My first Build - AB763 Hoffman Twin Reverb

[sjwood3]

Whilst I wait for a few resistors from the post I thought I'd share a few updated photos from my build.  I'd like to thank all who've given me input.  I've been familiar with Fender amps for a while, but decided to take the plunge into building one after I came across a Quad Reverb with various parts left in it.  The board, OT, a few pots and wiring are all my work.  I read for days trying to figure out specific schematics, layouts, specs on parts for this amp...Then things started to click.  The RobRob site and Hoffman sites are incredible sources of information (I used more sources, but those were the most frequently visited).  The abundance of quality encouraging advice has helped me tremendously to this point, as well.  Even reading posts about how to troubleshoot possible problems  has made me go over and over this amp to see if problems exist.  All I have left to do is install two 1.5K resistors, hook up the OT, NFB, and commence the Rob Rob start up procedure very carefully, of course.  Here are a few pictures....Please offer any comments concerning lead dress or anything else you notice, I'll know if it's good when I turn it on I guess.   I have a floating bus wire under the pot wires grounded at the far input end. Hopefully the magic electronic smoke stays inside.  I'm checking and triple  checking  everything before power up. 

[sjwood3]

A few more pics from earlier in the build...Last week. 

[vampwizzard]

All im really seeing as an issue are the white wires flying off the yellow mojo caps. Did you attach the other end of that capacitor to an eyelet and then wire off that or is it a direct connection? We dont want anything that can vibrate loose in the cassis

[sjwood3]

All im really seeing as an issue are the white wires flying off the yellow mojo caps. Did you attach the other end of that capacitor to an eyelet and then wire off that or is it a direct connection? We dont want anything that can vibrate loose in the cassis

The caps are directly into the center of the turret, while the wire is nearly circumferential at the base of the turret on all wire connections.    I used solid core 18g and bent it very very carefully.     

[p2pAmps]

Be a little careful grounding to transformer bolts as they usually loosen up over time.  I personally stopped doing that a long time ago and now drill a dedicated hole for my star grounds etc.  Just a thought.  Good luck with your amp :)

[mresistor]

loosen up over time. 

These can be used on transformer bolts to resist or prevent loosening.   https://www.homedepot.com/p/Hillman-8-32-Stainless-Steel-Stop-Nut-15-Pack-2989/204794711
Or the Nyloc  Nylok   etc.   

[sjwood3]

loosen up over time. 

These can be used on transformer bolts to resist or prevent loosening.   https://www.homedepot.com/p/Hillman-8-32-Stainless-Steel-Stop-Nut-15-Pack-2989/204794711
Or the Nyloc  Nylok   etc.   

My plan was to use Loc-Tite on the trannys, but I guess a dedicated bolt near the PT is just as good and less likely to be jolted loose.  Thanks! 

[p2pAmps]

Just one way to consider grounding.

Cheers

[sjwood3]

   SOOOOOO, I started the procedure for powering up the Twin Clone.   All connections were secure and in the right place before beginning.  I used a current limiter and a variac.   Measured all the AC voltages.   Check.  The limiter never glowed.   I have  very high wattage bulb.   
 Given the limiter was attached, the readings were close to expected.   Once I started to power up the amp and got to 45V on the variac, the B+2 cap blew up.    I waited about 10 minutes prior to each incremental increase of 10V.  I had taken appropriate safety precautions.   Could this be result of a bad choke?  Could the cap have been too small? It was a 22uf/500V?   Bad capacitor?    The polarity was correct. 
  Thoughts appreciated.   

[shooter]

got to 45VDC, the B+2 cap blew up

45vdc on a 500vdc cap, should never blow it up.
failure rate on new caps is probably < 1%

I would disconnect everything "downstream", Don't replace cap, bring it back up to 45VDC, THEN measure VAC where the cap was, should be ~<5vac

EDIT:
this is the cap/schematic?

[sjwood3]

I had the A section in series... 

[shooter]

It was the B tap that blew? NOT the series?

[sjwood3]

Yes

[shooter]

let's try this;

NO POWER, UNPLUG

set your meter to OHMS, auto-range
put a gator-clip where C35 was, so SHORT B+ to ground.
Gator clip the red lead to C35 positive point
gator clip the blk lead to C35 negative point.

WATCH the "range", ohms is WAY diferent than K ohms
disconnect the shorting gato clip and NOTE about the 1st value, should be K ohms real quick, let it settle for a couple minutes as the caps charge, should end somewhere north of 30-50K ohms

[sjwood3]

I do not have an auto range function.  But I turned it on with red connected to positive and black to negative on the 200M setting and it read 1.2.  Not sure of that's helpful. 

[sluckey]

Replace the cap and power up again

[labb]

any chance the cap was installed with the wrong polarity.

[Willabe]

For any ground bolt, either, PT bolt or separate dedicated bolt, I use a 2nd nut, with 2 wrenches to tighten them against each other and lock tight. 

Keps nut, (or star washer on the bottom against the chassis, then nut) then nylock nut on top, lock tight applied to threads.

[sjwood3]

any chance the cap was installed with the wrong polarity.

No. It was correct. 

[sjwood3]

Replace the cap and power up again

I did replace it, then  POOF at 30 on the variac.   The A  section caps were super hot and the C, D sections weren't at all.   

I think my limiter is ok.  But just for kicks, I removed it from the equation and powered.  Same result. 

[sluckey]

I just looked at your rectifier board. All six of your diodes are installed backwards and are applying reverse voltage to the filter caps! That's blowing up your caps. Those hot node A caps are probably damaged by now too. They've definitely been stressed.

[sjwood3]

That makes me feel like a REAL rookie.   So if that's the case,    Should I expect damage anywhere else?   Or test anything else?    I have an extra set.    The caps spent a total of about 40 minutes under those conditions.    This is an important lesson for me because I didnt ask what I suspected I was wrong about when reading the schematic.   I know diodes allow flow in one direction only, but I was obviously confused by the comparison of a cathode/anode direction based on the schematic.    Really kicking myself here.  I appreciate you graciously making the point.  This is why I took safety precautions, in case I made a dumb mistake.   

I found this thread explaining my error.
I should have asked!   ...https://el34world.com/Forum/index.php?topic=17971.0

[sluckey]

Should I expect damage anywhere else?

Probably. Depends on how high you cranked the variac. All the B+ filter caps should be replaced. I would replace the 1K and 4.7K dropping resistors too. The choke has been stressed but probably survived.

[sjwood3]

I'll replace them all.   Better safe.

[sluckey]

yes

[sjwood3]

Success!   Replaced the diodes, changed all the filter caps, and the 1k/4.7k resistors.   Powered up to 120V perfectly.   Preamp tubes checked out.   I used the the Tube Depot Bias scout to bias the power tubes to about 20 mA just to power it up.   My master volume mod didn't work so I disconnected it, I think the old pot I used was bad.   I'm in a much better place with it today.  More measurements and checking tomorrow.  Thanks for the help!