Trouble in amp land!

[Natman]

Hi all,

I just built a homebrew Gibson Scout-based amp with 2 x 6V6, 2 x 12AX7 and 1 x 12DW7 for reverb. I got her working for all of 10 minutes before the trouble started.

Now the reverb was the biggest problem at first but some online forumites helped me and I got it working fine for 10 minutes before the kids had to go to bed. The next day, nothing! There was a ticking sound in the tremolo which I was advised to change 2 resistor values and again nothing last night.

I'm still a novice having built 5 amps, 2 from scratch but troubleshooting is an issue for me. I usually have to bring an amp in to a tech when there's a problem, but I really want to progress past this kind of thing. Thanks if you can help!

Here's the problem:
When I turn the amp on, the pilot light does not always turn on. The standby switch
has to be flipped and usually it's ok (but not always).

Then, when the amp warms up, I do not always get sound. If I do not, I turn the amp off and there is a loud electronic "swooshing" sound that comes form the speaker, like a capacitor is discharging or something.

Last night I DID get sound (amp hissing noise), but I lost it as soon as I plugged the guitar cable in to the input jack. Weird.

Lastly, the reverb only worked briefly. It is in parallel with the 2 gain stages, so it adds noise/volume as I turn up the reverb knob even if there is no reverb coming through.

I am normally quite good at soldering and every connection looks fine. Of course I did not base this build on a schematic, but rather gut shots of a well-known amp. I literally copied component values where visible and for the rest, good educated guesses along with tips form online gurus (?) who helped.

Please advise where to start troubleshooting! I have no oscilloscope, just your standard multimeter.

[g1]

You will have to start by drawing a schematic. Otherwise I have may doubts anyone here will be able to help.

[booj]

I hate to sound obvious, but it's either a bad connection or a bad part.

[lowell]

The pilot light issue may be connected to the others. Double check all heater circuit solder joints. Heck reflow all of them if you're not sure. Is the heater secondary center tapped? Check too if it is. Report back. Let's get that fixed before worrying bout reverb.

[Natman]

I have an update: The amp is technically working fine. Reverb works. Trem works.

HOWEVER: the reverb is acting as though it is "gated". The 'verb only comes on if I hit the strings hard. If I pick lightly, I get a dry signal.

Any idea what would cause that?

This is an Ampeg/Traynor style 1-tube reverb even though they often use paired 12AT7's. This is the only schematic I can post at the moment. A few resistor values have changed and the coupling cap is now 0.1 uF instead of 0.68 uF.



Thanks if you can help!!!

[J M Fahey]

Unfortunately, it's on the edge of unreadable.
However: R7=1M is an impossible value.
The plate resistor which I *think* reads 82R is too an impossible.
Don't ask me the correct values, rush to the schematic you copied from and check for yourself, then repost with correct values for us all to see.
Also: what reverb tank did you use? (which code?)
Thanks.

[Natman]

I was afraid of that. I did fix those 2 resistors however it's not updated onthe schem yet. The tank is a 8FB3C1C. Thanks!

[Natman]

Bump -I want to see if anybody has input about this 'gated' reverb sound?

Thanks!

[J M Fahey]

Well, post the "corrected" schematic.
The only one posted will *not* work.

[loudthud]

I'm assuming the gating is occuring on the output side of the tank. Give the tank wack and you hear the gating, right?

Run the output of the tank to a different amp. Just a matter of getting the right cable and adapters if needed. This will tell us if the tank is good. While you're at it you can connect the external amp at the Reverb control to see if the gating occurs there.

If the tank is good it almost has to be some kind of intermittent connection. The pot is suspect, wire around it as an experiment. Monitor the plate voltage of the return amp, make sure it's steady, doesn't jump to B+ or ground. Chop stick all the connections between the return jack and the preamp R2.

That about covers it. Do you have any kind of footswitch connection?

Edit: Reality check. Do you still hear the gating if the volume control is all the way down?