|Author:||Steavis (registered user: 39 posts )|
|Date:||Wed, May 23rd, 2012 @ 05:28 ( . )|
On 05/23/2012 @ 01:18, Asdain wrote :
On 05/22/2012 @ 19:27, Dan McCormack wrote :
On 05/22/2012 @ 16:08, Steavis wrote :
: : yes I was thinking the same thing.
: : :
: : : Dan: just thinking out loud: is the amp fixed biased or cathode biased? A common mistake is not to ground the point where the 2 220K resistors meet in the PI section when it's cathode biased. Check a 5E3 deluxe schematic (cathode biased) vs a AB763 schematic..
: : :
: : : But since you were talking about adjusting the bias you probably have an adjustable fixed bias..
: : --
: : Yes, it's adjustable fixed bias.
: : Chris, it turns out a 6v lantern battery couldn't put out enough current either, but I unsoldered the heater supply wires from my pilot light and powered them from the 5v DC power tap on my bench power supply, and the hum was still there (It settled down at around 3.7 amps of draw at 5v, btw). So that seems to exonerate the heater supply.
: : You guys seem to be agreeing that the most likely location of the problem is in the heater network for the long tailed pair. I already went over it and it all looked correct, but I'll take another look and see if I can find a problem. Here's another [link] to the picture of my power section (giant version: [link] ) -- does anything stand out to you guys?
: : I appreciate the help in narrowing this down. Can't wait to solve the problem and hear this thing in its full glory.
: : Dan
: The heater wiring definitely doesn't look very tidy, keep the wires twisted tightly right up to the socket.
: How the Heater circuit is grounded is also important. Don't just ground one side of the heater supply. better use an artificial center tap and put it on an elevated potential (or ground if you have to).
: if you still have hum after that, at least your heater wiring is neat then ;)
As Asdain said.
I don't think your problem is caused by the heater wiring but judging from the picture you should redo the heater wiring anyway. Perhaps use solid core 18g wire as you can bend that in to shape easier and it stays in to the same shape. Twist the wires tightly. Either make heater wires float and keep other wiring close to the chassis, or keep the heater wiring close to the chassis and 'float' the other wires.
Does the PT have a CT for the heater wiring? If so you do not make a connection to ground.
> You guys seem to be agreeing that the most likely location of the problem is in the heater network for the long tailed pair.
We were thinking you forgot a ground connection in the PI circuit. Or I think possibly the reverb section as you get the problem when dialing in the reverb as well.
I notice from your picture there doesn't seem to be much solder in the turrets. I assume you soldered it from the bottom before installing the board. Are you sure the connections are all properly soldered? Maybe resolder them with a bit more solder.
Anyways, to sum it up, what I would do to try and tackle the problem:
- redo the heater wiring (see above)
- add some more solder to all turrets
- double check *all* ground conncetions, checking from schematic. (this is where alligator leads come in handy)
- bypass reverb section completely, see if problem goes away.
--* Loud hum in October Stage w/reverb
5/23/2012 @ 11:26--Dan McCormack
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