First time build with AB763 1 Chnl.

[Richs1979]

Have hum coming off the C node cap. When I chopstick it it goes away. When just touching it it goes away

[Willabe]

Set bias to 26.4 light bulb stayed a nice dim color after initial semi bright turn on.

26.4 what? DC volts, mA's?

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.

You don't test those things with the amp still plugged into the LBL. Just get the amp to pass the smoke test, LBL test. After amp passed the LBL test, plug straight into the wall, no LBL, then set the bias.

After bias is set, then you can plug a guitar in and test all the controls. 

[Willabe]

Have hum coming off the Code cap.

What is a "code cap"?

And why are you playing around with that?

Stay focused.

1st - LBL test.

2nd - set bias on power tubes.

The you can fool with the rest.   

[Richs1979]

26.4 volts

[Willabe]

26.4 volts

No.

You set it for MAX -bias before you put the power tubes in. Not the least -bias.

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

And you mean (negative) -26.4dcv?

[Willabe]

Setting the bias on power tubes is like setting the idol on a carburetor.

The more negative the bias the less current can flow through the tube. So -50dcv is more negative than -25dcv.

We set the bias for max -bias to protect the power tubes when we do the LBL smoke test.   

[Richs1979]

I redid the LBL test with the bias pot at -63. I watched the light carefully and it did dim. But not as much as the little 15 water did. I'm reading all this in millivolts. I'm stopping here so not to burn something up. The calculator came up with 26.4 being on the standard side. I disconnected the LBL and I turned the pot clockwise till I got
 to this reading using millivolts again. I have a pic of the tubes.

[Richs1979]

Yes -26.4 vdc

[Richs1979]

Ok. I'm exhausted. Redid the startup thing. With the bias pot completely ccw I put the tubes back in. Turned it back on with the LBL and it did dim steadily after turning it on. The calculator gave me a number of -26.4 after Inputting a 6L6GC. I will stop here with the LBL disconnected waiting for some guidance. There was a hum sounding like a bad ground and when i tapped on the 22 500 filter cap connected to the boards C-node it lessened a bit. Tubes had a red glow but not too bright. They seemed off colored compared to my 69 super bassman which had more of blue 4han red. Anyway appreciate everyone's help. Thanks

[glass54]

Your 69 Super Bassman has a +410V HT and -37V nominal Bias Supplies!!
Assuming your Mojo TX "Mojo752" with Secondary rated at 355-0-355 VAC will probably give you approx 450V DC.
Your Bias "safe starting point" should be no less than -50V DC!! Even if you aren't using the LBL.
You haven't told us you HT Voltage either with or without 6L6GC fitted  But we know you have 1R0 fitted to pins1+8 on O/P tubes.
At 450V HT and available -63V bias supply you should be able to dial in 40mV DC across you 1R0. ((Tube bias at approx 60%)
For whatever reason you are trying to dial in ?-26.4V, drop it and go back to Post 41 as Willabe kindly outlined 
-26.4V on Grids of 6L6 with HT> 400V will lead you to redplating and destruction!
...and good O/P tubes are getting expensive 
Regards
Mirek
 

[Richs1979]

Again, my inexperience comes to play. I typed in what I thought was general 6l6gc 18watt. Looked now and I was supposed type in 30 or 40 watts. Which I did and came up with -50 vdc as safe. Still have a slight hum, but my tubes look better. 22 500 cap on board node c seems the culprit. Seems microphonic when I tap on it. The bias is set though. Here's the best pic I can send you with this phone. Thanks again for responding.

[Richs1979]

The hum is only slightly worse than the sf super. Since the tubes looked normal I plugged in my strat set the volume to 3, mid scoop and a little top end and geeze..., I was blown away. With a small blackheart enclosed cab I blew the roof off the house. My wife loves me now. Anyway, other than something with that cap, and now I found that my number 1 input with the 1 meg resistor doesn't work, number 2 with the 2 68k caps works fine. Overall my 1st build works. As far as voltages go, please tell how to go about measuring them. I know how to keep one hand in my pocket, just need to know where and what measurements. Thanks again

[glass54]

Ok Richs, now that your wife loves you, can you please let us in on a secret.
What is ie can you please measure your +HT on idle, your current in both 1R0's on O/P tubes and just for fun, final Bias voltage on O/P grids (or grid resistors). With your two friends ie reliable Multimeter and 1R0 resistors, you can learn a lot and achieve good confidence in self builds.
?What calculator were you dialing numbers into?? Definitely NOT as useful as guidelines from mresistor and Willabe
Regards
Mirek

[Richs1979]

I had the meter on the whole time I was playing and the bias stayed at -50. As for measuring ht, please Instruct. Is it just the voltage off the 2 red transformer wires or the current. Would hate to burn another meter. I feel stupid asking this. But I'm just a guitar player with a soldering iron.

[Latole]

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

Wow !!!   

[Latole]

Erase

[Willabe]

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

Wow !!!   

Latole, you need to be a little more respectful to members. That reply was over the line.   

[Willabe]

I had the meter on the whole time I was playing and the bias stayed at -50.

Always put what the voltage is, AC or DC.

And if the -bias supply is new, it should/will be stable. What you want to measure after you set the -bias is across the K bias R. The current through that R will increase as you play through the amp. You just really want to know the current through the tubes at idol. (Unless you have some kind of problem.)

As for measuring ht, please Instruct. Is it just the voltage off the 2 red transformer wires or the current.

No. You have to measure the power 6L6GC plate dcv. It's pin 5.

Use you gator clips on your meters leads. Red lead goes to pin 5, the black lead goes to ground, set meter for dcv. Make sure the red leads clip is NOT touching anything but that pin 5. You don't want to short pin 5 to anything else!

Post the readings here of both tubes.

[Willabe]

In reply #63, the screen shot you posted of the bias calculator, you put the wrong dc plate voltage in.

That B+ power supply must be putting out more than 350dcv. So you biased the wrong.

You have to measure the 6L6GC plate dcv 1st, and put that into the calculator. Then calculate the -bias. After you set the new bias, you have to re-measure the 6L6 tubes plate dcv again, if different put the new dcv into the bias calculator and calculate it again.

The more current the power tubes draw, they will drop the B+ dcv. The less current the power tubes draw, the less they will drop the dcv. The current draw is a load on the B+ power supply.

[Richs1979]

So take reading off pin 3 on both tubes, correct? What should those voltages be? I know from reading it's never what the transformer rating is.

[Willabe]

Yes, you measure both tubes, pin 3, they will be close in their readings, within a few dcv's.

Probably will be ~ 450dcv or a little more.

Then put that number, average the 2, into the bias calculator.

[Richs1979]

Got it. I appreciate you sticking thru this with me. I noticed a comment from one of the other guys. Not cool. At least I got this running without smoking anything. Thank you. I'll post the numbers and redo the bias again.

[Willabe]

I noticed a comment from one of the other guys. Not cool.

I called him on it, just not necessary.

[Richs1979]

Checked voltages on the power tubes.
V6 was 426
V7 was 428
Do I average these before calculating bias, or do I use the higher reading?

[Willabe]

There 2 volts different, doesn't matter.

Go with 427dcv.

I get; 427dcv, 6L6GC @ 30w, Class AB fixed bias, 60% dissipation, 42.2mA. You can try 70% dissipation. Tubes will run 10% hotter.

After you set the bias, go back, measure the plate dcv again. If 5 to 10 dcv higher or more or 5 to 10 volts lower or more, re-calculate the bias again. Should be good then.

[Richs1979]

Now v6=439, v7=438
So re-do bias with 438, correct?

[Willabe]

Now v6=439, v7=438
So re-do bias with 438, correct?

Yes, I would. Easy to do, doesn't take much time. 

DCV went up, so the tubes were pulling a little more current before.

[Richs1979]

Set the bias to -45 gets more hum if I go lower.

[Willabe]

Set the bias to -45 gets more hum if I go lower.

Do you mean -45mA, measured across the K 1 ohm sensor R?

[Richs1979]

Yes ..... Pin 5
The hum seems to dissipate when I turn up the volume slightly. I know the noise floor goes up and maybe veils the lo frequency hum, but it seems it does lessen when the volume gets turned even slightly

[Willabe]

Set the bias to -45 gets more hum if I go lower.

Do you mean -45mA, measured across the K 1 ohm sensor R?

[Richs1979]

Yes -45mA

[Willabe]

I'm not sure how your setting the bias.

You put your meter probe's across the 1 ohm K R with the probe gator clips. Set the meter to dcv.

1mv = 0.001

And then 0.001dcv converts to 1mA.

So lets say, 0.042dcv = 42mA. 

I get tangled up in numbers, but I think that's right. 

[Richs1979]

I'm using pin 5 on the 6l6 and black to ground

[Richs1979]

Pins 1 and 8 go to a 1 ohm. Then to ground

[Richs1979]

With my probe across the 1 ohm on v7 I get 22.0mv

[Richs1979]

V6 I get the same 22.0 mv at that 1 ohm resistor

[Willabe]

I'm using pin 5 on the 6l6 and black to ground.

No.

Pin 5 is the tubes grid, that's where you measure for -dc bias voltage. Bias dcv is not the tubes current. When setting the bias you want to know how much current is flowing through the tube.

The tube calculator is telling you how much current you run through the tube. You check that across the 1 ohm bias current R.

Pins 1 and 8 go to a 1 ohm. That ;looks like ground

It's not ground, but it is grounded. As current is pulled through that 1 ohm R, a voltage will develop across the R. The more the current, the larger the dc voltage at idol. We use that dcv to set the bias.   

To set the bias with the bias calculator number;

Put your meter leads across the 1 ohm K R.

Set the meter for dcv.

If the calculator says, lets say 40mA, then turn the bias pot until you see 0.040dcv. 

[Willabe]

V6 I get the same 22.0 mv at that 1 ohm resistor

What does the bias calculator say to set for mA's for 60% dissipation? Set it for that #.

[Richs1979]

If I set it that low the hum gets annoyingly loud. If I set it at 70 percent it is acceptable. Is this safe or do I live with the hum and set it at 60 percent. I noticed also if I put my hand on the reverb input jack, the hum is lessened. I am trying to keep this on the cool side to stretch the life of the tubes. I wish I were more knowledgeable. I appreciate your patience with me. I wish I could buy you a drink. Thank you.

[Willabe]

    I don't drink anymore. 

We'll get to the hum. Lets just get the bias set correctly.

What's the 6L6GC plate dcv, pin 5, now? 

Edit; Pin 3.

[Richs1979]

-43.5

[Richs1979]

I'll be back

[Willabe]

-43.5

I'm sorry, I meant pin 5, B+ high dcv.

Edit: Sorry, I'm tired.     Pin 3! The plate.

[Richs1979]

So I did the following.
Measured pin 3 on both V6 and V7
V6 = 418v
V7 = 417v
At 1 ohm resistor
V6 = 31.0mv
V7 = 31.0mv
Pin 5
V6 = -49
67 = -49
This actually sounded the best with a single coil going through it. Beautiful full sound crystal clear with no artifacts.

[Willabe]

Ok, good.

So the bias calc says; 418dcv, 30w tube, 60%, 43.1mA. So your getting close.

So set it at 43.1mA.

Then re-measure the plate dcv, pin 3, then put that dcv in the bias calculator. Re-set the bias. Then measure the plate dcv 1 more time, pin 3, if dcv is close, your fine, if it moved up/down more than ~5dcv, run it again.   

[Richs1979]

Will do. Fingers crossed.

[Willabe]

Your B+ is low. Should be more like at least 330dcv to 350dcv, fully loaded.

You don't have the amp still plugged into the LBL do you? You don't need that any more. It will throw off ALL your measurements.

You sure you have a GZ34/5AR4?

We need the acv on pins 4 and 6 of the rectifier tube and the dcv on pin 8 and pin 2.

Measure each of those 1 at a time, red probe to the pin, black probe to ground.

Then measure the B+ dcv at each filter cap node and post those #'s. 

Post the #'s.

[Willabe]

I'm going out to get a sandwich, be back in a while. 

[Willabe]

I'm back, did you take those measurements?