This has me scratching my head??? NKT275's

[captntasty]

So I just got my hands on a few NKT275's and they all check out ok -- not too leaky, gain ranges between roughly 85-95.  They were tested so it felt good to confirm they actually were good specimens.  So I pop em into a FF and can't get the voltage more than .75V on Q2 C.  Hmmm... that's a bummer.  So I pull em and test em again - sure enough they're good.  So I start experimenting - pop a known good tranny into Q1 and measure - voila!  I have to dime the the two trimmers I have in place of the 100k resistor (250k) and the 8k2 (25k) but it comes up to 4.1 v.  Whenever I put an NKT in Q1 it's back to .75v or thereabouts.  Any ideas?  I'm stymied....

Here are the measurements with 2 NKT's Q1hfe=85 Q2hfe=95 Battery=8.79v with trims at max 

Q1 NKT275                 
C = 8.37
B = 8.40
E = 8.49                   

Q2 NKT275
C=  .63
B= 8.36
E= 8.38

Here are the measurements with 1 2N1305 and 1 NKT Q1hfe=85 Q2hfe=95 Battery=8.79v with trims at max 

Q1 2N1305
C = 7.63
B = 8.39
E = 8.48

Q2 NKT275
C= 4.07
B= 6.16
E= 7.70

Do either of those sets look right?

                       

[tiges_ tendres]

i think you might have a short somewhere.  the voltage shouldnt be the same at the collector, base and emitter of the same transistor.  check for solder bridges.

[captntasty]

I checked for bridges and everything is good.  I forgot to mention it is a negative ground version.  I've seen values for v measurements from the trannys that are very low voltage for the most part - .5v etc.  I always attributed the higher voltages in my pedal to the neg grounding - an inverse relationship to source voltage.  Also has one of the Fuller mods - the 1k pot in series with the 470ohm resistor.  I did use a 10k pot as it gave me a swing of about 1v either way on Q2 collector.  Anybody?  I just don't understand why two good trannys of differing types would give such different results...  why do the nkt's work in Q2 but not Q1?

[billings]

I doubt this is much help at all, but I'll throw it out there anyway...  on some of the Ge transistors I have, the metal can conducts with one of the leads.  So it's possible that you might be getting a short from that.

[petemoore]

Q1 NKT275                  
C = 8.37 high
B = 8.40  "  "
E = 8.49  "    "                  

Q2 NKT275
C=  .63
B= 8.36
E= 8.38
 These readings look like the Q1C is connected properly to Q2B, I think the battery dropped .01v during testing...
Here are the measurements with 1 2N1305 and 1 NKT Q1hfe=85 Q2hfe=95 Battery=8.79v with trims at max  

Q1 2N1305
C = 7.63 high...**this one should be connected to/= to Q2B Voltage
B = 8.39 high
E = 8.48 high no swing room with these proportions

Q2 NKT275
C= 4.07  well, could be ok but
*B= 6.16  is higher than the collector
E= 7.70  is higher than base
 These readings show Q1C/Q2B connection as not made...

[captntasty]

Thanks Pete.  This definitely gives me something to run with...

[captntasty]

Well, I did get it to work with the NKT's although it still has some issues.  First, decided to change it to pos. ground - poof! no more oscillation.  The only way I could get the voltages close to the magic 4.5v was to increase R3 & R5.  By putting in a 500k trimmer in place of R3 (normally 100K) and a 100K trimmer in place of R5 (normally 8k2).  With the trimmers set to max I can get a voltage of 5.6V on Q2c but that's as low as it goes.  What is the effect on "the rest of the circuit" by increasing these values?  I guess the pedal sounds okay but does gate toward the end of the decay - also crackles a bit which is really noticeable by picking very lightly, just another component of the gating I guess.  Doesn't seem to have the dramatic effect when dialing back the tone knobs on the guitar I've noticed on other FF's but does clean up when the guitar volume is dialed back.  I guess I'm still scratching my head

Here's the measurements: Battery 9.93v (probably one reason I can't get any closer to 4.5v)

Q1 NKT275 hfe=85                 
C = .23
B = .09
E = .00               

Q2 NKT275 hfe=95
C=  5.58 (noticed it is drifting slowly upward, that's not good news, huh?)
B= .18
E= .14

 

[captntasty]

This just gets weirder...  measured everything after noticing that the Q1c and Q2b weren't =.  They are in fact =...

Q1 NKT275 hfe=85                 
C = .15
B = .09
E = .00               

Q2 NKT275 hfe=95
C=  this is highly variable - as it cools it drifts up to about 5.7v.  Might make you cringe but a very brief pass with a lighter brings the voltage down to around 4v then it creeps up as it cools.  I know Ge is heat reactive but never thought in terms of cooler temps increasing the voltage readings.
B= .15
E= .12

Anyhow I think I'll put this thread out to pasture.

[Stephen]

No matter what you try these are positive + ground transistors PERIOD!!!  Dont mean to be blunt...

All of the caps need to be relative to a positive ground.. AGAIN...transistor are (pnp) transistors..

Any readings otherwise are bogus

If you rearrange the caps right and you cant bias the collector of the second trans unless the resistor is real high I think it means it is very gainy.. Like way to much gain OVER...120....probably leaky... Check that very carefully.....

I dont think you damaged the transistors having them negative grounded but maybe???

[captntasty]

That's the weird part - I've tested and retested and retested.......  they measure consistently as far as gain goes (85 and 95) and they are also stable - once they settle in the measurements remain constant.  I know the problem is in the trannys as I tested them in a boutique FF circuit and got the same problem.  Is it possible the trannys measure good for leakage, gain and stability but are in fact no good?

I tried the other 2 NKT's I have (hfe 80 and 90) and they work much better.      I can bias to wherever I like 3.5v - 5.8v and the other measurements seem to be within the aceptable range.  One of the other trannys must test good but be a dud.  Perhaps the neg grounding did fry them somewhat?

The negative to positive ground change I did after looking for any info I could about the circuit and noticing that the way to eliminate oscillation was to go positive - using the negative was just me being stubborn and wanting to use an adaptor.  I didn't really think it was going to fix the tranny problem.  Perhaps it caused it?!

Thanks so much to you guys for responding. 

[Stephen]

I never messed with the NKT275...what did you pay for them ....if you mind me asking?

what do they sound like there is so much MOJO about them ...

I have some Ac128 sound good to me....
I have used NTE 102A if you can check them...at the store ohm the base to coll base to emitter and see if it is 2-3k than they are cool to buy otherwise they are low gain!!

[captntasty]

I paid 4.75 gbp = about $9.  It was a total splurge - I figured that at least once in my life I had to check out the mythical NKT275 for myself.  I questioned the seller as to whether they were tested and they said they were.  I don't have any reason to doubt them because they all tested good for gain and leakage.  None were above 100hfe which was a bummer.  The two I used in the -gnd FF didn't hold up in this resistance test - I assume they were a bit fried in that setup as the other two test good using that method     Thanks for that, by the way. I can see that method coming in very handy.

The NKT's sound smooth compared to some AC128's I had (which don't sound bad either, just different).  The AC128's were a bit edgier, a little more bite if that's the correct terminology.

When all is said and done, they sound good (but I wish I had a higher gain tranny for Q2) but probably not worth the price and the frustration (unless you're an anal perfectionist/purist) but in the process I learned more than I knew before.  There are plenty of decent Ge trannys out there for less dinero.  I guess my philosophy is if you don't go down a road, how will you ever know what's there to discover?

 

[Stephen]

I always look at my purchases of germanium trans as half will be Okay......But buyer beware they say!!

[analogmike]

those skinny transistors the German guy sells in batches of five on ebay are not NKT transistors. I dont know what they are, maybe rebranded AC128? NKTs are not so thin and have no "hat rim"

[captntasty]

Thanks analogman...  That's what they look like.  When I got them I sort of suspected something funny.  But as they say "Denial ain't just a river in Egypt"!  I'd never seen NKT's in the tophat style case.  I figured I'd throw caution to the wind once.  Live and learn.  The supplier I got them from is Vintageparts.com out of UK (or so I was led to believe).  I'm a bit embarrased to be snookered but knew I was taking a risk - thankfully a fairly benign one.  Anyhow, thanks for the confirmation - if anybody knows a thing or two about NKT's it would be you!  Should I post a warning about this supplier?  I also got a few AC128's (which appear authentic) - most of 'em dead but a couple with low gains - 40 & 60.  I suppose the bunch was actually planted very carefully.

[analogmike]

vintageparts.com has some really cool old transistors on his website.

I bet the 2SB175 are amazing. The only way to try is to pay your money and take your chances.

Have fun!

[Stephen]

I was to the understanding 2sb176 would have a better gain to it.. I have some coming from a supplier called Centerpointe in CA.  They have other germ trans too....Look them up..
They are a store so I dont think they nick pick their trans.

  Use the Search Inventory button!!   You can find AC188 etc....kind of expensive


www.cpcares.com/transistors/2SB176.html

[mac]

Recently I posted about the ACs and NKTs. I gave up on them, mainly because I found other Ge that do the same job, are more relaible, available and cost a lot less.

2sb176 (r) I have, have more gain than the 2sb172 (a) and 2sb175 (b), but they are classified in several hfe ranges, so a 175 can have more gain than a 176 depending on the class. Most 172 (a) and 175 (b) on my box fit on a FF, TBMII and RM.

Of about 50 of this, and other 50 different Matsushitas, most of them have less than 200uA, and I have to trash only one!!!

Still I prefer the 2sa102 (ca) or (da) from Matsushita.

2sb54, 2sb56, 2sa49 and 2sa52 from Toshiba are killer Ge with less than 100uA. They sound brighter than the Matsushitas.

http://www.diystompboxes.com/smfforum/index.php?topic=46060.0


mac

[GibsonGM]

I used a 2sb54 & 2sb56 in my FF, scavenged from an authentic '60's clock radio....they sound awesome!  I did also have some biasing problems, just used an internal trimmer on Q2 and a pot for Q1 that I can adjust as thermal drift sets in.  No problems so far, it's been 6 mos!

[Fret Wire]

2sb54, 2sb56, 2sa49 and 2sa52 from Toshiba are killer Ge with less than 100uA. They sound brighter than the Matsushitas.

I had up to 500 TI's (182505's) that ran from 20 ua to 100 ua. Nice score. I've been saying this awhile, and recently You've been saying the same thing: Guys, don't get caught up in mojo Ge's. If you want an excellent Fuzz, use stable, low leakage Ge's of proportional gain, and bias them right. That's it. When I say proportion gains, I mean Q1 & Q2, not that they have to be 70 - 130hfe. I've gone quite a bit over, and it's not gainy or piercing, and I've gone alot under 70-120, and it's just as fat and ballsy. Again, if it's biased right.

They all might have sounded great 40yrs ago when they were made, but some don't stand the test of time. Try to pick your brands based on their track record of reliability, stability, and low leakage, not reported "mojo sound". Everybody's ears are different. The frequency response difference of Ge's is very small between brands.

Spend more time concentrating on stability, leakage, and most of all, biasing.