First time build with AB763 1 Chnl.

[Richs1979]

Hello again, and good Sunday morning to you all.
This has been some learning curve so far. Since this is my very 1st build, I had a lot to learn.I have gained quite a bit of knowledge in the last month or so since starting this project. I have actually started to understand and read schematics. It's too bad that the circuit flow doesn't follow a physical layout more closely. Anyway, attached are some pics of what I've done so far. I do have some questions though.
First, I stopped at soldering the first 2 filter caps, so I could ask if they are correct. I followed Mr. Hoffman's diag and schematic and placed both those caps on the a A-B+ node. Is this correct?
My 2nd question concerns the Mojo752 - 125p5d power trans that I will be installing tomorrow. I'll be running both 5v yellow wires to a GZ34 rectifier with diodes. Do I need to run the red/yellow center tap to ground? I've read conflicting statements on this one. I think that it should be tucked away along with the 6.3v grn/yllw wire. Help is much appreciated.
Thank you.

[glass54]

Hi Richs
I looked at the Mojo build documents and Sch, and I would have to agree with past comments of Sluckeys ie Rubbish!! I understand your confusion 
So simply, Yes. you could fit the HT CT (Red/Yel) with the Heater CT (Green/Yel) wire to a good Earth Lug near Power Tx BUT don't use the Tx  mounting screws. Or you could use 100R ristors to ground as shown in Mojo Sch BUT not shown in their layout.
Please, next time include the Tx wiring details to help our colleagues. And please note the second attachment is from Mojo 
Regards
Mirek

[Latole]

Nice build

From Fender Deluxe Reverb;

[Richs1979]

Thanks for your help. Since I'm using two 100 ohm resistors with the heater wires I'm pretty sure the green and yellow should not be hooked to ground. The red/yellow center tap should. My first question was if my filter caps are correctly installed. Thank you for the compliment btw. This started as a bare 20.250 x 8 inch chassis and I just designed it to taste.

[acheld]

Since your heater winding is center tapped, you could just ground the green/yellow and NOT bother with the 100ohm resistors to ground. 

It's fine either way, just don't do both.   If I have a center tap, I'll use it.  Saves space . . .

[Richs1979]

Good evening. I finally finished the Hoffman AB763 1 channel amplifier kit with 2 6l6 output tubes and a GZ34 rectifier. Since this is my 1st time building anything electronic from scratch, I'm a little nervous to fire it up. I went over the circuit dozens of times and it seems everything checks out with Mr. Hoffman's diagram. The only changes I made were placing the bass pot first, and adding a future NFB switch. Also a future Master pot and an extra (V6) preamp socket for future modification. Everything else is a copy of the Mr. Hoffman's design. Is there a way to check my work without powering up? And how do I go about making sure the cathode bias is correct once I do fire it up? I do have a Variac which I got just for this project. Since all my other amps have solid state rectifiers, I don't have a spare rectifier tube. I have one on order which should be here in a few days. Anyway, I'm a few months shy of the big seven-O and I'm hoping i can show this thing off to some of the younger session guys I play with. Help is much appreciated. Thanks.

[Latole]

The first fire up

I'll use bulb lamp current limiter, not variac. 2 tests
1- No tubes
2 - Only preamp tubes. NO power tubes
__________________

   "  And how do I go about making sure the cathode bias is correct once I do fire it up "

-if the amp starts up correctly with the bulb lamp current limiter. Only preamp tubes. NO power tubes

 1- Pug amp in the wall outlet ; power on , stanby in play .
 2- Read negative bias volts at pins 5 on each 6L6 socket
     Turn bias pot to reach than maximum negative voltage ; You must have more than -30 volts dc.

 3- Il you can't reach -30 volts ; issue in the bias circuit ; DON'T put power tubes in the amp

 4-  If you have -30 and more ( it is better) leave the bias pot at its maximum.

 5 - To adjust at the right bias, you must have a bias probe or 1 ohms resistor from each 6V6's cathode to ground.

Read here ;https://www.mozartproject.org/how-to-bias-your-amps-tubes-for-the-best-sound/

[Calboy]

Is the photo in reply#5 the finished amp.
It looks like a couple connections are missing.  Rectifier pin8.
I would ground the PT center tap red/yellow to the A node caps negatives.

[Richs1979]

Hello -
Posted is a closeup of the PT wiring. I copied this build from one of Doug Hoffman's post's. The PT wiring is identical to his build. My main question was how to make sure my wiring is good. On paper it's all correct. Nothing was missed. I have a Variac and I was wondering if I can do a lo voltage circuit test before installing any tubes. When I ordered this kit there were parts Doug no longer carried but I followed his recommendations to build a 6l6 based single channel ab763.
Thanks again.

[SEL49]

On paper it's all correct. Nothing was missed.

Maybe it looks good on paper but you are missing a very important wire inside the chassis. You MUST connect a wire from pin 8 of the rectifier socket to point A on the board! Hoffman puts a standby switch in line with this wire. Without this wire you will have no high voltage anywhere in the amp (except the rectifier socket).

I noticed in the first pic you posted you had a red wire correctly connecting the standby switch to point A on the board, but this wire disappeared in later pics. What happened?

[Richs1979]

You're right. Stupid beginner mistake. I took out the standby switch out since after reading dozens of post's saying it's only necessary with ss rectifiers. Thank you for pointing out that blunder.

[Calboy]

Using a lightbulb limiter is correct procedure for first  startup as outlined in reply #6.
Can't see the A/C wiring to check if it's correct.

[Richs1979]

It's tucked under the lip. Blk to fuse to switch to transformer. White to tranny white(neutral), green to ground.

[Willabe]

Light bulb limiter to test for shorts not variac for start up.

https://el34world.com/Forum/index.php?topic=32424.0

How to build a light bulb limiter;

https://www.sluckeyamps.com/misc/Amp_Scrapbook.pdf

[Richs1979]

I have an inline multi ac power meter. I thought I could use the Variac, the display meter, then the LBL. I need to make the LBL first. Can I rig a table top lamp instead of buying a switch box panel? Also, I bought that variac to use with my old Fender's since the voltages in my house get up to around 123, 4.

[Willabe]

Put the variac aside, use just the light bulb limiter. You don't need the ac power meter for 1st time start up.

The bulb limits the current and that will protect the amp from damage if there's a short. For 2 x 6L6GC use 100w bulb.

Read this;

https://el34world.com/Forum/index.php?topic=32424.0

[Latole]

 I need to make the LBL first. Can I rig a table top lamp instead of buying a switch box panel?

The principle is there; Picture .
Use or build the way you want but respect this wiring and use an incandescent lamp, not a led.

[Richs1979]

Will do. I know there's a box of 60s. Would that do if I dont have a 100? Thanks for your help.

[acheld]

60's are fine.

Also, you can use halogen bulbs instead of the more common "incandescent" bulbs.

[Richs1979]

Thanks again. I'll be back after start-up. I'll use a 1 amp slow blow for the 1st test. Then I'll put the 2.5 back in if it didn't smoke. It's like my first small block build breaking in a new cam. That 1st fire-up is nuts. Fingers crossed. Thanks again.

[Willabe]

I'll use a 1 amp slow blow for the 1st test. Then I'll put the 2.5 back in if it didn't smoke.

No, that's what the light bulb limiter is for.

Use the correct size fuse.

[Richs1979]

Ok.... Will do.... Got the lamp done. Bulbs were LEDs. Need to get the right ones L8r. Thanks.... I'll be back. 

[Richs1979]

Hello again.
Made the light bulb limiter and had buy bulbs from Amazon. Bulb works... Tested it with another amp to get an idea how it works. Bright for a second then dim. Anyway, powered up the AB763 and the bulb did not turn on. The pilot light is working and there was no smoke, hum or anything unusual. I'm thinking that my B+ wire to the rectifier tube is wrong. Shouldn't it be on pin 7 and not 8. Because I added the diodes everything shifted one over. Here's a pic. Help is much appreciated as always. Thanks

[mresistor]

NO! your rectifier is wired correctly. Do you have any tubes installed? If you looked over the documentation of this Hoffman circuit you would know that.   https://el34world.com/Hoffman/files/Hoffman_AB763_1.pdf
the exception being you added safety diodes.   


   https://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-yellow-sheet-diode-mod

[Willabe]

I'm thinking that my B+ wire to the rectifier tube is wrong. Shouldn't it be on pin 7 and not 8. Because I added the diodes everything shifted one over.

No, everything is not shifted over 1. You only shift the red B+ acv wires over 1 pin to mount the diodes.

You still keep the rectifier yellow 5acv heater wires on pins 2 and 8, and take the B+ dcv from pin 8. 

[Richs1979]

No tubes..

[Richs1979]

I got the 0 volt ted/yellow center tap going to ground. Can that be wrong?

[Willabe]

Are you using a 100w incandescent bulb? Not LED.

Read this on 1st time start up;

https://el34world.com/Forum/index.php?topic=32424.msg359486#msg359486

[Richs1979]

Incandescent Edison  bulbs from Amazon. Tried it with my 15 water and it worked liked they said it would

[Willabe]

I got the 0 volt ted/yellow center tap going to ground. Can that be wrong?

Yes, B+ CT goes to ground.

Did you have the power tubes in when you tested with the LBL? Or just the just the rectifier tube?

[SEL49]

PLUG THE TUBES IN!

[Richs1979]

No tubes. Should I have put the rectifier in? And why did I not get a complete circuit even without the tubes?

[Willabe]

Read this on 1st time start up;

https://el34world.com/Forum/index.php?topic=32424.msg359486#msg359486

You have to set the -bias to max before you put the power tubes in.

[SEL49]

Pilot light ON proves you have a completed primary circuit.

[Richs1979]

So max out that 10k tremelo pot. I read the startup instructions over and over. And it still tells me that the limiter bulb should glow from the get go. I tried it on another tube amp and it works. It did not even flicker when I turned on the new build. The pilot light did go on. But that's it. No complete circuit back to the the limiter bulb. Am I misunderstanding something?

[Willabe]

So max out that 10k tremelo pot.

No, not the trem pot. The little white -bias pot on the eyelet/turret board.

You measure for -dcv at the power tube pin 5 of both power tube sockets.

[Richs1979]

Correct, since it's wired to the intensity pot I just assumed it was for that. Since this is cathode biased I thought the value was fixed by a resistor. Please forgive my ignorance. Never did anything this deep before.

[Willabe]

I read the startup instructions over and over. And it still tells me that the limiter bulb should glow from the get go. I tried it on another tube amp and it works. It did not even flicker when I turned on the new build. The pilot light did go on. But that's it. No complete circuit back to the the limiter bulb. Am I misunderstanding something?

When you tried it with a different amp, that amp had all it's tubes in it. The tubes will draw current, that causes the bulb to light up.

The new build, you have no tubes in it, so no current draw, bulb will not light up.

I don't see how you could have read the LBL start up instructions "over and over" and are not understanding what to do. It's very clear.

"Yes, just plug the amp into the LBL, no variac, no tubes, no rectifier tube, if amp has 1.

This tells you that all the PT secondaries, and whats wired up to them, on/off switch, fuse, pilot light, heaters, 5acv rectifier tube socket, and -bias supply, if amp has 1, have no short. If SS rectifier, this also tells you power supply, B+ filter cap string should be good, no shorts.

If the amp passes that test, then add the rectifier tube, if it's got one. This tells you power supply, B+ filter cap string should be good, no shorts.

If the amp passes that test, then if adjustable fixed -bias, verify -dcv at power tube socket. DCV present, set for max -dcv.

Now with the speaker plugged in, or dummy load, put ALL the tubes in.
Passes that test, your should be good. Unplug the amp from LBL, and now if amp has adjustable -bias, set the -bias."

[SEL49]

So max out that 10k tremelo pot. I read the startup instructions over and over. And it still tells me that the limiter bulb should glow from the get go. I tried it on another tube amp and it works. It did not even flicker when I turned on the new build. The pilot light did go on. But that's it. No complete circuit back to the the limiter bulb. Am I misunderstanding something?

I bet the tubes were plugged in on the other amp. The fact that the pilot light is on means you have a complete circuit on the limiter bulb. But, that 1 watt pilot light does not draw very much current. That's why you don't see the limiter bulb lit up. PLUG IN ALL THE TUBES!

On another note... It looks as though you have the black wire from the PT connected to the end terminal of the fuse holder. For safety sake, connect the power cord black to the end terminal and the PT black to the side terminal.

One more note... This is not a cathode biased amp. It is a fixed bias amp. And the 10K trim pot on the board sets the fixed bias grid voltage.

[Willabe]

Correct, since it's wired to the intensity pot I just assumed it was for that.

Forget the trem intensity pot.

The little white pot on the power tube end of the eyelet/turret board is the -bias pot. That's the pot to set the -bias with.

Since this is cathode biased I thought the value was fixed by a resistor.

This amp is not a cathode (K) bias amp. It is an adjustable fixed bias amp. That's why you have that little pot on the board.

Take your meter. Set it for DC voltage, with alligator clips on your meter leads clip the black lead to ground and the red lead to pin 5 on 1 power tube socket. Turn on the amp, no LBL, measure dcv at full counter clockwise (CCW), then measure at full clock wise (CW). Post those readings here. 

[Richs1979]

Did measurement at pin 5.
Counter clockwise = -63.0
Clockwise = -25

[Willabe]

Ok, that's good.

Now set the -bias full CCW, -63. Now put ALL the tubes in, including rectifier tube.

Plug amp into the LBL, turn on the amp. Be ready to turn it off if the bulb stays bright after turn on and does not dim back down. Just like when you tested it on the other amp.

If it dims back down you should be good. Take the amp off the LBL and set the -bias.

Set the bias for fixed AB with this; https://robrobinette.com/Tube_Bias_Calculator.htm

I like to set the bias for 60% dissipation.   

[Richs1979]

I appreciate you putting up with my inexperience. I just wish I knew how to do this stuff 50 years ago I'll be turning it on in after the dishes are done. I'll look at the link you sent me. Thank you. 

[Richs1979]

One more quick question.
Do I need to put a load on the output now?

[Willabe]

Do I need to put a load on the output now?

Yes, always a load when you turn on an amp.

It's in the instructions I posted for you.

[Willabe]

Did you use 1 ohm resistors on the power tube cathodes (K)?

They go between the tubes K, pin 8 and ground. Then you can measure the tube current safely at low voltage.

[Richs1979]

Yes..... 1 and 8 to 1 ohm

[Richs1979]

Wow..... I just looked at this thread. Could write a book. Thank you all for your help. Set bias to 26.4 light bulb stayed a nice dim color after initial semi bright turn on. Pots work. Only Volume por is scratchy in a few spots. A bunch of rotations back o'and forth should fix that. Will plug in my Tele and see what happens. Didn't check the tube color though. Will do that now. Thanks again. I'll post the final results L8r in the evening. 

[mresistor]

If the LBL went dim with all tubes installed then turn the amp off and take the LBL off the amp and put it aside.  You will now have the bias set to max negative voltage. Then turn the amp on and then set the bias.   You cannot set bias when on the Light bulb Limiter  LBL.


Reread Willabes post 41 again..  and the posts after that and make sure you know what you are doing with setting the bias to 60% of max dissipation.   If not ask some questions.  We don't want to burn up some expensive tubes.

[Willabe]

You will now have the bias set to max negative voltage.

You set the bias pot to max -bias before you put the power tubes in and test with the LBL.