hi all its me again.
finished my firefly, and its not quite operational.
the power supply works fine, me and peter snowberg can verify that :)
the heaters glow, and i get voltage on the tubes, but no sound from the speaker, and i have no idea where to start looking. ive checked my tagboard layout over and over and im sure its fine.
suggestions?
Cheers
Jim
The first thing is to install only the preamp tube and build an audio probe using a 400 volt or greater cap with a value like .01uF or LESS. Don't use a higher capacitance cap.
I'm going to do some yelling in this post, but only 'cause I want to see you live. Bear with me. I've almost killed myself a couple times in tube amps.
CAUTION: You're messing with lethal power now and the only probe designs I see floating around are for low voltage effects. You can still use the basic audio probe, but use a high voltage cap and an insulated probe. ALWAYS hardwire your ground, and then probe. Try to stay away from hitting any high voltage point. Basically, avoid the plate contacts and anything before the output coupling cap for that stage (C1 or C7 in this circuit). If you hit HV, you could easily kill the amp you're tracing with.
Take the circuit one stage at a time.
First, use your meter to look at the plate and cathode of stage 1. If you see high voltage on the plate and only a few volts on the cathode, great. Now check the grid for stage 1. You should see just about zero volts on the grid. If you see HV on the grid or cathode, STOP!, and check all the components and solder joints for that stage. Assuming everything is cool... Set the guitar down and hit the strings to make some signal. DON'T TOUCH THE GUITAR AFTER THIS POINT! Audio probe the grid... if you have signal.... time to check stage 2.
Do the same thing as for stage 1. Make sure you have ground on the grid, HV on the plate, and a few volts on the cathode. If not, STOP!, else hit the guitar again for some signal, and then probe the grid of stage 2. If you have louder signal great.
Install the 2nd tube. Look for HV on the plates, a few volts on the cathode, and roughly ground on the grids. Hit the guitar again and probe for signal on the grid connected to stage 2. You should hear it.
Now probe the secondary of the output transformer (assuming one side of the secondary is already tied to ground.)
If you have a bad component you can get some nasty voltage in the wrong place, so always look first with a meter before audio probing.
The big question is where does the signal stop?
I'll be up for a little. Hopefully we can get this thing running tonight, BUT DON'T HURRY! Life is better than debugging. Sorry for yelling so much, but this is playing with fire and caution is what keeps you alive. Even with the power unplugged, HV caps can store enough juice to kill you DAYS later! Doug's design doesn't have a bleeder resistor, so I would also advise installing a 470K or 680K 1W directly from the B+2 to ground. That will bleed off the charge when the power is off.
Oh, what is your heater voltage now that the PS works? And what do you see for B+1 and B+2?
-Peter
hey peter if youre still up click chat up there next to reviews. me and aron and joep are trying to work it out, and we have encountered an odd problem.
see you soon
Jim