i bought a bag of ac125 ge transistors to use and i was checking the gain/leakage today when my radioshack dmm stopped giving me readings on mA... :P guess i have to go buy a new one...anyways, in the mean time i checked all of 3 (out of 25) using the trasistor checking method from rg keen. i let each one sit for a minute or two before checking it. my battery was at 9.5v and my resistors were around 2.4k and 2.2M...
# - hfe - leakage
1 24 41.2
2 15.5 19.5
3 24.9 48.3
any idea why these are all so low? i checked a specsheet which said these should be at minimum 50 hfe... the only problem i see is (and i don't know 100% how to read a specsheet...) that the Vce V Max (which i assume is the voltage) is 12 compared with an ac128, which is 16. the @Ic mA is 2 compared to 50 (ac128). the Cob pf Max is 50 compared to 100 (ac128). i tried finding some information as to how to read these terms, but i was having a bit of trouble.
any thoughts? help on those terms?
Germaniums are much lower gain in general than today's silicon.
There's no good way to tell from that info whether you may have tested incorrectly or whether you just got low gain devices.
This is one reason buying untested germanium is such a crap shoot. The pros spend a lot of time ferreting out leftover caches of germanium and testing them. When the good ones are tested out of the batch, there are usually about 2/3 leftovers. These can be thrown away and be a complete loss, or they can be sold for "leftovers after testing" which is about the same thing. But they can also be sold as just germanium devices. I know for a fact that there are some lots from surplus places and individuals on ebay which are 100% bad, and pre-tested to be so. I was the recipient of some of these.
So be careful with your buying and testing.
Then there's the other thing - just because the datasheet says that they all had a gain of over 50 doesn't mean that's what you'll get.
Trust your meter, not the numbers printed on the outside of the can.
yeah i was aware of the risk, but there's not many resources available so i figured it was worth a shot. they weren't that pricey...
i thought i might have tested wrong at first, but double and triple checked my connections... tried them in a ff circuit. pretty low output but sounded decent...i'll have to tweak the values a bit more. or get some new transistors!
any help with those terms, though?
Vce V Max - i'm guessing collector/emitter max voltage before the transistor gets damaged...?
@Ic mA - ?
Cob pf Max - ?
oh yeah i was wondering as well...has anyone added a tone control to a ff? i was considering just adding the muff tonecontrol but it might pull back the output somewhat...maybe add another transistor stage after? or is this pretty much the same as the 3 knob tonebender??
QuoteVce V Max - i'm guessing collector/emitter max voltage before the transistor gets damaged...?
Vce V Max is the maximum working voltage, as you guessed. You might have to restrict the voltage even lower if you pull a lot of current and the V*I product exceeds the power maximum.
Quote@Ic mA - ?
Means "At a collector corrent of this many milliamperes", and is probably there to specify the minimum gain at the given condition.
QuoteCob pf Max - ?
"Capacitance from output to base in picofarads; all devices guaranteed to be less than this number:" Only really useful for computing the high frequency response of the device.
thanks rg!
I bought lots of 2n2614s from Summit electronics in Fla.
very very few had to be thrown out due to leakage, and 90% had gain over 110
I never looked up the datasheet on the 2n2614 (Steve Daniels said the orig ampeg scrambler had that part# IIRC)
but I suspect that the part was a high quality hi gain part.
seems that summit might be one of the sellers who stocks large quantities and may be outside of the reseller/unscrupulous loop
they have a pretty high minimum order and are pretty no-nonsense on the phone. don't call em to chit chat about the pedal you are buildin if you knowwhatta mean
once again, the small bear option seems like a good one if you are just building a couple ge based peds
Get some SB Tranny's, even comparing whatever to Si's, once you get used to gains [Si's gain can be reliably measured with a DMM that does so because they don't leak much], this technique of 'interviewing tranny's...if it sounds good it's good, low noise and good gains, otherwise...crap.
But if they're low leakage you might try darlingtonizing them, [See Bazz Fuss article for darlington diagram], this will make them suitable for higher gain positions, leakage/noise gets multiplied as gain does.
For further tweeking, take a look at the Rangepig, it's a darlingon with a gain reduction method, two transistors of enough gain provide yet another gain control, it's just a pot between the first collector/second base of a darlington pair...I used a 6 or 8 pin IC socket for each darlington pair.
The Rangepig 'method' can be used in a FF, using gainset darlington for Q2 [and or Q1], I'd consider an Easy Face, the Ge/Si hybrids sound great and hold bias nice.
i thought of making them into darlington pairs. i'll give that a go tonight and see how it turns out. i'm not too concerned about the bias becuase i'm going to use an external pot so i can just tweak it on the fly...
Quoteoh yeah i was wondering as well...has anyone added a tone control to a ff? i was considering just adding the muff tonecontrol but it might pull back the output somewhat...maybe add another transistor stage after?
I've done this and I wasn't impressed with the results. It made the gain and volume controls pretty much useless since they had to be pretty much dimed to get unity gain. If you normally run your FF with the gain all the way up anyway it won't be as much of a problem but then you might as well just get rid of that pot.
I tried adding a gain recovery stage - I just tacked an LPB circuit after the tone control - which worked reasonably well but now you're two stages away from a pure FF. Not necessarily a bad thing but you should know what you're getting into.
i'm not really concerned with a pure ff really...i love hendrix, but i play keyboards and don't really want to try and emulate his sound or playing style. definetly not his technique! :icon_eek: anyways, i used a darlington pair (thanks pete!) with a ge>100k pot>si for both Q1 and Q2. the pot in between the 2nd pair did not make any changes worth mentioning so i just omitted it. i'm also just going to forget about the tonestack...the modifications i've added change the tone enough.
i'm leaving the other 100k pot external, along with the 10k bias and a voltage starve pot (really useful in the ff build!).
i also found an interesting modification which i'll post in a schematic. in it's most *basic form* it's a cap/pot from the emitter of Q1 to the collector of Q2 (or in my case, Q3/Q4). very interesting sounds like different octaves down depending on cap sizes, octave up, fizziness, weird sucking motorboating...definetly not a pure ff! :D
i'm not really concerned with a pure ff really...i love hendrix, but i play keyboards and don't really want to try and emulate his sound or playing style. definetly not his technique!
The guys I love, I think went after their own tone.
anyways, i used a darlington pair (thanks pete!) with a ge>100k pot>si for both Q1 and Q2. the pot in between the 2nd pair did not make any changes worth mentioning so i just omitted it. i'm also just going to forget about the tonestack...the modifications i've added change the tone enough.
Glad you could use it !
i'm leaving the other 100k pot external, along with the 10k bias and a voltage starve pot (really useful in the ff build!).
Lotsa tweeks on a FF, bias and gain are two good ones to have, it's hard to keep from adding more pots to it.
One pot I found useful as 'tone' knob is just a cap/resistor from volume control input to ground, to shave the last bit of highs off, that 'grow' when the FF is boosted, I often use the FF last-ish, that's where I like this kind of TC.
Another 'tone control' I use w/FF, and others of course, is a treble bleed cap bigger one on my neck pickup volume control, dropping the volume back makes a larger tonal than volume difference [turns trebly] than without when turning guitar volume down, with the right value cap/pot it makes a nice tone mixer.
i also found an interesting modification which i'll post in a schematic. in it's most *basic form* it's a cap/pot from the emitter of Q1 to the collector of Q2 (or in my case, Q3/Q4). very interesting sounds like different octaves down depending on cap sizes, octave up, fizziness, weird sucking motorboating...definetly not a pure ff!
Interesting, a form of + feedback I think !.
don't forget that you can still use "bad" ge's as diodes.
well i tried out the fuzz again this morning with mixed results. i was using a dual oscillator as my test signal last night and tested with a yamaha vss30 sampling keyboard and an old lotus guitar this morning. i am getting pretty intense motorboating (not good) when i turn the fuzz up and the transistor sounds seriously misbiased. i tried a handful of different transistors with no discernable change in sound...i have a feeling it's because of the darlington pairs amping up the gain in such an extreme way. i'm going to try changing the 100k resistor to a 100k trim and also try a 100uf cap across the power supply to see if either of those help out. might try tweaking the 33k as well...hmmmmm.
It might be bad transistors -- not wrong gain or too much leakage but just unmusical and tin sounding. I think I got a batch of AF125 or something like that
My experience: if it doesn't work, don't try to bias at max gain.
You can always also try the noise fighting hint on runoffgroove even though it's in the silicion fuzz article
"To block RF and other noise, Gus Smalley suggests a 10k resistor in series with the input and incorporating a power supply bypass cap.
A schematic of the Lovetone Big Cheese shows a 47pF cap between the collectors of Q1 and Q2, which supresses oscillation produced by high-gain transistors when Fuzz is at maximum."
http://www.runoffgroove.com/sili-face.html
Quote from: caress on February 22, 2007, 12:49:04 PM
i bought a bag of ac125 ge transistors to use and i was checking the gain/leakage today when my radioshack dmm stopped giving me readings on mA... :P guess i have to go buy a new one...anyways, in the mean time i checked all of 3 (out of 25) using the trasistor checking method from rg keen. i let each one sit for a minute or two before checking it. my battery was at 9.5v and my resistors were around 2.4k and 2.2M...
# - hfe - leakage
1 24 41.2
2 15.5 19.5
3 24.9 48.3
any idea why these are all so low? i checked a specsheet which said these should be at minimum 50 hfe... the only problem i see is (and i don't know 100% how to read a specsheet...) that the Vce V Max (which i assume is the voltage) is 12 compared with an ac128, which is 16. the @Ic mA is 2 compared to 50 (ac128). the Cob pf Max is 50 compared to 100 (ac128). i tried finding some information as to how to read these terms, but i was having a bit of trouble.
Remember that hFE is a function of ic, hFE = hFE(ic).
I've posted some ago about some texas instr 2n388 that are lower gain than expected. But they are lower gain at 4uA base current, which is the current rg method uses. I've also posted the math behind rg method to re-size it, ie, to test ge under different base current. When I used 10uA the gain of the 2n388 jumped to 1.7X or more.
http://www.diystompboxes.com/smfforum/index.php?topic=45481.0
mac
you know, i took out my little mofidifcation and the bad motorboating and hf oscillation went away. it's still pretty noisy, but i think that's going to be expected when amplifying ge transistors that much.
i changed 100k to a 47k and it improved the tone a bit. also a .047uf from C of Q1/Q2 to batt - acts like a LPF without taking away too much gain. it almost makes it sound like a big muff with the tone knob turned fully counterclockwise...i might add a switch for this.
still some issues with this setup, though. i can't seem to get Q2 (or in this case Q3/Q4) down to 4.5v or even close...the 10k pot in place of the 8.2k helps to bring it down, but nowhere near 4.5v. i even added a starve knob which brings the whole circuit down to around 7v and even with that, the bias pot still only gets the V down to 5.5 or so at the collector. i'll mess with it a bit more. i think this is useful for anyone who has a batch of low low gain ge transistors. funny, this thread more or less turned into another FF discussion... :)
well i think i have it to where it sounds pretty amazing. i changed the 10k bias pot to a 100k and it works really well. just got to box it up and i'll post a pic and a schematic with the modifications!