Hoffman Blues Junior Power Transformer

[etliharr40]

Currently rebuilding my 95 blues junior with the Hoffman tube and turret board. I've got the stock power transformer and an Allen T020MT output transformer. I've got a few questions on the build that I thought I'd put out there. 1) I've got an old Classic Tone 40-18027 Power transformer laying around from a previous 5F1 champ build that I've since disassembled. From what I've researched it seems to provide quite a bit more voltage than the stock BJ PT, so I'm wondering, how much voltage is too much? Or, with the hoffman build diagram, what values would you change to make the higher voltage PT work with the circuit? Question 2) I'm planning on replacing the LED in the stock BJ circuit with a jewel/bulb setup from the same 5F1 build. Hookup is simple enough, but will I need to change any resistor values to make up for the difference from LED to filament bulb? Question 3) I'm interested in the parallel triode setup for more gain, but wondering what to expect from simply jumpering the pins on the 12ax7? Is V2 the best option for this for an increase in clean headroom? I've read some posts about the possibility of V1 being used similarly.

I'm essentially trying to end up with a stable rig that has some decent headroom in the clean arena.

I have a very basic understanding of electronics from some entry level courses taken in for an audio engineering program in college. Done a few builds in the past, nothing major. Comfortable with a soldering iron, but not always so comfortable with circuit design/analysis. Any tips/help is welcome. sorry for the long winded post.

Cheers

[nandrewjackson]

1) the classic tone website literature for the 40-18027 shows no secondary tap to get the necessary negative DC power tube bias that would be needed for the Hoffman bluesjunior. The higher voltage would be ok, *IF* you could dial in the neg DC voltage to bias the output tubes. 


2) I don't know about the indicator bulb.


3) simply jumper the pins on V2 like the Hoffman schematic.  Parallel triodes sound nice, and if you don't like it, easy enough to undo.  To my ears, having parallel triodes versus a single triode is like having your pickups close to the strings versus having the pickups far away from the strings . Its *basically * the same tone, with a bit more volume and dynamics .

[2deaf]

As nandrewjackson pointed out, the 40-18027 has no bias winding so that you will need to design a bias circuit using one of the high voltage legs and then try to wire it up using the existing turrets.  The 40-18027 also uses a different rectifier than the BJ transformer and the board doesn't have enough turrets to accommodate this.

The jewel/bulb just hooks up to the heaters with no resistor or diode.

[sluckey]

This project will go smoother and have a much greater chance of success if you forget about that Classic Tone PT and just use the original BJ PT. The original is a good PT. You have nothing to gain by using the Classic Tone PT except a big challenge and probably headaches.

[etliharr40]

Thanks for the help! Definitely going to omit the classic tone trans. Appreciate the advice.

Another question, the kit comes with a few different types of wire. I see annotation on where to use the shielded cable, and assuming the solid core green wire is for the tube board wire up, but wasn’t sure on the correct use for the two different size buss wires (my guess is the thinner gauge is for the turret board jumpers, and the thicker for the pots/ground buds?). Also what are the proper locations for using the stranded vs solid core? Thanks again!

[acheld]

I've built two of these, and you will love the tone.  It's warmer than stock BJ, and the end product is much less noisy than my stock BJs.

The "stock" incandescent indicator bulb is designed for the 6V filament buss.  That voltage is too high for the LEDs, and thus it is dropped by using a resistor in series.  Hence, no need for a resistor when using the standard Fender bulb.  One simple trick is to simply place the LED inside of a clear or red jewel.  Looks great, and they don't burn out.  The neon bulb works off of the mains voltage, not the filaments. 

I had the same questions about wiring.

Strongly recommend that you print out and read all of the pages of instructions on the BJ conversion section.  To my chagrin, I have not always followed these instructions.  LOL, it's all part of the "learning" process. 

Anyway, contained in the document you will see that the "blue" board lacing is 24ga, and the orange is 20ga.   The smaller wire is easier to wrap, but not by a lot.

One last comment -- the BJ power transformer can be smaller because it is using solid state rectification, as opposed to the tube rectifiers used in most of the amps on this forum.   SS rectification is more efficient, and works well in this amp.

Best of luck!

[sluckey]

There is a LED replacement for the #47 6.3V bulbs...

https://www.ebay.com/itm/10-Pinball-6-3-Volt-LED-White-Round-Replacement-Bulbs-44-47-Bayonet-Base-BA9S/311973284988?epid=0&hash=item48a30e8c7c:g:7rQAAOxy0x1TWWlE

[etliharr40]

Tried posting another thread and somehow messed it up. New to the whole forum scene, so excuse my ignorance. Trying again here.

I recently did the Hoffman blues junior conversion, turret board, tube board and all. Loved the new sound from my point to point blues junior but decided it would be nice to squeeze out some more headroom and get a better low end response. I went from a stock power transformer and the Allen OT20 to an Allen PT24 (power) and Allen OT26 output transformer. Got everything wired up, fired up the amp and played it. Sounded great, with a big low end and seemingly much more headroom. However, after a few days, it seems that the output is quite a bit lower, even more than stock. My B+ reads 372, X=290, Y=321, Z=363, and my max C- is around 26.6/27. Seems that the bias point isn't quite high enough for a 6L6, but I'm also wondering if the higher voltages are negatively effecting my preamp stage? Or maybe my plate voltage is too high on the power tubes? (was getting some blue glow). Possibly damaged tubes causing the perceived lower output? The circuit is exactly the same as the original Hoffman conversion with the addition of the new PT, OT (Using black and yellow leads from Allen OT26 as recommended by David Allen for 8ohms with the 6L6 Setup) and switch to octal sockets.

[shooter]

I'm late as always 
this is what I'm using for Blues junior conversions

https://el34world.com/Hoffman/files/Hoffman_BluesJunior.pdf

you have an SE 6L6?
if so can you link to or post your idea of blues jr?

[etliharr40]

I'm late as always 
this is what I'm using for Blues junior conversions

https://el34world.com/Hoffman/files/Hoffman_BluesJunior.pdf

you have an SE 6L6?
if so can you link to or post your idea of blues jr?

Shooter,

thats the correct link. Only difference now is that I've converted to octal, switched power and output transformers to better suit the standard operating voltages of a 6L6 pair

[etliharr40]

This just in... My voltage across the 1ohm resistors is WAY too high. Assuming that is what has damaged my tubes and caused the significant decrease in volume. Ideas on lowering the bias with changes to the original Hoffman blues jr circuit? Im currently sitting at 70mv at my lowest bias setting. Double the generic recommendation for a 6L6.

[sluckey]

my max C- is around 26.6/27

THAT'S YOUR PROBLEM!

You need between -40v and -55v to properly bias those 6L6s. You burned them up by using too little bias.

You will have to modify the bias circuit to give proper voltage for those 6L6s.

[etliharr40]

my max C- is around 26.6/27

THAT'S YOUR PROBLEM!


You will have to modify the bias circuit to give proper voltage for those 6L6s.

Any ideas on bias circuit modifications to get my max C- up to par? I tried a few different values for the 22k resistor in the bias circuit and didn't seem to make any difference. Should I be changing the value of the two 220k resistors at the C- location (in the original Hoffman circuit)?

[sluckey]

That TP24 will not produce enough bias voltage for 6L6s. Here's what David says about that PT. I underlined the part that applies to you…

TP24 power transformer requires a full-wave-bridge type solid-state rectifier. It does not support a tube rectifier. It is rated 120V to 280V at 200mA with a 6.3V center-tapped heater winding rated 4A. It will support a power supply rated approximately 365V DC at 150mA DC. This is sufficient for a pair of EL34s in fixed bias or a pair of 6L6s in cathode bias or a single ended amp using a big 6550/KT88 power tube - plus 3 or 4 preamp tubes.

You can get a PT that's appropriate for fixed bias 6L6s or you can convert to cathode bias. I suggest converting to cathode bias. Just need a 250Ω/10W resistor and a 50µF/50V cap on the cathodes. And connect the junction of R31 and R32 to ground.

Either way, you need a new pair of 6L6s.

[shooter]

Only difference now is that I've converted to octal, switched power and output transformers to better suit the standard operating voltages of a 6L6 pair

ok, so you took a vw n converted to a chevy 

[etliharr40]

You can get a PT that's appropriate for fixed bias 6L6s or you can convert to cathode bias. I suggest converting to cathode bias. Just need a 250Ω/10W resistor and a 50µF/50V cap on the cathodes. And connect the junction of R31 and R32 to ground.

Probably a dumb question, but I may as well pile it onto the mess of ignorance I've already spread on this thread... are the two cathodes to be jumpered and the 250 Ohm and 50uF cap ran from the cathodes in parallel to ground? Or when you say "On the cathodes" do you mean that the cathode cap and resistor span the two cathodes in parallel?  Are there any other connections that need removed from the original setup aside from grounding the junction of R31/32?

Thanks for digging me out of a hole.

[sluckey]

Connect the two cathodes (pin 8) together and connect the resistor/cap from pin 8 to ground. You just need a short board jumper to ground R31/R32. Disconnect the PT bias wire from the bias diode and tape the end of the wire. That's about the minimum you need to do. You can remove the two 1Ω resistors and all bias supply components from the board if you want. They will no longer be used.

[sluckey]

Maybe like this...