Gibson G70 repair.

[gtrplayr1976]

Warm up of the power amp parts? How long was the amp on? I usually allow for a 15-20 minute warm up before setting final bias.

Remember no signal and on some amps no load either when setting the idle bias.

The other bias resistor reads higher. If one is reading 10mV the other is 25mV. The first one I had the meter hooked to has a loose solder joint. Maybe that's causing the issue ?
I don't have any signal ,but I'm not suppose to have a speaker connected either ? I always thought you had to have a load on an amp ,or it would fry the xfmr ?

By the way Bill it was on for 20-30 minutes at least ,and thank you for the help.

[loudthud]

For a solid state amp with no output capacitor there might be a small DC current flowing in the speaker and that will make the bias across the emitter resistors look unbalanced. That's why you usually disconnect the speaker when you set the bias. That bias may also drift around as the heatsink changes temperature. Solid state amps don't need a speaker connected in the same way a tube amp does.

[gtrplayr1976]

For a solid state amp with no output capacitor there might be a small DC current flowing in the speaker and that will make the bias across the emitter resistors look unbalanced. That's why you usually disconnect the speaker when you set the bias. That bias may also drift around as the heatsink changes temperature. Solid state amps don't need a speaker connected in the same way a tube amp does.

Good to know. Thank you.

[gtrplayr1976]

Found the pad for the one resistor had come loose. Got the resistor changed ,and powered back up. Bias voltage is still drifting maybe 3mV between 16-19 ,but it's now the same on both resistors. I checked the bias with the speaker disconnected ,and no signal.

[Jazz P Bass]

If you truly want absolutely no bias drift, I would suggest changing out the LTP transistors. (Q3 & Q4)

[gtrplayr1976]

If you truly want absolutely no bias drift, I would suggest changing out the LTP transistors. (Q3 & Q4)

It doesn't sound bad at all though. When I first plugged into the effects channel I had a very bad hum ,so I connected to the Normal channel to see if it did the same. It was noise free. I went back to the Effects side ,and it was mostly noise free ( I could hear some tremolo tick). The hum sounded like ground type hum?

[gtrplayr1976]

Reverb Tank ?

Does anyone know what type reverb tank is needed in this G70 ? All I see on the schematic is Type H ?

[g1]

As far as I know the type H is a high impedance tank, so some variant of the 4FB should work. You would need to alter the grounding scheme to match the original.

[gtrplayr1976]

As far as I know the type H is a high impedance tank, so some variant of the 4FB should work. You would need to alter the grounding scheme to match the original.

Thanks g1 ! The grounding I see inside the amp have one shielded ,and one unshielded cable soldered to the circuit board. Kind of how a Fender does with cables ?

[52 Bill]

The grounding I see inside the amp have one shielded ,and one unshielded cable soldered to the circuit board. Kind of how a Fender does with cables ?

Fender uses grounded/shielded cable for both input and output line and has a non-grounded input jack on the tank.

If you have one shielded and one non-shielded cable you need to use a tank with both jacks grounded. The unshielded cable goes to the tank input.

[gtrplayr1976]

Fender uses grounded/shielded cable for both input and output line and has a non-grounded input jack on the tank.

If you have one shielded and one non-shielded cable you need to use a tank with both jacks grounded. The unshielded cable goes to the tank input.

Maybe I'm thinking of the Fender footswitch ? Something I repaired that was Fender had an unshielded wire on one side. So I need to get a tank with uninsulated jacks ?

[52 Bill]

So I need to get a tank with uninsulated jacks ?

Yes, if the reverb cables are as you posted, the tank needs to have both input and output jacks grounded.

And yes, Fender footswitch cables have one plain wire for the trem and a shielded one for the reverb.

[gtrplayr1976]

Yes, if the reverb cables are as you posted, the tank needs to have both input and output jacks grounded.

And yes, Fender footswitch cables have one plain wire for the trem and a shielded one for the reverb.

Think I found one Bill. 4FB3A1B. Sounds like what it should be. Thank you !