Selmer Little Giant MKIII Clone Build (couple questions)

[Skev]

If you pull the preamp tube out is the hum still there?

Does the level of hum change with the volume control setting?

Are your output and power transformer cores at 90 degree angles to each other like they should be, or are they inline?
If this is wrong many times even with no tubes in the amp it will hum.


Sometimes the magnetic 50hz flux of the PT will couple right into the speaker with no tubes in the amp.

Are your filament wires twisted tight, are any of the preamp tube grid wires (pins 2 and 7) laying on, Or too close to the filament wires?

Are the B+ wires feeding the recto twisted as they should be?

Try a known good non humming preamp tube as this could well be your issue!

With your meter set for low ac volts how much ripple do you read at the power supply side of the preamp tube plate load resistors ?

Does the center tap of the power transformers filament winding have a solid ground ?


I list these in no set order, just a bunch of things to check out!

Hello. The amp is still working very nicely...excepted the 50hz hum, that i still can t eliminate.
The hum is still present without the preamp tube
It slightly decrease while lowering the volume pot
the two transformers are at 90degree opposite.
The "0" tap is directly soldered to the "-" of the first 32uf cap itself linked to the chassis.
I ve even change that first 32uf for a 100uf !!
I ve tried to make the "fake" center tap at el84 heaters with two 100ohm resistors.
All the wires are twisted tightly...
Very strange...i tried different el84 and 12ax7...
No difference. I have just 1 ez80 so i can t compare with another
Except all that, the amp behave really good and seems reliable.

[Leo_Gnardo]

Single ended power amps - hum unless the power supply is extremely well-filtered. There's no hum-cancellation as in push pull amps.

What I've done in Champs, you could do in your Selmer build. I pre-filter the power supply with a 22 uF cap and 4 Hy 90 mA choke - the same choke that's used in a Twin reverb and similar amps. With 90 mA rating, that's enough to handle all the bias current of the output tube plus drive tubes, with some left over. The hum level goes from apparent, to a mere whisper. "Is the amp on?" Like that. Choke value isn't all that critical. I figure anything from 2 Hy to 5 Hy should be sufficient, and 60 mA up current rating should be enough to run an EL84.

With the pre filter in place, you take the output transformer supply from the second node now, not the new first node. Some mid 50's Fender tweed amps did this. Credit Leo Fender for the idea which I merely nicked.

[Skev]

One more time, thanks for your help.
I ve a 4h 90ma? Am i ok?
Should i plug it betwwen the two 32uf cap or just after the ez80 ?

[Leo_Gnardo]

One more time, thanks for your help.
I ve a 4h 90ma? Am i ok?
Should i plug it betwwen the two 32uf cap or just after the ez80 ?

You're going to insert a CL filter section between your rectifier and the existing filter, see my sloppy schematic.

You're going to disconnect the rectifier from the stock first filter cap and wire the new CL section as in the schemo.

Your choke fits the bill perfectly. Whatever cap you have on hand should suffice, 20 to 50 uF with voltage rating in excess of amp's B+ voltage.

On testing, you may notice the B+ going to the OT is lower by a couple volts, probably 5V or less. No big deal. Hum should be noticeably diminished.

[Skev]

Thanks for the advices and the shemo. I m gonna try that...and close this chapter with a very nice little beast!
I ve finally made all the improvement ( 250v plate voltage, 1k resistor on el 84 screen, 270ohm cathode biased... The amp has made a big jump to next level now ( i mean for a 5w !) , time to solve minor problem!
Thanks

[Leo_Gnardo]

Thanks for the advices and the shemo. I m gonna try that...and close this chapter with a very nice little beast!
I ve finally made all the improvement ( 250v plate voltage, 1k resistor on el 84 screen, 270ohm cathode biased... The amp has made a big jump to next level now ( i mean for a 5w !) , time to solve minor problem!
Thanks

Something we should all remember, the "small" amps, champs & similar, were intended as "student" or "beginner" amps. The manufacturers didn't go to a lot of trouble trying to perfect their operation. It hums, it buzzes, so what. Leaving it to us who appreciate the tone of the little guys to perfectionize, because they're terrific for recording and very small gigs.

Good luck with yours. One of these days I'd like to build up one of those little Selmers.

[sluckey]

Another factor is the speaker. These little 2 or 3 tube beginner amps usually had a small cheap speaker too. So even though the amp was happily humming along, the speaker could not reproduce the hum very well so you didn't notice it as much. But today it's very common to connect one of these little hummers to a big speaker that can reproduce the hum very well. I've even seen a champ head sitting on a 4 x 12 cab.

[Skev]

You are right ! I ve noticed that too. And since i will probably use this amp a little bit more seriously , i would like to cancel that hum a little bit more.


Leo Gnardo, you are right , i won t make that amp sing like an ac30 ! But i m a bit proud of hearing that amp singing better each time i mod it thanks to you . So this hum canceling is the last mod i will do... And most of my buddy already love that little thing!😉

[Rodcaster]

Hi everyone !! javascript:emoticon(':D')

Let's first introduce quickly myself !

I am a 24 years old french musician located in Toulouse, who built 3 electric guitars and I am now going a step forward. Indeed, I am building my own Selmer Little Giant mk3 using parts from an old tube radio (Radialva super but 57).javascript:emoticon('')

I get back the power transfomer, the three 9-pins socket, the EL84 and the EZ80 (miniwatt-dario).

I ordered the other parts on ampmaker.

I read several times this thread and I thank all of you for the different advices you all gave.

You can discover with the pictures below my project. I feel a bit ashame about the quality of the soldering but I do not have the magic soldering product which helps the tin to stick on everything...

I've finished today the wiring and I've done the different improvements (10uF-400 V cap, 200 ohm resistor, 1.5 Kohm resistor between the pins 1 and 2 of the EL84, switch for output impedance)

However, I tried today to power on and I had few troubles. Here is the chronological description of what happened.

-Switch on
-pilot light OK
-Warming of the tubes OK (glowing slightly)
-hearing of a loud hum but I do not recognize the 50 Hz freqency
-no sound produced while guitar plugged in
-Blowing up of the 250 mA fuse
-swiching off =s

Then I did few test :

None of the capacitors are charged, while I put the multimeter on any cap, I read 0 V. So I guess the current did not go beyond the EZ 80.

Here my problem, could anyone check my wiring ? I am not pretty sure of the output transformer connection but I do not think that it could be the main issue.

I thank you for having a quick look on it !!! =)

Cheers !!

[martin manning]

Bonjour! Toulouse is a beautiful city.

Did you build to the schematic at the beginning of this thread? Do you have a wiring diagram for the OT you are using? Are you sure that the tubes you salvaged from the radio are good?

I would remove the tubes except for the rectifier and get the power supply sorted out first.

[Rodcaster]

Yes I used the selmer little giant mk3 shematic of the beginning of the topic.

The tubes come from an old radio and the radio worked before unmounting it. So I deduced that the tubes was OK...maybe it is a first mistake.

The output transformer is the one bought on ampmaker, therefore the only technical documentation I have is this :

http://www.ampmaker.com/store/5W-single ... ormer.html

What test must I do while I remove the 12ax7 and the El84 ?

Thanks !!

[Rodcaster]

I just tried to power on without the 12ax7 and without the EL 84, and I had exaclty the same results.

Hum and stop !!

[martin manning]

Ok so the tubes are at least functional. Since you have no DC, start at the beginning. Measure the AC voltage across the EZ80 anodes (pins 1 and 7) and the DC voltage to ground from the cathode (pin3). Also make sure that the transformer's center tap is grounded.

Edit: if the fuse opened again then there is a short somewhere. Take the EZ80 out and measure resistance to ground from pin 3. That value should be quite high. The rectifier itself may be shorted.

[Rodcaster]

Okay,

I read 238 V AC across pin 1 and pin 7
I read 47 V DC across pin 3 and ground.

My center tap is grounded yes !

I have to precise a little something

When I say that the radio worked, it was not exactly as easy.

Actually, the radio lighted on, but I did not received any broadcast, there were just a slight hum.

But I plugged my guitar directly on the volume potentiometer and I succed to obtain sounds, it was a clean and quite loud sound during 2 minutes and then it turned into a crunchy and quiter sound...

Maybe my EZ80 is simply over aged ?? (After 58 years of loyalty, I think it could take a rest... )



EDIT : I have 131 ohm across pin 3 and ground.

Good value ?

Once again, thanks a lot !!

[martin manning]

131 ohms is too low... It should be tens of thousands, the resistance of the filter caps. This measurement was taken with the rectifier out? If not remove it and remeasure. If it was out perhaps you have a wiring error.