Selmer Little Giant MKIII Clone Build (couple questions)

[Rodcaster]

Yes I read 133 ohm across pin 3 and ground of the rectifier socket while the rectifier is out.

So wiring error ?


I do not know if it is related to my problem :

-About the output transformer, I see on the selmer schematic that the secondary wiring has no wire connected to ground, but when you have a look on a fender schematic, one the the two wires of the secondary is linked to the ground. Might it be an issue ?

-Another question concerning my potentiometer, I already checked in internet before doing my wiring, but are you agree with my configuration ?

Maybe could you have a quick look on my wiring....?

https://imageshack.com/i/f0npO3XXj

Thanks a lot again.

[martin manning]

You should ground the common lead on the output transformer secondary, but that isn't causing the problem you have now. I'm reading on my phone so I can't see your photo very well. I'll have to look later.

[Rodcaster]

Ok for the common lead, I'll do it.

I checked again my wiring, it fits to the schematic.

You did not tell me what you think about the 47 V DC across the pin 3 and the ground of the rectifier. Is it enough to load the caps ?

What I really can not understand is why these caps are not loaded as I have a current at pin 3 of the EZ80....

I let a little bit more the amp switched on while I did my measurments, and it stopped to hum and it did not blew up the fuse (which is now a 1A, I do not have 250 mA anymore..) But I still did not have any sound, which is normal if caps are not loaded.

I feel really sorry to bother you with my problem, but I have to say that I do not know how to procede to test the different parts step by step..

[Rodcaster]

It works !!!!!!! No 50Hz Hum, but few little sound cracths while attacking a bit more on strings. Idea ?


Actually the big problem was on my output tranformer !!

I shorted it while I plugged the central pin of the primary on the ground ! It is not supposed to be connected. I thought that it was a kind of center tap.

Sound was great during 10 minutes, except a big sensitivity to vibrations, sound sometime just cut off and reappear.

Moreover, my tone pot does not do anything, it even cut the sound when it is turned on the max value...!!

After 10 minutes of playing, the sound became crunchy when it was not supposed to...with the volume pot at 1 or 2. And the volume decreased.
I think I will change the EL84 and the EZ80...

[martin manning]

Ah good! I was just about to post and suggest that you test just the part of the circuit with the power transformer, the EZ80, and the two filter caps.

I agree trying new tubes is a good idea if you have no idea about the condition of the ones you have. Are they easy to get? For now you could measure voltages on all the tube pins and post them. That will give some idea of how things are working.

[Rodcaster]

Great ! I'll do that !

I will post my different voltages later !

Do you have an idea for my tone pot problem ?

[martin manning]

I don't know about the tone pot... wiring error most likely.

In your layout you have the EZ80 on one side of the chassis and the EL84 on the opposite end. It would be better to have the EL84 next to the EZ80 to shorten the lead wires. Also, it looks like you have used steel screws for terminal posts on your circuit board, which are difficult to solder to. If you like the amp, you could rebuild and improve it fairly easily.

[Rodcaster]

I did not found any wiring error..=(

Yes it was almost impossible to solder on screws !! The tin did not stick to it !

Yes, I really thing, once all my improvments done, to do again the wiring with new wires and with a nice layout.

I would like to add a Negative feedback loop like "Uncle doug" did it ont his 5F1 fender.

Do you thing it is possible ??

Check this out : https://www.youtube.com/watch?v=PtoPma2-N1I

=)

[Rodcaster]

Hey I did few measurements of voltages :

EL84 :

Pin 3 : 8,3 V
Pin 7 : 255 V

EZ 80

Pin 1 : 228 V
Pin 3 : 270 V
Pin 4, 5 : 6,4 V
Pin 7 : 230 V

What do you think about these ?

I also measured the voltage across all my caps, and all of them display something between 70 and 250 V excepted my 100 uF cap of the Tone!!
0 V across it !

I thing it is maybe a defective component !

[martin manning]

Looking pretty good, actually.

Pin 1 and 7 (anodes) on the EZ80 are AC volts to ground, yes?

What is the screen voltage on the EL84 (pin 9)?

What about the ECC83?

The tone pot shouldn't have any DC voltage on it since it is blocked by the 0.1uF cap.

[Rodcaster]

Yes it is working fine do I did not measure all the voltages.

Really nice sound !

I just have a main problem with the tone pot.

I did like on the selmer schematic but it does not do anything when I use the tone pot. Just at the end, it decreases the general volume. It is really strange.

I tried to adapt the Fender 5F2 schematic but it was worst. DO you have an idea to have a tone pot working correctly ??

Thank you very much !

[Rodcaster]

Hi everybody !

I finished my custom amp, and it sounds great! Volume, tone and negatif feedback knobs all work perfectly.

I would like to thanks you for the thread and for your responses.

Here few pictures of the amp with my home made guitars.

Enjoy !

[martin manning]

The amp looks great, and the guitars too! What was wrong with the tone control?

[Rodcaster]

Thanks !!

It was the selmer schematic which was not a good choice. I just adapted the tone and volume schematic of the Fender 5F2 and now everything works perfectly.

Thank you very much, and maybe see you for a next amp project !