Hoffman Pro Jr Turret board

[EL34]

I am getting request for a replacement turret board for the Fender Pro Jr amps.
It's a pretty simple circuit compared to a Blues Jr

Here's what I have so far on the layout and hookup document
http://el34world.com/Hoffman/files/Hoffman_Pro_Jr.pdf

As usual, I start with another document as a template and modify it, so no doubt I have missed some editing
Hopefully it's pretty close

I included the original schematic on the last page.
It has some red rectangles where I modded the original circuit.

I added a adjustable bias circuit.
The 3 watt bias range resistor value is unknown.
I would have to see what value gives you an good sweep on the bias pot.

I changed the phase inverter plate resistor to the more common 82k / 100k.

I dropped the 47p cap on the phase inverter plates.

I dropped the cap and resistor across the plates of the EL84's.

The LED pilot lamp circuit will be different/ or maybe not since I have not seen one in person.
The LED may be re-useable?

Anyone that feels like having a look at the document is welcome to see if I have missed anything

[PRR]

Your plate-load on the long-tail driver is different from the Fender plan. Yours is like most other Fender LTPs, I do not know why this Pro Jr does it different. Unless you feel strongly, or know the production amps don't use the drawing plan, I'd toss on the one extra odd-value resistor and ape what Fender did.

I do not understand R40. It is not on Fender's plan. It makes the calculations very different. The stock scheme drops HV down toward 10V mostly with the 0.047uFd cap. This is 56K impedance at 60Hz, though the load current is far from pure 60Hz and hand-calculations are torturous. A particular benefit is that the capacitor heat dissipation is "zero". A disadvantages is that this cap needs to be surprisingly high voltage and very reliable.

You DO need Fender's R28. Without that bleeder the action is different again, and IIRC may not work as expected or at all.

Surely there is some pre-designed plan you can steal? But watch out for the several ways the HV windings may be connected to main and bias supplies.

I may try a simulation, unless I must shovel snow, or Pete does it first.

[sluckey]

I highly recommend temporarily building just the bias circuit and making it work. That circuit can act funny when you change values around.

Oh, and remove the jumper on the board that's shorting out your bias diode. 

[PRR]

> That circuit can act funny

Indeed. "Small" changes make very large differences. 10% parts are treacherous. 5% resistors are now easy/cheap, but caps are sloppy.

I _think_ this gives enough range to cover most requirements. The bias-range tends to too-cool, which is much better than coming up hot first time.

[uki]

IIRC

I keep bumping on those tags and I don't know what most of them mean, what this one mean ? Thanks.

[sluckey]

IIRC = if I recall correctly

[JB]

Hi,

I have a Pro Junior, bought it new when they came out in about '93, so mine is a Rev C board, same as your schematic.  They are great sounding amps when you wind them up.  Jeff Beck has been gigging with a pair of them.  They suffer from noise/hum problems and excessively hot output valves, same hot bias problem with the Blues Junior.  I've added additional PSU filtering to mine and a couple of tweaks to make it like the later board revisions.  I've recently replaced the on-board jack/pots with panel mount and run screened cable back to the board - made the hum worse!

Anyway, I'd be very interested in swapping out the board for a turret version, maybe I can help?  Photo's, measurements??

A few thoughts on this:

1  You've gone for the Rev B/C version - later ones increased the PSU filter cap values, added a cathode bypass cap on the first stage (I've done that but made it switchable - useful for some earlier break up as these amps are loud without any master volume), and tweaked the network between V1A and B.  I'd suggest looking up the schematic for the later versions and basing your design on that.  Certainly provision for a V1A cathode bypass cap would be good.

2 For the bias I removed R29, drilled a couple of holes in the board and fitted a 47K trimmer in it's place, wired as a variable resistor - one end shorted to the wiper.  This gives sufficient range to increase the bias (more -ve).  Yes, ideally it should have a series resistor to set a minimum for safety, but that would have involved more PCB surgery than I wanted!

3 I'd keep the conjunctive filter across the output.  EL84 amps can be probe to fizz, I'd at least make provision for it so people can choose to fit or not.

4 Likewise I'd keep the LTPI 47pF cap.

5 The amp is constructed on two PCB's - the main one and another for the valve bases.  Solid core ribbon cables link the two.  The LED mounts remotely from the valve base PCB on twisted pair cable.  There's a nasty plastic holder that clips it into the chassis/faceplate.  the clip is easily broken (guilty) and I've thought of replacing mine with a Fender jewel lamp.


Hope that helps, let me know if you want more details.

[EL34]

I highly recommend temporarily building just the bias circuit and making it work. That circuit can act funny when you change values around.

Oh, and remove the jumper on the board that's shorting out your bias diode. 

Thanks, removed the jumper
That bias circuit was copied from the 5F6A visio file and then I needed to move the bias diode up :)

I may have to mess with the bias circuit
It's the same one that I use on several other of my boards and seems to work fine
Except for the .047, which is not used
I just tacked it in there temporarily

[EL34]

> That circuit can act funny

Indeed. "Small" changes make very large differences. 10% parts are treacherous. 5% resistors are now easy/cheap, but caps are sloppy.

I _think_ this gives enough range to cover most requirements. The bias-range tends to too-cool, which is much better than coming up hot first time.

Thanks PRR,
The bias circuit may have to be modded
I just copied and pasted the same one I use on several other boards like AB763, 5F6A, princeton reverb, Blues jr

[EL34]

1  You've gone for the Rev B/C version - later ones increased the PSU filter cap values, added a cathode bypass cap on the first stage (I've done that but made it switchable - useful for some earlier break up as these amps are loud without any master volume), and tweaked the network between V1A and B.  I'd suggest looking up the schematic for the later versions and basing your design on that.  Certainly provision for a V1A cathode bypass cap would be good.

You just add a cap over the top of the cathode resistor if you want that, it's easy on my turret boards

I'd keep the conjunctive filter across the output.  EL84 amps can be probe to fizz, I'd at least make provision for it so people can choose to fit or not.

It's easy enough to add if you want it

4 Likewise I'd keep the LTPI 47pF cap.

It's easy enough to add if you want it.
The two things you mention up above can be added right to the tube sockets or across a couple turret on the board if you want to add them

That's the nice thing about my turret boards
You can mod your amp the way you want it easily

[EL34]

Your plate-load on the long-tail driver is different from the Fender plan

This is exactly how the Blues JR was set up and I changed it to the simpler classic 82k / 100k plate load

The Blues Jr sounded great and the classic plate load setup was way easier to do on the turret board

[PRR]

> copied from the 5F6A

5F6a has a 50VAC tap on the PT. Simple DC supply.

The Pro Jr plan you link seems to have no 50VAC tap. So low-volt bias must be got from a high-volt point.

The waveforms in this plan with large series cap are wonky. Narrow +150V spikes which somehow average-out to small negative DC voltage.

For more fun it uses a FWB rectifier, so no point of the HV AC winding stands still.

Marshall has a similar bias supply in several models.

[EL34]

The bias range resistor is the only thing that I alter on many of the already existing boards I make using the same bias circuit


Some boards use a 470 ohm, some use a 220k
Depends on the AC voltage level source


The pro JR would be closer to what the Princeton Reverb board I make uses
http://el34world.com/Hoffman/files/Hoffman_PrincetonReverb.pdf

[PRR]

> using the same bias circuit

SAME bias circuit? 50VAC or 234VAC source?

The "usual old Fender" bias supply is not the same as shown in the Pro Jr plan because there is no 50VAC tap in the Pro Jr.

[EL34]

Thanks PRR


I will have to wait until I get the test donor amp carcass here, and then I can dial in a proper bias circuit

[JB]

I'd be interested in converting my Pro-Junior to turret board.

Any news on when you might have them available?

[EL34]

No, not until things slow down here at work


I have been slammed for a couple months

[Willabe]

I have been slammed for a couple months.

Good. 

[DummyLoad]

that bias supply in post #1 is not going to work. at most you'll get a few (-) mV.


--pete

[EL34]

reply #1
PRR's reply?

[DummyLoad]

reply #1
PRR's reply?

the fist post. yours.

[EL34]

The cap is not going to be there

[DummyLoad]

The cap is not going to be there

it has to be if you want to tap the FWB HW winding. a couple of schemes that i have prototyped in the past and recently run on a SIM. the SIM for either indicate a bias range from about -7.5V to -21V

--pete

[EL34]

My Pro Jr info is here
https://el34world.com/Forum/index.php?topic=24077.0

[scstill]

My Pro Jr info is here
https://el34world.com/Forum/index.php?topic=24077.0

Cannot find the info on why you physically separated the 22uf filter caps from the 47uf.
The current layout is different than the board you used to build in the picture section.

Did you experience a quieter PJ after this conversion? What other advantages?

[sluckey]

Cannot find the info on why you physically separated the 22uf filter caps from the 47uf.
The current layout is different than the board you used to build in the picture section.

Doug did not say why. Just consider it an improvement. Here's a quote from his build page...

Note: I have made some changes to the pro junior Turret Board shown below after doing this build
The pictures below are not the build instructions
Always use the layout diagram and BOM on the library page to assemble Hoffman turret boards

[EL34]

My Pro Jr info is here
https://el34world.com/Forum/index.php?topic=24077.0

Cannot find the info on why you physically separated the 22uf filter caps from the 47uf.
The current layout is different than the board you used to build in the picture section.

Did you experience a quieter PJ after this conversion? What other advantages?

All the instructions say that you do not use my build pictures to build my boards