I've been looking at buying some MP15As and GT308Gs, but i cant find anything about how they actually measure (like how the hFE measures and how leaky they are) or anything like that.
So Does anyone have any experience with these transistors?
If you can't find anything about they actually measue WHY you're looking at buy them..??
I have to echo antonis' comment. "Why" is a valid question.
As for "actual vs datasheet" information, I've found that whatever the published gains are, 90% of low leakage (0.1mA or better) transistors will sit in the lower 25% of the specified range spec'd in the datasheet.
If you haven't already read it, this is a good primer;
https://www.diystompboxes.com/smfforum/index.php?topic=117061.msg1086042#msg1086042 (https://www.diystompboxes.com/smfforum/index.php?topic=117061.msg1086042#msg1086042)
Quote from: antonis on June 25, 2022, 07:03:01 AM
If you can't find anything about they actually measue WHY you're looking at buy them..??
because the MP15A is a super cheap lower gain transistor (theres 20 for $7 on ebay rn), and the GT308G is one of the only higher gain russian germaniums that i can find for a reasonable price.
Sorry but I still have to ask WHY..??
Quote from: antonis on June 25, 2022, 03:43:12 PM
Sorry but I still have to ask WHY..??
'cause i want to have some germanium transistors of different gains on hand.
also, i want to build a catalinbread karma suture, which calls for a 1T308A, but theres only 1 lot of that on ebay, and the MP15A seems like it might be a good substitute while maybe being more usable for more projects (with the hFE being 50-100 instead of 25-75), and its also cheaper. so im just wondering if the MP15A is cheap for a reason (like actually being super leaky and super low gain), or if its actually worth using.
And for the GT308G, I'm just wondering if it's actually worth using for something like a tonebender.
Snow with germanium — what could go wrong? :icon_wink:
Quote from: pacealot on June 25, 2022, 04:09:12 PM
Snow with germanium — what could go wrong? :icon_wink:
i mean, ive built a rangemaster and that worked fine.
Quote from: snow123 on June 25, 2022, 04:07:17 PM
also, i want to build a catalinbread karma suture, which calls for a 1T308A, but theres only 1 lot of that on ebay, and the MP15A seems like it might be a good substitute while maybe being more usable for more projects (with the hFE being 50-100 instead of 25-75), and its also cheaper. so im just wondering if the MP15A is cheap for a reason (like actually being super leaky and super low gain), or if its actually worth using.
My good old fellow.. :icon_smile:
We did agree in the past that any circuit based on BJTs particular h
FE definitely is a BAD circuit, didn't we..??
Quote from: antonis on June 25, 2022, 04:22:06 PM
Quote from: snow123 on June 25, 2022, 04:07:17 PM
also, i want to build a catalinbread karma suture, which calls for a 1T308A, but theres only 1 lot of that on ebay, and the MP15A seems like it might be a good substitute while maybe being more usable for more projects (with the hFE being 50-100 instead of 25-75), and its also cheaper. so im just wondering if the MP15A is cheap for a reason (like actually being super leaky and super low gain), or if its actually worth using.
My good old fellow.. :icon_smile:
We did agree in the past that any circuit based on BJTs particular hFE definitely is a BAD circuit, didn't we..??
well, when I say "super low gain" I mean <50 hFE, which i don't think super usable in a lot of circuits.
Quote from: snow123 on June 25, 2022, 05:11:41 PM
well, when I say "super low gain" I mean <50 hFE, which i don't think super usable in a lot of circuits.
Circuits are designed by people..
People should be aware of MINIMUM h
FE of particular devices implemented in respective circuits..
e.g. I should be a bad designer if I use a single 2N2222 of 5mA Base current to drive a 2N3055 Emitter follower for 15A load current..
There was a Russian website with a lot of info, it's down now. If MP15A is showing 50 hfe as the lowest, that's what you will get. Most all will read on the low end of the spec sheet. If your device needs leakage to work , the Russian trannys have very low leakage and might not work.
Quote from: mozz on June 25, 2022, 07:54:40 PM
There was a Russian website with a lot of info, it's down now. If MP15A is showing 50 hfe as the lowest, that's what you will get. Most all will read on the low end of the spec sheet. If your device needs leakage to work , the Russian trannys have very low leakage and might not work.
well according to this post: https://www.diystompboxes.com/smfforum/index.php?topic=108054.0
the actual hfe can get a bit lower than the published hfe.
and i actually want low leakage, 'cause one of the pedals i want to build (catalinbread karma suture) calls for a 1T308A, which has pretty low gain and super low leakage, and im wondering if the MP15As leakage is comparable to the 1T308A, and if the MP15As actual hfe is a bit higher than the 1T308A.
Measured some MP15A. First number is gain, second number is leakage in ua. Room temp is 75f. I have no GT308G only the A version.
65/83
62/165
65/155
61/93
64/77
Actually kind of hot here today, and i did use my fingers to hold them while putting in the socket on the DCA-55. I do expect the leakages would actually be lower.
Quote from: mozz on June 26, 2022, 01:58:32 PM
Measured some MP15A. First number is gain, second number is leakage in ua. Room temp is 75f. I have no GT308G only the A version.
65/83
62/165
65/155
61/93
64/77
and how does it compare to the GT308A?
GT308A
35/0
37/0
37/10
27/0
24/0
Here is something else. MP15A
Transition Frequency (ft): 2 MHz
Collector Capacitance (Cc): 50 pF
GT308A
Transition Frequency (ft): 90 MHz
Collector Capacitance (Cc): 8 pF
GT308G
Transition Frequency (ft): 120 MHz
Collector Capacitance (Cc): 8 pF
You can see the GT are more of a RF transistor. higher freq and a lot lower capacitance. You may have to put a 47pf cap across C-B when used in a fuzz. Making good RF transistors with higher gain/less leakage was always a problem back then.
Quote from: mozz on June 26, 2022, 08:06:28 PM
MP15A
Collector Capacitance (Cc): 50 pF
@snow123: Don't consider the above mentioned capacitance as a constant value.. It can (and will) increase by more than an order of magnitude as the collector current is increased from a low value into the quasi-saturation regime at a fixed collector-emitter voltage of 1 V. :icon_wink:
(you can look for transition & diffusion capacitance and "base-widening"/Kirk effect, also..) :icon_wink:
https://www.diystompboxes.com/smfforum/index.php?topic=129306.0
here's a thread for you, snow.
Also, how do GT313V transistors actually measure? Asking 'cause the GT313 series as a whole has a super wide hFE range, and I'm wondering where within that range, they actually measure.
Quote from: mozz on June 26, 2022, 08:06:28 PM
GT308A
35/0
37/0
37/10
27/0
24/0
Here is something else. MP15A
Transition Frequency (ft): 2 MHz
Collector Capacitance (Cc): 50 pF
GT308A
Transition Frequency (ft): 90 MHz
Collector Capacitance (Cc): 8 pF
GT308G
Transition Frequency (ft): 120 MHz
Collector Capacitance (Cc): 8 pF
You can see the GT are more of a RF transistor. higher freq and a lot lower capacitance. You may have to put a 47pf cap across C-B when used in a fuzz. Making good RF transistors with higher gain/less leakage was always a problem back then.
the karma suture already has that 47pf cap across the C-B, and im assuming i would remove said capacitor when using the MP15A, correct?
Hello - given the low price, just buy some and give it a try on a breadboard. You might be overthinking this. :icon_wink:
Quote from: pacealot on June 25, 2022, 04:09:12 PM
Snow with germanium — what could go wrong? :icon_wink:
I suppose we should be getting worried by now.. :icon_cool:
Not come across the GT308G before.
I did try some 1T308B (the military spec version of the GT308B) in a fuzz before.
Capital B is "V" in Russian, so really should have called this "1T308V".
Note datasheet (link below) has info for "a","b","v" versions of the 308, with "v" (i.e. capital "B") version being the highest gain.
Leakage was very low, but hFE was not as high as I expected.
When I got hold of the datasheet I saw why.
Graph shows hFE close to 200 but that is for 60mA of collector current.
At the much lower currents of a typical fuzz design, hFE is way lower.
https://alltransistors.com/adv/pdfview.php?doc=gt308a-b-v_1t308a-b-v.pdf&dire=_russia
There were also lots of short bursts of "fizz" noise that would randomly flare up out of nowhere and then die away.
I never got to the bottom of that. I ended up using AC128s.
Quote from: snow123 on June 28, 2022, 07:23:11 PM
Also, how do GT313V transistors actually measure? Asking 'cause the GT313 series as a whole has a super wide hFE range, and I'm wondering where within that range, they actually measure.
@snow123: Jeanne d'Arc is considered the 1st person burned at the stake due to repeatedly quoting herself.. :icon_wink:
Hello,
I found someone on eBay selling the + 50pcs 1T308V ~ 2N2048 (Russian1Т308B) - Germanium vintage transistor USSR - https://www.ebay.com/itm/174668870744
I contacted him, and he said he has one lot left that is not listed at this time. Just email him and he will provide information.
Dmytro, is great to talk to and has a lot to offer on his ebay store. The price is right!!!
I've made a few pedals with GT308s.
The last one had two with hFE of around 60.
I say "around" because collector current, temperature, DC vs audio signal and more affect hFE, and people fail to compare on an equal basis. Mine were measured at Ic = 1 mA DC and at 18C. Around hFE = 70 at 28C?
In any case, they don't leak (significantly) and are very quiet.
In my opinion, better than NKTs, OCs, ACs and the like, that although they reportedly have voodoo magic, that presumably refers to just the good ones. Whether "good ones" are three in ten or two in fifty, I leave to others. But in this context, I'd say 8 out of 10 GT308s are good.