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|18th May 2007, 02:15 PM||#1|
Join Date: Oct 2006
grounding and 12AV6 diodes
It's me with my Fanton mono amp conversion.
The grounding scheme appears to be odd. Please ignore the resistor values on the attached schematic.
Almost everything is connected to the AC return, except for the anode resistor on the 12AV6, which connects to the chassis via a wax capacitor (a Good All Type M503) that also connects to the AC heaters and the diodes on the 12AV6. This is the only place that is connected to the chassis.
It's probably a good idea to replace that wax capacitor, but I thought it might be a good idea to revise the circuit. Here's what I'm considering:
Ground the chassis with a wire to the N68X isolation transformer return.
Clip the lower leg of R2 and connect it to the ground.
I'm not familiar with AC heater schemes and am looking for advice.
(i.e. ground pin 3 and disconnect the diodes and discard the cap)
You'll notice a second speaker. It's a thin tweeter mounted underneath the main speaker.
I'm also considering ordering a Mod15-6 6" speaker to replace the old radio speaker. I may also remove the tweeter, but will have to see what it does to the sound.
|19th May 2007, 07:37 PM||#2|
Join Date: Oct 2006
Well, I checked and there is a ground wire tucked underneath everything.
The cathode resistor and diode straps are grounded.
The wax capacitor connects the ground to the chassis and the 12AV6 shield.
I've updated the schematic with some resistor values and fixed some mistakes, such as NFB.
I removed the tweeter and now the main speaker sounds harsh. I'm pretty certain it needs to be replaced. I've already glued two tears.
Here are the latest issues ...
1. A new speaker (possibly a Mod15-6).
2. There are a pair of Aerovox square capacitors connected to pin 7 of the 50C5's in series with a 470 ohm resistor. Does anyone know how to read these? The dots are white red black, brown white red. I'm thinking that the may need replacement.
3. Replacing all the .01µF coupling caps with something better. They are currently red ceramic disks.
4. Removing the .01µF cap connecting the guitar input to the volume pot. This may, however, add even more treble, which is currently a problem.
This is turning out to be a rather nice project!
Thanks again for all your advice.
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