ruby amp

ubertar · October 29, 2004, 07:43:58 PM

Hi, I just made the ruby amp from www.runoffgroove.com. It has a great, dirty sound. The only problem is the volume control doesn't do anything. I've swapped out the pot to see if that was the problem, but it doesn't help. The gain control seems to work fine, but not the volume. I checked the schematic and apparently it's wired correctly. Any ideas of what could be wrong?

Paul Marossy · October 29, 2004, 11:39:22 PM

Have you checked out the pot? Maybe it's faulty...

ubertar · October 29, 2004, 11:51:17 PM

I tried a different pot, but there was the same problem.

ubertar · October 30, 2004, 09:59:33 PM

I rechecked everything again, and there's nothing wrong with the wiring.

When I turn the volume knob, there's no change in volume, but when the gain knob is all the way down, there's almost no volume. All the way up and it's loud and distorted. Is it possible there's something wrong with the jfet?

B Tremblay · October 31, 2004, 07:56:13 AM

How did you build the circuit? On perf or with a PCB?

How are you powering the circuit? If it's a battery, make sure it is new.

What FET did you use? Are you certain of the pinout?

If the Volume pot does nothing, then there has to be something wrong in the wiring surrounding it.

Turning the Gain down will not reduce the output to near-silence unless the Volume pot is already set quite low.

You'll need to triple-check all the wiring and component orientation.

If you have a breadboard (and you should get one if you don't), build the circuit on it and see if the same situation occurs.

ubertar · October 31, 2004, 11:04:14 AM

It's on perf.
The battery is fresh.
MPF 102, just like in the schematic. Maybe it's backwards? I followed the picture... is what it's doing what you'd expect from a backwards JFET? I'll see if I can reverse it and see what happens.

this is the schematic: http://www.runoffgroove.com/ruby.html

Thanks for the help.

B Tremblay · October 31, 2004, 11:20:38 AM

I believe that a backwards FET would produce either no sound or oscillation, rather than render the Volume pot ineffective.

ubertar · October 31, 2004, 01:12:00 PM

I checked the FET orientation with the specs on the package (from a different brand, but still a mpf 102) and it's installed correctly. I triple and quadruple checked all the connections, and can't find anything wrong. Could it be a bum component? Any suggestions for how to test to find the culprit?

B Tremblay · October 31, 2004, 02:46:02 PM

Are you certain that lug 1 of the Volume pot is connected with all the other ground points?

Also, try this:

Remove the Volume control entirely. Connect the buffer output cap directly to pin 2 of the 386. Set the Gain at minimum and test the circuit through a speaker.

If the circuit functions normally, then there is either a wiring issue with the pot or the pot is faulty.

ubertar · October 31, 2004, 03:06:20 PM

Thanks, I'll try that.
I just tried swapping a 1 meg audio pot for the 10k volume pot. It works, but the volume still drops so much when the gain is turned down, I wonder if the amp is really supposed to be that quiet. The volume control seems kind of pointless if you're always going to want it on "10" (11 if you're Spinal Tap).

B Tremblay · October 31, 2004, 03:23:54 PM

I assure you, the Volume control is not pointless. The output volume (and overall distortion) should increase when using a pot larger than 10k. Since you are experiencing the opposite, it indicates that your Volume wiring is in error.

If you cannot determine the error(s), then I suggest that you make a fresh start and build the circuit again.

ubertar · October 31, 2004, 05:20:39 PM

I'm not sure I understand you when you say the output volume should increase with a larger pot. The larger pot has more resistance-- the top volume should be the same as with a smaller pot, but as you turn the pot down, you get more resistance and less volume. That's what I'm getting. Apparently 10k wasn't enough resistance to significantly affect the volume. I'm using 200k now and it's fine.

I think the problem was that I was using a 10k pot for the gain because I didn't have a 1k pot, so it gets turned down too quickly and too far. Also, I think it's just not as loud of an amp as I expected.

Thanks for the help. I appreciate it.

B Tremblay · October 31, 2004, 06:40:00 PM

The output volume will increase with a larger pot because there is more resistance between the signal and ground. The 10k value was selected to bleed off some of the signal to prevent overdriving the input of the 386.

As you have already noted, a 10k Gain pot will not provide satisfying results.

In the future, you may want to mention any component substitutions when posting about having trouble with a circuit.

siaoguitar · October 31, 2004, 07:48:43 PM

hI TREMBLEY, UNFORTUNATELY, i still couldn't get my ruby amp to work

, i noticed a little difference between yor wiring and that by grinko(pdf) his wiring has got wiring to all three terminals of the volume pot (R3), while yours don't (it only show wiring to volume lug 2 & 3) is that why mine didn't work?
Sorry if it sounds stupid to you, but i'm really perplexed as to why my ruby don't work, i'm really looking forward to make it work, can you help me?

ubertar · October 31, 2004, 08:06:47 PM

Quote from: B TremblayIn the future, you may want to mention any component substitutions when posting about having trouble with a circuit.

Sorry. I didn't think it was significant. I was wrong.

B Tremblay · October 31, 2004, 08:12:38 PM

True, the perf layout doesn't explicitly show that lug 1 of the Volume pot is connected to ground, but the schematic does indicate that. On our perfboard layouts, all the necessary ground connections are usually not shown. Lug 1 of Ruby's Volume pot is a perfect example of this.

Our perf layouts are simply a guide for physically arranging the circuit when building on perfboard. They are not an alternative to the schematic, but rather a companion.

ubertar · October 31, 2004, 08:15:51 PM

Maybe something else is wrong with mine, then, because if I connect lug 1 of the volume pot to ground, it shorts out the circuit and I get nothing. It seems to be fine otherwise.

I was also using the perfboard layout diagram as my main reference, though I was using the schematic also.

Paul Marossy · October 31, 2004, 08:18:56 PM

To me, it sounds like something was wrong with the FET section of the circuit - maybe an FET all around with poor qualities - I have had a few of those before. Replacing the volume pot with higher values should only serve to make it louder to some degree.

ubertar · October 31, 2004, 08:20:56 PM

So replacing the FET might do the trick?

Paul Marossy · October 31, 2004, 08:35:55 PM

QuoteSo replacing the FET might do the trick?

Possibly. I built the BSIAB 1, which uses five J201s. No matter what I did, the gain control didn't work properly. I put it away for probably a year, and one day decided to revisit it. I removed all of the JFETs and put sockets in their place instead. Put all new FETs in the sockets amd it worked beautifully.

My hunch is that perhaps part of your problem with the circuit is due to the high variability of FETs. Might be worth a try to pop a different one in there and see if there is an improvement.

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