Build a Princeton into a Blues Jr Chassis?

[BobL]

Anyone done this? Challenges/roadblocks?


Thinking Blackface style Princeton... with or without Reverb.  I just have a Blues Jr sitting here that gets no use and has a few issues, and it might be a fun project with a more useful amp at the end.

[EL34]

I don't understand what the roadblock is?


If you have DIY skills, you can fit just about anything into any amp as long as you have the space to do it and the proper planning

[PRR]

Sluckey could probably build a 5F6a, or a radar'scope, into a toaster oven.

I worked a furnace controller into a surveillance video interface box. (First, steal a surveillance box....)

Blues Jr is pretty-much a much-changed 2-6V6 Princeton. Oughta fit. The devil is in the details. If retaining the transformers, retain the rectifier scheme, as it will be right for that PT and 2-6V6. Etc.

[BobL]

Finally got around to researching this in more detail and I think it'll work just fine... replacing transformers w/ Mojo. I think the power transformer should fit w/o modification of the chassis.


As of now, it looks like the only major changes I will need to make are:


Expand one tube hole to fit a an octal socket;
Add two more octal socket holes;
Add a hole for the can 40+20+20+20uf/500 cap.


The rest should just be screw holes to mount stuff up - board, reverb driver, etc.


My one remaining question is about reverb tanks that will fit in this cabinet and sound nice... because of the size of the cabinet, a 16.75" tank won't fit in there, so I need a shorter one - I'm unclear if any tank that will physically fit will work, or if I need to get something that specifically works with a tube driven reverb, as in the description for an Accutronics unit I saw mention of 'will work in amps with solid state driven reverb'.


Do people have recommendations for reverb tanks that would fit this amp and work with this circuit?  I've not built an amp with reverb in it before...

[BobL]

This looks like the right size, but is the impedence wrong for this circuit?  I'm not sure how to decipher those numbers...


https://tubedepot.com/products/accutronics-8ab2a1b-reverb-tank

[sluckey]

Impedance is correct for the Fender PR circuit.

[DummyLoad]

Sluckey could probably build a 5F6a, or a radar'scope, into a toaster oven.

yes, but can he fit marshall 1987 in 50cal ammo can?   


--pete

[BobL]

Impedance is correct for the Fender PR circuit.

Thanks for the info... I'm guessing this isn't something a layman could look at the schematic and easily figure out?

[sluckey]

You probably can't figure it out just by looking at the schematic unless you are experienced with the typical Fender reverb circuit. All of the blackface era Fender amps use the 4AB3C1B long reverb tank. Knowing that fact you should be able to use this pdf and determine which short tanks will work.

[PRR]

> easily figure out?

98% of the time:

If the tank is driven by a small transformer, you want 4-10 Ohms input

If the tank is driven from a plate via a capacitor, you want a 600-2K input

[BobL]

Good to know - thanks to you both!

[traynor]

I built a Princeton Reverb in a Blues jr cab. I gutted the chassis and installed diff transformers. I was able to squeeze in a full sized reverb pan by cutting off the ends of the pan.

[BobL]

I built a Princeton Reverb in a Blues jr cab. I gutted the chassis and installed diff transformers. I was able to squeeze in a full sized reverb pan by cutting off the ends of the pan.

You are the guy I need to be bugging!


What did you use to cut the extra tube/can holes in your chassis?  Was there anything you ran into during that build that would be good to know before I get started?

[EL34]

Most likely, for any large diameter holes, you will need a greenlee type metal punch
Or
Metal hole saws will work if you can use them on such a tall skinny chassis and support them properly so that the bit can do it's work

[BobL]

Yeah, I'm not sure I know anyone with a greenlee punch local, though my brother is an electrician, so he probably has one a few hours away.


I was thinking hole saw - the chassis on the Blues Jr is pretty dang thin.

[sluckey]

I suggest a step bit. Much easier to handle than a hole saw. Greenlee is the goto brand. Irwin is another decent brand. Both available at Lowes. The cheap ones at Harbor Freight work OK in aluminum.

The greenlee conduit size punches like your brother has are usually the wrong size for tube sockets.

[BobL]

Looks like a 1/2" conduit punch will give you a .885" hole, which is just about perfect for a preamp tube, and between a 3/4" and 1" one of them should be about right for an octal socket.


Saw a used greenlee kit w/ these sizes on ebay for $40, so I grabbed that... I guess if it doesn't fit I can step them up a bit, but for $40 it seemed like a useful thing to have around anyway at $190ish new.

[Sonny ReVerb]

Yes, the 3/4" conduit punch will make a 1.115" hole which should work for an octal socket. I would punch a scrap piece of aluminum to see if you're happy with the fit before punching your chassis.

The large step bits will cut a 1" or 1 1/8" hole which will also work. Check the specs for your particular octal socket (or measure one) to see what you need.

[galaxiex]

My Blues Junior is kinda Princeton-ish... minus the tremolo.

Has all the stock BJ front panel functions plus foot-switchable tube reverb.

[BobL]

My Blues Junior is kinda Princeton-ish... minus the tremolo.

Has all the stock BJ front panel functions plus foot-switchable tube reverb.

That looks like a nifty face plate for the controls - where did you come up with that?

[BobL]

I'm seeing several PR builds that show wires running up over the top of the circuitboard when I would imagine you might want to actually run these underneath the board:


Examples:


'65
Other '65


I've also seen this on a few builds people have done - on the layouts these are the wires that just say 'B' or 'D'...


Is this the preferred way to run them? Up over the top and twisted w/ each other, and then only things actually shown w/ dotted lines would go on the bottom of the board...


This is my first BF style amp, so it's new to me...

[sluckey]

That's how Fender liked to do it. That style works well in the original board layout. Hoffman's layout is quite different from the original, so a different style of wiring is used. IOW, the board layout may determine how you run the wires. Which board layout will you be using?

[galaxiex]

My Blues Junior is kinda Princeton-ish... minus the tremolo.

Has all the stock BJ front panel functions plus foot-switchable tube reverb.

That looks like a nifty face plate for the controls - where did you come up with that?

Thanks, that is the stock BJ chassis and chrome face plate with a black and white vinyl sticker on there.

There is a guy on ebay selling these for the older BJ, Hot Rod series amps that have the *hard to read* chrome with white lettering.

The sticker is very good quality and has a satin clear coat to protect the lettering.

This guy...

https://www.ebay.ca/sch/jpcguitars/m.html?ssPageName=&_trksid=p2057872.m2749.l2654

[BobL]

That's how Fender liked to do it. That style works well in the original board layout. Hoffman's layout is quite different from the original, so a different style of wiring is used. IOW, the board layout may determine how you run the wires. Which board layout will you be using?

It'd be the original layout with the eyelet board from Mojo, not the Hoffman turret board.  I didn't actually know about that board until after I'd ordered. :/


EDIT: Looking at it, I think it'd be quite large for the Jr chassis, so the OG board/layout will probably be the best anyway.

[BobL]

My Blues Junior is kinda Princeton-ish... minus the tremolo.

Has all the stock BJ front panel functions plus foot-switchable tube reverb.

That looks like a nifty face plate for the controls - where did you come up with that?

Thanks, that is the stock BJ chassis and chrome face plate with a black and white vinyl sticker on there.

There is a guy on ebay selling these for the older BJ, Hot Rod series amps that have the *hard to read* chrome with white lettering.

The sticker is very good quality and has a satin clear coat to protect the lettering.

This guy...

https://www.ebay.ca/sch/jpcguitars/m.html?ssPageName=&_trksid=p2057872.m2749.l2654

Thanks, I emailed him to see if he does anything custom.

[BobL]

Should have all my parts for this today... got started.

Used the used greenlee punch I picked up and expanded one tube socket hole to fit an octal socket, and cut two more for the other two octal sockets.  Expanded the existing lamp hole to fit the 1/2" lamp mount. Waiting to cut screw holes until I can line up the sockets themselves, as well as the can cap hole, as I'm not sure of size yet.  Will also need to cut holes for the reverb connectors and fuse holder, but I want to see how things look space wise before deciding where to put those... probably on the back (front?) of the chassis.

I used Front Panel Designer to create a layout for a custom control plate, then used a nameplate maker to print it on black plastic, then lined it up w/ the existing holes and used a stepping bit to cut holes that line up exactly with the existing ones. Worked like a charm and I think the controls look good (and also 'right side up' to the user). 

I'm sure I'll have some questions as I get into this...

[BobL]

First question:


Things are going to be a bit tight in here... how far do I need to keep the can capacitor from the PT? Where I want to put it in terms of simplifying layout gets it really, really close to the PT, and I suspect this could be noise inducing?

[BobL]

Next question:


I've seen a bunch of different stuff on how to add a bias pot into this circuit... enough to confuse me as to what I should really do.


Looking at the article here on EL34, I see this specific bias circuit being discussed, and it makes sense me what is going on in that article w/ the last setup:





My question is that the trim pot I have doesn't have a 'back' leg, it has three legs off of the front:





With this pot, I'm unsure what does what, so I'm not sure where each thing should be connected... the single side lug to the end of the cap, and the left side of the two leg side to the resistor?

[Sonny ReVerb]

The inside of your pot looks something like this:



The outer lugs are across the total resistance. The center lug is the wiper which changes resistance as you turn the "knob". Hook up a meter and see what you get if in doubt.

[Sonny ReVerb]

The 'back leg' is your center leg (wiper). They just take it out the other side. I would say outer lug connected to the 10k and center lug connected to the cap.

[BobL]

Thanks, Sonny! That was what I thought, but you know... Measure twice.

[BobL]

So... what I had originally done was this:





And I'm thinking it's functionally equivalent to the Hoffman version above, and I'm wondering if there would be any benefit in changing my version to match the Hoffman version?

[BobL]

Well, to be safe I wired it as per the Hoffman diagram... but something seems off, as I'm getting 65mV on pin 8 (1 ohm, 1 watt to ground), and adjusting the bias pot does nothing except introduce a TON of noise unless I have it fully off - the bias value doesn't change when adjusted.


I've got 336v on pin3, so I figure I want 29 or 30mV on 5, yeah?


I've got the back side (single lug side of the pot) going to the connection of the bus wire and 10k resistor.  I've got the left lug going to the end of the 47/100 capacitor:





Am I missing something?

[sluckey]

Well, to be safe I wired it as per the Hoffman diagram

No you didn't. Compare your actual wiring to Hoffman's diagram and you will see some big errors.

[BobL]

Well, to be safe I wired it as per the Hoffman diagram

No you didn't. Compare your actual wiring to Hoffman's diagram and you will see some big errors.

Yep. That took me a minute.  This was the result of trying to change it back from the previous way I had it and not noticing the location switch on the resistor/cap. A bit embarassed over here...


Will the differing value of my cap be an issue?  I've seen several designs that have this as a 100uF and not 47uf, and my actual electronics knowledge is poor enough that I'm not entirely sure what affect this would have - from what I understand, the value I need to worry about changing w/ the pot to adjust the bias is the value of the resistor, yeah?

[shooter]

since you're unsure, I'd recommend pulling the PA tubes til you have the pot adjustable.  You can monitor it at the grid pin, guessing there should be a range of 10 - 20vdc Negative.  the cap size won't be an issue, unless it's wired backward 

[BobL]

Bias fixed.


Now to figure out what all is being noisy. :)


Appreciate all the help... I haven't built anything in about 6 years, and so much has exited my brain.  The older I get, the harder it becomes to cram stuff in there. I just wanna play the damn guitar!

[Willabe]

Now to figure out what all is being noisy. :)

It's noisy when turning the -bias pot, or the amp is noisy all the time?

There's dcv on the -bias pot, it will be noisy turning/adjusting that pot.

[BobL]

Now to figure out what all is being noisy. :)

It's noisy when turning the -bias pot, or the amp is noisy all the time?

There's dcv on the -bias pot, it will be noisy turning/adjusting that pot.

It's kinda noisy all the time (noisy = lots of hum, buzz). I think that some of it is due to shoehorning it into this chassis and making some decisions w/ the chassis early on that resulted in a less than ideal layout.  I'll get a chopstick and see if I can do anything to tame it a bit.


I threw it in the cab and it does sound pretty great otherwise, though... so that's nice.

[BobL]

Fixed!  Bad grounding on the pilot light... which was to say I hadn't installed the 100ohm resistors from the filament wires on the pilot to ground.  Did that... dead silent.  Sounds great... pretty stoked to see this through from 'I wonder' to actual final product!

[PRR]

> big errors.
> embarassed over here...
> hadn't installed the 100ohm resistors

If you do this enough, you eventually learn that the parts are 99.99% good, designs are 99% good, and "most" troubles are those darn inexperienced assembly bench workers.

[BobL]

Yeah, I've always been the constant in my own mistakes. ;)

[BobL]

Took the amp out to band practice and it did great, though I do have one issue:


If I turn the reverb up past about 5, I start to get a feedback sort of tone as I play, which then increases and stays until I turn the reverb down for a bit.  This seems to be regardless of the volume of the amp.


I wonder if this is because I am temporarily using unshielded wire to run from the chassis to the tank, and it's feeding back from proximity to the speaker magnet?  I've ordered some braided/shielded reverb wire to make these runs w/ the old style RCA jacks anyway, so I guess I'll find out, but I'm also curious if there is a common cause of this.  The in/out for the tank are very close to the pre-amp tubes since that was, space wise, a good location for them, though I wondered if it might cause me some issues when I was putting them there.


I typically don't have the reverb any higher than 3 anyway, but...

[pompeiisneaks]

Yes reverb send/return rca wires should be shielded.  If you buy the common ones at the store they're shielded, did you build custom ones yourself? 

~Phil

[BobL]

I had some standard, cheap RCA cables sitting around that I plugged in while waiting for the materials to build my own.  No idea if these ones would be shielded... they were from a red/white/yellow set sitting in a box here.

[sluckey]

That cable is shielded and should perform just fine.

[BobL]

Looks like people run into this when the tank is vibrating too much.  I used the original rubber washers to mount the new tank, but I'll try loosening those up, or setting the tank on something extra cushy to see if that fixes things.

[BobL]

Well... I'm a little stymied.


The reverb will cause this feedback under some interesting circumstances.


The tank is out of the cab, so it's not vibration.


The feedback only starts once the amp has been on for a couple minutes.


If I tap the reverb tube, it changes to a different tone.


If I tilt the amp forward, it stops...


I've tried two tubes - the 12AT7 I had in there, and a 12AU7.  I thought it was the tube because swapping made it go away... but then I found that the issue takes a couple minutes of operation to show up.


Vid:

Anyone have any ideas of what I could try?


As mentioned, the connections for the reverb tank are fairly close to the preamp tubes - on either side of V1, to be exact.  The reverb output is between V1 and V2, and the input is between V1 and the side of the chassis.