Fired up my first build today...

[Madkrafter]

Not without its problems. I started it up with RobRob’s procedure. Everything went well. The limiter (250 watt bulb - per Uncle Doug) never lit up. I slowly formed the caps with my Variac in 10 volt increments.

I put it together in the cabinet and found the speaker given to me (broken in Jensen C12K) had much more damage than I thought. I originally fixed a tear with Permatex Ultra Black Gasket. Turns out the cone was torn halfway around right where it meets the rim. I put some more Permatex on and will see if it holds while I debate the ever-opinionated speaker option. I’m considering the Eminence GA-SC64.

The tremolo ticks a bit, but I will try the .02uF cap fix on the left side of the roach. Wires are as far away as I can get them from others.

The biggest issue is the reverb doesn’t seem to work. There’s a tonal change but little reverb effect. I took out the tank and resoldered the output transducer wires. I measured wire continuity on the input side between the white and black transducer wires with a resistance reading of ~1 ohm. The output side did not have wire continuity and a resistance reading of 216 ohms. I have triple checked my wiring but just not sure what the issue could be. The 12AT7 in V3 is new as is the MojoTone M4AB2C1B tank.

The build details: Filter cap and main board built with flying leads by Mike @ Marsh Amps. The cabinet in charcoal and oyster tweed & Sahara Grey grill cloth with my design from Peter@ Mather Amp Cabinets. Transformers from Mercury Magnetics. Assorted extras from Hoffman Amps. I made one modification - RobRob’s 3-way NFB switch in place of the ground switch.

[bmccowan]

Beautiful amp - nice clean circuit work and love that cab. You will get the reverb straightened out no doubt with help from some of the super knowledgeable techs on the forum. Have you recorded voltages? That would be a good place to start.
Congrats on the amp - love it! 

[Madkrafter]

Thanks. Peter did an awesome job on the cabinet. I love it. The not-so-traditional aspect of my ‘65 BFDR. Meanwhile, I’m over at a friends and plugged the tank into his Twin reissue and the tank works. I have a little more studying to do about what voltages on what pins. I adjusted the bias to -35 DC. I have checked rectifier voltages and thought about adjusting my variac. I was around 345v (going from memory) I was around 7v on the preamp filaments. I might build the bucking transformer if it brings everything down in line.

[mresistor]

MMMmmmmmmmmm,,,,,nice!   that's a pretty DR         Did you try swapping the reverb cables? Also can use some other rca cables to see if its a cable issue.

[Madkrafter]

Thanks mresistor. I took the tank and cables to my buddy’s house last night and it was all working on his Twin RI. Tonight I will dig in with the schematic and meter, swap the 12AT7 from V6 and move some other tubes around and see what happens.

[mresistor]

Post up the voltages on the reverb AT7 and the 1/2 AX7.     

[Madkrafter]

Post up the voltages on the reverb AT7 and the 1/2 AX7.     

  I pulled the two 12AX7's from my PrimaLuna Stereo amp which I was having a problem with last week, I suspected one of the preamp tubes (12AU7 & 12AX7). I deliberately used those 12AX7's in V1 & V2 in case something catastrophic happened on startup. I didn't want to sacrifice the vintage Telefunkens that I have which are now in my PrimaLuna. Maybe I'll pull those back out of the PrimaLuna since I just listened to that system this morning and everything sounded good. I'll be sure to post my numbers tonight.

[Madkrafter]

Here we go. I have not changed anything. I wanted to keep it constant and get a baseline reading to see if there is something wonky.

@ 115v AC on my variac, 6V6 heater voltage: 5.0vAC, preamp heater voltage: 6.29vAC

Rectifier Tube GZ-34
HV-P4,6: 327.5v Ea leg
P8,2: 412v

B+1: 412v
B+2: 408v
B+3: 326.5v
B+4: 275.2v

6V6 V8-P1: -34.19v
6V6 V8-P3: 410v
6V6 V8-P4: 409v
6V6 V8-P5: -34.25v
6V6 V8-P6: 411v
6V6 V8-P7: 0.3mV

6V6 V7-P1: -34.19v
6V6 V7-P3: 412v
6V6 V7-P4: 410v
6V6 V7-P5: -34.19v
6V6 V7-P6: 410v
6V6 V8-P8: 0.1mV

Normal Preamp 1&2-12AX7
V1-P1: 185.1v
V1-P3: 1.346v
V1-P6: 179.5v
V1-P8: 1.417v

Vibrato Preamp 1&2-12AX7
V2-P1: 175.4v
V2-P3: 1.287v
V2-P6: 182.1v
V2-P8: 1.416v

Reverb Driver-12AT7
V3-P1: 404v
V3-P2: 21.5mV
V3-P3,8: 8.18v
V3-P6: 404v
V3-P7: 21.6mV

A Reverb Recovery & B Vibrato Preamp 3-12AX7
V4-P1: 187.7v
V4-P3: 1.475v
V4-P6: 182.5v
V4-P7: 1.9mV
V4-P8: 1.476v

A Tremolo Oscillator & B Tremolo Driver-12AX7
V5-P1: 397v
V5-P2: -34.97v
V5-P3: 0
V5-P6: 346.6v
V5-P7: -35.29v
V5-P8: 3.0mV

LTP Phase Inverter-12AT7
V6-P1: 189.6v
V6-P2: 44.4v
V6-P3: 71.2v
V6-P6: 181.6v
V6-P7: 47.5v
V6-P8: 71.5v

Some more detailed shots

[jordan86]

Man, good on you. AB763 for your first build. That’s no joke.

How long did it take you?

I just finished my first build. Blackface Princeton from Mojotone with some mods along the way (ala Rob Robinette and David Allen). Took me nearly 18 hours. Got lucky though. Fired up clean and quiet.

Hope you get it all dialed in. I’m new here but everyone is really cool.

[Madkrafter]

Thanks jordan86 and welcome from another newbie. Congrats on your build - 18 hrs... I feel like a slowpoke! This whole thing really started 8/26/18 when I purchased the face plate and back plate on eBay and bartered with a good friend for a chassis and speaker. It sat until the start of Covid and I needed a project and I started compiling my own kit. I had to redrill and modify the face and back chassis holes quite a bit. The transformers arrived May 18th and the major parts from Mather Amps a couple days later. I've been assembling it for about a month now. Taking my time and building other things too. I modified my variac with an amp meter, built a lightbulb limiter, a speaker dummy load box with variable impedance. I was missing some parts and had to wait for multiple shipments. Decided to do a mod and waited for parts... It's been slow... Mine isn't dead quiet.

[sluckey]

6V6 heater voltage: 5.0vDC, preamp heater voltage: 6.29vDC

Do you really have dc voltage on the heaters? Looks like a typical 6.3VAC filament string to me.

Tremolo tube should be a 12AX7, not a 12AT7. The gain of a 12AT7 is not high enough for reliable tremolo operation.

LTP PI tube should be a 12AT7.

[Madkrafter]

Do you really have dc voltage on the heaters? Looks like a typical 6.3VAC filament string to me.

Tremolo tube should be a 12AX7, not a 12AT7. The gain of a 12AT7 is not high enough for reliable tremolo operation.

LTP PI tube should be a 12AT7.

Sorry sluckey, the amp is correct, list was wrong (I've corrected it). Heaters do have AC voltage and I do have an 12AX7 for the Tremolo and an 12AT7 in V6 LPT PI. I will replace the Tremolo V5 tube since I have 0v on V5-P3. The Tremolo works but has a tick. It's the Reverb that's dead. Requires further digging.

[sluckey]

I will replace the Tremolo V5 tube since I have 0v on V5-P3.

Nothing wrong with V5. 0v on v5-3 is very normal when v5-2 has -34v. This is what you expect when the tremolo is switched off. And the tremolo works when switched on. The ticking is an issue. Search the net for "ticking tremolo fix" and you will find an answer.

There are some other voltage readings that look wonky. Maybe recheck...

V3 pins 2 and 7 should be zero. I doubt they are 31v. Maybe that was 31mV, which is really like zero.
V4 pin 7 should be zero. I doubt it is 4.1v. Maybe that was 4.1mV, which is really like zero.

[Madkrafter]

You were right sluckey - they were mV. I’ve updated my numbers again. My meter doesn’t have a backlight and was probably in the shadow of the amp... and I just didn’t notice the mV reading on the display. Next upgrade will be a Fluke... and keep learning! I did order all new 12AX7 tubes this morning. The AT7’s are new but I’ve had the AX7’s for years, if not decades. I have no idea of their condition. Those and a .02uF cap for the tremolo should be here Friday.

[Madkrafter]

VICTORY! I received the preamp tubes and .02uF cap today and installed them. I had emailed Mike Marsh last night and he suggested reflowing the solder on the reverb tubes cathode circuit (2.2K resistor and 25/25 cap). I turned the amp on and the tremolo tick was gone but the reverb had no change and the speaker was still blown (I ordered an Eminence GA-SC64 last night too)...

Mike also said a reversal of the reverb tank wires would stop the signal flow through the tank I assumed (you know what happens then) that reverb tank OUT > In @ the amp and vice versa. I swapped the wires so OUT>OUT and IN>IN and gave the strings a thwack ...Check the simplest solutions first. Is that what you meant by swap the cables @mresistor?

There is a good hum when the reverb knob hits 4 but I'm calling this a victory tonight and having a beer.

[Madkrafter]

Now that the thrill is gone I'm anylizing a bit more. The speaker has that typical amp speaker hiss. By itself, totally acceptable. With the tank unplugged, the reverb amplifies that hiss past 3 on the reverb knob - it's like a hiss volume knob. I noticed tonight that the hiss oscillates with the tremolo. When I plug the reverb tank in, the tone changes to sounding like a ground buzz. I saw this thread and will follow but am not nearly as smart.

https://el34world.com/Forum/index.php?topic=17645.0

[Madkrafter]

To recap - I still have the noise as the reverb knob gets turned up and the tremolo oscillation in the reverb signal. It also starts to get some feedback howl that doesn't go away by turning down the amp or guitar volume. It is coming from the reverb circuit and only goes away by turning down the reverb knob.

I have now tried:
1. Moving the reverb pedal resistor ground connection from the chassis (w/ the reverb TR ground) to the jack ground lug.
2. Soldered a .0022uf Cap in parallel with the reverb pedal resistor.
3. Wrapped the tremolo wire bundle in copper shielding tape - tied it to the chassis... and not.

Would anyone have a clue as to what can cause this?
Next thing that I will try is bringing it to an amp tech in Salt Lake City who is a friend of a friend and will maybe work on my DIY amp.

[shooter]

only goes away by turning down the reverb knob.

is the reverb n trem foot switched?
IF so;
with both switched OFF;
does the amp play normal at LOUD volumes?

[Madkrafter]

I forgot that detail. Yes the amp plays normal at loud volumes. With the reverb switch off - the hum and howl go away. The tremolo by itself seems to be totally fine other than the hiss oscillating too. The amp noise (hiss) is fairly low with the reverb switched off.

[shooter]

guessing you have a mis-wiring issue in the reverb circuit.
a relaxed evening and you can have it stripped down, then build it back up slow.
as you're tearing it down, note each wire you remove against the schematic, if you want to know where you went wrong.

I did a trem circuit twice before I put "that cap" where it belonged, not where I had it 

[Madkrafter]

Thanks shooter. I have been working on this for a while and trying to fix it and I just want it to work. I need to step back, and take the time to analyze. One thing that haunted me in building it was gathering all the clipped ends of wire and resistor legs. One got away that I never found - it might be in the amp or across the room. The other thing is my boards were built by Marsh Amps. I’ve ASSUMED that there are no errors... we’re all human. Time to grab a beer, clean up the work area, get the chassis on the bench and study the schematic and layout.

[shooter]

One got away that I never found

 

My dayjob was fixing MRI's, I spent many an UN-paid night hunting dow 3mm washers 
The cool thing, I learned to NOT watch, but listen, very few freebies after that 

[acheld]

I've built two AB763s with Reverb, and can say that -- at least in my hands -- it is tough to get rid of the noise in the reverb circuit.  The only thing I've found to work is to make sure your grounding scheme is good, and that you keep the signal wires into the reverb section as far away from high voltage as you can. 

This forum has real experts who are good guys as well. 

Also, an AB763 as a first build is not a simple one. So well done on that!   But, be prepared for the dialing in process, which can take time and can be frustrating.  It is just part of the build.

[Madkrafter]

I knew the AB763 was a deep first build. The fact that Mike Marsh built the boards with flying leads (main, filter cap and bias boards) took a lot of anxiety out of the build. Still, I studied and took my time, watched repair videos from Uncle Doug and D-Lab from May 18 to now.

I’ve tried quite a few things as previously mentioned. Last night I traced all of the wires from the tubes to the board. Everything was correct. Today, I tried a shielded wire from V3p7 to the V3 grid leak resistor. Also tried RobRob’s noise filter fix of a .002uF cap to ground from the reverb pan input RCA. No change (I quickly tried a .0047 and it killed the reverb but not the noise). I‘m beginning to think it is noise/voltage interference of some kind. I’ve been watching Dr Carlson’s Lab too. He has a Super Probe project that traces noise and oscillation problems. Maybe I need to build that next... and no, I don’t have an oscilloscope.  There is a local shop I went to on Friday with a used one 

[acheld]

Check out Picotech.com.  --  work well, fairly inexpensive, and don't take up a lot of room.

[Madkrafter]

Thanks acheld. Those are pretty cool. Besides the cheapest at $139 with probes, the price goes up pretty quick. The guy I talked to said the used ones are usually around $150-300. Still, starting at $139 the PicoScope® 2000 Series might do the trick for something I wouldn't use too often (yet). I'll have to research those.

[shooter]

Always take into consideration the aggravation factor of NOT knowing what's wrong and the time n money spent swapping parts on guesses.  besides, it's something new to tinker with 

[ac427v]

I don't have a good understanding of your answer to Shooter's foot-switch question. The reverb will hum if the tremswitch (unshielded wire) is plugged in to the reverb RCA jack. I'm pushing for clarity on this cause it is a very easy solution to some reverb hum problems. Can you verify that the shielded footswitch wire is plugged in to the reverb RCA jack? Thanks for checking.

[ALBATROS1234]

i dont know what to say about the reverb because i havent built an amp with reverb but one side note when you measured voltages on the power tubes 6v6s dont have anything on pin 1 and 6 so it should be negligable. as you surely know 2 and 7 are filament pins , the ones you need to test : 3 is plate ,4 is screen and 5 is grid but you really need to know cathode voltage from pin 8 to calculate plate dissipation. unless you have a bias tester you plug in the socket and tube in to that or you could use the output transformer method

[Madkrafter]

Always take into consideration the aggravation factor of NOT knowing what's wrong and the time n money spent swapping parts on guesses.  besides, it's something new to tinker with 

I have definitely considered and have realized that NOT knowing is the most aggravating part as well as my ignorance on the subject (first build). And ordering new caps and such and having to order... wait... test... order... wait... test  There is a part of me that enjoys the hunt but it's being overshadowed by the aggravation of a month and a half of work and still no progress on the problem. I distract myself by building other things - my new bucking transformer from RobRob's design. I will make a new post as soon as I get the last two parts... I'm waiting!

I'm pushing for clarity on this cause it is a very easy solution to some reverb hum problems. Can you verify that the shielded footswitch wire is plugged in to the reverb RCA jack? Thanks for checking.

The shielded reverb wire is plugged into the reverb footswitch RCA.

i dont know what to say about the reverb because i havent built an amp with reverb but one side note when you measured voltages on the power tubes 6v6s dont have anything on pin 1 and 6 so it should be negligable. as you surely know 2 and 7 are filament pins , the ones you need to test : 3 is plate ,4 is screen and 5 is grid but you really need to know cathode voltage from pin 8 to calculate plate dissipation. unless you have a bias tester you plug in the socket and tube in to that or you could use the output transformer method

When I get home, I will take those readings. I will have to learn about the cathode voltage and calculating calculate plate dissipation. That is the one thing I haven't dug into - the math.

[shooter]

the math

easy, 4 function calculator, 7th grade dust up and you're there 

PP PA;
ALWAYS insert 1ohm resistors from cathode to ground, each PA tube, EVEN if it wasn't there in '64!
no math, just read vdc at cathode, converts to mA because OHM said so 
measure plate vdc * I = close enough dissipation

SE - self biased - cathode biased; (the easy method without G2)
measure vdc at cathode, do V/R = I
measure vdc at plate
(vdc @ plate - vdc @ cathode) * I = close enough dissipation

once you master that math, read more and have fun

[Madkrafter]

Thanks shooter.

PP PA;
ALWAYS insert 1ohm resistors from cathode to ground, each PA tube, EVEN if it wasn't there in '64!

Do you mean temporarily clip a 1 ohm from cathode to ground to do the reading? Work was fun enough today. Now I've got some learning to do... and order some 1 ohm resistors. Luckily last weekend I found out an electronics shop that closed years ago reopened under a new name, same owner (what I mentioned previously about the oscilloscope). I can get that stuff same-day if I make the drive.  I did see this procedure on RobRob's site.

[mresistor]

No he is saying install them permanently. They make bias and power checks much easier.

[Madkrafter]

Looking at the output transformer method, reading voltage at both plates - P3 I have V7p3 @ 409 and V8p3 @411. Next reading the center tap voltage (B+1, standby switch) @ 412.  a 1v & 3v drop. Is that right? Also, I read voltages on cathode V7p8 @ 0.3mV & V8p8 0.1mV. The voltage is negligible and the cathodes go to ground, what is the purpose of the 1 ohm resister? I see a similar method in Jack Darr's book putting a 1 ohm 5 watt resister in series on the primary side of the OT to read wattage. I read all voltage again and updated the OP.

I admit, tonight I'm not feeling smarter than a 5th grader, let alone a 7th grader  and I'm just not getting it... but I can take a decent picture. I took this because I noticed the blue in V7 and wasn't sure if it indicated anything critical. And I will be honest, I’m not sure what PP, PA & SE refer to (push-pull, preamp, single-ended??). These acronyms are new to me. I will have to look for an acronym definition page.

[Madkrafter]

So I read through the output transformer method and realized that my meter does not read with enough digits to maybe give an accurate milliamp reading. Today I went back at it with a determination that the problem may be interference/noise based. I started with the doghouse bundle. The wires were simply laid down on the chassis. I unsoldered and twisted them and followed that with a wire wrap like they used to do as a shield and soldered one end to the power ground lug.

Next, I took the two reverb pot wires which were loosely twisted and tucked under the pots. I unsoldered and tightly twisted them. I kept them suspended over the board and crossed them perpendicular over the tremolo bundle to the pot lugs.

Then I noticed the green reverb transformer wire had crept up towards the filament wires so I pushed it down. All of this was an improvement but there was still the ground hum. I moved the tank out of the cabinet as far as I could and the hum went away. I then tried something that I had tried before but didn't work - I rotated the tank 180 degrees... No hum, no howl. 

There's still some hiss. I have actually never owned a reverb amp so I can't compare the noise level. I have a Groove Tubes soul-O 75 with a Peavey Valverb in the rack space but it's not the same circuitry. I talked to an amp tech recommended by a friend. I travel this coming week and I am going to bring it to him to check over when I get back.

[shooter]

 

get play time in so you "understand" the amp as is.

I’m not sure what PP, PA & SE refer to

while you're traveling here's a fun read
 http://www.aikenamps.com/index.php/what-do-the-terms-push-pull-and-single-ended-mean

[Madkrafter]

So I did bring my build down to the amp tech. He called me last Monday and said it was ready. Commended me on how clean it was, solder joints were all good. He didn't touch anything in the circuit. He found that that V3 & V4 were noisy tubes - Mullard reissue 12AX7 and TAD 12AT, and changed them out. He also took the feet off of the reverb tank and strapped a piece of foil-wrapped cardboard to the bottom of the tank. I'm happy to say that I can call this build DONE!  Thank you for all the help and tips.