The bulb is intended for testing.  It lets you apply power to the system while protecting it in case of excess current draw, which lights the bulb instead of burning up the amp.   This process does limit the mains voltage some and that is a problem for operation.   But the bulb unit is not intended for operating the amp.  Once we know the unit will work without blowing fuses, we ditch the bulb.

Then there is the variac,  if you dial it up slowly, the amp passes through unstable periods at the reduced voltage.   and with a speaker load, it won't always settle down.

You worked on the amp, fired it up, and it worked.   Then oddly you decided to fix it some more and put it back under test conditions.   The amp works, now is the time to go over it and make sure no adjustments remain to be made under real operating conditions.

OK, so don't plug in the phones any more.  Open it up.  Is there DC voltage on the phones jack?  Is anything loose inside that an inserted phones plug could make touch something else?

Do yoi have different phones to try?

Sure, there are other ways to get home.  I was addressing the OP question.  He apparently wants to design his own with these features.  I mean, how can I add a gain channel to my amp is a separate question from what pedal should I put in front of my amp.

Link worked for me, four pages.

Switching is simple, it was his desire to have a remote reverb level control that was a challenge.

RElays are clean, and can switch signal at 40mv or 40 volts  Relays let you control completely isolated from the signal path.  I have found them darn reliable over my decades.  I have replaced a few, but that is in thousands of amps. 

But I expect relays to be here a long long time.  DG419?   And when they stop making them?  Remember the useful TL604 that were used in large numbers in the 1980s?  GOne now.

Don't get me wrong, if you have a useful design based on them, go ahead.   I just didn't buy the backup argument

A common approach to using a FS and panel switch is to run them in series.  The FS jack between panel switch and ground, with its normal shunt completing the circuit when no FS is used.

A remote level control?  I'd hate to run the signal path outside the amp and back.  If you want to run a control voltage, you'd have to have a supply out to the FS, and make up a control circuit there.  VCA?  Would work, I don't know what is current in VCA chips.  On th other hand a simple digital pot would likely work too.

https://www.icollector.com/L-M-Sound-Reader_i9447359

A picture doesn't tell us much.   I am not even sure what the thing is.   WHat do you want to do with it?

Projects are great, but if you only do a few gigs a year, consider renting something for them.

Help us out, please describe the problem.

Transformer is likely just fine, it makes AC.  I was concerned with the DC supplies.

15v 16v whatever.  It is the low voltage supply for the op amps.  SOme amps use 12v, I see 17v a lot on some mixers.   Your problem is that the preamp is just not working at all, and the lights don't even work.  This is not going to be because voltage is off by one or two.  I'd be concerned one or both supplies are MISSING.

Iy is just a place to start.  NO circuit works without good power supply.

Start with the low voltage supplies, +15v and -15v.

What if it DOES work?   If it doesn't, we can determine what is wrong and fix it.

You have 13v on the output pin of your op amp instead of zero.  Doesn't matter that the amp is different, that still sounds like a bad op amp, so change it out and see if progress has occurred.  For small practice amps like this I rarely get out a schematic.

This calls itself a signal detector, not a signal generator.   I have a guitar next to me in the shop, but mostly I just use a signal from my little shop stereo receiver tuned to a music station, or sometimes a CD player.   All I need is a signal for testing, it doesn't matter a lot what it is.   Of course you could use an actual signal generator (oscillator) if you have one.  It keeps your hands free.


And honestly, any amp running on 9v will likely be very simple and basic, meaning there won't be many stages in the half built steps where listening tests will lwork.

And without looking at a schematic, I'd ask:  are the power supplies right?  I expect the op amps run on a pair of 15v supplies or similar.  f they are lopsided or have poor filtration, you can get distortion.   And certainly FX circuits need quality power.  Within a channel, I will take the mere moments it requires to go down the ICs and check their output pins for unwanted DC offset.  it is possible your signal runs into that DC offset and clips extra early.  Like that.