Please help. (Weird acting input resistance)

[ju.stin]

A while back I tried my hand at some minor amp mods on my Mesa Boogie Dual Caliber 50. (DC5) One of the mods involved removing a 470k resistor that was ran from v1-7 to ground. I did it and didn't like it, so I reversed the mod. After doing so, the amp sounds totally dull and lifeless, on both lead and clean channels, UNLESS I use some sort of pedal. With a pedal, it acts like the amp sees a different impedance or resistance.(forgive my ignorance on the difference between the two, I always get them confused.) With a pedal on, in this case a MXR Zakk Wylde OD or MXR 10 band eq, you can hear how the amp is really supposed to sound, but with the addition of the coloring you get from the pedal, and without it sounds flat, dull, muffled, no highs, almost no gain at all, and all around really dang cruddy sounding.

Any idea what happened or how to fix it?

Thanks in advance for any and all help.

[printer2]

Any chance of a picture of the area?

[ju.stin]

Any chance of a picture of the area?

Sure. Give me a moment.

[Firestorm]

Have you tried replacing V1? If you removed the first triode's grid leak, you might have killed half the tube. Without a ground reference, they can run away.

[oldhousescott]

Did you put back the same resistor you took out? It sounds like you put back a much lower value resistor, maybe 4k7 or 470 ohms. The pedal has a low enough output impedance that it can drive that low of an input impedance, but it would make a regular guitar output sound dull and weak. 470k should be Yellow-Violet-Yellow, if it's a four-band resistor, Yellow-Violet-Black-Orange for a five-band resistor.

[ju.stin]

It is located on the row of resistors running east/west, right in the middle of the photo, just south of the red/gray/purple wires.

[IMG1944]http://i.imgur.com/MwFC3d0.jpg[/img]

[ju.stin]

Have you tried replacing V1? If you removed the first triode's grid leak, you might have killed half the tube. Without a ground reference, they can run away.

I have tried, but can't be sure that they are good backup tubes. Though, if it was a bad tube, why would it still sound good with the pedal engaged? Just asking, since I don't know much. haha

[ju.stin]

Did you put back the same resistor you took out? It sounds like you put back a much lower value resistor, maybe 4k7 or 470 ohms. The pedal has a low enough output impedance that it can drive that low of an input impedance, but it would make a regular guitar output sound dull and weak. 470k should be Yellow-Violet-Yellow, if it's a four-band resistor, Yellow-Violet-Black-Orange for a five-band resistor.

I tried to put the original back in, but after it still sounded dull, I pulled it and tried another. It is very possible that I have the wrong value in there. Though, looking at the picture, it looks to be correct. Do you mind confirming this? Could it be anything else, if it isn't a wrong value resistor? I am at my wit's end trying to get my amp just back to normal. haha

[ju.stin]

Also, here is the schematic. It is the 470k that runs from v1-7 to ground that I fooled with. Could be anything in that area as far I know.

[IMG684]http://i.imgur.com/Vz482ni.jpg[/img]

[Reeltarded]

It's something else.. can you measure that 470k from the 7 pin to ground?

I think it is something else. Still looking.

[martin manning]

...can you measure that 470k from the 7 pin to ground?

Good suggestion. Any chance the LDR could have gone south?

[Reeltarded]

You'd have to imagine it is an ldr problem, or a connection towards the input from the 470k.. but..

Bothers me a little that it's dull on both channels. I think (me as an obsessive freak) would hardwire across the 1M and 470k then throw a 1M over the input directly just to see... maybe even jumper another input beyond all that on both channels and work back towards the input.

[alvarezh]

I am not familiar with this amp, but after closely looking at the picture I am under the impression you have a double layer board, which means that if that's the case, you will need to lift the board and solder the resistor on the back also.

Again, I am not sure, but it's a possibility, perhaps someone familiar with this rig (or more experienced with the brand) can confirm this.

All the best.