This Week on the Breadboard: The Tube Bender Deluxe

[Mictester]

One question i have is would we get even more tonal options if we use Germanium for Q3 and Q4?

What is this obsession people have with Germanium components? They're noisy, thermally unstable, getting expensive, low gain, and really really NOISY! For real!

I've built circuits with both germanium and silicon devices, and with the tiny modifications I sometimes make to the silicon variants, NOBODY who've tried them in A / B tests can hear the difference.

Just for amusement, a couple of years ago, I built a couple of Fuzz Faces in identical diecast boxes, painted the same colour, with the same knobs. One was built with germanium transistors and was temperature compensated (the compensation circuit was ~12 times as complicated as the Fuzz Face circuit!). The other was built with silicon transistors, and had small value capacitors from base to collector of each transistor to tame the "fizziness". Each had a "serial number" on the base, so that I could identify them without opening them up.

I loaned them both to a well-known London Rehearsal / Recording Studio. NOBODY who tried each of them could tell the difference - and we're talking about professional musicians who are always picky about the way they sound!

Properly configured silicon effects can sound just as good as germanium one, without the thermal instability and noise!

 

[Chuck D. Bones]

Ge trannys have a well-deserved reputation for all of the things you mentioned. But, not every Ge transistor is noisy, and not all Si transistors are quiet. I have built pedals with Ge transistors that are dead quiet. Like JFETs, Ge transistors must be hand-selected for the desired properties.

Thermal stability can be an issue, depending on the circuit. I have used a few simple and very effective methods to stabilize the bias of a Ge transistor.

Adding a feedback cap to reduce the bandwidth of a transistor circuit is useful in both Ge & Si circuits. Some Ge transistors, such as the 2N130x series, have a lot of bandwidth and they benefit from a feedback cap the same as an Si transistor.

I have compared Ge & Si transistors on a curve tracer and I can tell you that the transfer function of many Ge transistors has "features" that Si transistors lack. The sonic difference will be apparent in some circuits and not others.

I don't doubt that the musicians who tried your Si vs. Ge FF comparison could not tell the difference because it's a highly subjective test. Did they hook them up to an A/B box and switch back & forth, playing the same passages on the same guitar? What do you think would happen if you provided two electrically identical pedals, except one was painted red and one was painted green and you asked the same musicians to pick which one sounded the best?

I do agree that most of the hype surrounding Ge transistors is just that: Hype. But IMO there are some places where Si just won't do.

So what's your opinion on carbon comp resistors?

 

[steviejr92]

What is this obsession people have with Germanium components? They're noisy, thermally unstable, getting expensive, low gain, and really really NOISY! For real!

I've built circuits with both germanium and silicon devices, and with the tiny modifications I sometimes make to the silicon variants, NOBODY who've tried them in A / B tests can hear the difference.

Just for amusement, a couple of years ago, I built a couple of Fuzz Faces in identical diecast boxes, painted the same colour, with the same knobs. One was built with germanium transistors and was temperature compensated (the compensation circuit was ~12 times as complicated as the Fuzz Face circuit!). The other was built with silicon transistors, and had small value capacitors from base to collector of each transistor to tame the "fizziness". Each had a "serial number" on the base, so that I could identify them without opening them up.

I loaned them both to a well-known London Rehearsal / Recording Studio. NOBODY who tried each of them could tell the difference - and we're talking about professional musicians who are always picky about the way they sound!

Properly configured silicon effects can sound just as good as germanium one, without the thermal instability and noise!

In my humble opinion, I can hear a difference. Simply put a Ge transistor has characteristics that a Si transistor doesn’t have. Sonically I believe Ge have a much smoother tone than Si.

I agree with you that ge transistors are noisy but that’s in generalities. I’ve had plenty ge transistors that were quiet as any Si I’ve tried. I’ve breadboard plenty of circuits where the ge component behaved as it should have with no compromise to the harmonic output and had no background noise.

I only use that cap on the 2nd transistor and have always been successful in taming the circuit (for both si and ge). While those capacitors do help I believe they dull the tone in an unsatisfactory way. I only use them if it’s necessary.

 

[MattG]

Is there anything special about the MPSA13 transistors in this circuit, other than they are Darlington transistors? More specifically, could each be replaced by a pair of commodity BJT NPNs (e.g. 2n3904 or bc549) configured as a Darlington pair?

 

[Chuck D. Bones]

We need the high HFE. You can make a Darlington pair from two transistors. The only caveat is that the two Darlingtons (MPSA13 or a pair of transistors) should be closely matched for Vbe. We want the transistors to operate at nearly the same collector current. Why not avoid the hassle and buy MPSA13s from Mouser? I bought a bunch from AliExpress, but I have not checked them for matching.