Problems with TheAirTightGarage AB763 board.

[sigkarl]

We are building an amp from TheAirTightGarage AB763 board.

We have three issues with this amp.

1. Hum on idle with all pots at zero. It does not change if you crank up
volume, tone or other pots.
2. Extremely weak tremolo, almost impossible to hear if it is on or off.
3. The sound is acceptable at low levels, but is very harsh and ugly, and
the amp farts and cuts out at high volumes. Changing the speaker does not
cure this.

On this amp we decided to wire the heater "Hiwatt style". Heater wiring could be the cause of issue number one, but not issues two and three.

The voltage charts are @
http://www.el34world.com/forms/valvedata/AB763%206V6.htm
&
http://www.el34world.com/forms/valvedata/AB763%206L6.htm

Is anyone familiar with these problems?
Does anyone have a schematic for the TheAirTightGarage AB763 board?

Following are some pictures of the wiring.

[FYL]

1. Hum on idle with all pots at zero. It does not change if you crank up volume, tone or other pots.

Check your wiring and grounding. Revert to a standard twisted-wire heater wiring.

http://www.el34world.com/charts/grounds.htm

2. Extremely weak tremolo, almost impossible to hear if it is on or off.

Lower the 470K R in the trem section as per the instructions (last paragraph). Run the amp with cooler bias - very important with "hard" 6L6s.

http://www.el34world.com/Hoffman/instructions.htm

3. The sound is acceptable at low levels, but is very harsh and ugly, and the amp farts and cuts out at high volumes. Changing the speaker does not cure this.

Sounds like oscillation and/or blocking disto. Use lower gain tubes beginning at V1, use lower value coupling caps. Check wiring and stoppers, chopstick your leads. Some components could also have been damaged : check coupling caps and plate resistors.

Full layo: http://www.el34world.com/Hoffman/images/AB763Old.gif
Doug's AB763 build (no dog house for filtering caps): http://www.el34world.com/Hoffman/images/IMG_0633.jpg

BTW, Triode, head honcho at TAG, is a frequent contributor here and very helpful, you may contact him here or thru his web site.

[stingray_65]

Nice clean build!

[Shrapnel]

Personally, I'd address the other issues before I redid the heaters. If getting the other issues fixed still leaves undesired hum, only then would I consider redoing the heaters.

[sluckey]

There's an easy way to tell if filament wiring is causing your hum. Disconnect the PT filament leads from the filament string and power the filaments with a 6V lantern battery.

[Throstur]

Hey sigkarl, does the reverb work? I installed one of the ATG boards two years back and it worked fine. These boards are quality work IMHO. I would suspect a faulty component/components after making sure the wiring is good.

[sigkarl]

Thanks all.
The amp is still huming. When preamp tube 2, V2 is removed the hum disperses.
Throstur : the reverb doesn’t work.
Replacing the preamp tubes doesn’t change anything.
Regards.
Sig