Unusual problem with new build

[jon]

Im having an issue with my new Gibson GA-100 build. At low volumes the amp sounds perfect. There are no unusual noises hisses pops or squeals. What I do have is a strange charging and discharging effect happening. When you hit a note that would draw some current the amp cuts out instantly then comes back. I checked voltage drop during the phenomenon and there is none to speak of. Any thoughts?

[markr14850]

Measure all the resistances in the power supply dropping string, and check that the power supply caps are properly connected to ground.

[jon]

Grounds good as are filter caps. I'll check the dropping string again, but I'm pretty sure it is correct.

[markr14850]

Another thing to check is that every grid has a dc path to ground.

[Firestorm]

Grid blocking?

[jon]

Grid blocking?

On the schematic it shows the cathodes and one of the grids on the pentode anode drivers connected together. I ran a wire from cathode to grid to cathode to grid to the 470 ohm cathode resistor. Upon looking at the photos of the original amp I cloned the tied the cathode to the grid on each tube and then ran a separate wire to the resistor.

Could this be the problem?

Here is a link to my build thread it has schematics and such
http://ampgarage.com/forum/viewtopic.ph ... bson+ga100

[markr14850]

Measure the voltage of each grid to ground, while the problem is occurring. See if taking this measurement fixes the problem a bit.

[jon]

Looks like the grid bias in the anode drivers is constantly changing. The schematic shows a value of 1.6vdc I'm getting anywhere between 2.5vdc and -22vdc.

[bluefireamps]

Did you build it with the compressor circuit? If so, does it do it with the compressor switched off?
Dave

[jon]

Yes and yes

[markr14850]

Looks like the grid bias in the anode drivers is constantly changing. The schematic shows a value of 1.6vdc I'm getting anywhere between 2.5vdc and -22vdc.

That might be it. Is it 2.5v at idle, and -22 when you hit it with signal?

What's connected to those grids?

Can you post the schematic? Which valve(s) is this?

[jon]

Check the link a few posts up.

[jon]

Bump

[jon]

schematic

[markr14850]

You'd mentioned a large bias shift, where the spec was supposed to be 1.6. Was that on v4/v5?

What happens if you simply remove v4/v5? Does the problem change?

Is there also a large bias shift on the PI - V2/v3? If so, try clipping in another 22m resistor in parallel with the one that's there in the grid leak area of V2/V3. See if that reduces the problem.