Hum in Marshall 18 Hoffman build

[Oddvar]

Voltage on pin 3 is 12.89 v

[Leevi]

That's fine maybe a bit high but that's not the reason for "no sound"
/Leevi

[Oddvar]

Follow Hoffman's layout exactly!

I belive I did so, but the grounding issue is a bit uncertain, if you are, like me, an hobbyist.

[Oddvar]

I resoldered the input jack with a Cliff jack, do you see any reason for no sound here?

[Leevi]

The picture is not so sharp but have you soldered the shielded cable in a correct way?
I mean shield to the ground?

/Leevi

[Oddvar]

The "hot" is attached to the lug closest to the chassis input?

[Leevi]

It's then wrongly wired!


This is a correct way


http://www.geckoamps.com/milliAmp/cliff-wiring.jpg


/Leevi

[Oddvar]

Just a question about the 130ohm resistor, the leads broke on it and I have a 150 ohm wire wound 14w, it delivers 16,5 volt on the tube side, could I use it and could I reduce the voltage in anyway? (I have ordered a new one from Hoffman, but it takes a while.)

[Leevi]

Your cathode current per tube will be 16.5V/150 Ohm/2=55mA which is too high.


I wonder the big current since 150 Ohm resistor should be OK?


Can you check the wiring of the power tubes especially the screens, pin#9.


What is the plate voltage?


/Leevi

[Oddvar]

V1 1 260v,
     6 242v
V2  1 242v
      6 357v

PT
1 1.1 mv on both tubes
3 154v on both tubes.

[Leevi]

PT1 1.1 mv on both tubes3 154v on both tubes.

Measure the voltage on pin#7 of EL84s (= plate voltage)
if pin 3 is 154v there is something wrong in the power amp.


Check that the common cathode resistor is properly connected
Check that grid leak resistors 470K are properly connected
Checlk that screen resistors 100R are properly connected


/Leevi

[labb]

Would be a good idea to measure the voltage on each pin of the EL84's and post them..one place you say 154 volts on pin 3 and another place you say the cathode resistor (150 ohm) has 16.5 volt on the tube side..Pin 3 is the cathode. If you Have 154 volts on pin 3 you either have the EL84's wired wrong or the tube has failed.

[Oddvar]

The values:
A=397
B=394
C=359
D=351

Cathode res= 13.69

V1
1= 279
2=1.99
6=262
8=1.89

V2
1=279
3=2v
6=262
7=82
8=82

PT1 and 2
1=1.1
3=13.69
7=391
8=391

As accurate as I could measure.

[Leevi]

Could you measure power tubes pin 9 as well


There should not be any voltage on pin 8 since it is empty

/Leevi

[Oddvar]

Sorry, that should be nine.

[Leevi]

The values are OK, is it still "no sound"?
/Leevi

[Oddvar]

No, I hear the occasional scratching in the speaker when I touch the preamp tubes with the meterstick, but no sound.  I am stuck here, gone through the parts several times, I guess it's one little something that is not ok.

[Leevi]

Send a picture of the current setup
/Leevi

[Oddvar]

This is the latest version, I have resoldered every point, must be an obvious detail I have missed,  As often.

[Oddvar]

I am using this OT, only 8 ohms output;
Info:
Bolt-on upgrade output transformer rated 6,600 ohms to 8 ohms with 2-13/16" mounting centers. Wider 1-1/4" lamination "fat stack" gives this unit more than 60% additional core mass than typical 3/4" stack units (see photo) for improved performance. Special grain-oriented M6 core steel provides maximum output and punch (over the whole guitar spectrum) from a pair of EL84s or 6V6s while its low profile easily clears speakers in combo amps. This unit is a direct bolt-on upgrade for amps such as the Blues Jr.™ and Princeton Reverb™ amps and a good choice for 2xEL84 or 2x6V6 projects using an 8 ohm speaker load and where a compact unit is desired. Great for Princeton Reverbs™ with a 12" speaker installed. 2" tall, 2-5/16" deep and 2-5/8" wide across end bell covers. Black oxide finish with 8" leads. 1.6lbs. Custom paper stick-wound in the USA by Heyboer!

[Leevi]

Looks like the input jack is wrongly wired
/Leevi

[labb]

Leevi---run his bias calculation...Looks to me that he is at 17 watts per tube. ...The 391 plate voltage is really high for a EL84. That is not his no sound problem but it is a problem he will have to correct. Helpful little tool that one can build. http://www.diystompboxes.com/pedals/debug.html


can be used to inject a signal at different points in the amp to find where the no sound problem...Think you are right, the input jack is wired wrong...


The high tube dis may be what is causing the original hum.

[Leevi]

Leevi---run his bias calculation

Yes, you are right. The tubes are running hot but first things first, he has no sound;)


Max. plate voltage for EL84 is 300V but it seems that no one is following that rule nowadays.


/Leevi

[labb]

Yep. I normally run 350 to 360.

[Oddvar]

The hot is now on the right side away from the chassis, 1 m resistor fom there ro ground to the left pin away from the chassis?

[Leevi]

This is the right way to wire it


http://www.geckoamps.com/milliAmp/cliff-wiring.jpg


/Leevi

[Oddvar]

So what could be wrong if the components are ok, the tubes all make noise in the speaker when touched on the relevant pins?

[Leevi]

In the last picture you sent the hot wire is connected to wrong side of the jack.
There is switch inside the jack and it's important that the wires are connected to right lugs.


Your jack should be connected like in the picture (A).


/Leevi

[labb]

Maybe this can be of some help..turn power off of amp...Plug a guitar cable into the input jack. check for continuity from tip of the cable to the terminal you have the signal wire of the amp connected to. If you don't get it then you have it wired wrong. continue checking until you find the terminal that the cable connects to and connect your signal wire to this terminal. Then you are on your way. The drawings should be clear then.

[Oddvar]

I have wired it like the A version, I get 1m when metering it.

I have also now changed all the components on the board, still just a very low hum in the speaker, all the wires to the tubes are correct, just a little scratching on every tube.

I am a bit uncertain as to how I should ground it.  Enclosed is a picture of the grounding. What is going in here is: 300v ac ct, 5 vac ct, Heater ct, board ground and the speaker jack ground.  The neutral from the inlet is going by itself to the lug next to the rail.

[Oddvar]

Transformer part.

[Leevi]

It's a bit difficult to see from the picture since the cap groundings are not visible there.


You have power stages A, B, C and D on the layout.


I propose that you try with with two grounding points as follows:


C and D: Connect all the grounds (=V1, V2) including the minus pins of filter caps C and D to this point
A and B: Connect all the grounds (=V3, V4) including the minus pins of filter caps A and B to this point


/Leevi

[Oddvar]

This is the power connections,

[Oddvar]

The schematics

[Leevi]

Ok, it looks that everything is grounded to the same point.


If that works without hum then let it be as it is.


If you still have ground hum some changes needs to be done


/Leevi