Ax84 P1 101004 stops working but the day after it works again

[Tanazzo]

Hi, I build a Ax84 P1 101004. I bought everything on the site ampmaker.com, the PT is the 190-0-190 5watt. The first time I powered up, I was glad to hear that the hum was almost nothing. The voltages are a little below the values of the P1 diagram, but they are in acceptable limits. When I put the gain and the master on maximum, suddenly I don't hear nothing more. But the following day it works again. It happened another time. I don't have a electric guitar, I'm building a Les Paul, but it's not ready yet. I use the silent guitar Yamaha SLG 100 powered with a power supply. Does anyone knows why the amp suddenly stops working when I raise the gain and the master at ten, and after a day it works again? I didn't find a solution or a similar problem on the web.
Thanks
Tanazzo

[Willabe]

Hi Tanazzo, and Welcome!

That's not enough information as it could be a lot of things.

Can you post some hi-def pictures of the inside of the amp build and a schematic?


               Brad    

[Tanazzo]

Hi, here are the schematic and a picture of the fiberboard.

[Tanazzo]

Here're other pictures.

[Willabe]

It looks like you have the EL84 cathode bypass cap wired backwards. You have it's + going to ground. It probably is dead.

Your schematic lists this cap at only at only 25v. I don't think that's enough, should be at least 50v I would think. And it lists the cathode resistor as only 1w, probably need at least a 5w R.


                 Brad    

[Tanazzo]

Thank you for the answer. I will try what you suggest tomorrow, and I'll let you know.
Thanks again

[HotBluePlates]

Your schematic lists this cap at only at only 25v. I don't think that's enough, should be at least 50v I would think. And it lists the cathode resistor as only 1w, probably need at least a 5w R.

Notice the voltages indicated on the schematic.

6.36v on EL84 cathode across a 130Ω resistor. The 25v cap is then obviously more than enough. 6.36v / 130Ω = ~49mA. 6.36v * 49mA = 0.31w, so a 1w resistor is plenty.

[Willabe]

Whoops, sorry, my mistake.


            Brad     

[Tanazzo]

A lot  of information I never hoped.
I'm just a beginner, my first amp. I never didn't notice de cap that was turned, maybe after a good check.  Thanks Brad, and also to "Hotblueplates". I tried two used caps. I have to go to the big Town to find electronic components: Liège, Luik or Lüttich (French, Dutch and German).
With all the caps the same story, but I think I ruined the cap with desoldering it, and after they had such little pins to solder that maybe I ruined them.

[Willabe]

I'm not sure what your saying.

It's ONLY the power tube cathode bypass cap that looks like it's wrong, it's +/- are  backwards. Just change that cap the rest look fine.

Do you know how to take test voltage readings on the B+ line with a meter? If so then check the dc voltage at each + end of the filter caps. That will tell you if they are working. They are listed on the schematic you posted as, B+1, B+2 and B+3.


                Brad      

[Tanazzo]

Hi Brad, I have to go at 60 km, from where I live, to find new caps, so I tried used caps, but I think they were damaged.
I put here a picture to let you see how I take the readings on the meter. I think I'm right. In this way I took the voltages, that are written in black on the schematic, the first time I powered the amp. 

[HotBluePlates]

... I think I ruined the cap with desoldering it, and after they had such little pins to solder that maybe I ruined them.

That's possible. But if you remove that cap, the amp will continue to function, so you need not worry if it's a while before you can replace it.

What is the electrical tape near the EL84 socket? Some kind of splice of the heater wires? If it's to hold together some kind of splicing of the wires there, I would suspect that splice first. Maybe remove the tape and splice, and run a new pair of wires?

[Tanazzo]

Hi HotBluePlates, on the picture I took away the electrical tape to let you see what  happens under the tape. Yes, it's a splice of the heater wires. So you think that is better to run a new pair of wires from the 4th and 5th pins of the EL84? Two wires on the 4th pin and other two wires on the 5th. And the 4th pin is also connected with the third pin, according to the schematic.

[HotBluePlates]

... So you think that is better to run a new pair of wires from the 4th and 5th pins of the EL84? ...

Well, the amp didn't work, then it did. Often, that means an intermittent problem. And any splice draws attention as a possible intermittent connection.

So it couldn't hurt to run the wires properly.

But has the amp stopped working again since the first time (and then began working again)?

[Tanazzo]

Each time it stopped working when I changed the cap, I didn't wait to power it up again with the same cap. Yesterday I found a 400v 100uf cap, I tried and after 30 minutes it still works, maybe  the problem is solved. The wires of the splice are soldered together, the tape was used to avoid that the wires would touch together. But it seems that after all they are not touching anyway.
Gaetano

[sluckey]

Each time it stopped working when I changed the cap, I didn't wait to power it up again with the same cap. Yesterday I found a 400v 100uf cap, I tried and after 30 minutes it still works, maybe  the problem is solved. The wires of the splice are soldered together, the tape was used to avoid that the wires would touch together. But it seems that after all they are not touching anyway.
Gaetano

Maybe it is fixed now. After all, the cap was installed BACKWARDS to begin with.

[Tanazzo]

Hi, here you can see my amp that's finished. I put also photos of the cab and some guitars that I build.
Ritchie200 and super&plexi asked me if I could post some pictures of the amp and the jumbo. Here they are.

[Ritchie200]

Beautiful work on the Jumbo!  Did you do all the inlay?  Post another pic when you get the cabinet finished.  Nice job!

Jim

[Tanazzo]

Beautiful work on the Jumbo!  Did you do all the inlay?  Post another pic when you get the cabinet finished.  Nice job!

Jim

Thanks Jim. I did the inlay on the bridge, on the palette and the binding. The pickguard and fretboard were bought with the inlay already done.