• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

JE Labs 300b round 4. Wiring, HELP!!!

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I have eight diodes, would it be possible to see a diagram of how the connections should be made?
Use 4 per channel like this:
 

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Hi,

Thank you for all the help and the fun.

John, here is a pic of a layout for the bridge rectifier. Wanted to make sure my polarities are correct. Also, when I use my DMM on the diode, the continuity goes from the + pin to the metal on the mounting hole of the diode. Does this change anything?
 

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diyAudio Senior Member
Joined 2002
THEY SCARE ME.

Hi Paul,

Just make sure they don't short anything will you?
We'd like to see you alive&kicking.

If ever you run tests on your amp remember you have the full B+ on the heatsinks too...

It can't be stressed enough...a couple of hundreds of volts,a few amps will kill you and anyone else for that matter.

I fail to understand why they arranged it like that...lethal stuff Paul.:xeye:

Cheers,;) Errrr....I am not kidding really.
 
Hi Frank,

Thank you very much for your tip and concern, it's nice to be loved:hbeat:. I do want to keep that thing ticking. I do have one question, the tap from the tranny to the bridge is 7.2v 1.5a, then rectified to 5v 1.5a. the board is isolated from the chassi on standoffs, how does B+ come into play here? Just trying to understand, not miss anything.
 
Hi,

Questions,

1. Because the 300b filament is rectified to DC, should I still use the 50 ohm hum pot that connects to the 300b filament? Will this short the circuit? What is the correct polarity if it should be connected?

2. On the power supply schematic, the 100k 2 watt resistor that connects after the choke, Shows a connection to ground. this is to earth ground? connect to IEC socket ground?

3. The choke for the B+ has three tabs for connecting, rated 10-15h 125ma. How do I determine which is which? two connections have continuity while one does not.

Thank you again. Making progress, slowly and carefully.
 
diyAudio Senior Member
Joined 2002
DC.

Hi,

1. Because the 300b filament is rectified to DC, should I still use the 50 ohm hum pot that connects to the 300b filament? Will this short the circuit? What is the correct polarity if it should be connected?

You should leave that pot out.
Polarity on the heaters doesn't matter.

2. On the power supply schematic, the 100k 2 watt resistor that connects after the choke, Shows a connection to ground. this is to earth ground? connect to IEC socket ground?

Chassis ground.It is called a bleeder resistor since it bleeds off the caps' charge when you switch it off.


3. The choke for the B+ has three tabs for connecting, rated 10-15h 125ma. How do I determine which is which? two connections have continuity while one does not.

Connect for continuity,in an out that's all.
You should measure some DC resistance between the two points.
No idea what the other one is for.

Thank you again. Making progress, slowly and carefully.

You're welcome.Take your time.

Ciao,;)
 
diyAudio Senior Member
Joined 2002
GROUNDING MATTERS.

Hi,

Well,personally I wouldn't connect the IEC ground wire at all.
If you feel inclined to, connect it to a single point on the chassis.
Close to the powercord inlet.

The bleeder resistor (100K) + the -ve leg of the 47microF cap can be connected to a single ground point close to the resistor.
Connect the xformers CT grounds to the same point.
That would than constitute PSU ground.

A simple bolt (brass prefarably) and nut + the required amount of solder lugs ( I count four for that point) fit on the bolt are than to be used as solder posts.

Repeat the same procedure for circuit ground, choosing a point at mid distance is often best.
I explain: RCA ground to 300B cathode R ground distance divided by two.
From looking at your circuit a point close to 6SN7 would be fine.

Some experimenting may be called for but in any case it won't prevent the system from working.

Check with a DVM that your groundpoint is working (conducting) by measuring between lugs,bolt and chassis for 0 Ohm resistance.

Cheers,;)
 
Hi,

More questions, but almost done.

Are the centers of RCA jacks posistive connection?

Also, my OPT has multiple leads that you connect in certain combinations to achieve the required ohms for output. I have two leads left for the binding posts. On the instructions for OPT wiring it states to "ground the F output lead/s" I assume that one wire goes to one post and the other wire to the other post, then run a ground wire from the "F" lead to ground? or should both binding posts be grounded?

The attenuator needs to be wired. there are three holes labeled " in / out / grnd " Now the 'in' wire is from the RCA? the " out" wire to grid of first signal tube? ground goes to ground. This would put the attenuator in series with the signal coming into the amp?

Would it hurt anything to wire up the hum pot? or will the hum pot not work with a DC filament? I know it adds to the signal path but want to try the hum pot if I can use it.

:scratch1:What should I double check, besides everything, before pluging the amp into the wall socket and throwing the amp switch closed( WITH A LONG BROOM HANDLE:scared: )?

I should post a pic of the wiring, before turning it on, so everyone can take a look to spot anything I might have overlooked.

Thanks again and have a musical new year.
 
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