Crawls Backward (When Alarmed)

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Chassis and Input Wiring on the Tweed Bandmaster/Super/Pro Clone

There is a fair amount of wiring to be done.  Most of it involves taking each 'flying' lead, trimming it to length, dressing it, and the soldering it to the appropriate place (usually a tube pin).

Here's a typical small part of the process - I'm soldering a 100K resistor across the second preamp tube.  You can see a couple of other leads from the board already in place.

Snip, solder, snip, solder.  Whoo hoo.

You may recall that we're using shielded wire - RG-174 - for the input jacks to the first preamp grids, and for the long runs over to the volume pots.  The wires on the coax shields are pretty small, so I'm using a bit of heat shrink tubing to try and ensure they don't contact the core wire. 

Or something like that.

This is a closeup of what I did.  I learned one lesson with this stuff- heating them up too much when you connect them *will* melt the insulation between the core and shield.  Need to be careful and not do that, lest we have a short and defeat the purpose of using shielded wire.

The input jack ends of the coax.  The shields are pulled back - I'll connect them together, and then run a line to the preamp side ground.

I'm going to run the shields to an .01 uf capacitor, then to ground.  This will trap any RF that's on the cable shields on the instrument cable coming into the amp.

There are a lot of things I've put into this build in an effort to keep hum and noise to an absolute minimum.   I'm as anxious to hear (or not hear...) the results as I am to hear what the amp sounds like. 

Instead of leaving the pairs of 68K input resistors on the board, I've elected to mount them directly on the input tube grids.  Again, in an effort to reduce noise.

I twisted one end of each of the pairs together, and then attached the hot lead of the input cable to the other end.  Doing it this way eliminates a bit of what would be unshielded wire in a stock tweed amp.  There's about 4 inches of wire - it may not make much of a difference, but this is the highest gain part of the circuit, so we'll find out.

The twisted end of the resistor pair goes to one of the preamp tube grids.  The one in the picture is for the normal channel and goes to pin 2.


Post a Comment 2 comments:

  • December 4, 2013 at 8:44 AM
    Hi, I have a clone of 5F6A building myself and I feel a hum when turn up the vol pot.
    Can you see this video anda give me a suggestion?
  • Yr Fthfl Blggr said...
    December 6, 2013 at 1:06 PM
    Ciao Vincenzo -
    Is that cable ok with other amps?

    If the cable is good, next step is to check the grounds to chassis in your amp. I would guess the problem is before the volume control (preamp) since you can control it with the volume.

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