Lil Gem/Ruby amp

[rimo]

Hi,

I've build the Lilttle Gem Amp from ROG, it's very cool. I've mounted it in an old audio speakerbox driving an 8 ohm speaker. It is also very loud (no problem waking up the kids at night). I don't have the 25 ohm rheostat to regulate the output. I can't get one either here in Holland (neither 50, 100 or 500 ohm).

My question is: Can I use a 1K pot with a 25 Ohm resistor in series over lug 1 and 2 of the pot to mimic the 25 ohm rheostat ? And if this can would I need a linear, log or reverse log 1K pot ?

Second question: I'm thinking of also building the Ruby amp in the same speakerbox (and same output speaker) with a switch to select either powering lil'gem or ruby. So bassically using a On-off-on switch (lil'Gem-off-Ruby). Would this be possible.

Thanks in advance,

Rimo

[B Tremblay]

I'm not sure if a 1k and 25 ohm resistor set up as you describe will function correctly for the Master Volume.  It may be possible to place the 1k pot, wired as a variable resistor, in series with the positive speaker lead.  Peter Snowberg has described this approach using a 500 ohm pot, but a 1k may work as well.  At the same time, you have better results by simply adding the input buffer and volume control found in the Ruby.

The Little Gem and Ruby are so similar, it seems (to me) like a waste of components and time to build both.  If you want to switch between the two, just use a DPDT switch for bypassing the buffer.  It's your choice whther the volume control gets bypassed as well.

I personally prefer Ruby over the Little Gem.

[rimo]

Thanks,

Maybe I'll just build the Ruby with your suggestion of a switchable input buffer. (If the sound isn't really different I can always remove the switch again   )

I don't have a MPF102, but I do have a BF245A. Does anybody know the pinout of this FET ?

I really like this small Amp. It allows me to practice without waking up my children. I don't like to practice with earphones very much when I'm alone in the house and the baby is sleeping.

Rimo

[KORGULL]

Hello,
The BF245 pinout (and some others) is here://www.muzique.com/lab/trans.gif
Good luck w/your project.

[siaoguitar]

Hi B Tremblay & anyone, built the ruby amp, don't know why there is no sound coming from the speaker(8 ohm, 1W).
trying the audio probe(rd. keen debugging page) to check where the problem is(there is sound at the input to the 2n3819 tranny that i'm using but not at the out to speaker) to , also having a DMM to check volts around.
why is it like that?
there is just no sound coming from the speaker, (i'm using a stereo jack as my input jack to turn on the amp.

1) did i use the correct trans?(i switched 2 pins around to match the circuit,coz it's different from mpf102)

Any help?

[rimo]

Thanks KORGULL,

I actually build the Ruby the otherday, but it did'nt work. I checked the pinout of the BF245 at some (apparently) dodgy webpage. I don''t remember the webadress but they gave the exact opposite pinout as shown on www.muzique.com/lab/trans.gif  :x  .
So probably (or most certainly) I have the FET the wrong way around. When I find some time I'll try it again in the correct orientation.

Only goes to show that more than half of the information on internet can't be trusted  :wink: .

Tanks again everyone for great advice, I'll let you all know if (no, when) I've succeeded.

Rimo

[B Tremblay]

siaoguitar,

Re-check the pinout of your FET and audio probe lugs 3 and 3 of the Volume pot.  If there is no sound at the Volume pot, then the FET is improperly oriented.

If there is sound at the Volume pot, check the orientation of the 386 and the wiring around it.