1976 Bassman 100 - BIAS and Converting to PPIMV

[topbrent]

This is the first time I have had a later 70's fender across my bench.  I am much more familiar with the way fender did the bias circuit on the blackface amps and the earlier silverface amps.  The customer has requested that I install the type II larmar PPIMV master volume and convert to traditional adjustable bias arrangement. 

I have successfully installed this very PPIMV master volume circuit in many amps, but none have been with this style bias circuit.

The bias arrangement on this 76 bassman 100 looks closer to what comes on a marshall,(which I am not too familiar with.)
Unless I am mistaken, I don't see a specific bias tap.  I am curious about converting this to a standard style bias arrangement.  

Bassman 100 Schematic link:  http://www.el34world.com/charts/Schematics/files/fender/BASSMAN_100.pdf

 The LarMar master volume will take the place of the 68K resistors that feed the 10K balance pot.  
-  Just not quite sure how to proceed from this point on with converting to an adjustable bias setup.

 My thinking cap must not be working so well today....

[chocopower]

I have made that work maybe 5-6 times, and after a lot of brainheadakes, I just get out all the bias stuff and redone fron cero.

just my newbie opinion...

[Willabe]

The bias arrangement on this 76 bassman 100 looks closer to what comes on a marshall,(which I am not too familiar with.)
Unless I am mistaken, I don't see a specific bias tap.

It has a bias tap on the PT, it's labeled as Red/Blu and goes to a 470R that's right before a reversed bias diode. The first part of an older Fender bias is already in place, it's the last part that is different. Bias balance instead of a raw bias control. The 68K R's are the OP tube grid return R's and are 220K value in the older Fenders. This is where the -bias is injected.  

Look at an AB763 Super Reverb, Twin Reverb, etc, and compare it to the amp you have. You'll see it. Just study it for a little while to see the difference.

The one your working on adjusts the (bias) balance between the tubes, so the 2 tubes would draw the same current at idle, but that over all idle is already set/fixed.

The older style Fender bias control allows you to adjust the current that the OP tubes idle at, but not the balance of the 2 or 4 OP tubes. A lot if not most guys use matched OP tubes to keep the OP stage in balance with this type of bias control, which is by far the most used type on MI amps.

And from Dougs library, read this on bias, (in here is info on changing bias balance to adjustable (fixed) bias, and lots more on bias)

http://www.el34world.com/charts/fenderservice5.htm

and this,

http://www.el34world.com/charts/Biascircuits.htm

and this,

http://www.el34world.com/charts/bias_conversions.htm



           Brad          

[Colas LeGrippa]

your thinking cap is too weak for the brain you have, try a 600V .

[stratele52]

Topbren,
Yes as say Willabe the power transfo have a bias tap. Your bias pot is a "balance pot" useful if your tubes are not matched but you can't change bias voltage.  You can put a "regular" bias pot , as blackface amp , between the wiper of your balance pot and the bias voltage coming from where is write -57 volts in the schematic.
With this you'll have the best of both world; balance and bias adjust pots.

[DummyLoad]

use AB763 twin reverb schema. all of it relating to bias supply, etc.; including the PI grid leak and tail R values. ditch the .022 and 1M-J MV leave the .01 PI input cap and graft in your "lamar" PPMV.

the bassman 100 uses the same model of PT, choke, and PS cap values as the twin reverb so it's fairly straight-forward mod.

http://www.el34world.com/charts/Schematics/files/fender/TWIN_REVERB_AB763.pdf

--DL

[topbrent]

your thinking cap is too weak for the brain you have, try a 600V .

I might have one of those higher voltage thinking caps around here somewhere....it was on my last Mouser order...

It has a bias tap on the PT, it's labeled as Red/Blu and goes to a 470R that's right before a reversed bias diode. The first part of an older Fender bias is already in place, it's the last part that is different. Bias balance instead of a raw bias control. The 68K R's are the OP tube grid return R's and are 220K value in the older Fenders. This is where the -bias is injected.  

Thanks for the tips on the bias circuit. After looking at the schematic and the amp, I think I now see what needs to be done.

[Willabe]

You probly already know this but, what ever you do, make sure you have a -bias dcv on the OP tube grid pins (pin 5) --- BEFORE --- you put the OP tubes back in.


          Brad      

[crashtm1]

Im still a bit confused.....When installing  PPIMV in a AB763, where does the wire coming off the center tap from the balance pot go?
 I have a late 70's MV Twin that i'm converting to a non-master, then adding the PPIMV.... Once I remove the 2x220K resistors, where does this wire go?
 I've looked here..

http://www.el34world.com/charts/bias_conversions.htm

 Is there a layout somewhere I can view....

[crashtm1]

Or do I leave the 220K's in,,, like this?
http://blueguitar.org//new/schem/misc_mod/pi-m-vol.pdf

[crashtm1]

Here's what I've got.

[J Rindt]

Most guys on this forum have WAY MORE knowledge/ability with amps than I do, but.....
It seems kind of funny to even want to install a MV on a BF/SF Fender. Especially a high power version. The preamp is so clean. Do they really work/sound good.?
best

[TIMBO]

Hi guy's, We touched lighty on the PPIMV subject here http://www.el34world.com/Forum/index.php?topic=14577.0 it might give you a bit of info you need. 

[crashtm1]

I've used it several times, but never on this layout. Mostly need confirmation of my wiring.
 Hate to power it up and have an early 4th...;)

[crashtm1]

lots of reading done.. check..:) 
  Looks like my wiring is correct.. Now what Im not sure about is... on the dual pot, where I have the 2M resistors between the middle and left side of the wiper,, do I have to take the left side to ground, or leave it as is?