My Express clone has lost all of it's gain

[robtone]

Well it was too good to be true. I finished "Lucinda" one month ago and fired her up without any problems. The amp has been awesome. I turned it on 2 days ago and the amp had lost all of it's gain. It has about 20% of the gain it originally had. I checked the voltage on the plates, grids, cathodes, bias, B+(2,3, &4) and everyone checked out ok. I chop-sticked the entire amp and couldn't find anything. I changed out all of the pre-amp tubes. I don't have any more output tubes to try at the moment but they light up and even at 20% gain the amp sounds pretty good.

Can anyone give me advice on where to look next.

Thanks in advance.

Rob

[Ian444]

If all the voltages check out, and the tubes are OK, then I would be looking for a dry solder joint. A pair of cheap x3.5 reading glasses and good lighting (reading lamp) work well for checking solder joints. Look for tiny cracks in the solder joints, grey solder instead of shiny solder joints, or sometimes you get a joint where the solder has not properly taken to the wire yet still surrounds it. It could be something else but that is where I would start, especially on the volume and tone control pots, then on the turret board. Other outboard things like guitar output, guitar lead, and speaker lead are worth checking too. Hope that helps Rob.

If you get stuck a signal generator and a scope will find the source of the problem very quickly.

Ian.

[robtone]

Thanks. I will give that a try.

[robtone]

This is strange. Now 1 day later when I check the plate voltages. All of the preamp plate voltages are 22 to 30 volts less than what they are supposed to be. However the power tubes are right on at 415volts. Any suggestions now on what to do next?

Thanks

[RJ Guitars]

Hey Rob,

Sounds like you are making progress... failure is a critical element of success. Since your amp now seems to have a consistent voltage issue you can start tracking it.

Can you follow your dropping string voltage chain through the 5 nodes? If you can post those values for us it would be useful.

Also, pull your preamp tubes one at a time and measure the voltages again.

Let us know what you find.

rj

[robtone]

Supposed to get 210V on the plate of V1a and I got 185.4
182V on the plate of V1b and I got 160.3V, 267 on the plate of V2 and I got 219V, 190V on V3a and I got 219, 193V on Plate of V3b and I got 165.2.
I'm supposed to get 415 on the plates of both power tubes and I got 417V on each. My heaters come out right as does the bias.

Is this what you were asking?

I'm sorry for the delay. I have been reading and trying to find an inexpensive Oscilloscope.

Thanks,
Rob

[Ian444]

Rob, it looks like your low gain problem is right there in the phase splitter V3. With no signal, the anode voltages should be very close as the 190V/193V specs suggest. V3a plate is about 30V high (turned off) and V3b plate about 30V low (turned on), which seems to indicate a source of DC on one of the grids of V3 that should not be there. Maybe some DC is leaking through one of the capacitors? If you measure the DC voltage on pins 2 and 7 of V3 they should be exactly the same, if they are not, then you are very close to solving the problem. Probably the best way to measure this is to measure directly across pins 2 and 7, e.g. black lead on pin 2 and red lead on pin 7. This will measure the difference between the 2 grids, I would be expecting something around 0.5V DC or more difference. Safest if you can measure these voltages on the turret board on the wires leading to these pins.