Distortion Booster in Thomas Vox V1032?

[MWaldorf]

I guess I'm a sucker for punishment. After many months, I've finally gotten my V1032 up and running. Even added a Top Boost. Now I'm on to the next challenge - installing the "big head" distortion booster. Yes, I know I could just use a pedal in front of the amp, but I'd like to get it all on board.

So, after getting all the parts, I built the circuit on a little PCB and added it to the V1032. No surprise, it's not working. None of the voltages are where the spec says they should be. At least I was able to confirm that the relay functions, so I can rule that out.

Starting at the top, the DB circuit expects a +20V source which is not available in the V1032, so I used a 100R/680R voltage divider on the 27V (25.6V in my amp) source. I read 19.6V where I should have 20V, close enough, but the voltages in the rest of the amp are thrown - the 31.5V drops to 30.8V, the 25.6V drops to 22.7V, and the 16.6V drops to 14.8V. Should I be using a higher resistance voltage divider?

Beyond that, none of the other voltages read as I would expect - Q401C reads 9.8V instead of 1.65V, Q402C reads 10.9V instead of 4.5V, Q402E reads 0.6V instead of 1.5V and the positive side of C404 reads 14.3V instead of 10V. I'm not sure how much of this might be due to the voltage divider issue.

Any advice? Thanks, Mel

Here's the V1032 circuit. In my amp, I removed the e-tuner.
vox-berkley-ii-v1082-1032.pdf

Here's the V1141 Beatle circuit where I got the distortion booster schematic.


Here's my PCB layout. Inelegant, but I'm using a RadioShack board I picked up on clearance.


Finally, here's the amp itself. The DB circuit replaces C1, and I made a jumper so I could use the posts on the board. The white cable clip runs to ground to shield the leads to the DB board. Also, you can see the Top Boost pot in the background where Input 3 had been.

[J M Fahey]

Starting at the top, the DB circuit expects a +20V source which is not available in the V1032, so I used a 100R/680R voltage divider on the 27V (25.6V in my amp) source. I read 19.6V where I should have 20V, close enough, but the voltages in the rest of the amp are thrown - the 31.5V drops to 30.8V, the 25.6V drops to 22.7V, and the 16.6V drops to 14.8V. Should I be using a higher resistance voltage divider?

No big deal, all voltages are close enough, but if you want certified +20V source, I can suggest a resistor+20V Zener regulator, but I need to know your K401 relay coil resistance (and voltage rating) .

Beyond that, none of the other voltages read as I would expect - Q401C reads 9.8V instead of 1.65V, Q402C reads 10.9V instead of 4.5V, Q402E reads 0.6V instead of 1.5V and the positive side of C404 reads 14.3V instead of 10V. I'm not sure how much of this might be due to the voltage divider issue.

Itīs not a voltage divider problem, that "distortion pedal" expects "about 9V" so 10V is fine and even 14V no big deal but what you measure is gross, most probably wrong layout or construction.
Build the distortion circuit on Protoboard and remeasure, please post Q401 and Q402 C/B/E voltages.
In fact post the ones you measure *now* , basically Q401 is not passing current , thatīs why its C voltage is high, and Q402 can/should not have such a high B voltage, but post the full set of 6 values to have a better idea.

As a side note and looking at that Super Beatle schematic: those 4 x 12" speakers must have been LOUD and the horns must have been ear piercing or worse, does anybody have some audio or YT video showing how they sound?

[olddawg]

Test the distortion unit sepearate from the amp with a bench power supply or two 9v batteries. Use a 20vdc TIP type regulator instead of a voltage divider to get the supply you want in the amp.

[drewl]

Check out any Beatles us concerts from 65 or 66, their using Super Beatle amps and cabs.

Having worked on a couple they are loud even more with the 4x12's with horn.

Only got to play through that once, but it was brutal


Since the "distortion booster" is pretty much a Fuzz Face or Tonebender you could make the fuzz and level adjustable if not with pots (if there's no space) then trimpots on the board.

[olddawg]

Check out any Beatles us concerts from 65 or 66, their using Super Beatle amps and cabs.

Having worked on a couple they are loud even more with the 4x12's with horn.

Only got to play through that once, but it was brutal


Since the "distortion booster" is pretty much a Fuzz Face or Tonebender you could make the fuzz and level adjustable if not with pots (if there's no space) then trimpots on the board.

How times have changed... back in the day when I was young you could pick up a complete, working Super Beatle and cab with frame for $50-$75 because nobody wanted one. We would pull the speakers out to make monitor wedges and toss the rest in the dumpster!

[52 Bill]

Mel, I'd suggest that you remove the voltage divider that you added. It really isn't needed, but if you feel that you must create a 20 volt supply tap, then use larger value resistors so that the divider won't load down the main power supply.

This circuit is basically just a Fuzz Face or Tone Bender and runs at voltages similar to the battery version voltages. I haven't been able to check all of your work on the pc board, but are you certain about the relay contact positions?

[The Dude]

Agreed. Or, you could just up the value of R402 slightly and add a similar dropping resistor on the way to the relay.

[52 Bill]

Agreed. Or, you could just up the value of R402 slightly and add a similar dropping resistor on the way to the relay.

Yes, increase the value of the 4K7 resistor and of the 300 ohm resistor from the relay to the footswitch.

Everything on your pc board looks right, but because of the too high voltages, I'd check the transistors and their lead orientations.

[MWaldorf]

OK, an update. I've looked and I've looked and I don't see anything wrong with my circuit or my wiring, including the 2N3906 pinout. Not to say there's nothing wrong, just that I can't find it.

Based on OldDawg's suggestion, I removed the board from the amp and isolated the fuzz circuit by decoupling R402 from C404 and inserting a 9V battery on the positive side of C404 where the schematic says there should be 10V.

Here are the voltage readings I get:

Battery voltage: 9.52V
Q401E: 0V
Q401B: 7.03V
Q401C: 7.65V

Q402E: 0.25V
Q402B: 7.65V
Q402C: 8.25V

Relay/C403:0V
R406/C407:9.49V
C407//R408: 0V

According to the schematic, there should be 1.65V at Q401C/Q402B and 4.5V at Q402C. I know all the resistors and capacitors are the correct values, so am I looking at one or both transistors being bad? I'm using 2N3906 transistors.

FWIW, I put a 440Hz/500mV test signal in from my oscilloscope:


And the output is this. Slightly hairy, and a 200mV drop from the input signal.

[g1]

Those 2n3906 are PNP. Try 2N3904 or equivalent NPN.

[J M Fahey]

Those 2n3906 are PNP. Try 2N3904 or equivalent NPN.

Good catch,that explains it all.
A PNP transistor will never ever work instead of an NPN one and viceversa.

[g1]

The 7 volts B to E just didn't seem right.

[MWaldorf]

Duh, thanks guys! I'll give that a try. PNP, NPN, what's the difference, right?

[MWaldorf]

SUCCESS!!! - I swapped in 2SC1815 transistors and tested it. Using a 9V battery at the positive side of C404, I get the expected voltages at Q401C and Q402C. Then I tested using two 9Vs in series at the positive side of C401 and still get the expected voltages. So far, so good.

After playing around with a few other resistor combinations and not getting the right voltage, I tried an LM7818 and that did the trick - 25.6V in and 17.7 out. I've got to install it properly and set up the footswitch, but I'm in business. Once I've got it all in place I'll post final schematics and some pictures.

Thanks everyone for your help!

[MWaldorf]

Update on the V1032 - I added lights to the footswitch and it's all back in the cabinet. I've used it for two band rehearsals and so far, so good. Here's a link to a quick sound test:
https://www.youtube.com/watch?v=t29O...ature=youtu.be

Also for the record, here are the last few changes.

Here's the distortion booster PCB with the 2SC1815 transistors and an LM7818 18V regulator to supply power. On the left you can see where I've marked the chassis for mounting holes.


Here's the final layout inside the amp. You can see the the in/out to the fuzz on the upper right side of the PCB. The green wire coming from under the board next to the in/out runs to the push/pull on the volume pot and from there to pin 2 on the foot switch jack, allowing the fuzz to be engaged by the foot switch or the push/pull. The red wire running from the +31V source on the lower left of the board up to pin 6 on the foot switch jack supplies power for the pedal lights. Also note the top boost pot in place of Input three, with the orange wire running to the bottom right of the board.


Here's the foot switch wiring.


Here's the foot switch showing off the lights.


I want to thank everyone who helped and gave advice over the course of this seven month project. I couldn't have done it without you. I still don't know a lot of the hows and whys, but I've got a much better sense of troubleshooting.

Given the added "big head" features, I call this the Cambridge Reverb BMOC - "Big Man on Campus"