Homemade EL84 Push-Pull amp doesn't work

[neptunes4you]

Hi! Yesterday i built an EL84 push pull amplifier. When i turn it on, and the tube heaters start working, a really weird noise comes out from the speaker; a buzz, around 50-100Hz. Any thoughts on where i can start the troubleshooting?
PS: sorry for my english.

 

[neptunes4you]

Oh, and that's the only sound, that comes out from the speaker, no music at all.

 

[NZkeith]

Hello there, sorry to hear that there is no music..... Very difficult to work out your problem without 'seeing' and knowing just what and how you have built the amp....
Maybe you could post some photos of both the top and bottom of both the amp and powersupply section.....

You could start by checking that the powersupply actually does produce a well smoothed 250V DC. Are you using smoothing capacitors or a choke ????? Heater wiring method and placement also a possible !!!!

Also earthing could be an issue. Also an EF86 is very prone to pick up stray signals when not shielded...

OK then, very best with your project and I am sure that someone will be able to help, but with the VERY LIMITTED info., it is very difficult to even start to imagine what you have there....

Keep well and keep us informed...

 

[neptunes4you]

The psu is ok, i use 2 capacitors, to reduce the buzz. And also a resistor. So it's a CRC filter, i guess. But, i don't know what the shielded EF86 look like. How can i do that? I heard that the EF86 has an internal shield connected to pin 2 and pin 7.

 

[Yvesm]

Does noise still there whith the EF86 removed ?

Pse, show the PSU schematic, this may help !

Yves.

 

[neptunes4you]

This is the schematic: http://diyaudioprojects.com/Tubes/EL84-Push-Pull/EL84-6BQ5-Push-Pull-Power-Supply.png
A toroidial transformer with 2x190V, and 1x6.3V on it. 2 way rectifying with 1N4937 diodes, and CRC filtering with 2 470uF 200V capacitors in serial, a 100Ohm 5W resistor, and 2 330uF 200V capacitor again.

 

[neptunes4you]

Yes, the noise is still here without EF86.

 

[neptunes4you]

And without the ECC82.

 

[DF96]

No, you don't need DC heaters, especially on the output stage.

You probably have a wiring error, or a heater-cathode short in an EL84. Measure the DC voltage at the EL84 cathode with both valves in, then with each separately - but do the 'one EL84' measurement quickly as the valve may be taking too much current.

Check the output transformer wiring too. Some P-P circuits rely on hum cancellation in the output transformer, but miswiring can stop this from happening.

 

[Yvesm]

Right.
I suppose you have used the 6.3v winding to feed the heaters instead of the SMPS drawn ? If so it MUST be referenced to the ground.
Other probablity is you've make a mistake somewhere . . .
Should be fine to look at if you can attach a photo.

Yves

 

[neptunes4you]

Yes, i have 6.3v AC. "referenced to the ground" What does that mean?

 

[neptunes4you]

No, you don't need DC heaters, especially on the output stage.

You probably have a wiring error, or a heater-cathode short in an EL84. Measure the DC voltage at the EL84 cathode with both valves in, then with each separately - but do the 'one EL84' measurement quickly as the valve may be taking too much current.

Check the output transformer wiring too. Some P-P circuits rely on hum cancellation in the output transformer, but miswiring can stop this from happening.

I have 5 EL84 tubes, and all of them creates this annoying sound.

 

[Yvesm]

Yes, i have 6.3v AC. "referenced to the ground" What does that mean?

That means that one side of the heathers must be tied to ground.
Better: use two equal resistors in the 50 to 100 ohms range from each side of the heater to ground.

Yves.

 

[neptunes4you]

That means that one side of the heathers must be tied to ground.
Better: use two equal resistors in the 50 to 100 ohms range from each side of the heater to ground.

Yves.

So that means, i need to connect both side of the heater winding on the transformer to the main 250V power supply ground through a 50-100Ohm resistor? I can do that. I'll be right back after a few minutes.

 

[neptunes4you]

I did it, nothing changed. :/

 

[gokite]

You probably have a wiring error, or a heater-cathode short in an EL84.

Agree. If you get nothing but buzz you wired something wrong using that schematic it should at least play music. Recheck all your connections and solder joints. Any detailed shots of the interior of the amp?

If you have a DMM which I assume you do, what are the voltage readings on the tube pins and your rail voltage supplies feeding each plate? This should give us/you a good idea of what's going wrong.

 

[TheGimp]

Looking at the reverse recovery characteristics of the diodes, I would put a 10nF cap in series with a 10R resistor across each diode (from cathode to anode) in the ps. It does not matter which end the cap goes to. This is a snubber or Zobel and will help with switching noise.

 

[Yvesm]

So that means, i need to connect both side of the heater winding on the transformer to the main 250V power supply ground through a 50-100Ohm resistor? I can do that. I'll be right back after a few minutes.

No, more precisely to the negative pole of the hi voltage supply wich is the "reference" for all other.

 

[neptunes4you]

No, more precisely to the negative pole of the hi voltage supply wich is the "reference" for all other.

I did exactly what you said. Nothing changed. I decided to make a PCB instead of "air mounted" design. This is just stupid. I thought if i made the circuit "in the air" it's going to be easy to modify. Well, it turned out, this is not true.

 

[DF96]

It might be better to persist for a while with the point-to-point version. The reason is that if the mistake is due to a misconception on your part then the PCB will probably include that misconception and then be harder to correct. For example, you have misread the EL84 pinout.