Faults from a boutique Ge pedal builder

[Mark]

I’m in the midst of building a Tone Vendor MK1 pedal out of circumstance rather than choice. Knowing the hassle with finding the right transistors let alone the “right” components I thought I would buy one. This should have been the end of it.

I saw collector effectors on Facebook and he had some good clips of his pedals. So I decided to get the Song Machine which is a Tonebender MK1 with different input caps to simulate the sound of a Zonk. Perhaps I should have sprung for something from the British Pedal Company in retrospect, the pedal had issues from day one in that annoying Velcro sounds was all that it could do and it’s best Mick Ronson tone was incredibly mediocre.

So after finding the pedal to be unusable after some months I decided find out why is this the case, perhaps it’s a minor thing that’s easily fixed. This wasn’t the case, the Hfe of the transistors was Q1 = 30 leakage 100uA, Q2 = 118 leakage of 80uA, and Q3 = 50 leakage of 101uA. He also used a 180K to bias Q2 when a 470K was the vastly more effective resistor. When I mentioned these problems he said he would charge me to repair it. I thought that was outrageous as the product clearly had issues.

I looked at the pcb again today and found yet another problem he miss wired a 56K resistor to the collector of Q3 rather than the coupling cap. This guy takes no responsibility for his workmanship as far as I can tell and perhaps I should have just sold it, but then someone else would have to fix it and I didn’t like that at all.

Should you come across a Song Machine try before you buy, I have heard clips of pedals sounding great but that was not my experience.

If you have to repair one check the Hfe and leakage of the transistors as well as checking the 56K resistor is soldered to the correct position.

I built up a Tone Vendor pcb as a means of auditioning suitable transistors. I started off with using the transistors from the Song Machine and the set works better with the 470K resistor setting bias/gain but it’s nothing special as the decay of the note sounds fizzy and noisy.

I’m finding voltage across the B-E junction is 0.037v with the attack on 0 and 0.161v with the attack up full. The meter on diode mode shows a voltage of 0.214. It would seem the circuit doesn’t provide enough voltage to turn the transistor on and yet the Attack control works fine. I suspect the leakage is biasing this stage and the additional current flow is increasing the gain. It seems to me that if one were using a very low leakage Q2 transistor then a bias resistor as low as 180K might make sense, though generally higher value resistors should be used.

I did notice that the attack/gain control was affecting Q3 (which is impossible as it’s isolated), I lifted the coupling cap and Q3 was quite stable, I suspect I have m a leaky Panasonics coupling cap, I replace that tomorrow.

I’m wishing to record as many stats as possible on the transistors and well as listening tests using a Ditto and recording results. Hopefully I will get what I should have to paid for and get a great Tone Vendor pedal in the process.

If I’m incorrect please feel free to correct me.

Thanks for allowing me to vent and I appreciate your patience.

 

[HamishR]

In my experience the best way to build a Tonebender or any Ge transistor pedal is to get some sockets and just try a bunch of transistors. It can be tedious but it works. Getting a transistor tester can help weed out transistors with too high leakage but some leakage is usually necessary to get the pedal to work. FWIW my favourite Tonebender is the Marshall Supa Fuzz - it has a fatter sound.

The best TBs I have built have been with Russian MP38 transistors which are NPN so you don't need a positive ground or anything too fancy. I have a few dozen MP38s and sorting through them has got me some decent sounding fuzz pedals. Don't worry too much about HFE - it doesn't really seem too important in these pedals. And usually the high HFE transistors are also the ones with the highest leakage.

A lot of the time the high prices for Ge pedals is probably for the time spent sorting transistors. The parts are not expensive. MP38s are not expensive. Don't get sucked in by mojo part numbers - just because some original Tonebenders used NKT275 transistors doesn't mean you need NKT275 transistors. It just means that NKT275 transistors were what was available when the original TBs were made. Having built a lot of Ge pedals I have found that the Russian transistors are actually easier to find good examples of and easier to get a good sound from. You can get them from Ebay sellers in Bulgeria amongst other places.