Help with the bias circut of my SF Fender showman

[krankguitarist]

OK, so, I've been lookin through schematics, and I'm trying to figure out what the dizzle with my bias circut is.

It looks like somebody tried to change out the "bias balance" pot with a "bias adjust" pot. Which is cool by me. But the way they did it is perplexing.

Now, here's my bias pot:

I can't make heads or tails of what they were tryin to do here. I've been lookin it over with my dad (who's a bit more experienced than I) and he cant make heads or tails of it either.

My Showman should be running between 51mA and 68mA with the JJ/teslas. Right now its running real cold at about 22mA, and adjusting my bias pot gets it anywhere between 20mA and 24mA.

So, I'm thinking about removing those extra 10k and 100k resistors, and doing a bit more component swapping to make the bias circut look a bit more like it does in the BF schematics.

I'll post exact details in my next post...I need to edit a few of these chassis layouts so I can show ya'll what I'm (thinking I'm) gonna do.

 

[krankguitarist]

Re: Help with the bias circut of my SF Fender showman

Ok, these pics explain it pretty well:

I'm gonna get rid of those two 10K resistors, that 100K resistor.

Next, I'm gonna swap out that 15k resistor on the pot with a 27k resistor (as per the blackface schematics)

After that, I'm gonna swap out that 68k resistor with a 220k resistor, and I'm gonna add another 220k resistor between the contact point for the yellow wire and the first blue .1uF cap. (as per the blackface schematics)

Here's the chassis layout that I'm goin by...

I'm an amature, I can't say I know much about electronics, so these ideas are simply fuzzy interpretations.

I could go the other route as well, by switching it back to the original specs and adding a bias adjust pot in addition to the bias balance pot. I have no need for a bias balance pot however.

Lemmie know what you guys think...

 

[kmcguitars]

Re: Help with the bias circuit of my SF Fender showman

Re: Help with the bias circuit of my SF Fender showman

HOLD IT !!! Those 220k resistors are your bias split resistors. I've read your post 2xs and I'm still not sure I understand what you are trying to accomplish? The stock Fender bias circuit should give you plenty of sweep range.

 

[kmcguitars]

Re: Help with the bias circuit of my SF Fender showman

Re: Help with the bias circuit of my SF Fender showman

O.K. on a 3rd look, There is some weirdness in how your bias pot is wired? Go to www.hoffmanamps.com and check his library of information. He has awesome diagrams on adding or repairing the fender bias board.

 

[krankguitarist]

Re: Help with the bias circut of my SF Fender showman

Looks like its got some good info, thanks!

As far as what I was thinking I'd accomplish with the 220k resistors...

There's a red and a yellow cloth wires that you can see underneath the first blue .1uF cap going underneath the board. The yellow one meets up at the top of the first blue .1uF cap, and the red one meets up at the top of the second blue .1uf cap in the picture.

The yellow cable goes to two power tubes, the red goes to the other two.

If I snip out those 10k and 100k resistors, the yellow wire will not be connected to the bias pot in any way. Which, to me, seems to indicate that there won't be any power goin to those two tubes(?).

So, in looking for a solution to that problem, I came up with this:

There's a yellow cloth wire coming from the center tab of the bias pot. The yellow arrow I've drawn underneath it indicates where it connects. On the blackface schematics, there are two 220k resistors that branch off of that, each meets up with one of the cloth wires from the power tubes.

My idea was to add in that 220k resistor where it is on the BF schematics, which would solve the problem of getting power to the "yellow wire" set of power tubes. Then I saw that there'd be a 68k resistor between the bias pot and the "red wire" set of power tubes, and for the sake of keeping things even I figured I'd swap that out with a 220k resistor as well, keeping with the BF schematics.

****, I really hope that made sense. :smack:

 

[kmcguitars]

Re: Help with the bias circuit of my SF Fender showman

Re: Help with the bias circuit of my SF Fender showman

Just be careful buddy. Your dealing with the power section of a 100w amp. Very dangerous if you wire something wrong. Hoffman also has a real good forum board with knowledgeable tech heads who maybe able to sort this out for you.

 

[krankguitarist]

Re: Help with the bias circut of my SF Fender showman

Heyyyyyyyyy, I got it right!

CHANGING BALANCE SYSTEM TO ADJUSTABLE BIAS:
The balance system is very easy to change over to the adjustable bias system. We will use the super reverb schematic AB 568 on page 462 of the tube amp book volume 3. If you compare this bias circuit with the super reverb AB763 circuit on page 463 you can see that the all we have to do is rewire a few things and change some resistor values. Basically we are just going to copy the AB763 circuit. First you must disconnect all wires that are attached to the bias pot. Some bias pots have a center tap on the back of the pot, across from the three normal tabs. We will not use this center tap when we rebuild the bias pot. Any resistors that are soldered to the can of the pot can be unsoldered from the pots tabs but do not unsolder them where they are attached to the bias pot can. After all wires have been disconnected from the bias pot, you must find the wire that comes from the bias circuit board where the bias diode and bias filter cap are soldered. Solder this wire to the most clockwise tab on the bias pot. We will call this the right hand tab. The resistor that used to be soldered to the middle tab must now be soldered to the left hand tab. The other end of this resistor is soldered to the pot can itself. Now we must rebuild the section of the board where the two .1 caps will join with the two 220k resistors that we will install. We must first unsolder the two .1 caps on the end that is towards the front of the amp. Unsolder one end of each cap a lift it up out of the way. Any resistors and wires that are attached to these two points must be unsoldered and removed from the board. There should be a hole that is between the two eyelet's where the .1 caps were attached. It is between the two eyelet's but closer to the middle of the board. Remove all components and wires from this eyelet also. Use a solder sucker to remove all solder from the three eyelet holes on the board. Now you will need two 220k 1/2 watt resistors. Both resistors should be inserted into the middle hole on one end and each resistor should go to one of the eyelet's where the .1 caps where removed. The resistors are shaped like a V when finished. Now a wire must be inserted into the point where the two 220k resistors come together. This wire will go to the middle tab of the bias pot. Solder the wire into the board at the two 220k resistor point and solder it to the middle tab of the bias pot. Now find the two wires that used to be soldered where the .1 caps were. These two wires lead to pin one on two of the power tubes. Sometimes these wires go under the board and come out near the power tubes. If these wires were under the board and never removed they may already be poking up through the two eyelet's. You can unsolder and pull both wires out from under the board just to make sure that you are making the right connections and no mistakes will be made. Once you have located the two wires that lead to pin one on both halves of the power amp, insert each wire into the point where the two 220k resistors come together. Do not criss cross the wires. The wire leading to the right power tube must go to the right 220k resistor and the left wire to the left 220k resistor. Now insert each .1 cap back into the eyelet where the 220k resistor and the wire leading to pin one on the power tube are located. At this point you should have a wire that comes from the center tab of the bias pot and goes to the junction of two 220k resistors. Each 220k resistor should go to a eyelet that has the 220k resistor, a .1 cap and a wire that goes to pin one on each half of the power amp. If this is all correct then you may solder the two eyelet's where the .1 caps and resistors and wires all join. After everything has been checked you may go to the (powering up the amp after rewiring the bias) section.

This guy explains it a whole lot better than I coulda .

Muchas gracias on the linkage amigo.