The Amp Garage

A Traynor YBA-1A can make the screens of EL34s glow, even with no signal. You can see it around the top mica spacer if the shinny silver getter around the top of the tube is small enough. On some tubes there is a round hole in the plate that also lets you see the screen grid. It's basically a thin w...

The cause of grid current is usually low plate Voltage. Check the dropping resistors in the B+ string and the plate and cathode resistors. If those checkout, a conductive board is a strong possibility. Move the 68K resistors off the board either on the input jacks or at the tube socket. What is the ...

I measured the AC voltage across the speaker jack and got a max value of about 3.5vac, but it seemed to hover around 2.0vac which is consistent with the kit guide/schematic. That 2.0VAC reading is supposed to be under the conditions detailed at the upper left of the schematic. You set the Volume co...

One could expect some buzz from ripple voltage on the B+ in a single ended amp, so another interesting measurement would be the AC voltage at the speaker with the preamp tube removed. It was previously stated that the hum disappeared when the preamp tube was removed, so I concluded that the issue w...

One of the problems that comes up from time to time in a thread like this is that hum is relative. Very few tube amps are dead quiet. This amp, being of the low cost, novice builder variety will surely have some hum, but how much is normal for this amp ? It's difficult to judge the amount of hum whe...

In comparison the 6L6GC (used in the champ article) running at a grid No.2 voltage of 250V (plate voltage =350V) in class A, requires 54mA = 250 x 0.054 = 13.5W, plus 5.67W for heater current when it pulls 0.9A at 6.3V. This tube used in the champ article is clearly "greener" putting out around the...

This is just an experiment to see if we can find the path of how the hum enters the amplifier. Set the controls so you get a fair amount of hum. Turn the amp off and let the tubes cool down. Turn the amp back on, let it warm up until you can hear the hum, cup your hand around V1, the 12AX7. Any chan...

I suggest the following change to grounding. Remove the wire on the terminal strip that ties the preamp filter cap to the other filter caps and ground it in the preamp.

Let's review the symptoms. There should be no hum between the time when you turn the power on and when the tubes warm up, please confirm. Does the Volume control affect the Hum ? Is the hum very low or zero when the Volume is all the way down ? Does the hum gradually get louder as you turn the Volum...

The wire going to pin 9 of the 12AX7 is laying right across the wires going to pins 1, 2 and 3 which is the input triode side. I suggest you unsolder the wire from pin 9, untwist about 1 turn from the wire that goes to pins 4 and 5, and re-route that wire along the chassis, as far as possible from t...

An earlier version of the schematic shows two 100 Ohm resistors on the pilot light holder, but on this version, one side of the heater supply is connected to the cathode of the EL84. If you have added any resistors per suggestions above, remove them, they will alter the bias of the EL84. I think a "...

Is the opening going to be on top, on the back, or both ?

Perhaps what you want is more like this:

The grounded resistor on the jack is the 'input resistor.' It sets the amp's input impedance to 1,000,000 ohms (1M) or 2,000,000 (2M) to boost the signal voltage from the guitar. The 2 x 33k resistors are 'grid stopper' resistors. They help stabilize the amplifier by removing much of the audio sign...

Now I believe the PT wire might be too short to reach the switch lug if I try to rewire it. Are you saying that having to "extend" the PT wire with new wire would be a bad idea? If you must extend a transformer wire, it's best to use Heat-Shrink tubing to insulate the splice. Don't be tempted to us...