Hoffman Fender Blues Junior Conversion

[EL34]

The layout Diagram, Schematic and BOM and other build information can be found on the library page here
http://el34world.com/schematics.htm

Layout and Schematic - Hoffman Blues Junior Schematic and Layout PDF - Newest version uploaded on 04/14/2015

BOM - Parts list - Blues Junior BOM PDF

Hoffman Board install instructions

Blues junior Turret board Build Page Link - http://el34world.com/Hoffman/Blues_Junior_Build.htm
Blues Junior Tube board assembly page - http://el34world.com/Hoffman/Blues_Junior_Tube_Board.htm

Sound clip #1 - Humbucker cranked
http://el34world.com/Misc/Music/files/BluesJuniorDemo1.mp3

Sound clip #2 - Single coils with the Revibe unit
http://el34world.com/Misc/Music/files/BluesJuniorDemo2.mp3

Hoffman Turret Boards are on this page
http://hoffmanamps.com/MyStore/catalog/TurretBoard.htm


The parts list for this build is here
http://hoffmanamps.com/MyStore/catalog/PartsListIndex.htm

[EL34]

Here's some previous help topics by people installing the BJ boards

http://el34world.com/Forum/index.php?topic=18679.0

http://el34world.com/Forum/index.php?topic=18376.0

http://el34world.com/Forum/index.php?topic=18359.0

http://el34world.com/Forum/index.php?topic=18378.0

[Bret608]

This is long overdue for Blues Jr. players and looks really cool. I just modded my B Jr. recently but still may take this on at some point.

Tell me more about the meat control. I did the Bill M. mods on my amp, and the changes he makes on the tone stack increase the bass content quite a bit (maybe more than I needed at 0.1 uf for the bass cap). I can see where his mods would go on your layout but just wondering if they would still be relevant.

[EL34]

Already selling some of the New BJ boards
Will be nice to the new BJ's up and running with hand wired guts

The meat control is a low end resonance control
I used it on my later amps and it was pretty cool

The owner of the BJ I converted really likes it

Here's the schematic of how the meat control tied into the phase inverter

[Bret608]

That does sound cool! Now I'm off on a tangent trying to figure out what classics the B Jr. PI draws from, though...

Would I be okay just keeping my stock transformers if I did this? I am thinking about David Allen's TO20 but thought I should ask.

Also, I am not set up to install the turrets myself. If I went with the kit function in your store, could I basically drop the separate board and turrets, and add back in a board with turrets installed?

My Blues Jr. is one of the really early "green board" models, from 1996.

[EL34]

You can use the stock transformers for EL84's
I would upgrade to the Allen TP24 for 6V6 use and just as a way better upgrade
http://www.allenamps.com/parts.php#transformers

It's pretty massive
This is the PT I installed on the BJ conversion I did
The customer sent it to me here





You can change any of the parts on any of my parts list to anything you want
That's why a parts list is way better than a kit
Just zero out the stuff you don't want to add to the cart


Then go add any changes to the cart

[smackoj]

Great idea Mr. H. Is there any interest in making a similar swap-out board for the plethora of Fender Hot Rod amps?  A fair number of people are selling them low ball because there are thousands out there and a good amount of negative feelings about the lack of performance. Lots of reports from repair techs that the boards are flimsy and prone to breaks, loose solder joints etc.

Just thought I would ask. I don't own a Hot Rod but I might some day if I find one selling muy cheepo.

jack 

[afireworship]

I have several questions in regard to the BJ conversion;
1. does this conversion come with instructions and wiring diagrams / schematics??

2. does this include the use of a choke?
    a) if so is it included or can I use a mercury mag choke?

3. will a mecury mag ot tranny also work with this mod?

4. I am hoping to build this as a head kind of unit into my pedalboard - any suggestions?

Thank you

[EL34]

I have several questions in regard to the BJ conversion;
1. does this conversion come with instructions and wiring diagrams / schematics??

2. does this include the use of a choke?
    a) if so is it included or can I use a mercury mag choke?

3. will a mecury mag ot tranny also work with this mod?

4. I am hoping to build this as a head kind of unit into my pedalboard - any suggestions?

Thank you

1) See post #1 at the top, all the links you need are there
2) Only if you add a choke
3) Any OT that is correct for the tube config will work
4) This is a conversion of a BJ, you must have a BJ carcass
I don't understand the pedal board statement?

[afireworship]

Can anyone tell me, if I wanted to add a Mercury magnetics choke do I place leads in place of the 2.2K 3watt resistor?? If not where would it go? Thanks!!

[sluckey]

Can anyone tell me, if I wanted to add a Mercury magnetics choke do I place leads in place of the 2.2K 3watt resistor??

yes

[jim]

I have a BJr for conversion and am checking my parts inventory to place an order.  When I order the board is the tubeside board also included? Is it necessary or can I use the existing chassis holes to mount sockets? Jim

[EL34]

All the parts are separate

The tube boards and the turret board are separate

Use the parts listing, it's way easier than trying to pick out all the parts
http://hoffmanamps.com/MyStore/catalog/PartsListIndex.htm

No, the sockets do not mount in the chassis holes, that's why I made the tube board
If the sockets mounted in the chassis, there would be no need to have the tube board

The build page with pics is here
http://el34world.com/Hoffman/Blues_Junior_Build.htm

[EL34]

I had a customer hook up the brown bias winding leads to totally wrong places on the turret board.
The layout diagram, schematic and build pics all clearly show where those two brown wires go.


Advice for you people that are building any of my turret boards
Take your time, don't rush and look at all the links for this build at the top of this page and on the library page
The biggest problem I see from people who are having problems with their builds is that they are not taking the time to look over all the information I provide.


Just go slow and do not miss any wires or steps

[Bret608]

Hi Doug,

I've been thinking about doing the tube board only option as I modded by main board in my Jr. pretty recently. One funny thing about the version I have--a green board version from 1996 (revision 'B')--is that there are actually components on the tube daughterboard. Just checking with you to make sure my version would be compatible with the tube board replacement.

Here's what appears on the schematic and layout diagrams:

CR6 - 1n4448
LED indicator wiring
R34 - 150r (CLR for LED?)
R35 - 47r, 1/2w
R36 - 47r, 1/2w
CR10 - R3000
CR11 - R3000

R35 and R36 just appear to be the artificial center tap, so no worries there. But what about CR6, CR10, and CR 11? It looks like CR6 has to do with the LED (along with R34, I think), and CR10 and CR11 are associated with the brown and blue OT leads on the schematic. Would there be a way to deal with all this if I did the tube board replacement? And would the rewiring of the ribbon cable follow the cream board diagram you've created?

Thanks and sorry the long reply!

[EL34]

Hi Bret,
I have never seen one of those models, so I don't know if my tube board is the same dimensions or not regrading screw holes


Post a link to the schematic and I'll have a look at what those other components are doing
You could probably add them on a terminal strip if they are actually really neccesary

[Bret608]

Hi Doug,

Here's a link I found for a reasonably clear image:

http://www.blueguitar.org/new/schem/fender/blues_jr.gif

I will see about scanning mine if that turns out to be too hard on the eyes! Also, here is a slightly blurry photo of the guts of my amp post-mod; I think it's similar dimensions to the more recent ones:

image gone

Let me know if this helps! Thanks again.

[EL34]

The image does nothing for me

[Bret608]

No problem; I managed to get a decent-looking PDF scan of the schematic and layout and have attached them. The scanner I used didn't have a big enough glass plate to get the outer edges, but it does at least encompass the part I was asking about. Let me know if this works out better.

[EL34]

CR6 - 1n4448
LED indicator wiring
R34 - 150r (CLR for LED?)

You can put this stuff anywhere you want
It's just for the LED

R35 - 47r, 1/2w
R36 - 47r, 1/2w

These are on my tube board

CR10 - R3000
CR11 - R3000

You don't need these
I left them off my BJ turret board

And would the rewiring of the ribbon cable follow the cream board diagram you've created?

Probably, but I don't know for sure
I can't see why they would change that
The images I took should be able to sort that out
http://el34world.com/Hoffman/Blues_Junior_Tube_Board.htm

[Bret608]

Thanks for the help and for pointing me toward those images--looks like all the ribbon cable connections are exactly the same. It looks like this should work well for the green board versions in general. Let me just make sure I've got a good plan for the LED wiring and I will most likely order this.

[EL34]

You can just put the 150 ohm resistor in line with the led wire
I doubt you need the 1n4448 diode

[Bret608]

Thanks--yeah, I also doubt that diode is necessary. It's probably just there to protect the LED or something.

[EL34]

I uploaded a new PDF file today
http://el34world.com/Hoffman/files/Hoffman_BluesJunior.pdf


Make sure you hit refresh after clicking on the link above

[EL34]

Doug I made good progress bringing up the B Jr. Conversion.
Used the second half of V2 in cathode follower mode.
Substituted a Lar Mar master volume and had some oscillation at high settings.
Seems to be related to the feedback/Meat control routing.
Made some progress, but not quite there.
I need to up the bias supply for my octal conversion and am looking at a 36v or 44v Hammond transformer to replace the power transformer secondary.
Any suggestions in this area? Thanks.

You are using the stock BJ power transformer?
Maybe you can raise the value of the 22k resistor in the bias circuit?
see my diagram
Maybe some of the other guys can help?

[sluckey]

...some oscillation at high settings.
Seems to be related to the feedback/Meat control routing.

Reversing the OT plate leads may fix that.

I need to up the bias supply for my octal conversion

What is the max negative dc voltage you get with the circuit as is? I'm guessing not enough.

There are two easy, low cost ways to get the voltage you need. Modify the existing bias circuit to look like one of the circuits at the bottom of page 5 of this pdf...

     http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

And a third option... Get the smallest 120 to 12.6v transformer that Radio Shack has. Connect the 12.6V secondary to your 6.3V filament string. This will give ≈60VAC on the primary. Now just connect the primary in place of your present bias winding.

[jeffs]

Thanks.

Using the Allen TP24. The 20V winding gets me about -30V.  HT is about 390V.

Starting with 6V6s and really need a few more volts to bring the dissipation down long term.
Have 6L6s and 5881s so will need even more adjustment for these.

Will try the 36V Hammond which should give me close to -50V with the existing circuit.

[EL34]

The Allen PT has a separate bias supply winding for bias just like the stock tranny?

Maybe a full wave bridge rectifier instead of a single diode?


I know I have seen guys saying they use the Allen PT for 6V6's
not sure I have seen anyone say that they used 6L6's with that PT

[jeffs]

Was using the BillM voltage doubler bias mod on the 20V winding.
Want to solve the problem fully. Much cheaper to replace the bias winding with a $20 transformer than to replace the entire power transformer.

Installation should be like mounting a choke on the chassis.

[airmech50]

I have the TP24 transformer and am using 6V6 tubes and had the same problem with biasing. I used the schematic at the bottom of page 5 of the noted PDF, marked as marshall JCM 900. I used the same values as noted on the schematic and was able to bias in the mid range of the pot supplied with the board.

[EL34]

Thanks for the feedback
This is the circuit you used?

[bakerlite]

would this circuit be of suitable quality to use as an fx loop so as to get a reverb pedal after the pre tubes?


https://www.tube-town.net/info/datenblaetter/kits/tt-fxlooplnd150.pdf


would you be inclined to insert it between R18 and the master pot or would it be better suited elsewhere?


thanks

[airmech50]

Yes, that is the circuit I used. I also added a tube reverb circuit using one 6111 and one 6021 micro tube.

[EL34]

Ok, just asking so that others can figure out exactly what you used :)

[jeffs]

Installed the Hammond 36V filament transformer and using the stock BJr bias circuit.

Works as expected - I am able to bias 6V6s, 5881s, and 6l6s.

Now on to taming the oscillations in some settings.

[EL34]

Go over everything again to make sure it is wired correctly


Hard to say about the oscillations
Flip the plate (pin3 for 6L6 and 6V6) OT leads around and see if that is the problem

[jeffs]

The oscillation was at full preamp gain and the fat switch on (cathode cap ground lifted).

I added a 47K in series with the 10K grid stopper (R7) and the oscillation was gone.

[jeffs]

I like the gain structure better with a 12AT7 in V3 (PI), but the feedback/Meat/Presence doesn't have much affect.

Should I try adjusting the feedback resistor (R27)?  Currently it is the stock 75K. Thanks.

[EL34]

It does on my marshall 4 x 12 cabinet
Not sure if the speaker cabinet makes a diff or not since it is low end presence

[jeffs]

I was reading a thread on The Amp Garage about Lar Mar PPIMV and had an "ah ha" moment.

With any PPIMV setting other than full on, NFB is also reduced resulting in less signal to 2nd input of the PI.  Since the PI gain is fixed, the effect of NFB/Presence/Meat is reduced correspondingly. This should have been obvious, but took a while to sink in.

Bottom line is that the amp is behaving as built.

[jeffs]

Put a stock pre-PI master volume in and the NFB/Presences/Meat behaves as expected.

[EL34]

Good deal
Your build looks great from here.

[Willabe]

Yeah, very nice!


        Brad   

[EL34]

How do you like the tone?


That circuit has a nice clean and super nice overdrive

[jeffs]

I changed the PI coupling caps back to the stock value since it was a bit boomy with the 15" speaker. Will cycle through the guitars before declaring it done (for this round).

May look at using the open space between the PT and turret board -
Foot switch relay for Fat
  or
+/- 15V supply and reverb op amp driver / recovery

There is always something to modify or tweak.......

[jeffs]

Noticed the oscillation was back with the input Volume all the way up and the Fat switch on.  Seeing a couple volts of about 50KHz.

Increased the grid stopper to 68k and tried moving some wires without much affect.

Then I found that removing the tube shield eliminated the problem with the Tung-Sol 12AX7.

Next I tried a Fender/GT 12AX7 and it didn't oscillate with the shield installed.

Since the tube sockets are not mounted to the chassis, should they be grounded?

[EL34]

Try running a ground wire to that tube socket mounting screw and let me know if that solves it

[jeffs]

Did some experiments with the Tung-Sol 12AX7, Volume all the way up and toggling the Fat switch -

1. With 68k grid stopper, shorted the chassis to the shield and the oscillation stopped.

2. Changed the grid stopper back to 10k - oscillated with or without the tube shield. Grounding the tube shield did not help.

3. Changed grid stopper to 56k and there was no oscillation without a tube shield. Putting the tube held on resulted in oscillation again.  Grounding the shield eliminated the oscillation.

V2 is a 12DW7 and V3 is a 12AT7 and don't seem to be affected.

[EL34]

Weird

[EL34]

yeah, that may be a good idea


I forgot that the sockets are all mounted to g10
But when the shield is on it touches the chassis


I'll add a note to the build info to ground the pre amp sockets