Gain staging

[passfan]

Has anyone checked the signal voltages through one of these ODS circuits. I'm getting no real output through a new build unless I start diming the stack and volumes. Voltages and bias is spot on. I'm looking for signal voltages through the gain stages.

[ToneMerc]

Has anyone checked the signal voltages through one of these ODS circuits. I'm getting no real output through a new build unless I start diming the stack and volumes.

What % taper pots are you using?

TM

[passfan]

Just standard taper pots ,off the shelf. With the gain and master at 50% this thing should be loud. I'm getting up to 6 volts of signal gain from V1a. By the time I get to V1b I'm lucky to have a volt. The pi is sending 12 volts max to the grids of my power tubes with everything dimed. I'm losing signal somewhere. If I want to hear it at all at 50% I have to kick all the boost circuits on. When I get a chance I'm going to compare it to my twin.

[splgeo]

check your shielded cables, problem I had

[norburybrook]

I had exactly the same thing when I built my 100w Bluesmaste and it turned out to be a cold solder joint. Re flow the solder connections on the V1 pins.


Marcus

[Structo]

Check possible shorts in the shielded cable to ground.

[glasman]

Typically you will see a minimum of 35dB at around 800 hZ and 45dB+ at 1.5kHz measured on the the output coupler of V1B. Assuming all controls are centered. Treble, Middle, Bass and the "Volume" controls (30% taper). Will be less for 10-15% taper pots.

This is measured from input to V1A (grid resistor input) to the output coupler of V1B.

Gary

[passfan]

Thanks all. Gary , just what I needed. Hopefully I will get a chance to crack back into it soon.

[John_P_WI]

I'd suggest you investigate around the od relay and effects jacks. The wiring can be confusing to say the least. Throw a test signal and see where it is being lost

Having to dime the tone controls could also indicate a problem with the tone section and associated relays...

[passfan]

Thanks John , first thing I verified as the relay cards I bought were laid out backwards and I had to draw a conversion schematic. I suspect something around the pi as I plugged into the power amp in with a preamp and couldn't get any real output. I don't think it's biased correctly for some reason and its sitting at 48 volts to the cathodes. A twin is closer to 100 volts.

[martin manning]

Sounds like you have isolated the problem, but around 50V on the PI is reasonable. You are forgetting that a Twin has a 12AT7 PI.

[splgeo]

http://bmamps.com/Schematics/dumble/dumblenotes.pdf

alttie help with a voltage chart. PIC cathode here is around 60v. but 50v should be more the enough

[passfan]

Sounds like you have isolated the problem, but around 50V on the PI is reasonable. You are forgetting that a Twin has a 12AT7 PI.

youre right.

[passfan]

Well after doing a comparison with my twin reverb I believe I have a bad OT right out of the box. I am driving a good 2.4 volts in both amps into the output tube grids. The twin is loud , the Dumble is barely audible. A primary winding comparison between the two shows the twin split closely between 72 ohms, and the Dumble spit at 15 versus 20 for a total of 35 ohms. I am going to do some further testing and eventually swap out the transformers before I get in touch with Webber.

[passfan]

Brand new OT no good. Sounds beautiful on the vintage one out of the twin. Thanks to all.