Need Help With Bias Problem

[Axer57]

I know my amp is a little out of the ordinary here but I really need expert advise. I have a Marshall JTM60 guitar amp. It has an odd bias pot configuration . I am trying to use 6L6GC tubes in it even though it was built for EL34'S. To bias the 6L6GC'S I need to set it at -43ma , but with the pot turned all the way it will only go to -40.5ma. I can't locate a sweep adjustment resistor. Can anyone look this over and tell me how to accomplish this.

[phsyconoodler]

Ok,first off you are measuring the bias how exactly? Bias is not measured in (-) , it's in ma but you need either a bias meter or a 1 ohm resistor to ground in place of the cathode ground wire.

 So,whoever told you or whatever you are reading,you need to understand what it is you are setting before you go trying to change the range resistor(NOT sweep resistor) .

 Who told you it should be set at 43ma? And in truth is that is what you are measuring,the difference between 40ma and 43ma will be un-noticeable in terms of tone.

 So we need to know what you are talking about,how you are measuring it,and when you show a schematic,show the whole thing,not part of it.We can't see the whole bias supply in that diagram.

[sluckey]

Make R147 smaller. Or make R141 larger.

[Axer57]

I measured voltage at pin 3 and got 484 using a multimeter. I measured voltage at pin 5 and got -40.5. Using the weber bias calculator online it said the bias should be set at 43ma. I can't load the entire schematic it's too large of a file. I took a snapshot of more of it.
 This amp doesn't have a range resistor. It uses a zener diode in some configuration I have never seen.

[Axer57]

R147 is just a jumper . there is no resistor there.

[sluckey]

R147 is just a jumper . there is no resistor there.

I gave you two choices based on the info you posted. Can you post the correct schematic!

[Axer57]

The schematic is in pdf format. It's 977kb. I tried to post it but it said it was too big. I will try to post a better version.

[Axer57]

here's part 2 of 3

[Axer57]

here's 3 of 3. There isn't a good schematic for this amp. I searched for quite a while just to find these. It's hard to read but R147 says "link". these are the correct schematics.

[Willabe]

It has an odd bias pot configuration.

It looks like the PT has a separate wind for the bias. H107, H108 (CT), H109.

The bias acv from the PT's bias wind, which has a CT going to ground, goes to 2 caps then to a full way bridge rectifier.

I don't see why they would have a PT made with a separate bias wind with a CT then couple it through a pair of caps so they can use a FWB rectifier? Cost them a CT and 2 caps plus PCB space for the caps. Why not leave out the CT (and the 2 caps) if they wanted a FWB, would cost less.

Then it looks like R147 is the range R, like Sluckey said.

ZD101 is a 51v zener diode so with it in place the -bias will never go above/more negative ~-51vdc.


                   Brad      

[Axer57]

Well if R147 is the bias range, then there is no adjustment. I have the board laid out on the table and R147 is a factory metal wire jumper.

[Willabe]

Sometimes an amp company will change the PCB layout and not up date the schemo to the public.

You'll have to find where the 3 traces that connect to the bias pot go. If 1 outside leg of the bias pot goes straight to ground through a plain wire jumper when you turn the bias pots wiper to that jumper the amps tubes would have NO bias at all and would go into runaway instantly and burn up. There has to be an R somewhere to make sure there is always a minimum -bias voltage on the output tube grids. They won't make an amp with adjustable -bias without it.

You have to find it.


              Brad        

[Axer57]

What you just described is exactly the way this board is wired. The right leg of the bias pot [when facing the adjustment side] is connected to the jumper wire [R147] which goes to ground. I have never turned it past about 1/4 turn from the left. That's where the EL34's bias at around 35ma. My problem is I need more room the other way. when turned all the way left it shows a bias of 40.5ma. I need 43ma to use 6L6GC tubes...
alot of people say this amp was one of Marshalls greatest mistakes. The engineering leaves alot of questions. I agree though , there should be someway to extend the range towards more negative voltage. I haven't been able to find it yet .

[jazbo8]

Take out the jumper and install a ~22k resistor in its place, that should give you a bit more range.

[Willabe]

That's where the EL34's bias at around 35ma. My problem is I need more room the other way. when turned all the way left it shows a bias of 40.5ma. I need 43ma to use 6L6GC tubes...

Ok,first off you are measuring the bias how exactly? Bias is not measured in (-) , it's in ma but you need either a bias meter or a 1 ohm resistor to ground in place of the cathode ground wire.

 So,whoever told you or whatever you are reading,you need to understand what it is you are setting before you go trying to change the range resistor(NOT sweep resistor) .

 Who told you it should be set at 43ma? And in truth is that is what you are measuring,the difference between 40ma and 43ma will be un-noticeable in terms of tone.

 So we need to know what you are talking about,how you are measuring it,and when you show a schematic,show the whole thing,not part of it.We can't see the whole bias supply in that diagram.

phsyconoodler is right.

1. How are you measuring the bias current?

2. And are you talking about -dc bias voltage OR the tubes current which is measured in mA's?

The Weber calculator in not a way to measure bias current. It's just giving you a number to get to without red plating the tube at a certain dc plate voltage. You can always set the bias for the tube to draw less current if it sounds better to you. But you still have to measure the tubes current to know how much current it's drawing. Every output tube will measure different. That's why you have an adjustable -bias so you can set the -bias voltage for the current with different tubes.

If you set the -bias at a certain -dcv that does not tell you how much current the tube is drawing. And every tube will draw a different amount of current with that -dc bias voltage on it's grid.

There is NO exact current number for a given output tube to draw.

Besides if you are trying to set the tubes to draw more current, 43mA not 40mA, then if the bias pot is standing on a jumper and not a resistor, then you should be able to draw as much current as you want to the point of burning up the tubes. Less -dc bias voltage lets the tubes run hotter/more current. More -dc bias voltage allows less current to be drawn by the tube. If the bias pot is set up like you say you should be able to get to 0 -dcv.

Something's wrong.

                            
                        Brad    

[Axer57]

I measured voltage at pin 3 and got 484 using a multimeter. I measured voltage at pin 5 and got -40.5. Using the weber bias calculator online it said the bias should be set at 43ma. I can't load the entire schematic it's too large of a file. I took a snapshot of more of it.
 This amp doesn't have a range resistor. It uses a zener diode in some configuration I have never seen.

Like I said before. I used a multimeter to measure plate voltage at pin 3. I also measured the negative voltage at pin 5 , which is -40.5 when the bias pot is turned all the way to the left. Then I used the weber bias calculator by inputing my tube type and plate voltage to find the correct bias setting, which according to the calculator , is 43ma.
 All I can tell you for sure is. My tubes need a higher negative current. At -40.5 on pin 5 , they are getting too hot. They are not red plating but , they are definetly running too hot . I simply am trying to find a way to get a higher negative voltage on pin 5.
 I usually set my bias by the negative voltage on pin 5 , the sound, and the heat of the tubes. I realize that negative voltage and ma's are two sepperate things. I am sure there are better ways to set the bias , but with only a multimeter this is the way I have always set it. I cannot get the negative voltage high enough on the 6L6GC tubes because I run out of adjustment on the bias pot.
 I agree with you about the jumper on the right side of the pot. If I kept turing to the right I am sure it would get such a small negative voltage that it would burn up the tubes. I have no Idea why Marshall designed the amp this way, but if you google this amp you will read story after story of people having overheating issues. The tubes are mounted horizontally in these amps . So all the heat goes straight to the internal board. I have installed a top side vent and a cooling fan on mine so once I get it set it runs pretty cool.

[jazbo8]

Have you tried replacing the jumper with a resistor? Seems like a simple solution...

[Willabe]

I usually set my bias by the negative voltage on pin 5 , the sound, and the heat of the tubes. I realize that negative voltage and ma's are two sepperate things. I am sure there are better ways to set the bias , but with only a multimeter this is the way I have always set it. I cannot get the negative voltage high enough on the 6L6GC tubes because I run out of adjustment on the bias pot.
 I agree with you about the jumper on the right side of the pot. If I kept turing to the right I am sure it would get such a small negative voltage that it would burn up the tubes. I have no Idea why Marshall designed the amp this way, but if you google this amp you will read story after story of people having overheating issues.

Ok, now where getting some where. You want the tubes to draw less current.

The problem is that the bias pot is not standing on a resistor. One outside leg of the bias pot goes straight to ground through the jumper wire. This is why people are having over heating problems with this amp. They can't get enough -dc bias voltage to control the tubes current. So......

Do what jazbo8 said.

Take out the jumper and install a ~22k resistor in its place, that should give you a bit more range.

                       Brad      

[Willabe]

You can do a simple test to see if there is really no resistor between ground and 1 outside leg/ground end of the bias pot.

Take out the power tubes. Clip your positive meter lead on pin 5, set meter for dcv, turn on the amp, rotate the bias pot all the way to both far ends and take readings there.

If there's no bias pot tail R then it should go all the way to 0 (zero) dcv at 1 end.

If you conclude that there is no bias pot tail R with this test and put in a tail R like jazbo8 said make sure you test for -dcv at pin 5 on both power tube sockets BEFORE you put the tubes back in.

When you know you do have -dc bias voltage at pins 5, set the bias for MAX -dcv before you put the tubes back in and then adjust from there.


                          Brad      

[Axer57]

I replaced the jumper with a 22k resistor. It had no affect on the amount of negative voltage when the pot is turned all the way to the left. The jumper is on the right leg of the bias pot. I have the entire pot in range when turning to the right , which produces lower and lower negative voltage readings. With the pot turned all the way to the left I still show -40.5 on pin 5. With the 22k resistor in place of the jumper , pin 5 shows -5.5 dcv when the pot is turned all the way to the right.
 The only thing I have found online about this problem and this amp , was a guy who said what I am trying to do could be accomplished , but he said it required changing a few resistors. He just didn't say which resistors.
 By the way. I always check the negative voltage on pin 5 with the tubes out before I put them in to bias them.

[Willabe]

Take a -dcv reading where I marked in red and post the reading.

Keep the power tubes out until you get the bias fixed.


                   Brad    

[Axer57]

It reads -60.5dcv

[Willabe]

Ok.

I'll stay on line with you for a while if you have time.

Now lift 1 end of the 22K R you put in, either end doesn't matter and take a -dcv reading at pin 5 and post that.


                  Brad     

[Axer57]

This will take a while. As you can see from the photo I attatched the amp is not easy to access. I have to pull it completely appart , desolder the leg of the 22k resistor and then reassemble it to check it.

[sluckey]

Take a -dcv reading where I marked in red and post the reading.

Keep the power tubes out until you get the bias fixed.


                   Brad    

Yes. And also measure -dcv at the blue circle. There should be more than -51v at the red circle. If so, then there should be -51v at the blue circle. And that -51v would be the max that you can set on pin 5 of the output tubes.

But since you can only get a max of -40.5, that must be the voltage that's present at the blue circle. Remove the zener. Does the negative voltage at the blue circle increase? You can also decrease the size of R141 to increase the negative bias voltage.

If you have plenty of negative voltage available at the red circle, you should be able to get what you want by removing the ZD101 zener and/or decreasing the value of R141.

[Willabe]

This will take a while. As you can see from the photo I attatched the amp is not easy to access. I have to pull it completely appart , desolder the leg of the 22k resistor and then reassemble it to check it.

     PCB amps can be fine but when they make them so you have to take it apart just to change a part........

I think you need to use a bigger bias pot tail R. Problem is I don't know how big. If you put in too big of a value when you adjust the pot it will move to much with just a slight turn.

I'd be very tempted to put a 100k in there right now.

I replaced the jumper with a 22k resistor. It had no affect on the amount of negative voltage when the pot is turned all the way to the left. The jumper is on the right leg of the bias pot.

With the pot turned all the way to the left I still show -40.5 on pin 5. With the 22k resistor in place of the jumper , pin 5 shows -5.5 dcv when the pot is turned all the way to the right.

This seems odd to me. If you get -40.5dcv with the bias pot without a tail R to ground and when you add a tail R between the bias pot and ground it should lift/raise the -dcv.

Your new reading says -5.5dcv that the pot is now standing on above ground, so that -5.5dcv should add to the -40.5dcv and now give you -5.5dcv + -40.5dcv = -46dcv.


                  Brad        

[Axer57]

after lifting 1 leg of the 22k resistor I installed , pin 5 reads -40.7dcv.
 I have already tried lowering R141 and it had no affect. I even tried lowering R145 and R146 and nothing so far has had any affect. I had considered removing the zener diode but wasn't sure if this would hurt anything.

[Willabe]

There should be more than -51v at the red circle. [/quote]

It reads -60.5dcv

If so, then there should be -51v at the blue circle. And that -51v would be the max that you can set on pin 5 of the output tubes.

But since you can only get a max of -40.5, that must be the voltage that's present at the blue circle. Remove the zener. Does the negative voltage at the blue circle increase?

I was getting to that.      

But I suggested this 1st,    

Now lift 1 end of the 22K R you put in, either end doesn't matter and take a -dcv reading at pin 5 and post that.

He put a 22K in place of the jumper that goes from 1 outside leg of the pot to ground and it did not raise the -dcv. He says there is no R147, it's a wire jumper.


               Brad    
    

[Willabe]

Put the 22K R back in and try what Sluckey said. Measure where he marked in blue and post that reading.

If you don't get ~ -51dcv there then take out the zener and see what you get now.


                  Brad    

[Axer57]

That will take a little while. The article I found about this subject online , said , the zener diodes are there to lock in the negative voltage. And that changing a few resistors would get around this.

[Willabe]

The article I found about this subject online , said , the zener diodes are there to lock in the negative voltage.

Yes but it locks in/limits it from having more -dcv not less and you need more.

Something's wrong because in the red mark you get -60.5dcv. But when you lifted the bias pot tail R you still only have -40.7dcv.

That should have raised up to where the zener limit is set, around -51dcv, but it didn't. This is why Sluckey said to measure at the blue mark he posted and if it's low to take out that zener.


                      Brad      

[sluckey]

The zener regulates the voltage that is applied across the bias pot. A 51v zener would regulate to 51 volts, provided the voltage at the red circle is greater than 51v (and you said it was 60.5). Therefore, there would be a constant 51v across the zener (locked in). There would be 60.5-51=9.5v dropped across R141. A properly working zener would set the max bias voltage to -51v.

I would just snip/clip the zener out. It's not necessary. There is no problem with "unlocking" the max available negative bias voltage and allowing that voltage to go up to -60v. In fact, since you want to use 6L6s, it's desirable to be able to go up to -60vdc.

[Axer57]

The voltage where you specified was -50.5vdc. I am curious , If I removed R141 the 4.7k resistor and put a jumper there would it not do the same thing?

[Willabe]

Something is dragging down the voltage after that zener.

Sluckeys right you should snip out that zener anyway, it's not needed.

But what is dragging down the -dcv after that zener?


              Brad     

[Axer57]

do I need a jumper where the zener is or would you leave it open?

[Willabe]

I am curious , If I removed R141 the 4.7k resistor and put a jumper there would it not do the same thing?

That's not the problem. The problem is after that R141, besides it's part of the voltage divider formed with the bias pot and it isolates C140 from C139 so you have a 2 stage ripple filter.

It needs to be there.


                Brad    

[Willabe]

do I need a jumper where the zener is or would you leave it open?

No leave it open.



               Brad     

[Axer57]

OK I will remove the zener and see what happens. There's no way to just snip it. You can't get to it once the amp is assembled.

[Willabe]

Sluckey do you think it might be C139 is bad and dragging down the bias voltage?

The zener seems to be fine.


              Brad     

[sluckey]

The voltage where you specified was -50.5vdc.

That's a crucial clue. If I had known that back up at reply #24, I would never had even mentioned the zener.

The zener is OK. Leave it alone.

I agree with Willabe. Remove C139 and see if you can now get more bias voltage on pin 5 of the output tube. If so, replace C139.

If you still can't get about -50vdc on pin 5 of the output tubes, we will need to look at possibly leaky coupling caps from the PI.

[Axer57]

Too late. I already removed the zener diode.Now I can get -48.9 on pin 5. Pin 3 reads 500vdc. If nothing else is wrong this should work well. I only needed about -43vdc on pin 5. Is there any other reason Not to leave the zener out?

[phsyconoodler]

Sluckey and willabe are offering solutions to your negative bias voltage. You need to listen to them,especially Sluckey.

 but....that still doesn't address this statement:

I measured voltage at pin 3 and got 484 using a multimeter. I measured voltage at pin 5 and got -40.5.

 Your minus bias voltage is NOT the milliamps bias current. You cannot just plug in those figures to the weber bias calculator and get the proper current.You do not understand the difference.

  You are heading in the right direction to reduce the curent and I'm thinking sluckey does not want to expend the effort to teach you and would rather just get you in the safe zone first.
 
  You still have an issue with likely R139. But at least you are getting closer with -48.9v. With that kind of B+ voltage on the plates you need to actually learn how to measure the bias CURRENT so maybe you can get where you really need to be.

[Willabe]

Now I can get -48.9 on pin 5.

And that's with the 22K bias pot tail R? If so and you still think you might need a little more -voltage, then change that tail R to say ~48K or so.

Too late. I already removed the zener diode. Is there any other reason Not to leave the zener out?

Nope.

If nothing else is wrong this should work well.

Depending on how old that amp is you might want to buy a couple of filter caps for the bias supply? Maybe even for the whole amp?


                    Brad      

[Axer57]

Yes the -48.9vdc on pin 5 is with the 22k R resistor in place.
As long as it will run safely without the zener , I will try it for a half hour or so and see how it does. I have a new set of caps for C139 and C140. Just hadn't installed them yet. as far as the Power filter caps go they are almost impossible to find. They are 47uf 550v. You can find all kinds of 47uf 500v and even some 47uf 600v but that's about it. Is there a way to measure bias ma's with just a multimeter?

[Willabe]

I have a new set of caps for C139 and C140. Just hadn't installed them yet.

Put those in! Make sure you put the positive lead to ground. (It's a negative bias voltage supply.)

I agree with phsyconoodler and I bet after you put both of those new filter caps in, the -bias will go up at least little.

as far as the Power filter caps go they are almost impossible to find. They are 47uf 550v. You can find all kinds of 47uf 500v and even some 47uf 600v but that's about it.

You can always go with a filter cap who's voltage rating is higher than what's in the amp now. So 47uF 600v will work fine.

Is there a way to measure bias ma's with just a multimeter?

Yes, phsyconoodler told you way back in the beginning of your thread. You put a 1ohm R in between the power tubes cathode and ground. Then you can measure across that R for mV's and that will transfer directly to mA's.

Our host Doug sells the 1ohm R's in his online store.


               Brad        

[sluckey]

but....that still doesn't address this statement:

I measured voltage at pin 3 and got 484 using a multimeter. I measured voltage at pin 5 and got -40.5.

 Your minus bias voltage is NOT the milliamps bias current. You cannot just plug in those figures to the weber bias calculator and get the proper current.You do not understand the difference.

I agree with phsyco's statement above. I think you have misunderstood how to use Weber's bias calculator. You plugged in your 484v for the plate voltage and then clicked the 'Calculate Bias' button. And the answer was 43.3mA. That 43.3mA is the CURRENT flowing thru the tube. IT IS NOT THE NEGATIVE VOLTAGE ON PIN 5 OF THE OUTPUT TUBES!
The negative bias voltage on pin 5 controls the current thru the tube, but it's not the same thing. You must have a method of measuring the actual current flowing thru the tube. There are a couple easy ways to do this. One way is to buy Weber's Bias Rite and follow the instructions.

Another way (my personal choice) is to install 1Ω resistors between pin 8 of each output tube and ground. Then measure the mV across that 1Ω resistor. Adjust your bias pot to get a voltage reading of 43.3mV. That corresponds to the same 43.3mA current that Weber's bias calculator told you to use.

[Willabe]

Here's a link to Doug's store for his bias checker kit, it works great too.

http://www.hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=Tools&ORDER_ID=236265838



                  Brad     

[Willabe]

Your minus bias voltage is NOT the milliamps bias current. You cannot just plug in those figures to the weber bias calculator and get the proper current.You do not understand the difference.

I tried to explain this to him in reply #14.


              Brad     

[Axer57]

I understand what you are sayin.
 I have taken a few shortcuts that I probably shouldn't because this amp is so hard to work on once it's assembled. I much prefer my old 66 Fender princeton. It's like a walk in the park compared to this one.
 I greatly appreciate all your help and patience.

[Willabe]

To be fair, our friend Axer57 did say this earlier;

I usually set my bias by the negative voltage on pin 5 , the sound, and the heat of the tubes. I realize that negative voltage and ma's are two sepperate things. I am sure there are better ways to set the bias , but with only a multimeter this is the way I have always set it.

              Brad