6l6 bias pot value question

[Sven]

Hi,

I have an amp that's based on a tweed bassman using two 6l6GCs which has a bias trim pot.

I've installed two NOS JAN Sylvania 6L6WGB tubes which are the same specs as 5881s and can be used in place of 6l6gcs. They are fine with the plate voltages in the amp and can handle 30W.

The bias section in the amp is: 50v bias tap -> 1k resistor -> Rectifier Diode -> 10k Bias divider; 2x 10uF Bias Filters and then a 50k bias pot with a 22k resistor in series.

When setting the bias pot to the coldest setting I'm getting about 25W with those tubes installed. With the 6l6GCs it's much lower.


That wattage is fine in general for me and I'm not worried about the tubes but in theory, could I change the bias pot to a higher value to get more range on the bias or maybe change the 22k to a higher value? just want to understand how this works better.


Thanks!

[Lectroid]

Have you Sluckey's  Amp Scrapbook page?  http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

Pull up that doc and skip down to page 9.  He explains Fender vs. Marshall and show how each operates pretty clearly.

Ohm's law is your friend here.

[Willabe]

I've installed two NOS JAN Sylvania 6L6WGB tubes which are the same specs as 5881s and can be used in place of 6l6gcs. They are fine with the plate voltages in the amp and can handle 30W.

No, 6L6WGB/5881's can only handle 26w, depending on which companies spec sheet you look at, some only 19w, 20w, 23w.
Those NOS JAN Sylvania 6L6WGB tubes were made later and should handle 26w.   

The bias section in the amp is: 50v bias tap -> 1k resistor -> Rectifier Diode -> 10k Bias divider; 2x 10uF Bias Filters and then a 50k bias pot with a 22k resistor in series.

This is not very clear, we need a schematic to see what you really have and to be able to tell you what to change. You can draw it out by hand if you have to. 

When setting the bias pot to the coldest setting I'm getting about 25W with those tubes installed. With the 6l6GCs it's much lower.

Those are different tubes, they will bias up differently with the same -bias setting.

......but in theory, could I change the bias pot to a higher value to get more range on the bias or maybe change the 22k to a higher value? just want to understand how this works better.

Need the schematic. The circuit can be changed to get better range and center it better on the range needed. You don't change the pot. 

[mresistor]

this is helpful too https://el34world.com/charts/Biascircuits.htm#google_vignette

[Willabe]

Have you Sluckey's  Amp Scrapbook page?  http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

Pull up that doc and skip down to page 9.  He explains Fender vs. Marshall and show how each operates pretty clearly.

Ohm's law is your friend here.

No, that just shows where current flows in the -bias circuit. It doesn't go into how to change the range and center it better on the range needed.

[Willabe]

this is helpful too https://el34world.com/charts/Biascircuits.htm#google_vignette

Those are fine, but their for BF Fender's and for a BF/SF Princeton Fender.

He said he built a tweed bassman, little different -bias.

We need to see a schematic from him of what he really built.

[Sven]

Thanks for the quick replies!

No, 6L6WGB/5881's can only handle 26w, depending on which companies spec sheet you look at, some only 19w, 20w, 23w.
Those NOS JAN Sylvania 6L6WGB tubes were made later and should handle 26w.   

Maybe I got some wrong information from my tube supplier but this is what I got about these tubes:

" The JAN-Sylvania 6L6WGB tube has the identical internal construction as the Sylvania 7581A tube (the industrial version of the 6L6GC tube). The difference between these two tubes is only the size of the glass bottle. Although you may not get a 7581A's 35 watts out of the 6L6WGB, you can certainly get 30 watts without any problems. These are super durable tubes made for the US military in the mid '70's. Their specs are not the same as earlier 5881 and 6L6WGB specs from the 1960's. So to get to the point, the JAN-Sylvania 6L6WGB tube will work perfectly in any amp requiring a 6L6GC tube. "

I drew out the power supply here:

[Sven]

Power transformer is a Pacific Audio Tweed Bassman Transformer PAM-PT-45-17R

[Willabe]

No, 6L6WGB/5881's can only handle 26w, depending on which companies spec sheet you look at, some only 19w, 20w, 23w.
Those NOS JAN Sylvania 6L6WGB tubes were made later and should handle 26w.   

Maybe I got some wrong information from my tube supplier but this is what I got about these tubes:

" The JAN-Sylvania 6L6WGB tube has the identical internal construction as the Sylvania 7581A tube (the industrial version of the 6L6GC tube). The difference between these two tubes is only the size of the glass bottle. Although you may not get a 7581A's 35 watts out of the 6L6WGB, you can certainly get 30 watts without any problems. These are super durable tubes made for the US military in the mid '70's. Their specs are not the same as earlier 5881 and 6L6WGB specs from the 1960's. So to get to the point, the JAN-Sylvania 6L6WGB tube will work perfectly in any amp requiring a 6L6GC tube. "

Never heard this before, but very well could be.

[Willabe]

Now, with the power tubes out;

1. What is the dcv at pin 5 on both power tubes, measure at the socket.

2. What is the dcv at pin 5 both power tubes, measure at the socket, with -bias control turned full clockwise (FCW) and full counter clockwise (FCCW) We need both readings.

3. We need the acv reading from the PT's bias tap wire where it goes to that 1st 1K R. Take the reading from before that 1K R. We need to know what the PT gives as a raw bias acv. 

[Sven]

Now, with the power tubes out;

1. What is the dcv at pin 5 on both power tubes, measure at the socket.

2. What is the dcv at pin 5 both power tubes, measure at the socket, with -bias control turned full clockwise (FCW) and full counter clockwise (FCCW) We need both readings.

3. We need the acv reading from the PT's bias tap wire where it goes to that 1st 1K R. Take the reading from before that 1K R. We need to know what the PT gives as a raw bias acv.

Pin 5 DC FCW Both tube sockets read the same - 58.55V
Pin 5 DC FCCW Both tube sockets read the same - 42.18V

AC on bias tap before the 1K is 54.65V

[Willabe]

Ok, great. But I wanted to also know where the tubes -bias dcv was set at.

Need this to center the overall -bias adjustment.   

[Sven]

Ok, great. But I wanted to also know where the tubes -bias dcv was set at.

Need this to center the overall -bias adjustment.

Do you mean where it was set at for the 6L6Gcs?

I also just as I'm measuring noticed that I made a mistake in my schematic, forgot that I have 1.5k resistors going to pin 5. Corrected now here

[Willabe]

The 50K pot with the 22K tail R form a voltage divider.

If you increase the value of the 22K R to ground it will cause the dcv to go up. The larger the value of that R, it stops dcv from going to ground. Lessen that R's value, and more dcv bleeds to ground.

The 10K is in series with the -bias dcv and increase it's value it will decrease the dcv. Lessen it's value the dcv will increase.

I'd probably increase the 10K if I wanted less dcv than decrease the 22K adjust pot tail R.   

[Willabe]

Do you mean where it was set at for the 6L6Gcs?

Yes.

We want to know where it was between the lowest and highest dcv pot adjustments. So we can center it.

I also just as I'm measuring noticed that I made a mistake in my schematic, forgot that I have 1.5k resistors going to pin 5. Corrected now here.

Ok, those are the grid stoppers.

[Sven]

We want to know where it was between the lowest and highest dcv pot adjustments. So we can center it.

Ok, 6L6GCs in, biased to 31mV across the 1R I get 49.8V on both pins 5. Same measurement with power tubes removed: 50.6V

[Sven]

The 50K pot with the 22K tail R form a voltage divider.

If you increase the value of the 22K R to ground it will cause the dcv to go up. The larger the value of that R, it stops dcv from going to ground. Lessen that R's value, and more dcv bleeds to ground.

The 10K is in series with the -bias dcv and increase it's value it will decrease the dcv. Lessen it's value the dcv will increase.

I'd probably increase the 10K if I wanted less dcv than decrease the 22K adjust pot tail R.

Would I just move the range of the -bias dcv to make it better for this particular tube if I increase the 10k resistor? I guess it would be great to increase the range of the adjustment to be able to run different tubes and be able to go back to 6L6GCs without changing back the resistor.

[Willabe]

Ok, 6L6GCs in, biased to 31mV across the 1R I get 49.8V on both pins 5. Same measurement with power tubes removed: 50.6V

That's pretty good but, that's at only 31mV, that seems kind of cold to me. Those are 30w tubes.
How are you figuring out where to set the bias? 

[Sven]

That's pretty good but, that's at only 31mV, that seems kind of cold to me. Those are 30w tubes.
How are you figuring out where to set the bias?

I guess I always had the bias in that amp set to that setting. The manual for the amp specifies a range of 25-35mV (since the amp has biasing ports in the back)

[Willabe]

Would I just move the range of the -bias dcv to make it better for this particular tube if I increase the 10k resistor? I guess it would be great to increase the range of the adjustment to be able to run different tubes and be able to go back to 6L6GCs without changing back the resistor.

1st you need to set the NOS JAN-Sylvania 6L6WGB tubes correctly. If those are really a 30w tube, and I think they probably are after reading what you posted on them.

Then you can try popping in the set of 6L6GC's you have.

Then post the -dcv for both tubes sets that you set the tubes with. Then we'll know better where your -bias pots center of swing is at. 

[Willabe]

I guess I always had the bias in that amp set to that setting. The manual for the amp specifies a range of 25-35mV (since the amp has biasing ports in the back)

What manual?

Setting the bias for a set of power tubes depends on a few things,

1. Class of bias, cathode bias, -grid bias, class A, class AB.
2. Power tube plate dcv.
3. Power tubes max dissipation in watts.
4. For your amp, class AB/-grid bias, where you want to set the dissipation, 50%, 60%, 70%.   

[SEL49]

Pin 5 DC FCW Both tube sockets read the same - 58.55V
Pin 5 DC FCCW Both tube sockets read the same - 42.18V

That adjustment range should handle most any 6L6, 5881, or variants.

As for your original question...

but in theory, could I change the bias pot to a higher value to get more range on the bias or maybe change the 22k to a higher value?

Changing either of those as well as changing the 10K, will change the voltage sent to pin 5 (control grids) which will change the mV measured across your 1Ω bias resistors. More negative voltage on pin 5 results in less mV on your 1Ω cathode resistors, and vice versa.

[Willabe]

Try setting the bias with this -bias calculator, I like 60%, don't go more than 70% dissipation.

You set it, then you check the plate dcv and probably have to put in the new dcv in the calculator, be cause drawing more current will cause the power supply/power tubes plate dcv to drop. So you have to do this at least 2 times, maybe 3 before it settles in. 

https://robrobinette.com/Tube_Bias_Calculator.htm

[Sven]

Thanks for all the help!!

https://www.tedweber.com/bias-calc/

I've been using the Rob Robinette calculators!

I think everything should be fine! I was checking the bias again with the Sylvania 6L6WGBs:

- Bias pot set to full clockwise
- Plate Voltage @pin 3: 487V
- Pin 5: 57.4V
- mV across the 1R: 36mV and 34mV for the two power tubes.

That's averaged 17W (35mV) and that's 65% of the 26W. But since I'm pretty sure I could use 30W as my 100% I'm definitely on the safe side.

It's just that I'm at the max setting on my bias pot and I'm wondering if I would ever have to insert power tubes into this amp that need more resistance? But it's not worth to change anything just for that possibility anyway.

Seems like everything is fine here.

[Willabe]

That's averaged 17W (35mV) and that's 65% of the 26W. But since I'm pretty sure I could use 30W as my 100% I'm definitely on the safe side.

It's just that I'm at the max setting on my bias pot and I'm wondering if I would ever have to insert power tubes into this amp that need more resistance? But it's not worth to change anything just for that possibility anyway.

Seems like everything is fine here.

No, everything's not fine, your not done yet.

That's averaged 17W (35mV) and that's 65% of the 26W. But since I'm pretty sure I could use 30W as my 100% I'm definitely on the safe side.

No, that's too cold for those tubes. You don't bias 30w tubes as 26w tubes
"to be on the safe side." They won't sound very good, in part because of below;

That's why your plate dcv is so high. That's too high for a tweed Bassman. That power supply voltage will also raise the preamp tubes plate voltage, it's way too high.

And at start up your power supplies dcv will probably exceed 500dcv for a little while, too much stress for your B+ filter caps.

What are your B+ filter caps rated for?

Figure them as 6L6GC's.

And you bias pot is at max, you probably need to fix this.

We won't know for sure until you get those power tubes biased at at least 60% for 30w 6L6GC tubes. 

[Willabe]

A tweed Bassman 5F6a schematic shows only 432dcv feeding the OT CT, you have 487dcv.

And with a 12AY7 for the 1st preamp tube it shows only 150dcv on the plate. What does yours read?

What are you using for a rectifier? A GZ37/5AR4 tube?

[Sven]

A tweed Bassman 5F6a schematic shows only 432dcv feeding the OT CT, you have 487dcv.

And with a 12AY7 for the 1st preamp tube it shows only 150dcv on the plate. What does yours read?

What are you using for a rectifier? A GZ37/5AR4 tube?

Thanks for reminding me about all those things, might have settled too fast!

The 35mV is 65% of a 26W tube and if it's actually a 30W tube it's 57% so not too far off the 60%.

I just set them to 44mV which is about 70% now of 30W and I'm pretty well around the middle of my bias pot.

Currently the measurements are:

Pin 5: -52.5V
Pin 3: 481V

My V1 is a 12AX7 and the plates are at 188V

Rectifier is a new sovtek 5AR4

And yes all the voltages seem pretty high. I attached a spec sheet of the power transformer. And my voltages are a bit higher on all of them:

AC outlet: 121V
HV: 382.5 x2
Bias: 54V
5V: 5.3V
6.3V: 3.35V x2

The amp is not built by me and the manual tells me to bias the tubes between 25mV and 35mV which is 40%-55%. Why would they tell me to bias it so cold?
Any idea why my power transformer has higher voltages? I know that they change a bit depending on the load but that much? My wall voltage is pretty stable at 121V and it's designed for 120V.

Reading about the control grid voltages it seems like I'm in a good place with -52.5V

[Sven]

Filter cap voltages are:

2x 100uf 450v in series so working voltage of 900v
3x 22uf 500v
1x 8uf 450v

[Willabe]

Filter cap voltages are:

2x 100uf 450v in series so working voltage of 900v
3x 22uf 500v
1x 8uf 450v

If the amp was built with the 2x 100uf 450v in series, they knew the PT was running high.

[Sven]

Filter cap voltages are:

2x 100uf 450v in series so working voltage of 900v
3x 22uf 500v
1x 8uf 450v

If the amp was built with the 2x 100uf 450v in series, they knew the PT was running high.

It was actually built with the classic two 350v caps. I recapped it with the bigger ones because that's all I could get and there is enough space for them.

[Willabe]

I'm sorry, I think I accidentally deleted the part where you said the name of the amp. 

The amp is not built by me and the manual tells me to bias the tubes between 25mV and 35mV which is 40%-55%. Why would they tell me to bias it so cold?

Well, there's a lot of people building amps that don't always know what there doing.   

And the amp was named after an old jazz player who played clean.

Any idea why my power transformer has higher voltages? I know that they change a bit depending on the load but that much? My wall voltage is pretty stable at 121V and it's designed for 120V.

I don't know why Pacific Audio made that PT with such a high PT B+ secondary and called it a tweed Bassman PT. That's more of a BF SR PT. 

Part of why guys like the old Fender tweed amps is because of their warm sound. And that's in part because of their lower dcv's. The later BF Fenders were brighter and had higher dcv.

[shooter]

I just set them to 44mV which is about 70% now of 30W

I didn't see in the OP that this amp wasn't working so i'm assuming it's a runner?


If so and you have 1 ohm'rs to monitor current;
jam pretty hard for 10 minutes while just "watching" the meter, if it has a min/max feature, let it look while you play.  That's the "real number" I use to determine where to set the bias.  add that with Cost tolerance of new tubes, play style, "tone", and you come to the final setting....for now 

[Willabe]

For reference, Fender BF Super Reverb amps, which are the cleanest 2 X 6L6CG Fender amp, the schematic shows 6L6 plate dcv is 467 and 1st 12AX7 plate is 270.   

[Willabe]

I'd try a 5U4 to get the dcv down.

The PT spec sheet says it has a 3A 5v wind for the rectifier.