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The Newcomer's Forum / Amp repair?!
« on: January 23, 2019, 12:35:01 AM »
I have an Epiphone Valve Junior Combo amplifier that needs repair. It used to play fine until I decided to swap in/out a few different tubes both at pre and power section. I suspect the repeated removal/insertion of tubes and retention spring must have done something to the socket. I simply cannot turn the amp on and start playing and in less that a few minutes there is a weird crackling noise, then some hiss/static noise and then silence. I then check the tube with a tester it's dead, both the heaters (pins 4 & 5) show no continuity. I have ..."lost" two brand new tubes so far, so I am not risking putting another in there unless I identify the problem. This is a very simple design the only thing that could be the culprit is a resistor and cap and of course the voltage that is provided to the power tube. BUT, there is absolutely no problem at all with the preamp tube (which I also swapped in/out many times) so I am kinda stuck. I can follow instructions, I am good at soldering, I have a decent DMM and tube tester (albeit borrowed from a friend), I understand electronic schematics although definitely not an expert. I believe I can do the troubleshooting with someone's guidance. It is simply not worth paying a tech to look at it, it will probably cost me more than the amp's value. I am also posting the schematic, however at this early point I am primarily concerned about asking questions in a Solid State forum about a tube amp. Let me know, I hope I did not break any rules. Thank you!

EDIT (1/23): I uploaded another schematic I found online, the tube section is identical but there are some "interesting"(?!) differences in the power supply. Indeed the transformer outputs 6.3 VDC for the heaters obviously after the bridge it should be a bit higher, so indeed the first schematic is wrong (we already know that).  ;)

Hello -

I have recently come across a few old RadioShack mono PA amplifiers at my work which were supposed to be thrown away so I decided to take them home myself and see what's inside.

Each of these PA amps had (among many others) the following parts inside:

1. Two power output transistors of the D718 type mounted on a huge (e.g. worth saving) heatsink.

2. One NJR4558D dual op amp.

3. One output transformer with 4/8/16 Ω connections

4. One power transformer rated at approx. 80W.

I was wondering if these boxes are worth "cannibalizing" for parts. The RadioShack manual claims this is a 35W mono PA amp with 70V capability (not sure what that 70V means). The 4558 is of course a very cheap but certainly usable IC plus due to the age of these boxes it may worth a bit more (could it be an original 4558 ?). The D718 transistor, I was not able to find much on that, except the attached datasheet. It appears they are either used in power boosting applications or as output transistors in power amps (as in this case).

Can you please advise if there is some justification in salvaging some parts? IMO the boxes and the buttons/pots etc on them can be reused in other projects. Nothing serious, just amateur DIY stuff and mostly for fighting ...boredom while learning something new of course!

Thank you!

Preamps and Effects / Mono guitar to stereo effects to 2 amps !?!
« on: October 25, 2011, 05:17:22 PM »
OK, I know the subject of my topic is unusual, to say the least. I will also admit that I am a noob and I know little about electronics, well just a few basics. I also looked up as best as I could into previous topics and could not find something similar.

So ... my apologies if what I will say makes no sense or is not doable. Also, sorry if this has been covered elsewhere.

Here it goes:

I am currently building my first DIY amp using this design --> .

I used a Radio Shack board (see attached PDF) to build it thus I build two side by side.

These little amps are so easy to build, cost is very low, and each puts out no more than 1W of power. I tried both through an 8" speaker, and I also intend to build a 2x8" cabinet later.


I also have an old DOD multi effects unit (3 dimension) which has separate L and R input and output jacks.

I did some measurements and the dual-amp PCB can fit inside the DOD chassis, with all necessary in/out jacks and pots drilled on the DOD faceplate and backplate (after some creative artwork of course).

So ...  here's what I'd like to do IF possible:

Take my guitar's signal, split it in two, send one "half" to L and the other "half" to R input of the DOD, then take each output from the DOD (L and R) and send one to one amp and the other to the other amp. Finally, each amp drives each own speaker within the same cabinet. The DOD chassis will be mounted on top of the cabinet (slide-in, removable) and this whole contraption should measure about (18x22x8)" (HxWxL). Hopefully, it will sound OK. Not necessarily great but OK. I am not a professional musician, just for practice, jamming with friends and playing at (not-so-loud) parties.


So, are you still with me or looking for something to ...throw at me? (lol) (sorry, my jokes, as well my gadget ideas are lousy).


If you are still here, now it's time for some questions:

1. What is the best way to split the guitar signal in two equal/identical parts? Y-cord? Audio Mixer? small DIY circuit?

2. The DOD does not have separate volume controls for its L and R outputs. Therefore it makes sense to keep the amp controls separated too, having a L and a R side on my amp not only will look cool, but it can be practically useful to control how much/what effect goes to each side.

3. Since this simple amp design does not have FX in/out, what signal path is best?

a) Guitar to DOD to amp to speakers? (But how would this sound? Is n't it a wrong idea to insert your FX before the preamp?)


b) Guitar to amp to DOD to speakers? (would that work? I mean can the DOD drive the speakers? I do not think so).

c) If both a) and b) are not ideal, can anyone suggest if and how I can mod this little amp in order to use the FX unit?


So take your time, let me know if all this is possible or not. I understand it might not be worth the time or the money, and yes I agree, I do not intend to spend a lot of (more) money on it, but time, that I got, (no I'm not in jail) especially if this is going to be a fun project.

Thank you all so very much,


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