ROG Tube Reamer: Oscillations when I swapped the Volume pot.

[Yeahno]

Hi there.

Just built my second project, the ROG Tube Reamer, the output of which is simply insanely loud.  I mean, peel the chrome off my toaster, loud.  Chip is the NE5532, but I'm going to try a few Burr-Brown OPA series for fun.

I couldn't take the Volume past about 10 o'clock because I was afraid I'd cook the input on my little amp.  At lower volume, the Gain knob is very effective at providing everything from a clean signal, to a slight growl, to a full, tube-cooking sound, so I'm happy with the way it sounds.

So after reading about possible solutions, I decided to swap out the 100k-A volume pot for a 250k.  It's still pretty loud, but manageable.  However, I now get squealing when I have the Gain at max and the Volume near max.  With the Volume up, I can actually make it squeal by moving the tone knob on my Tele.  I assume the squealing is caused by oscillations in the circuit.

Questions:

1. Is the volume ROG TR normally this hot with a stock build?

2. By swapping the pot out, have I simply moved the problem?

3. Because I changed the resistance of the Volume pot, do I need to alter the value of the 100n cap before it?

I've got enough background to be able to read the schematics, but I'm clearly no expert.  All suggestions welcome.

Many thanks!

[DavenPaget]

OT : I had a pedal so powerful it was doing 1.5Vrms ... and it didn't cook my inputs , absolutely not 

[Yeahno]

Anyone else swap the pot and run into this?  Can anyone else confirm that this thing is *loud*?

[gregwbush]

Today i built the Tube Reamer...

Apart from using different diodes (1N4148 instead of 1N914) it's completely stock. Built it on perfboard as shown at RoG http://runoffgroove.com/tubereamer.html

I can confirm that it has a lot of gain in the last stage, as you describe. Quite loud. Very loud, even.

I once built a circuit that squealed in varying pitch depending on my guitar's volume knob. Had a kind of wah effect controlled by the volume knob. Weird! (and useless)

What i'll do now, is try a 250k POT and see what happens...

[gregwbush]

Ok...

I replaced the A100k with an A250k and there is no oscillation issue... However... i've just realized i'm not actually recreating what you've done. It's not stock, i changed the 2k7 resistor to 10k to reduce gain/distortion in the first stage. Therefor when i crank the gain control, i'm not reaching that point that you are where alleged oscillation occurs. Sorry.

Out of curiosity, do you like this circuit? I don't... I shoulda built a proper TS

cheers,
gregwbush

[Yeahno]

I replaced the A100k with an A250k and there is no oscillation issue... However... i've just realized i'm not actually recreating what you've done. It's not stock, i changed the 2k7 resistor to 10k to reduce gain/distortion in the first stage. Therefor when i crank the gain control, i'm not reaching that point that you are where alleged oscillation occurs. Sorry.

Hmm.  Sounds like you think it's missing something. But by reducing the distortion in the first stage, aren't you removing the very thing it's designed to do?

For what it's worth, I put in a chip socket because I have a few different dual op-amps floating around from a headphone project.  It's amazing how the sound of the thing changes when you substitute a different chip.  I've got a pile of Burr-Brown OPA2XXX chips of varying flavours and the sound really cleans up, becoming almost brittle with some of them.  For the moment, until I get the diode switch installed, I'm sticking with a NE5532 I had on hand.

Out of curiosity, do you like this circuit? I don't... I shoulda built a proper TS

I do, actually. Enough to want to add a switch to flip between the stock diodes and some Germaniums. 

And there are tons of threads that talk about other mods to change the sound.  I'm looking into this one next:

http://indyguitarist.com/forum/index.php?topic=2406.0