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TubeHound 23rd December 2002 03:18 AM

JE Labs 300b round 4. Wiring, HELP!!!
2 Attachment(s)
Hi Everyone,

Here is were I need major help, understanding the schematic to wire this baby up. I'll post a pic of schematic and pic of what it looks like underneath. Where would one start? Also, where should I put a star ground? Thanks for the help and there will be a ton of posts in this thread, so please check back often. Thank you!!!

TubeHound 23rd December 2002 03:21 AM

2 Attachment(s)

stigla 23rd December 2002 08:02 AM

Nice article about Stargrounding.

Good luck!


Tomatito 23rd December 2002 08:46 AM

Hallo Paul,

Start with the heater wires. Twist each pair tightly, keep the pairs away from each other and close to the chassis. Keep signal wires away from AC wires. If you have to cross, do so at 90 degree angles.

happy wiring ;)


fdegrove 23rd December 2002 10:37 AM


Make sure you have the socket layout for the tubes at hand.

Take a copy of your diagram and add the pin numbers of these sockets on it.

Also take note of the colourcoding of the transformer and add this to this copy as well.

Then you can start with heater wiring as Tomatito suggests,than the B+ from the 5U4 to the caps and choke.

From there you can add the anode and cathode resistors for the 6SN7.
Connect B+ (390V on the diagram) to the second half of the 6SN7,
the 36K 1W resistor from that point to the anode resistor of the first half of the 6SN7.
Add the bypass cap to the anode resistor.

Connect the B+ (405V) to the OPT and wire the other end to the anode of the 300B.

Solder the B+ resistor (1k8/3W) in series after the first filtercap and the 100K in paralel (this one needs to be grounded,see below) after the choke.

Connect the trimpot,cathode resistors and bypass cap (watch polarity) for the 300B heater.

Solder the 470R gridstopper as close to the grid of the 300B,the bias resisitor (220K) can share the same solder eyelet.

On the second half of the 6SN7 add the cathode bypass cap (100microF/?),again watch polarity.

You can now hook up the input,volpot and its' output to the grid of the first tube making sure to connect to G1.
Jumper the anode of the first half to the grid of the second tube.

From the anode of the second half you can now solder in the coupling cap to the gridresistor of the 300B.

On the diagram you will see a grounding point at the first cathode resistor and one at the 300B cathode resistor.
You can wire the ground from the RCA connector to the cathode resistor end,add the endwires from the second half of the 6SN7 cathode resistor and bypass cap towards this point.

Make a separate groundpoint at the 300B cathode resistor and solder there to ground.

A third groundpoint is required for the bypass cap at the first anode resistor.

Another one is to be made at the PSU end where you see the 100K/2W bleeder resistor.

As far as I'm concerned groundpoint 3 and 4 can share the same connection provided it is done at the PSU side.

Make sure any unused transformer leads are individually insulated and not touching the chassis.

Visually check for anything forgotten (me or you:) ) and you can now put in the tubes and fire up the amp.

Actually soldering on the tube sockets is often easier with the tubes in place but I think your sockets are special ones that don't move when you solder on the pins hence...

From what I see you're building these amps as monoblocks,so don't be surprised to measure a too high B+.
This will require adjustment later on especially since the 6SN7 is direct coupled.
Unless you use a different xformer of course.

BTW,I see a UX5 socket as well,is this for measuring purposes?

May I ask fellow members to correct me if I made mistakes or have overlooked something.:bawling:


Tomatito 23rd December 2002 11:57 AM

Only one half of the 6SN7 is used, driven by a 76

fdegrove 23rd December 2002 12:22 PM


Thanks Maarten.

I also forgot to state the obvious:the power on switch as to be wired up as well of course and the amp meter and LS posts.

Also in this case it would be a good idea to wire the 6SN7 on one of the monoblocks so it uses the second half of the tube.

That will allow for more useful life of these since you can than swap them after a couple of years use.


Tomatito 23rd December 2002 12:47 PM

It's Martin. I have killed for less :mad: ;)

Bas Horneman 24th December 2002 11:45 AM

Hallo Martin,

Any succes with the common mode filter on your amp?


Tomatito 24th December 2002 01:17 PM

Hi Bas,

I'm in Ibiza right now, recuperating from a hernia I got after lifting my amp. (dangerous hobby) so the filter has to wait a while.
I'll let you know. ;)


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