Crawls Backward (When Alarmed)

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Adjusting the Bias: Tweed Pro Bandmaster Super Clone

The filter caps have been charged up, and the amp works!  I will be doing sound clips at the end of this whole extravaganza, but I will say this much:  it sounds really good - tons of bass, typical fat tweed amp midrange and brother is it ever loud!

I want to set the bias and then make a few refinements based on some ideas that popped into my head. 

On the right you see my homebrew device for measuring cathode bias current.  It's two octal sockets mounted in a box with long leads going to two octal bases.  The leads go from each pin in order, e.g. pin 1 on the socket goes to pin 1 on the base, etc.  The only exception is that there is a 1 ohm resistor from pin 8 on the socket to ground. 

The actual wiring in the box is: pin 8 goes to one of the two banana jacks on either side from the center jacks.  Then the 1 ohm resistor goes from the red jack to the black jack, which goes to ground.  Each side is wired this way.  The two center "vertical" jacks are wired to pin 3 of one of the sockets (red) and to ground (black).

So what we have then is a way to measure the plate voltage (the two center jacks) and the cathode current in milliamps (across the two outer jacks for a given tube).  This makes it a lot easier and safer to measure outside the amp rather than trying to clip jacks on and off the tube sockets in the amp where there is a chance of slipping with a probe and zapping something (or someone).  With an ammeter set to measure milliamps, we get the current draw for each tube.   This also is a way to see the actual current draw of a specific tube and "match" them up.  It's good for me since I have a lot of good used tubes and I can pair them up this way.

The shot above is one I took during this process.  You do need to be careful since you have high voltage at the center jacks, not to mention you now have hot output tubes on your bench!

Here's the actual adjusting of the bias inside the amp.  I wound up biasing it a bit cold - about 25ma with about 340 plate volts.  Since the new owner plays with a clean tone, I want more clean volume and headroom if possible.  I'd bias it a bit hotter if I was playing it.

The Tung-Sol 5881s I'm putting in should last a long time at that conservative bias setting.

I need to decide on some knobs to put on.  I found some maroon Kurz-Karsch and Dakaware knobs in my stash. 

After playing the amp a little, I found the tone controls are a little 'wacky.'  The original 5E7, 5E5, 5F4 circuit is known to sound a little 'loose.'  I wired the amp with a Treble, Middle, Bass (TMB) circuit from the Weber 5E7M schematic, which is based on the original Bandmaster 5E7 circuit.  For one thing, with the Weber design, there is a ton of midrange boost - there is a 100K mid pot which is probably too high a value for what I'm after.  It's got a fantastic blues/rock/roots tone, but again, I want a bit cleaner.  Just a touch. 

So, I'm going to adapt the 5F6A Bassman tone stack - which should tighten it up I think.  So the section of the circuit board in the picture above is going to change.


Post a Comment 1 comments:

  • March 25, 2013 at 12:14 AM
    I'm in the midst of building a 5E7 and stumbled over your blog. Very nice. I do amp repair and I too scratch the itch from time to time. is the address.

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