Blues junior conversion, grounding!

[BobSmith]

Parts ordered, project waiting!

After studying the Hoffman schematic I am wondering how the grounding should go for the input jack.

The Hoffman schematic shows the input jack connected to the bus bar which has a ground symbol on the opposite end.  This seems to imply that the ground bus bar should have a separate connection to ground.  However, the standard switchcraft jack grounds directly to the chassis.  It seems like that would have 2 paths to ground...eek!  Certainly we cannot do that!

What do people do here?  Use fiber shoulder washers to isolate the input jack, then run the bus bar to a separate ground lug?  Or just ground the bus bar thru the input jack (I’ve built a few 5e3 that do it this way, and am fairly happy with results.)

[sluckey]

Ground the buss ONLY at the input jack. I prefer to actually install a ground lug very close to the input jack and run a short jumper from the jack sleeve lug to this dedicated ground lug. I don't isolate the jack from chassis.

[EL34]

The PDF layout shows it correctly
https://el34world.com/Hoffman/files/Hoffman_BluesJunior.pdf


The pots and jacks are all connected to the chassis, so there is your grounds
The ground symbol only reminds people that that whole buss is grounded

[dude]

Is that buss bar soldered to the pots

[shooter]

doesn't need to be, just keep it grounded 1 side only, near or at input jack

[dude]

I usually just let the buss float over the pots and like you say, only ground it near the inputs not the other end. But l have seen a lot of amps with a wire soldered to the pots, no problems either. Of course both ways the pots are grounded to the chassis through the nuts..? I always have two grounds in my amps, a star for the PS near the PT and all the way at the other end for the inputs, just floating a buss bar to catch the grounds as it runs to the inputs where it’s grounded near the input jacks that are grounded by their nuts, too. Never had any problems.

[EL34]

Yes the buss is soldered to the pots

All the info on installing my boards is here
https://el34world.com/Hoffman/instructions.htm

And in the PDF layout and BOM for each individual board on the library page here
https://el34world.com/schematics.htm

[BobSmith]

Thanks for the replies!  I’ll be grounding the bus bar thru the input jack then.  (Exactly how my 5e3 builds did).

As for the placement of the bus bar, I was planning to avoid directly mounting it to the pots themselves.  I would like to keep the pots more accessible for repair in case one needs to be replaced.  I’m still considering this as there are a fair number of pots to consider and therefore a large footprint is needed (and may not have!). If anyone has some pictures of their placement I’d sure like to see!  Thanks!

[sluckey]

I’m still considering this as there are a fair number of pots to consider and therefore a large footprint is needed (and may not have!). If anyone has some pictures of their placement I’d sure like to see!

You'll have the same number of pots that Hoffman used. Plenty of room. Hoffman also has lots of pics of his conversion.

[EL34]

All the documents on the library page and links are worth looking at
Like this picture of my build

[BobSmith]

Ground the buss ONLY at the input jack. I prefer to actually install a ground lug very close to the input jack and run a short jumper from the jack sleeve lug to this dedicated ground lug. I don't isolate the jack from chassis.

That would make 2 paths to ground, right?

[sluckey]

right

[BobSmith]

Aaaaaand, not grounding questions...but...

1.  It seems the rectifier has a hole in the middle.  Is this for mounting?  Any reason why I shouldn’t drill a hole and fix it to the board, rather than let it be held by the 2 turrets?  Seems easy enough to attach it. 

2.  Anyone have advice about cap selection?  Kit comes with xicons.  I got the 715p orange drops from Hoffman as well.  Besides the lead finagling that needs to happen to fit the orange elephants, any preferences out there(other caps used/fit?). I still need to use xicon for the 104, which doesn’t have a corresponding Orange drop equivalent.

3.  Ok, one “near grounding” question...  the artificial center tap for the heater filaments is two 47ohm resistors.  Why not 100ohm like seems to be usual?  I’m not using the Hoffman tube board, but rather this is going in another chassis all together.  I’m thinking of placing the artificial tap near the lamp and grounding with preamp section.  Bad idea?

Component and wire prep and dry fit photos below...

[sluckey]

1. If you don't feel the bridge is secure enough then bolt it to the board.

2. This board was laid out with Xicons in mind. You can mount orange drops but they won't look as neat. The Xicon 104 is a .1µF cap. Hoffman sells a .1µF Orange drop.

3. Hoffman used 47Ω resistors because the original BJ used 47Ω. Bad idea to ground them with the preamp grounds. Better to use one of the PT bolts.

[PRR]

> the rectifier has a hole in the middle.  Is this for mounting?

If you need to work it near the 3 Amp maximum rating, you bolt it to a heat-sink. But nearly no tube amps suck nearly that much current. In this build, more like 1/20th the maximum. It will hardly get warm (get more warmth from other parts than its own heat). He used that part because it is mass-produced and thus very inexpensive.

So here you don't need the hole for the intended heatsink bolt. Obviously it would be more secure bolted to "structure" (sturdy board, or chassis). I've never had trouble with these bridges just hanging on terminals. My builds, something else would fall off before the bridge even thought of breaking free.

[BobSmith]

1. If you don't feel the bridge is secure enough then bolt it to the board.

2. This board was laid out with Xicons in mind. You can mount orange drops but they won't look as neat. The Xicon 104 is a .1µF cap. Hoffman sells a .1µF Orange drop.

3. Hoffman used 47Ω resistors because the original BJ used 47Ω. Bad idea to ground them with the preamp grounds. Better to use one of the PT bolts.

Thanks PRR and Sluckey!  Very helpful!

I think I will leave the hole unmounted unless it feels like it moves around too much.

I realized why I didn’t get the Hoffman stocked 0.1 Orange Drop...they are 600v and super huge!  They would not fit in the layout.  No way.  The ones in the kit are actually Panasonic.  I think they will be good.  The OD that I did get seem to fit alright too. I suspect I’d have to do my own turret board if I wanted another cap...

Another Q:
Any idea if it’s worth using the Dale Vishay 1 ohm resistors that Hoffman shows in his build page?  I happen to have some Dales left over from another build.   The metal film he shipped with the kit seem to be very accurate.

[sluckey]

I use 1Ω 1W 1%. I don't know what brand they are. If I had Dales that meet those ratings I would use them.

[BobSmith]

I use 1Ω 1W 1%. I don't know what brand they are. If I had Dales that meet those ratings I would use them.

Sounds like I’ll be using the Dales in the 1ohm spots there.

Anywhere else you would recommend using them? I’ve heard the 1meg inputs resistor is a good place to sub in. 

[sluckey]

I think those blue resistors took pretty on a black board.

[BobSmith]

Board all soldered up. 

I added a stretch of shrink tube to the HV plate resistors as they cross over the ground terminal lacing.

I decided to place a hole under the rectifier.  Assuming it doesn’t get too hot, I will mount it with a black nylon bolt (good up to 220F).  I also will have my grounding bus bar mounted to the board.  It will occupy 2 of the 4 holes on the HV side of the board.

[acheld]

Nice work!  Looks great.

The grounding for this project is not as critical as you might think.  I've done this project several times (in one form or the other), and have used both grounding schemes without any difficulty.

I would not worry too much about the pots.  Whether they are soldered directly to the ground bus, or not, it's a pain in the neck to change them out. 

[EL34]

Board all soldered up. 

I added a stretch of shrink tube to the HV plate resistors as they cross over the ground terminal lacing.

I decided to place a hole under the rectifier.  Assuming it doesn’t get too hot, I will mount it with a black nylon bolt (good up to 220F).  I also will have my grounding bus bar mounted to the board.  It will occupy 2 of the 4 holes on the HV side of the board.

Very clean and neat work
I don't see too many people doing that with my parts

[EL34]

Nice work!  Looks great.

The grounding for this project is not as critical as you might think.  I've done this project several times (in one form or the other), and have used both grounding schemes without any difficulty.

I would not worry too much about the pots.  Whether they are soldered directly to the ground bus, or not, it's a pain in the neck to change them out.

I have alpha pots from the 90's still going
Even if I did have to change a pot with a bus wire soldered on the back, it's not a problem
Just snip the wire on each side of the solder blob
Put the new pot in
Solder the two wires to the pot again
Easy

[BobSmith]

Thanks for the excellent comments.  I appreciate all the idea!

I too have a TP24 from Allen.  It turns out it has a center tap already for the heater wires (green and yellow), but I noticed in the Hoffman build example the green and yellow wire is capped with shrink tube and there is an artificial center tap used instead.  I don’t plan to lift the heater DC voltage or anything else unusual. I was going to use the center tap already in the tranny and attach it to ground.  Any reason why I should reconsider this?

Here is the Allen schematic for the TP24:

[BobSmith]

Chassis is coming along as good as could be expected for a first time sheet metal worker.  GreenLee punches (used on tube sockets holes) worked like a charm!  See before and after below.  All other holes was done with incremental step bit and lots of deburring.

[BobSmith]

I’m hoping someone could comment on the TP24 heater center tap question in reply #23.  Thanks in advance!

Meanwhile here are some shots of the chassis development.  Holes knocked out, and control labels applied.  I now have the Meat next to the Fat (where else would you expect to find it!)

Labels done in old fashioned Datak dry transfer, Grotesque 216, 12 point.  Available online for fairly low in cost but high in time investment.  I coated it with Datak Hardkoat, 4 layers.  Now curing for a few days before attaching parts.  I’m satisfied with the results overall.

[PRR]

> center tap question

Are you trying to start a fight?? <g>

Using the CT lead is simple and clean. This was THE thing back in the day.

Using resistors means that a heater-cathode short may burn a resistor instead of tube or transformer.

[BobSmith]

> center tap question

Are you trying to start a fight?? <g>

Using the CT lead is simple and clean. This was THE thing back in the day.

Using resistors means that a heater-cathode short may burn a resistor instead of tube or transformer.

Opps.  I didn’t know this question could be treading in contentious territory.  (But then again I think ground theory tends to bring out a lot of opinions, so I should have expected it.)

I was just always under the impression that if a CT was there, it would be best to use it.  So I was surprised to see Hoffman go out of his way to cap it.  I guess there are several schools of thought here.

[Greg]

I don’t plan to lift the heater DC voltage or anything else unusual.

This procedure has nothing to do with elevating heaters, you can elevate your heaters by attaching your center tap to a dozen of volts DC source.

The center tap is for a ground reference to cancel hum. If your tube has a fault and shorts plate to heater, the current might burn the heater winding without the 2A main fuse blowing.

Now, IF you have a center tap and you would like to use it, use it. Like PRR said, It almost always been done this way. If you are worried about a heater short, you can just use just one 47 or 100 ohm half watt resistor IN SERIES with the center tap.



 

     

[davidwpack]

Hey. Very nice! I was wondering where you sourced your dry transfer lettering from. Thanks!

[66Strat]

Nice job on the project. I appreciate the attention to detail evidenced in your workmanship.

[BobSmith]

Hey. Very nice! I was wondering where you sourced your dry transfer lettering from. Thanks!

https://www.minute-man.com/

[davidwpack]

Thanks!

[BobSmith]

I made my ground connections to the chassis.  Cutting nice pads for grounding thru the powder coating was no trivial matter.  I think next time I’ll go with bare aluminum...

Wall ground to chassis is on its own dedicated lug.  Ground lead the longest of wires coming from wall.  Next to it I have a ground lug that will attach a jumper wire to the back cover and top cage.  A bit over fron there (basically under the fuse). I have the first major ground spot connecting the filter cap side of the board (and their back side connections), heater center tap (green-yellow), bias tap (brown), and the output jacks (I can’t assume they will ground thru their chassis connections).

I haven’t done the preamp grounds yet, but you can see the prepared bare circle near the input hole.

I also want to confirm the location of the choke connections (the two black wires on the top of the board.  I believe they should go in the place of the 2.2k dropping resistor.  (Which I will need to remove.)

Here are the pictures...

[BobSmith]

Here’s just a status report photo.

Trannies, sockets and main ground all hooked up.

I went with some overkill heater wiring that I’ve used before.  It’s 18awg 600v stranded acquired from Valvestorm.  I’ve used it before with excellent results, albeit not so easy to work with.

Next step are the pots and input.  Then either chassis work or testing.  (Who am I kidding...it’s testing of course!)

[EL34]

Beautiful work

[BobSmith]

Question:
Why does V2 (12ax7) use both sides for what appears to be a single gain stage?
Does it “add” anything?

The original Blues Junior only used one side.  I was hoping if I did this again, to add a classic fender tube reverb and to use this as the recovery for a tube reverb.  What changes would you recommend if I wanted to free up half of the 12ax7 again?

[sluckey]

Running the triodes in parallel gives a slightly fuller sound and maybe a slight increase in gain. Doug probably wired it like this mainly to utilize the unused triode. Simply remove the jumpers on the socket to free up pins 1,2,3. No other changes needed.

[BobSmith]

I finally did the startup and testing and biasing.  I used Billm’s recommendation for the Allen/Heyboer TO20 biasing, keeping it between 3.2-3.4 voltage drop across the output transformer legs.

Amp is has very low noise floor which I’m really happy about.  The Heyboer TP24 definitely gives more clean headroom.  You have to get pretty loud to get breakup, but I like that.  (That’s one reason my twin is my favorite amp).

Here are my Voltages in case anyone interested:
Line 117
B+ 375
Z 374 (using Mercury choke instead of 2.2k resistor, so only sees about 100ohms of DC resistance)
Y 325
X 295
Heater Filament 6.6

V1 (12ax7)
Pin 1 197
Pin 3 1.4
Pin 6 196
Pin 8 1.4

V2 (12ax7)
Pin 1&6 149
Pin 3&8 1.6

V3 (12ax7)
Pin 1 240
Pin 2 42
Pin 3&8 70
Pin 6 242
Pin 7 47

V4 (el84)
Pin 1 -16
Pin 9 374

V5 (el84)
Pin 1 -16
Pin 9 373

Last steps are are handle placement, cage cover ground screw, and consider some taller footpads.

Photos are of completed internals.  I show it on my test bench with a 1khz generated signal that I followed thru the circuit to output.  Finally next to my Fender blues junior (currently playing thru a Heritage greenback).  It sounds excellent.

[sluckey]

Very nice workmanship. Great test equipment too. I have a very similar variac. You may be interested in pages 4 and 5 of this pdf...

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[shooter]

+1

don't know when you carried your 1st scope, the TI is WAY lighter 

[sluckey]

My first was a 545 beast! I still have a scope cart for the 545 scopes. A 465 fits nicely on it.

     http://sluckeyamps.com/warbler/w_105.jpg

[shooter]

can't recall the scope before the 465, twice as heavy n I did mobile service 
the 465 made it til early 2000's then I got my digital TI 

It's no wonder I'm vertically compressed by 2" since those days 

[sluckey]

The 545 is a TUBE scope. Weighs about 50lbs. It was king until the solid state 453/454 and later 465 came along. Then the "tiny" 465 became king for a long time. FAA was still using them when I retired in 2012, but repair was no longer an option. If it broke it was replaced with a "teensy eensy" Fluke 199C scopemeter.