CV7001 - fun and gains?

[Greenwichpaul]

i've managed to retrieve my old Fuzzface that I loaned to a friend years ago. I didn't miss it that much as I never really liked the sound - buzzy and thin, probably because it's the silcon version.
However, since then another friend showed me a stash of germanium transistors he found. Most were useless - RF mixers or switching transistors - but there were also a bunc of CV7001 which seems pretty close in performance to the original germanium PNP. I figured I could buy one of the clone circuit boards (as that's cheaper than making one up) and simply replace the board in my pedal .
However, of the original 20 or so transistors, many had excessive leakage or low gain. I ended up with  two gain 80, one gain 100, and one gain 110 with leakage of 360microamps.
I've tested it, using the 80/100 pair and it sounds great. Much juicier than the silicon version, and more touch-sensitive (partly because it's less trebly and buzzy).
BUT web lore is that the gain for the second transistor shoudl be 120. Has anyone compared anything in those combinations?
Does it make that much difference. My inclination is to go with what I've got as it sounds good. reasonable?

[tonyharker]

No Germanium transistors are useless.  Switching transistors can be used for audio and if they are high leakage, they can be used as diodes.

[idy]

Gain choice is personal, more like "around 80-100 and around 90-120, but your mileage may vary..." If it sounds good to you, it is good.

You may want to check how these new transistors are "biasing" with the old resistors though, could be room for improvement...

[ElectricDruid]

Absolutely if it sounds good, it is good.

Someone once tried something and *they* thought that 80 and 120 sounded good. You might not agree, or your transistors might not be the same as theirs, or, or, or, etc.

Ultimately, there's a lot of variability and therefore a lot of "taste" in building with something like germanium transistors. You choose what you like, and you decide if it sounds good.

Anyway, since when was anything on the web the gospel truth?!?

Tom

[duck_arse]

welcome to the forum, Greenwichpaul.

Anyway, since when was anything on the web the gospel truth?!?

this will be the first time EVER: your input cap is backwards if you are using positive ground.

[Greenwichpaul]

welcome to the forum, Greenwichpaul.

Anyway, since when was anything on the web the gospel truth?!?

this will be the first time EVER: your input cap is backwards if you are using positive ground.

Haha, thanks! I have the ring/outside of the input socket going to positive ground and the tip going to the negative connection on the cap which I was thinking would be more negative, but I'll check.

idy i used a trim pot to bias to 4.5V, although I've not tried experimenting with different values .

Tony I tried a couple of the mixer and switching transistors as well as some unlabelled ones. All but one or two, if they worked at all, gave low gain. I was surprised how few of the CV7001 had decent gain.

All that said, since posting I realised I should test this by comparing with the slightly higher gain transistor with higher leakage.

BTW I should also thank the forum for leading me to all the necessary info.

I was interested to see that while the CV7001 are referenced here one or twice there's been no-one back reporting on how they sound to my knowledge. They do sound terrific. Can't see any easy way to post an MP3 but the sound is exactly as it should be to my ears.

[PRR]

> the cap which I was thinking would be more negative

Don't think. Use volt-meter.

And nearly all electronics is +/-20% tolerance. Even before we get to circuit interactions and measurement differences, "hFE=100" is exactly equal to "hFE=120" within 20%.

And "RF" transistors are better-grade audio transistors. In those days they made some transistors would sing 1MHz, and some that wouldn't go that fast. The fast ones were marked "RF" or "IF" (intermediate freq) and sold for 2 cents more. The rejects were "audio". "Switching" were selected from the same pot on different criteria. Today they are ALL fodder for your experiments.

[Electric Warrior]

BUT web lore is that the gain for the second transistor shoudl be 120. Has anyone compared anything in those combinations?

It's just web lore. According to the data sheet, the NKT275s Arbiter used in the 60's maxed out at 90. Many pedals have transistors that measure just arout 60. They can still sound great. Matched gains or mismatched is a matter of personal preference. Let your ears be your guide!

idy i used a trim pot to bias to 4.5V, although I've not tried experimenting with different values .

How do they bias with stock values?
4.5V is pretty low for a germanium Fuzz Face. They're usually hotter than that. The MK1.5 Tone Bender it was derived from was biased beyond 7.5V. The Fuzz Face was slightly tweaked to bias a little colder than that, but there is overlap.. My point is that they can sound good throughout a pretty wide range of voltages. Just experiment with different gains and leakages until you find something that sounds spectacular to you. You may not need to change anything - especially since you've got a big bag of transistors.

The voltages mostly depend on Q1's leakage. With higher leakage there, its collector voltage will decrease and Q2's will increase.
If you do end up outside the "good sounding" range, you can try tweaking Q1's collector resistor instead of Q2's to fix both collector voltages at the same time. Worked great for me. If you're using a trim pot in that spot it may be safer to limit its range with a little resistor, btw..

[Greenwichpaul]

No Germanium transistors are useless.  Switching transistors can be used for audio and if they are high leakage, they can be used as diodes.

Just an update for anyone who (like me earlier) is searching for the STC CV7001 and others.

You were right; I've tried the same circuit with other STC transistors, including one marked IF (whose gain I didn't measure) and a TK1002 (don't know what it's designed for), that gave a gain of around 120. The pairs sounds great - perhaps as good as a matched pair of OC75. I will try some of the others, too, and report back.