Hoffman Amplifiers AB763 - 2 Channel Build - Noob Questions.....

[GrampaMojo]

Good morning guys, I am in the process of very slowly building a Hoffman Amplifiers AB763 - 2 Channel Deluxe Reverb style amp.  I am having a blast and learning a lot!  I am seeing my soldering skills getting better, and believe this build will only lead to another on in the future!

So here are a few shots of my build and my "work area"....  I am in Japan and have to work on the floor at my house.  I decided a chassis jig would be the way to go so built one out of plywood and plexiglass!  Make working on the chassis so mush easier!

As you can see I have been procrastinating on doing the tube heater wires... I know I should have already done them but that was the single most frustrating part of the Mojotone 5E3 build I did and I'm hoping that I can must up the confidence to get to them shortly!

Also trying to make copper buss bar for grounding.  I used plexiglass to isolate the buss bar from everything and will eventually ground it to the chassis over near the input jacks.

Please let me know what you think, and I will be asking questions very soon!

[mresistor]

Nice work..    I typically wait until last to do the flyover filament wires so that's not a big deal really. 
Thats a very nice cradle too..     

[GrampaMojo]

Nice work..    I typically wait until last to do the flyover filament wires so that's not a big deal really.

Thanks for the reply and it's good o hear I'm not alone....  I'll get to it...... eventually!

[GrampaMojo]

Ok, here I go!  My first noob question, please bare with me!

So, I received my four Mercury Magnetic transformers in the mail yesterday.  I figure I'll start easy, Choke and then Reverb.  Both pretty simple and straight forward.

But after those two, the Output Transformer..... when looking around I noticed that it has a hookup to "A", no biggie right!  But also the Standby has a hookup to "A", the filter caps have a hookup to "A", and the board has the one hookup for "A".....

Is there a preferred way to hook up these four "A's"? 

-- All meet up on the board at the "A" post? 
-- Or filter cap and standby lead meet at the board "A" post and the Standby lead from board and Output lead on the Standby switch?

Or some other way?

[GrampaMojo]

And noob question #2 is in regards to the Power Transformer! 

I tried asking Mercury Magnetics questions on which Power Transformer I should get.  Unfortunately after several emails over a couple of months and no replies from Mercury Magnetics I figured I'd just try my luck and pick one myself.....

So with that I ordered their FDP-20X, it said it's an upgrade for the Blackface Deluxe Reverb......

Anyhow, this Power Transformer has a Red/Yellow Center Tap, and a Green/Yellow Center Tap....

I believe the Red/Yellow Center Tap just goes to chassis ground but can't find anything specific on the Green/Yellow Center Tap?  Do I just trim it down and heat shrink the end?  Does it also go to chassis ground?   

Thanks again!

[sluckey]

Is there a preferred way to hook up these four "A's"? 

-- All meet up on the board at the "A" post? 

The board is a good central location for all 4 of those wires.

I believe the Red/Yellow Center Tap just goes to chassis ground but can't find anything specific on the Green/Yellow Center Tap?  Do I just trim it down and heat shrink the end?  Does it also go to chassis ground?

The green/yellow center-tap wire provides a ground reference for the filaments to reduce filament hum. But Hoffman's board provides two 100Ω resistors on the end of the board that serve the same purpose. I see you have already connected these two resistors to the filament string so I suggest just heat shrink the end of the grn/yel wire and tuck out of the way.

[GrampaMojo]

Thanks a bunch Sluckey!  That clears up everything for now.  I'm sure I'll be back asking more!

[GrampaMojo]

Here we go again, question #3!!  And I’m not sure this is even a noob question!

So, I understand and agree with the use of the three-prong power cable, which of course I am using!  But here in Japan they still do the older two-prong plugs and outlets…..  In new house builds the two-prong outlet are polarized, but in older houses and a lot of gigging joints they are not…..

Because of that, I’m thinking of adding a ground switch, do you think it’s needed?  And if so, could I get some help on how it should be hooked up?

I have a toggle switch like the one in the picture, any input would be much appreciated! 

[sluckey]

Look at page 19 of this scrapbook pdf...

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

I thought Japan used a 100V power system. Is that so?

[GrampaMojo]

Look at page 19 of this scrapbook pdf...

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

I thought Japan used a 100V power system. Is that so?

Thanks for the link, I'll check it out!

And yes, normal power is 100V.  On the island I live we run 60Hz and on the main island they run 50Hz... 

At my house I run a Furman Voltage Regulator which boosts me from the 100v up to 120v~125v.  Then for my vintage amps I use a Brown Box to drop me back down to 115v (for my Hiwatt Custom 100) - 117v (for my Fender Dual Showman).......

[GrampaMojo]

Look at page 19 of this scrapbook pdf...

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

Lots of cool stuff in that scrapbook Sluckey!  I like that "Buckaroo!", I guess that's all the Brown Box is but much cheaper to build and seems pretty easy!

After looking at page 19, since I don't have the outlet I'm thinking it's as easy as this.....

[PRR]

...I thought Japan used a 100V power system....

There's at least four districts. Japan's electrification is a long story. Aside from reminding builders, we really should steer such points to local (Japanese) technicians with local knowledge and tradition.

[GrampaMojo]

All super points!  But let me be clear, my build is being designed to one day return to the USA!

So, even though I do live in Japan, please reply to me like I’m your next door neighbor!!

I’ve lived in Japan since 88’!  I know a few amp techs around here that do great work, but with that being said they are the reason I’m on here and not talking with them.

So many times their vision is very narrow, and they don’t allow themselves to experiment with new ideas.  The whole "We've always done it like this" mindset.

I once bought a pre-assembled Telecaster control panel from Fralin pickups.  And they made a few custom tweaks to it for me.  When I gave it to the guitar tech over here he couldn’t understand why it was set up like it was…. I told him to leave it as is and just install it, simple install just wire the new Fralin pickups to the prewired control panel.... did I say simple?  At the time I didn't have any soldering gear.

Well when I picked up my Tele, the tech let me know that he "fixed it for me"…. In other words he modified it how he thought it should be wired!

And since he was the “expert” I was expected just to accept it as he "fixed" it and say thank you so much!

Sometimes that’s just the way it is over here.  And that’s why I’m reaching out to all you gurus!!

I hope you understand.

[pdf64]

…, since I don't have the outlet I'm thinking it's as easy as this.....

That looks good.

As your mains connectors aren’t polarised and so either feed can be live, maybe consider a double pole mains switch.
How about not using a ground switch, rather hardwiring the death cap between chassis and the nominal neutral, unfused feed (which is how the switch would be set to minimise hum)?
Then if the amp hums on power up, you know you need to flip the mains plug. That way, you can be sure the fuse is on the live feed, rather than neutral.
By that, if there’s somehow a fault to earth, the fuse will blow.
I acknowledge that’s not how Fender did it on amps with no ground switch, but the modern environment is somewhat different, there’s a lot more installed stuff that might be earthed, eg heating etc system metalwork.

[sluckey]

After looking at page 19, since I don't have the outlet I'm thinking it's as easy as this.....

That's right. Please note the cap has a Class X type. (Technically the cap should have a class Y rating, but class x works too.) This is a special type cap made for this application. They are supposed to fail open rather than shorted.

Also be aware that this cap looks like a 56K resistor to 60Hz line voltage. When you flip the switch to connect the cap to the hot side of the line you will be connecting the hot line voltage through a 56K resistor to chassis ground and your guitar strings! This may result in a nasty shock if you are holding the guitar strings and your lips touch a mic that's plugged into a different amp. You've probably already experienced this but I just wanted to remind you. This is not an issue in a properly installed 3-wire grounded electrical system.

[GrampaMojo]

Here's another option instead of a ground switch, but I definitely want to do what's best for the amp and my safety!

What if I skip the ground switch, and just do the three prong if I use one of those three to two prong adapters?  This can work most of the time for me, but I play at a few older places and because this has the polarized tips I can't plug it into some of the older sockets that have two smaller socket holes...

This is kind of like running a three prong but yanking the ground out with a pair of pliers, and I know a lot of guys that do that over here!

[sluckey]

You can also modify that ground buster adapter to be used in a non-polarized outlet. Just use a lineman pliers (or heavy duty diagonal cutters) and a file to shape the wide blade the same as the narrow blade.

[pdf64]

Back in the 90s, I recall a company I was with selling a batch of kit to Japan, we sent them with these mains leads
Are those used in your area?

[GrampaMojo]

Back in the 90s, I recall a company I was with selling a batch of kit to Japan, we sent them with these mains leads
Are those used in your area?

Yep, I see them around.  But regardless you need someplace within 6" to attach the ground and it's not always available.

[sluckey]

The spade lug is typically just connected to the screw that holds the cover on the outlet. This means nothing if the house does not have a 3-wire grounded system. And none of this means anything if the grid is not referenced to earth.

[GrampaMojo]

The spade lug is typically just connected to the screw that holds the cover on the outlet. This means nothing if the house does not have a 3-wire grounded system. And none of this means anything if the grid is not referenced to earth.

Exactly!

[GrampaMojo]

Good afternoon guys!!

Probably a little late in the game to be asking this, but I just read on one of the "Instructions" pages that Doug (I believe) wrote "I like to use 18 gauge stranded wire for the fuses, switches and speaker connections", well I used 20 gauge solid core wire for EVERYTHING, is that ok or not so much?  I really remember reading someplace that solid core was better and that you could actually wire the entire amp with 20 gauge wire....  maybe even the board could be done in 22 gauge....   


So I plan on attacking the filament wires this weekend, hopefully my wife finds something else that she needs me to do!   The two wires coming from my board that has the two 100 ohm resistors, does it matter which tube I tie them into?  AND is 20 gauge solid core wire ok for the filaments?

[mresistor]

20ga solid core wire is rated for 6 amp and 18ga is rated for 9.5 amps.  20ga would work and 18ga would be better.


I've tied the filaments from the board to the nearest tube socket before using a hoffman board with the 2 100 ohm resistors and it was one of the octal sockets. Pins 2 and 7 were a bit crowded but it wasn't too bad. 3 filament wires tied to a 7 pin socket pin would be harder to do.

[pdf64]

The wires to the board mounted balancing resistors could be much skinnier. They’re only passing about 30mA.