Guitar Buffer Issues with Amp

[bal704]

Sorry in advance for the long post...

I was reading about how adding a buffer to your guitar can improve the tone, so I decided to build a buffer from general guiltar gadgets:

http://store.generalguitargadgets.com/p ... s-pcb.html

I added this a P90 equipped guitar, with a DPDT switch so I could run with and without the buffer. I plugged this into a Dr Z Mazerati clone, in non-buffer mode, and the amp sounded great. As soon as I kicked the buffer on, the amp went from roaring glory to sounding like a distance amp. Lost most volume and gain. Even when I flipped the switch back to non-buffer mode, the amp sounded terrible.

I tried a different guitar, different cable etc, and still lousy. I tried the buffer guitar in another tube amp, and it sounded fine in both buffer and non-buffer mode. I replaced a pre-amp tube in the Dr Z clone, decided the buffer didn’t make much difference, and moved on.

I just finished up a new guitar build with a Fishman Piezo bridge, and decided to use the buffer on it. It’s got a switch to switch the buffered piezo in and out of the circuit. I’ve been playing this on a solid state amp, and it sound great. Acoustic and Magnetic sounds with the flip of a switch.

Today, I’m playing my Gibson LP and the piezo partsocaster with the buffer. I plugged the LP into a Mesa Lonestar clone (2 channel, clean and crunch), and it sounded amazing. I plugged the buffered guitar into the amp, and it also sounded amazing with the P90’s engaged (no buffered piezo). I flipped the switch for the piezo, and it instantly sounded like ass. Lost most volume and gain on the channel I was on. The other channel sounded fine. I plugged the buffer guitar into the solid state amp, and it sounded fine. Plugged it into another tube amp, and it sounded fine.

So I’ve now had 2 instances where I had this buffer plugged into an amp, flipped it on, and lost most volume and gain instantly. I chalked the first time up to coincidence, but now it’s happened twice.

Is it possible that this 9V buffer is blowing tubes in amps? Is this just bad luck that happens to coincide with this buffer, and why only on certain amps?

[Stevem]

Is cap C2 on that board installed right, and or is it leaky

[bal704]

I'll double check the polarity. How do I check to see if it leaks?

Also, the Buffer isn't hooked directly to the output jack. If you look at Fishman's installation instructions:

http://www.fishman.com/files/powerbridg ... _guide.pdf

I've got the buffer going into a 250k volume pot through a resistor/cap combo. This knocks the level down quite a bit. Without it, the signal coming from the piezo is really hot. Too hot. So I've got:

Fishman -> Buffer -> cap/resistor to Leg 3 of 250k pot. Leg 2 of pot to selector switch that's shared with magnetic pickups.

The piezo sounds suprisingly realistic, when it's not causing me problems.

[Leo_Gnardo]

So I’ve now had 2 instances where I had this buffer plugged into an amp, flipped it on, and lost most volume and gain instantly. I chalked the first time up to coincidence, but now it’s happened twice.

Is it possible that this 9V buffer is blowing tubes in amps? Is this just bad luck that happens to coincide with this buffer, and why only on certain amps?

Very unlikely you're damaging input tubes. Tubes are very robust in the face of large signals. I'd worry more about anything transistorized but with only 9 volts available from the guitar at worst, and your proof the axe works with SS amps, nothing to be concerned about there.

I'm thinkin' possibly a loose connection on your homebuilt preamp may be sending into ultrasonic oscillation at times, that would make anything "sound like ass."

[bal704]

The problem is the amp permanently sounds like ass after this happens. Even if I plug another guitar into the amp, the amp has a problem. I tried 2 different guitars into this amp later in the evening, and the amp is hosed even with those guitars.

I'm having trouble believing what is effectively a stomp box can cause any real harm to a tube amp, but it's happened twice now. Just coincidence maybe, but the buffer is the common denominator.

[bluesfendermanblues]

Sorry in advance for the long post...
.........
Is it possible that this 9V buffer is blowing tubes in amps? Is this just bad luck that happens to coincide with this buffer, and why only on certain amps?

Would like to se a schematic of your buffer circuit. However, it doesn't sound likely, unless a capacitor on the buffer's output has failed, been left out or is shorted, but it doesn't sound likely.

[bal704]

Here's the schematic:

http://www.generalguitargadgets.com/pdf ... buf_sc.pdf

And as I said above, the output of the buffer is going into the fishman recommended cap/resistor and pot setup before it hits the amp.

[bluesfendermanblues]

I use that exact same buffer pcb from GGG for DIY "Suhr Minimix II" (in combination with GGG mini mixer) and that particular buffer is a tried and tested circuit.

You must have made some connection to the fishman that provide the problem.

[bal704]

That's basically why I posted here. I'm hooking the fishman directly into the buffer, then the buffer out through the pot and cap/resistor etc to the amp.

I'm having trouble imagining what this little circuit could be doing to hose a tube amp. Perhaps just a strange coincidence twice?

Plan B might be to run the fishman into the cap/resistor and pot without using a buffer.

[SilverFox]

Have you determined what component is being damaged in the tube amps?

silverfox.

[bal704]

No. This just gets more baffling actually. It only seems to effect one of my amps. Even the amp that had trouble the first time I tried the buffer seems to work OK.

Curiously, the amp seems to 'heal' itself. If I wait 24 hours or so, the amp works fine. I also removed the battery from the guitar so the buffer would be a non-factor. The amp works great without the battery, and flipping to the buffer via switch causes no issue. Putting in the battery causes the same issue as before.

So to summarize:

Amp sounds fine with magnetic pickups
Turning to the buffer robs the amp of almost all gain and volume
waiting 24 hours or so basically fixes the amp
This is repeatable

I'm just not going to use this amp with this guitar. I've got lots of amps and guitars so this won't be an issue. I was just hoping to understand the mechanism of failure here.

Have you determined what component is being damaged in the tube amps?

silverfox.

[JTU]

Hey, are you sure when you use the buffer that your input's grid still has a ground reference, ie. a resistor to ground? Maybe a bad solder joint on the 1M input grid leak or no resistor at all?

It would work with just the guitar but not with a buffer that has an output cap and no resistor to ground...

First post in this forum...

[SilverFox]

It sounds like the buffer circuit is charging up a capacitor at the input circuit of the amp...

silverfox.

[Rick]

Hey, are you sure when you use the buffer that your input's grid still has a ground reference, ie. a resistor to ground? Maybe a bad solder joint on the 1M input grid leak or no resistor at all?

It would work with just the guitar but not with a buffer that has an output cap and no resistor to ground...

First post in this forum...

That's what I thought, if there is a channel that relies on the guitar volume pot for a grid leak resistor, i.e., no grid leak resistor on that input, then that grid can become overly charged and not modulate the electron flow to the plate. Which is why it's always good to have a grid leak resistor on V1, otherwise, pedals and modelers and this "buffer", whatever that is, may not work very well.

[bal704]

I'll check for the resistor. I'd be surprised if there's not one there, but it wouldn't be the first time I was wrong...