Zero Loss FX Loop Problem

[Krinkle]

Hey Everybody,

I used to frequent this place regularly, then we got the old band together and we've spent the last few years practising and playing so not much building happening.

I have a break until after Christmas so I'm diving back in!

I'm having trouble with a Loop I bought from George a long time ago, probably 2007-2008, it's REV 1B.

I may go a little long here but here's what I'm doing. I built a 12000 head from a kit from 2007, best money I ever spent on gear. I wanted more gain so I changed the preamp to 2204 specs, sounds killer.

I liked it so much I recommended it to my friend, who bought a second hand MA - one that George built, 1959 specs and has the FX loop in it, REV 2A. I keep hearing something in my friends' 1959 spec that my amp doesn't have, plus I'm missing being able to jump inputs. So I'm thinking about changing back to original 1959 specs.

Then I had an idea. I had some extra parts laying around so I built a separate 1959 preamp. My plan is to SEND the signal from my separate 1959 preamp to the RETURN of the FX Loop in my friend`s MA and compare to make sure that I built an exact replica.

Then I'll probably build up a separate 2204 preamp, switch them back and forth, and have the best of both worlds! I'll buy another loop and install it in my MA and switch back and forth between 1959 and 2204 pre specs.

So I have verified that my preamp works, I have signal at the input of the FX Loop but no output. I'm a tech so I started poking around with my DMM, signal generator and scope.

I have signal until I get past the first resistor in the circuit, then nothing. I have about 330 volts at B+, about 214V at the drain, and about 0.45V at the source. Shouldn't I have a larger voltage on the Source, such that the Gate becomes negative biased like the Grid of a preamp tube?

I'm wondering if I've handled/fondled this thing so much that ESD has killed the MOSFET. I figured I'd post here before I bugged SDM, anybody familiar enough with the voltages around the MOSFET in the LOOP to comment?

Thx
Darrin

[SDM]

Voltages on buffer stage look fine. Signal after first resistor will be very very very tiny due to strong NFB/how stage operates. Have you checked for signal further than that first resistor (like on loop jack tip lugs or on output pad)?

Hopefully something as simple as a switch issue or such here.

[Krinkle]

Thanks for the quick reply Steve, I think I know what the issue might be, me, of course.

Reading my original post I can see that I should have taken more time, given more information, and been more clear. I also realize that I shouldn't have put the word "problem" in the description of my post, sorry.

I'm not using the switch, I'm just wiring from the wiper of the treble pot straight to the input of the FX Loop.

I have 4Vp-p coming off of the treble pot output (all front panel pots at max), and I AM getting a signal on the output of the Loop, 50mVp-p, I just thought that it should a lot higher. Is this the intended output level, for a pedal?

I don't know why but I thought it was a line level loop, just so used to putting pedals in front of the amp I guess. If this is the case, can I just change the pot (what value should I use?), to decrease the NFB? I have the rest of my preamps set up to run at a 0dB output, going into my FX.

If I can change the pot to make it line level, do I need to adjust the RETURN to make the levels match?

Thx,
Darrin

[SDM]

4V p-p is low incoming, but I'm not sure how you are driving the loop right now. Are you using the built in preamp of the amp (and if so is it stock 1959 and volume/s cranked)? Or are you running the separate preamp into the loop when you get the low voltage?

With a typical 1959, loop built into amp, signal input to loop should be much higher, around 80V p-p (at 1KHz). signal in the loop should be around .8V p-p (for a pedal level loop), and final output back to amp/PI around 80Vp-p again. Possible the loop trimmer is set incorrectly or such, but can't say for sure until I know how the loop is being used/tested here.

Also, for an external preamp to best interface with the loop return directly, the external preamp should have a buffer stage on its output. Feeding the loop return stage directly means the loop's buffer isn't used of course, hence why it's best for the external/interfacing gear to have its own. Don't know if that comes into play yet here or not, but does the external preamp have an output buffer?

[Krinkle]

Okay, then the problem is with my preamp, sorry to drag you into this.

I have built a standalone 1959 spec preamp (no phase splitter or power amp), with the signal to the input of the loop coming from the wiper of the treble pot, just as you would in a full amp. No buffer as I want it to be an exact copy of the 1959 preamp in the Metroamp.

I plan on buying another loop to put in my Metroamp and using the return on it to verify that my preamp sounds the same as the one in my Metroamp.

I'm now seeing that my preamp is definitely not working right with a signal of 4V with everything cranked. With all front controls set at full, I'm getting 4Vp-p at the wiper of the treble pot. I wasn't sure of the voltage I should see there, so obviously 4V is pretty far from 80V!

I'll start troubleshooting my preamp, thanks for the quick (and weekend) help!