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. Oh, and that's the only sound that comes out from the speaker, no music at all. Any thoughts on where can i start the troubleshooting?
PS: sorry for my english.

[Jack Hester]

I would start by going though your power supply very thoroughly and be sure it's built correctly. This can be the whole source of your hum. You didn't include it in your amp schematic.

Once you have eliminated this as a problem (polarity of caps, grounds), you can lift the 27nf signal cap, feeding the grid of your phase inverter. Separating the pre-amp from the power amp.

Something I do on my schematics is put a component label on each (R1, C1, L1, T1 etc.) so that when I'm troubleshooting, I can refer to them by this label. I would love to build more scratch-built amps, but do very few of them, as I spend my time troubleshooting and repairing the old ones that I have a passion for.

Anyway, see if the amp goes quiet when have tried these.

Jack

[Jazz P Bass]

Take your schematic & measure all of the plate & cathode votages.
Take the voltage measurements directly at the pin of the socket.
Write them down on the schematic next to each plate & cathode pin.
Scan it & post it here.
These measurements will help locate the trouble.

[loudthud]

Post a link to a schematic for the power supply. Does it include a center tap for the 6.3VAC heater winding or have a resistor from each side to ground?

[Bruce / Mission Amps]

You could get some DC voltages on the plates and cathodes of the tubes for us... frequently it's just input jack miswiring. Or, start by pulling the two power tubes out and then check to see if there is a volt or two of bias voltage on the cathodes of the preamp tubes. If no, you have bad tubes or a wiring problem that needs attention asap. If yes, then plug your guitar back in and turn the tone controls and volume pot(s) up at least half way. Switch your voltmeter to a lower AC voltage and go to the other side of the preamp tube's coupling caps and see if there is a swinging AC voltage coming out of those coupling capacitors when you strum the guitar. If yes, move over to the grids of your power tube sockets (still no power tubes) and do the AC test to see if any or all that amplified signal is getting to the power tube sockets. A good EL84 amp ought to be able to push 15vac-20vac into the grids of the power tubes when cranked full tilt. If yes... put the power tubes back in with a speaker load connected to the output jack. Check to see if there is any DC voltage drop across the high voltage supply to the output transformer and the actual plate lug/pins of the EL84s and the two screen pins.
If no.... you probably have a high voltage rail miswiring or something like that. You didn't say if the power tubes are cathode biased or fixed bias. If cathode biased, you should be able to find 12vdc to 15vdc of DC bias voltage across the biasing resistor on the EL84s.
If no... they're not turned on for some reason. Even if you don't find the issue right away, that should get you going in the right direction an eliminate some issues to make it easier to troubleshoot.