Blues Jr - Odd Topology

[Fresh_Start]

KnottScott's question about building an 18-Watt Stout or a Tweed Overdrive Special got me daydreaming again about what to build in my Blues Jr. chassis after ripping out the P.O.S. PCB, ribbon cables, etc.  I looked over a variety of Marshall 18-Watt variants with TMB stacks, DaGeezer's "Little Wing", the November from AX84, and other schematics.

Then I looked at the original Blues Jr. schematic and something struck me as unusual.  It has a pretty normal Marshall TMB tone stack between the 2nd and 3rd gain stages.  However, the Master Volume comes after the 3rd gain stage - not right after the tone stack like you usually see.

I don't think I've seen this arrangement in another amp.  Am I correct in thinking that the Treble and Bass controls are going to have an effect on the gain of the third triode?  IOW the grid return for that third gain stage is the combination of the Mid pot (full value), Bass pot (it's a variable resistor), and the Treble pot (effectively a variable resistor in terms of a grid return).  With Treble & Bass both on "1", the grid return resistor is only 25K.  With both on "10", it goes up to 525K.  That's a pretty big change, eh?  

Wouldn't that have a big effect on the gain from the 3rd triode as you turn up the Bass and/or Treble controls?

This is all leading to "What should I build in this box?"  

One possiblity would be to replicate the Blues Jr. preamp, using the "extra" triode in one of several possible ways, adding a 1-tube reverb, and using 6BM8 tubes for the PI/power tubes.  So I'm trying to understand the Blues Jr.'s preamp.

Thanks for your thoughts,

Chip

P.S.  Yes, I should measure the effect myself.  The problem is the amp is so damn noisy right now, I'm not sure how well I can measure things and I don't want to spend any more time trying to "fix" it.

[PRR]

Trying to figure out a 1999 Fender is probably a waste of time.

> "What should I build in this box?"

Can you punch for 6V6 and build a tweed Deluxe?

[jjasilli]

Some comments:  I agree that the Blues Jr. is built like a POS.  That's too bad, because IMHO it sounds great stock (unlike its bigger brothers in that model line) and comes in nice size combo.  It can be made to sound even better with quality signal caps, and the Bill Machrone mods.  I think there'd be nothing wrong with building a proper Blues Jr. into a Blues Jr!

The tonestack / signal strength issue you mention is due to the inherent insertion loss of the TMB design and would exist wherever the vol pot is located.  

The vol pot's location -- after the tone recovery stage and before the LTP PI -- is as good a place as any.  I think this amp has a great topology; it's just the build quality that's a problem.

However, the faceplate is ready to go -- numerous types of amps could be built into the chassis, making good use of the pre-printed faceplate.  

[phsyconoodler]

I took a Blues Jr and added one preamp tube and built it into a Princeton Reverb.It sounded fantastic.I used 6V6's.I used all the controls as laid out on the panel so I didn't have to make a new panel.I also used the stock transformers and speaker which sounded great as well.
  It sounded so much nicer than the Blues Jr it was unreal.It has some gain on tap because of the lower than Princeton voltages on the preamp and power stages.Still plenty loud.Sweet,sweet tones.However,the long reverb tank would not fit so I actually cut it down on the ends with a band saw so it just fits into the cabinet.
  This was one of the most satisfying builds I've done converting a stock Fender into a hand-wired tone machine.
The 18 watt Marshall builds are pretty cool too.I use the 'lite' version and add a post phase inverter master volume.
  I was quite surprised that the stock transformers actually sound quite good.I wonder what they would sound like with some real Heyboer/Hammond iron.
  I recently did the Bill M mods on a Blues Jr and was kind of under-whelmed.Sure it does sound more like a bigger Fender but it's EL84 magic was lost.I did all the mods,the OT too.I'm not  huge fan.My Princeton Reverb Conversion kills it dead for tone.IMHO.

[jjasilli]

I pretty much agree with psycho.  Still, what to build? Maybe build a type of amp that you don't already have.

[Cups]

I think I got a green light as far as gutting and rebuilding my friends Blues Jr. (havn't heard from him in a bit though ) I was thinking of keeping it really simple and using an everyday blackface preamp and eliminating a 12ax7 altogether. I'm just waiting to see if he liked the fat boost and wants me to keep it in the design. I figured switchable cathode bypass caps would be cooler.

[Fresh_Start]

Off topic sort of, but it occurred to me that I could re-create the basic Blues Jr. circuit, employ the un-used triode for reverb recovery and use an op amp or MOSFET (shudder... ) to drive the reverb tank.

Got to fix the vintage Princeton Reverb first though.  This is just daydreaming while I wait for parts from Hoffman, not that I expect to wait long!  It's just that I ordered the wrong switch and forgot a couple of other things.  Don't you just hate that?

Cheers,

Chip

[jjasilli]

use an op amp or MOSFET (shudder... ) to drive the reverb tank.  I agree that this is a viable option for this project.