PNP Germanium FF issue

[Elijah]

Hi there. Here's the thing. I've built pnp fuzz face from tagboardeffects with a little mods that Josh from JHS was talking(I simply swapped 33K to 47trim then changed 8k2 to 3k3 and 10k pot in series, swapped 1k pot to 1k res). Transistors are GT402I which are according to the datasheet equivalent to ac128. The output is very low I can barely hear it, but accidentally I touched the Q2 and the 2log of 500k pot which is the output wire and it worked just fine. What could be the reason for that? Any input is wildly appreciated.

[dennism]

In the past, when I've had similar issues, it was because I had a bad solder joint connection.   I'd start by reflowing the solder connections around Q2 and the output wire to the volume pot.

[Bunkey]

I would highly suggest socketing the Ge transistors instead of soldering them as they're easily toasted! That may or may not be your problem already - Which part of Q2 did you short to the output?

[Elijah]

Thank you guys. Checked everything again. I didn't know that the enclosure of this transistor is directly connected to the collector. So I was shorting the collector of Q2 to the output wire. That left me with couple of components in between. The problem was a bad 500k pot. It always shows 1 on multimeter. Changed that and guess what? Everything worked

[Bunkey]

Well troubleshooted.. (...troubleshot...?  )

I have an NPN FF with 65/90 MP38A's that sounds terrible! (Work well as a Ge woolly mammoth though..)
Might have to try the GT402's myself.

Welcome to the forum btw.

[iainpunk]

welcome to the forum, nice that it works!

Well troubleshooted.. (...troubleshot...?  )

''trouble well shot'' i guess

cheers

[Elijah]

Well troubleshooted.. (...troubleshot...?  )

I have an NPN FF with 65/90 MP38A's that sounds terrible! (Work well as a Ge woolly mammoth though..)
Might have to try the GT402's myself.

Welcome to the forum btw.

Thank you! So as I red from the Russian forums and according to the datasheet you'd only want GT402I. That letter at the end is important. Cheers

[Elijah]

welcome to the forum, nice that it works!

Well troubleshooted.. (...troubleshot...?  )

''trouble well shot'' i guess

cheers

Thanks!

[Bunkey]

Thank you! So as I red from the Russian forums and according to the datasheet you'd only want GT402I. That letter at the end is important. Cheers

Thanks, I have a few GT402B's here already, 80-200hfe <100uA tested, but I haven't built anything with them yet.

The B's are rated 25v, the I's 40v; it looks like the I's typically have less gain. I can't comment on differences with noise etc. but both are toted to work well.

Check out the GT308v's too; they sound great and the few I have are all negligable leakage.


There's lots more info about these on the forum here from people with greater experience, always worth a search for ideas.

[Elijah]

Yep. By the way, I don't know about other lettered Gt402's but "I's" are absolutely noiseless! Like... 0buzz, hiss or whatever

[Bunkey]

Yeah the old mil-spec stuff is really top quality. Cheap right now but one of these days it won't be!

BTW, I said the 65/90 MP38A pairing sounded terrible; I was wrong, it just needed re-biasing.
Trim both collector resistors. Q1 Collector = 1.4v
I think Q2 is probably better set by ear depending on the characteristic of the transistors used.
Also make sure to remember which way round you fit your pots in the enclosure

This thing has just come alive now and I'm loving it!
Ta for motivating me to pick it up again.

[Elijah]

Such a clean build tho. Yeah Russian parts are cheap for now. I've just got about 300 D9's for about 7$.

[Bunkey]

I just got a whole bunch of those K42's and axial tantalums so it's cool seeing them on your board.

Hope it goes well for you man, cheers.

[WoundUp]

Yeah the old mil-spec stuff is really top quality. Cheap right now but one of these days it won't be!

BTW, I said the 65/90 MP38A pairing sounded terrible; I was wrong, it just needed re-biasing.
Trim both collector resistors. Q1 Collector = 1.4v
I think Q2 is probably better set by ear depending on the characteristic of the transistors used.
Also make sure to remember which way round you fit your pots in the enclosure

This thing has just come alive now and I'm loving it!
Ta for motivating me to pick it up again.

Nice enclosure! I've never even considered using something like that but I will in the future! Thanks!

[ThermionicScott]

Yeah the old mil-spec stuff is really top quality. Cheap right now but one of these days it won't be!

BTW, I said the 65/90 MP38A pairing sounded terrible; I was wrong, it just needed re-biasing.
Trim both collector resistors. Q1 Collector = 1.4v
I think Q2 is probably better set by ear depending on the characteristic of the transistors used.
Also make sure to remember which way round you fit your pots in the enclosure

This thing has just come alive now and I'm loving it!
Ta for motivating me to pick it up again.

Nice enclosure! I've never even considered using something like that but I will in the future! Thanks!

One of my other hobbies leaves me with round tins about 4" in diameter... hmm. 

[Bunkey]

I was impressed with some of Eric's (EBK) cigar box amplifiers in the Pictures sub recently. Would make a great combo!

Thanks!  I had a lot of fun working with cigar boxes.  The Cuban box is my favorite one of the bunch.  Here is the updated line up.

My skills have definitely improved over time.  I need better tools though.  I'm still doing most of it with a hobby knife.