosonic]

I bought this pedal new probably 20 years ago. The original wall wart broke, the pots need to be replaced, and it is not true bypass (from the schematics I've seen it's not a buffered bypass, the 1 Meg input resistor is in parallel with the output when bypassed) .

The wall wart is a center-tapped 12vac one. I have had no luck finding online. I could make a separate AC power supply with a small transformer, but I would rather find a suitable wall wart if possible. Would it be possible to convert it to use a 12vac adapter without a center tap? Here is the schematic...
http://analogguru.an.ohost.de/193/schematics/Chandler_TubeDriver.gif

The plan is to rehouse it and replace the pots and move them off the pcb. (I have tried (and failed) to clean the originals pots and they are an odd sized pcb mounted type)  When I rehouse it I can then switch it to true bypass also.

Any suggestions?
Andrew.

[DougH]

I built a pentode driver based on the BK Butler schematic and use a non-center-tapped wall wart for it. It works fine. The bipolar power supply is not the same design as the tube driver PS but it will provide +-12 (15) v just the same.

[sjaltenb]

I'd be interested in this too.

Mine is actually 24V CT (or maybe that is the same as 12/0/12) Either way, I have removed the internal transformer as well. I don't really have any suggestions, other than, i'll be watching this thread. I'd like to know if we can pump DC into it and just let it get rectified again, too.

I also TB'd mine and added an additional LED above the switch. I don't like the way the other switching worked, either.




Luckily when you remove that darn power cord thing it leaves a perfect size hole for a power jack! PS if you haven't already done this, the easiest way to do it is cut the AC wire as close as possible on the inside and leave about 2 inches on the outside. Grab some needlenose pliers and wrap the wire around a few times, it should pull out pretty easily. It took me 30 mins the for the first one and about 2 the second one

[electrosonic]

As far as rehousing it I think I can squeeze it into a Sovtek 1790NS enclosure from Smallbear (5.70 x 4.70 x 1.50).

It might be possible to modify the power supply to a 12vac non-centertapped transformer. Like this BK Butler clone by Bajaman
http://i197.photobucket.com/albums/aa211/bajaman002/TubeDriverschematic.png

The tube heater is powered by 12v AC on the Bajaman version, while the my Chandler's heaters are after the bridge rectifier (ie 12v DC).

I guess a first step for me is to verify the schematic of my unit, before I think about changing anything around.

A.

[electrosonic]

I have spent some time looking over the schematic for my Chandler unit and Bajaman's BK Butler clone. It looks pretty simple to convert my 12vac center-tapped bridge rectifier to the 12vac non center tapped full wave rectifier that he uses. I need to remove two of the bridge rectifier diodes and connect the ac power correctly. I will get parts on Monday and take a photo when I get some time.

Andrew.

[electrosonic]

I also TB'd mine and added an additional LED above the switch

What does "TB'd" mean?

[nick d]

                 True Bypassed. (I think..)

[electrosonic]

Ok it worked.  I changed to rectifier part of the circuit to be like Bajaman's BK Butler clone.  I bought a 12vac transformer and a power jack, and it works. I replaced the power supply electrolytics also.

There are three other things I might consider doing to this circuit.

1) True bypass, maybe with a latching relay, using the existing switch. I could leave the LED part and us 1/2 of the DPDT to switch the relay.

2) Add the bias modification. Not sure where I would put the bias knob (internal or external) and how to do it neatly.

3) Maybe adding a mosfet follower to before the tone stack (as in RG's Mosfet Follies on the Geo site) - I think I could use a BS170 as a follower at the voltages here.

Andrew.

[joelshep3]

Ok it worked.  I changed to rectifier part of the circuit to be like Bajaman's BK Butler clone.  I bought a 12vac transformer and a power jack, and it works. I replaced the power supply electrolytics also.

There are three other things I might consider doing to this circuit.

1) True bypass, maybe with a latching relay, using the existing switch. I could leave the LED part and us 1/2 of the DPDT to switch the relay.

2) Add the bias modification. Not sure where I would put the bias knob (internal or external) and how to do it neatly.

3) Maybe adding a mosfet follower to before the tone stack (as in RG's Mosfet Follies on the Geo site) - I think I could use a BS170 as a follower at the voltages here.

Andrew.

I'm wanting to do the same thing (power supply conversion) to one of mine. It looks like I need to remove two of the diodes, and possibly change the two 1000uF electrolytics, possibly to 470uF. Am I correct?
Thanks!
Joel

[electrosonic]

My original schematic matches this - http://analogguru.an.ohost.de/193/schematics/Chandler_TubeDriver.gif but the wall wart CT transformer is hard to find so I converted to non CT wall wart.

Using this schematic - http://3.bp.blogspot.com/-AxwZ_1IljpY/T38Xg2ZeMbI/AAAAAAAABSE/c8L4beEA5ZA/s1600/TubeDriverschematic.png

I removed D3 and D4 and connect the AC power to CT and the junction of D5 and D6.

In the Baja design, the filaments are powered via AC connected via a 10 ohm / 1 Watt resistor - switched to that design (maybe unnecessarily), which is why R17 is removed and the 10 ohm resistor spans from one pad of D3 to one pad of R17, there is jumper from the D4 pad to the non CT AC lead to connect that resistor to power.



I tacked a 10uF capacitor on the end of R13 as suggested in the analogguru schematic to keep DC of the LO EQ pot.

The pots were all scratchy and exact replacements are not obtainable so I managed to fit 16mm long pin Alphas in there. I guess small bear didn't sell dust caps for the pots 3 years ago - you can see the double sided tape keeping the pots from shorting out the board.



3 years later, still works fine.

Andrew.