Beer of the Bass Posted July 23, 2012 Share Posted July 23, 2012 After playing around with overdriving the preamp of my valve head using a booster, I thought it might be handy to build a pedal to get that kind of sound when I'm using other amps. It will also be used in my guitar setup if I like it. I'm basing the power supply and general concept on Fred Nachbaur's Real McTube design, [url="http://www.dogstar.dantimax.dk/tubestuf/mctube.htm"]http://www.dogstar.dantimax.dk/tubestuf/mctube.htm [/url]but I'm adjusting the plate and cathode resistor values so that it behaves more like a conventional amp front-end. I'm also adding switchable cathode capacitors (to cut the low end, Marshall style) and a single knob tone control. The power supply is unusual in that it uses inexpensive low voltage transformers rather than a purpose built valve amp transformer. It uses one transformer to step the mains voltage down to 12v, which is then stepped up with a 230:24v transformer used in reverse to give a B+ voltage which should be somewhere around 140v after rectification. The 12V supply is also rectified to power the heater filament. Here is the schematic I've drawn up, but I may change things after I've put it together depending on the results. I've left the bypass switch out of the drawing, but it will have one. I have the transformers, ECC83 and box, but I'm waiting for the other parts to arrive before I can put it together. It's going into a diecast box which is about 180 x 120 x 55 mm. The power supply and audio circuitry are going to go on two small tagboards, with the valve mounted on standoffs inside the box. In the meantime, I have prepared the enclosure and painted it with Hammerite. If anyone is wondering, this is not my first valve project, as I have a couple of small amps under my belt (the last one was copied from a Fender Princeton Reverb). More to follow when I've got my parts... Quote Link to comment Share on other sites More sharing options...
dincz Posted July 23, 2012 Share Posted July 23, 2012 (edited) Looks like a worthwhile project. Just wondering why you decided to use a DC heater supply. EDIT: I just looked at the original design - problems with hum. A bit surprising given that ac filament supplies are the norm and don't cause problems. Edited July 23, 2012 by dincz Quote Link to comment Share on other sites More sharing options...
Beer of the Bass Posted July 23, 2012 Author Share Posted July 23, 2012 (edited) I'm going with the DC heater supply partly because it's in the design I'm basing the power supply on, and partly because it may reduce my chance of hum, especially in a fairly tight layout like this one. It'll be handy for the status LED also, as otherwise I'd have to run AC through one pole of my bypass switch to power the LED, and I'm pretty certain that would hum. Edited July 23, 2012 by Beer of the Bass Quote Link to comment Share on other sites More sharing options...
Beer of the Bass Posted July 24, 2012 Author Share Posted July 24, 2012 No pictures today, but my package arrived from Ampmaker and I've started putting the tag boards together. The next few days are a bit busy, but I anticipate I'll have it together and be testing it some time next week. Keeping the wiring neat in this small a box will be a bit of a challenge! Quote Link to comment Share on other sites More sharing options...
Beer of the Bass Posted July 27, 2012 Author Share Posted July 27, 2012 A little more progress in the past couple of days; the tag boards are populated and it's all test-fitted into the case, with the exception of the 3PDT bypass switch, which is in the post. It all seems to fit OK, but I've slipped up slightly as one of the jacks gets in the way when putting on or taking off the valve shield - not a major issue though. It's a tight layout, which may or may not cause problems, but the Real Mctube build from Nachbaur's article was similarly crowded and apparently worked OK. I'm going to used screened cable for most of the audio wiring to be on the safe side. Next will be the hard-wiring and testing... Quote Link to comment Share on other sites More sharing options...
umph Posted July 29, 2012 Share Posted July 29, 2012 looks good dude, i'd recomend putting another screw in the boards, you dont want them being able to rotate. Twist all the wires from each triodes together aswell, the cathode works as a shield and it'll make it easier to avoid oscillation Quote Link to comment Share on other sites More sharing options...
Beer of the Bass Posted July 29, 2012 Author Share Posted July 29, 2012 (edited) Good call on the twisted wires - I'd already wired up the socket to the board with straight parallel wires but might go back and redo them. The power supply board actually has a second screw, hidden from view by the caps. On the smaller board I left myself too little space to get another screw in (I probably should've planned that better!), but it seems firm enough when assembled with locking washers and threadlock. Unfortunately I'm down at the inlaws in sunny Barnsley for most of next week, so I'll have to carry on with it when I get back... Edited July 29, 2012 by Beer of the Bass Quote Link to comment Share on other sites More sharing options...
Beer of the Bass Posted September 27, 2012 Author Share Posted September 27, 2012 I got a bit busy through August, but I've dug this out in the last few days and finished wiring it. I checked the voltages today, and I need to tweak the power supply a little as the heater voltage is running too high at +/- 7.4v when it should be +/- 6.3v . All other voltages are about as expected, so hopefully I'll have it sorted at the weekend. Quote Link to comment Share on other sites More sharing options...
Beer of the Bass Posted September 30, 2012 Author Share Posted September 30, 2012 As of this morning, it's done and sounds excellent! I've tried it on both bass and guitar. On bass it goes from just slightly hairy to a much more aggressive overdriven amp sound. With the tone control up it's very much in the early Entwhistle mode. On guitar it gets well into crunchy rock territory but stops short of metal, which is not my thing anyway. It has plenty of mids, which is great for thickening up my tele-style guitar. Compared to my previous overdrive pedals (a Tube Sound Overdrive/Red Llama clone and a Tubescreamer), there is much less ugly intermodulation when playing chords and a complete absence of fizz even with the tone control up. I could quite happily leave this on most of the time. The wiring is a little more rats-nesty then I would have liked, but I'm not getting any hum so I guess all is well. I think if I build something like this again, I'll spend more time planning the layout and give myself a bit more space to work in. Also, in a premature senior moment, I have wired up the bypass switch without grounding the input to the valve, which is causing some oscillation in bypass mode when the gain control is right up. This should be easily corrected by rewiring the switch though. If anyone is wondering about the lettering, the small ones are white letraset transfers with some clear acrylic paint to seal them and my name is in adhesive vinyl letters from an eBay supplier, the kind you would use on a van. Compared to the schematic above, I have changed the resistor in the heater supply to a 22 ohm 3 watt to get the filament voltage correct, otherwise it is as per the schematic. Here are a couple of pictures; Quote Link to comment Share on other sites More sharing options...
untune Posted September 30, 2012 Share Posted September 30, 2012 Looks brilliant! Really wish I had the know how for projects like this. Any chance of some sound samples? Quote Link to comment Share on other sites More sharing options...
Beer of the Bass Posted September 30, 2012 Author Share Posted September 30, 2012 I might manage some sound samples in the week. Quote Link to comment Share on other sites More sharing options...
untune Posted September 30, 2012 Share Posted September 30, 2012 Cool, look forward to hearing it Quote Link to comment Share on other sites More sharing options...
voxpop Posted September 30, 2012 Share Posted September 30, 2012 Are you going to offer to make it for fellow basschatters ? Quote Link to comment Share on other sites More sharing options...
Badass Posted September 30, 2012 Share Posted September 30, 2012 Yeah good question...I maybe interested, after a listen to some sounds. Quote Link to comment Share on other sites More sharing options...
Beer of the Bass Posted September 30, 2012 Author Share Posted September 30, 2012 [quote name='voxpop' timestamp='1349006421' post='1820534'] Are you going to offer to make it for fellow basschatters ? [/quote] I'd be wary of making things for sale, given that it's mains powered and I'm an amateur working from my kitchen table with no bits of paper for safety certification, sorry! Pretty much any custom amp builder could probably run you up something similar though. Quote Link to comment Share on other sites More sharing options...
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