Homebrew HELP!

[lexluthier72]

Hey Folks,
I have a homebrew amp based on a fender pro jr. First, let me say, I did not build this amp, and the guy who did, has fallen off the planet. It has 2 problems which I think are related. First, the volume will not turn down all the way, and second, there is a hiss coming from the amp even when guitar cable is unplugged, and the volume pot all the way down. The input jack is a shunting type. I have included a photo of the innards. I did replace the volume pot to no improvement. I switched out the power and the preamp tubes. I checked all the grounds, they all visually look good. When I touch the shaft of the volume pot, with the volume at zero, the leaking volume decreases. It does this with the new volume pot installed. Im stumped. Any help would be greatly appreciated, as this has become my main amp....I forgot to add, I tried the chopstick trick with no improvement.
-Erin

[txstrat]

Hey Erin,

I would follow the signal path from input to tube 1 and so on. Maybe it's not a problem of the pot. Could the pot be bypassed by a wire?
You might be able to draw sort of a schematic as you follow the signal path. I mean just a sketch to where every wire leads. That could help alot finding the problem.

Matt

[PRNDL]

Grounding issues are common in those amps. Also, splurge on solder and fill up all those eyelets.

If you're interested, I built an amp based on the Matchless Spitfire and Vox AC15 from the same chassis. We put everything on the circuit board, and mounted it flat.

Here's a photo
http://www.naturdoctor.com/Chapters/Amps/Christine.html

[lexluthier72]

So, How do I fix the problem? Do I have the wrong value volume pot? Its a 250k audio taper. Im more concerned that the volume doest zero, than the noise.
-Erin

[PRNDL]

So, How do I fix the problem?

Grasshopper, you're training has just begun.

Continue to test everything, and take a break when you get frustrated.

Point-to-point jumbles are a nightmare to debug!

I know ... last Friday I spent 5 hours fixing a 50's Gibson GA-50T, which is cluttered as your amp times 50!

It's also possible that there was a mistake in the build, or something so simple that you're going to consider whether or not to post it. Humility helps!

[Twist]

Judging by the lead dress you may have some bleed over from a previous stage. I agree with the comment about grounding. I would try redoing the grounding scheme. I don't think the problem is with the volume pot.

[lexluthier72]

I was just going to ask what to look for next...I truly am stumped. If it is bleed from a nearby gain stage, what then? Change the ground path? Heavier/shielded ground wire? Any suggestions would be great. I did chopstick the grounds, and nothing made a noticeable difference.....
-Erin
p.s.- The alternative is to rewire it to a turret board layout, but I really like the sound of the amp

[Twist]

I would rewire it with a turret board. If anything it should improve the sound. I draw my layouts on a sheet of paper before I do any building. I use a eyelet press to make my own boards.

[lexluthier72]

I think that is where I am headed, I will most likely build it into an 18 watt lite at some point. Frustrated that I cannot seem to find the problem. I did get very minimal results from moving the yellow wire around with the chopstick from pin 9 (grid) Of the 2nd el84 (this wire is promenent in the foreground) but also found I got more squeal at higher volume...I did not re-route the grounding, and the amp is still noisy at low volume. Any other ideas before I rewire it into an 18watt lite?
-Erin
The only thing that concerns me about rewiring it into something else, is I will most likely loose the fender snap that this particular amp has. Plus I am still fairly horrible at building from a schematic, and have not found a good layout for a pro jr, but have been told an 18watt lite sounds better than a pro?

[lexluthier72]

I think I found a good layout for the pro jr. See the schematic below that Ive found. Any insight on how to adapt the ao-35's PT/can cap hookup to this would be great.
-Erin

[lexluthier72]

Hey Guys...Not fixed yet, but I tinkered with it today, here is what I did, and some observations....

I isolated the input jack with a fiber washer on both sides....No improvement
I increased the size of the buss wire to the back of the tone/volume...No improvement
In the process of isolating the jack, I broke the leads of the .01uf cap from pin 7 of the 12ax7 to the volume control (this is the BLUE cap in the photo)....This made the volume problem worse. I soldered in a .02uf and I am now back to square 1. Just for kicks, I put a 12au7 in the 1st preamp stage, and the volume problem went away. I know, WAY less gain....So, I am assuming it is somewhere in the 1st preamp stage, OR the signal path from the volume pot to the 1st 12ax7. I was trying to wrap my head around how to shield the signal path, but the path is basically made up of the leads from the various caps/resistors (too short of spaces between components.) From a noise standpoint, Ive read all the carbon comp resistors should be taken out, that they increase the noise? is this true? Now what? At least I think Im getting somewhere....Id appreciate any hints on what to do next.
-Erin

[lexluthier72]

aaaarrrrgggg....still cannot isolate the problem. Bump! Anybody?

[PRNDL]

I agree with Twist and recommend rewiring it based on the Pro Jr Layout. The main reason I posted a photo of my amp build was to provide an example of what your amp could look like with some work.

My experience with these amps is that they suffer from grounding issues and hum from minimal PS filtering (the Spitfire and AC30 both have a choke).

I notice there's an extra 35µF cap across the cap can.
Also, is that the original cap can? If so, that's most likely the issue.

And again, grasshopper, take a moment to relax, rewind and experience the joy of life outside of tube amp debugging.

[lexluthier72]

PR....Thanks. I posted a layout for a pro jr that I found, I have no idea how to adapt it to the ao-35 PT. Ultimately Id like to rewire it to a turret layout, but thats where Im stuck. Thanks for all the help so far. I just cant wrap my head around how to mate the layout I found to the ao-35....See an above post for the Pro layout pdf.
-Erin

Edit: I got a hold of the guy that did the layout, and am getting some good advice. Thanks to all who replied.

[Bruce / Mission Amps]

PR....Thanks. I posted a layout for a pro jr that I found, I have no idea how to adapt it to the ao-35 PT. Ultimately Id like to rewire it to a turret layout, but thats where Im stuck. Thanks for all the help so far. I just cant wrap my head around how to mate the layout I found to the ao-35....See an above post for the Pro layout pdf.
-Erin

Edit: I got a hold of the guy that did the layout, and am getting some good advice. Thanks to all who replied.

Don't forget to tell us about the advice... we like to learn too.