Another bias dilemma

[telentubes]

I'm fairly confused about the relationship between the negative bias setting and the cathode current measured across the 1 Ohm resistor. It feels like I'm in danger of wearing out power tubes rapidly (I may have already fried a set).

The amp is a 3rd gen 50 Watt ODS. I hope this explanation will make sense.

The only change from stock I've made was to replace the 27K tail resistor on the bias pot with an 18K, both to increase the cathode current and to give me more positive readings when measuring bias (by changing the pot I could adjust the bias to -29, where I could only reach -34 with the 27K resistor).

The plate voltage is 438V

Here are some sample voltage readings from both extremes of the bias pot...

With the pot rotated all the way counter clockwise (as seen from inside the chassis) the bias is -29V, V4 is 32.3mA, and V5 is 36.9mA. At this setting the Winged C 6L6s will REDPLATE (I've tried 2 different matched sets).

With the pot rotated all the way clockwise, the bias is -43, V4 is 12.7mA, and V5 is 15.7. The amp has way less power, but no redplating.

My understanding was that with the bias set cold (-29 is "colder" than -34 ?) your tubes would have a longer life, but I'm getting Redplating at the "colder" settings. There is obviously some relationship between the negative bias and the cathode current I don't understand.

The cathode current values are low (in some cases WAY low). If I can raise the cathode current to somewhere in the 34mA to 40mA range, then set the bias in the -32 to -36 range, will this solve the problem, or is there something else I'm missing? Or do Winged C 6L6s just glow like that?

Any tips for properly balancing this part of the system would be greatly appreciated?

[ayan]

Simply stated: the more negative the bias voltage, the less current will flow through the tube. For "average" plate voltages, you indicated 438 VDC, 6L6s will need "about" -50 VDC to bias right and EL34s about -40. Of course, in this day and age, tubes are all over the place. However, I think I have never seen a pair of 6L6s that will bias right with -29 VDC. Pay no attention to the bias voltage, however, just go by the bias current.

If you want to be conservative, follow this rule of thumb: have the tubes dissipate 50% of their power while idling. Power is calculated multiplying voltage X current. If you use your averaae 6L6s, they are rated at about 25W, so you want them to dissipate 12.5W at idle. So, in your case, 438 X current = 12.5. Solving for current gives you 12.5/438 = .029 A, or 29 mA. Aim for that and you will be fine with your 6L6s. The term hot VS cold are often misused, so I suggest you don't go by that.

Cheers,

Gil

I'm fairly confused about the relationship between the negative bias setting and the cathode current measured across the 1 Ohm resistor. It feels like I'm in danger of wearing out power tubes rapidly (I may have already fried a set).

The amp is a 3rd gen 50 Watt ODS. I hope this explanation will make sense.

The only change from stock I've made was to replace the 27K tail resistor on the bias pot with an 18K, both to increase the cathode current and to give me more positive readings when measuring bias (by changing the pot I could adjust the bias to -29, where I could only reach -34 with the 27K resistor).

The plate voltage is 438V

Here are some sample voltage readings from both extremes of the bias pot...

With the pot rotated all the way counter clockwise (as seen from inside the chassis) the bias is -29V, V4 is 32.3mA, and V5 is 36.9mA. At this setting the Winged C 6L6s will REDPLATE (I've tried 2 different matched sets).

With the pot rotated all the way clockwise, the bias is -43, V4 is 12.7mA, and V5 is 15.7. The amp has way less power, but no redplating.

My understanding was that with the bias set cold (-29 is "colder" than -34 ?) your tubes would have a longer life, but I'm getting Redplating at the "colder" settings. There is obviously some relationship between the negative bias and the cathode current I don't understand.

The cathode current values are low (in some cases WAY low). If I can raise the cathode current to somewhere in the 34mA to 40mA range, then set the bias in the -32 to -36 range, will this solve the problem, or is there something else I'm missing? Or do Winged C 6L6s just glow like that?

Any tips for properly balancing this part of the system would be greatly appreciated?

[telentubes]

Thanks Gil,

Here are the numbers now that I've made the adjustment...

plate 436

V4 28.7mV

V5 31.1mV

bias (I know I should ignore this but here it is anyway) -33

I will give this a try. I'm thinking that maybe I should not have changed the tail resistor as all I was looking at then was the bias voltage and not the current. Can't remember what the numbers were before the "mod".

And you are right about changing tubes. The numbers can be wildly different. Thanks again.

[ayan]

I though you'd said the original tail was 27K? Either way, now you probably have way more range, so long as you're careful when setting the bias. One thing I should have said is: use the rule of thumb to bias the tubes, but the ultimate test should always be to ensure that the pltes are not glowing red, regardless of what the tube current is.

Gil

Thanks Gil,

Here are the numbers now that I've made the adjustment...

plate 436

V4 28.7mV

V5 31.1mV

bias (I know I should ignore this but here it is anyway) -33

I will give this a try. I'm thinking that maybe I should not have changed the tail resistor as all I was looking at then was the bias voltage and not the current. Can't remember what the numbers were before the "mod".

And you are right about changing tubes. The numbers can be wildly different. Thanks again.

[telentubes]

The original tail was 27K. I tried 20K then 18K thinking I needed more range for setting bias.
I could get -34 as the highest setting with the 27K (meaning that -30 is higher, in terms of positive charge, than -34), and -29 with the 18K tail. I paid no attention to the current because I didn't know that that was important. I had always believed that bias was set regarding the negative current at the tube. Thanks for the clarification. I may revisit the tail resistor value.

Both sets of Winged Cs would glow when the current was increased into the mid 30s mA and the bias was -29. I imagine red-plating will permanently damage the 6L6s. They were glowing a while before I caught it. Bummer, as they are expensive. I guess we call that tuition.

[Bob-I]

The original tail was 27K. I tried 20K then 18K thinking I needed more range for setting bias.
I could get -34 as the highest setting with the 27K (meaning that -30 is higher, in terms of positive charge, than -34), and -29 with the 18K tail. I paid no attention to the current because I didn't know that that was important. I had always believed that bias was set regarding the negative current at the tube. Thanks for the clarification. I may revisit the tail resistor value.

Both sets of Winged Cs would glow when the current was increased into the mid 30s mA and the bias was -29. I imagine red-plating will permanently damage the 6L6s. They were glowing a while before I caught it. Bummer, as they are expensive. I guess we call that tuition.

Red plating for a short time won't kill the tubes. Don't let them go for more than a few minutes though. More than damaging the tubes, the sockets can get hot enough to melt solder, burn resistors and damage the socket.

[Chris333]

With the pot rotated all the way counter clockwise (as seen from inside the chassis) the bias is -29V, V4 is 32.3mA, and V5 is 36.9mA. At this setting the Winged C 6L6s will REDPLATE (I've tried 2 different matched sets).

With the pot rotated all the way clockwise, the bias is -43, V4 is 12.7mA, and V5 is 15.7. The amp has way less power, but no redplating.

These numbers seem a little goofy in a couple ways ways... your bias voltage should be higher (more negative), or the tubes should be idling hotter. If they're glowing at between 32 and 36mA at 438V, that's well under 70% dissipation, and I don't ever remember seeing a healthy pair of 6L6s redplating that low. Is there any way your 1 ohm resistors could be out of spec? That might explain the current readings.

There are other ways to check idle current, it might be worth trying an alternate method to see if the numbers agree.

This is from Randall Aiken's website, the URL for teh biasing page is

http://www.aikenamps.com/Biasing.html

This is the third bullet point under "Plate/cathode current method":

The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side.

[telentubes]

Thanks for the suggestions. I am going to try a few different measurement methods, both for cross referencing, and the education. I agree, the numbers seemed a little goofy.

[amplifiednation]

Did you test the 1R resistors? You want them to be very close to spec, within 1%.

Also snap a pic of the power section wiring if you can.

Thx,
T

[telentubes]

This amp was gigged before I discovered the problem, and although it roared at the gig, I'll go back in and check for heat damage (the bases of the 6L6s looked scorched) to the 1 Ohms and a few other components (the 1 Ohm resistors are 1% 3 Watt). I'll report back with pics as well.
I'm also wondering about the current difference from V4 to V5. V5 is always higher. Normal?

[amplifiednation]

This amp was gigged before I discovered the problem, and although it roared at the gig, I'll go back in and check for heat damage (the bases of the 6L6s looked scorched) to the 1 Ohms and a few other components (the 1 Ohm resistors are 1% 3 Watt). I'll report back with pics as well.
I'm also wondering about the current difference from V4 to V5. V5 is always higher. Normal?

Even when you switch tubes? Sorry if I missed that

[67plexi]

Look at the bias board if you used a 50 watt power transformer 3k3 is too high

Lower it to a 2k2-1k8 on the bias board, 50 watt bias 50 volts AC. 100 watt bias 60 volts AC. Fender PT.

A 10k pot 22k-27k tail resistor should be fine. If you look at a Fender 1969 Super reverb you will see a 1k8 is used.

Steve.

[ayan]

I guess we should find out what PT was used and what the bias tap voltage (AC) so as to be sure; I don't know whether it was a Fender PT. Regardless, if the bias voltage quoted was measured at the grids of the power tubes, it seems to be too high (I.E., NOT NEGATIVE ENOUGH... so the absolute value is too low) for 6L6 to yield the idle current quoted. I have seen weird behaviour from tubes before, so maybe everything is OK, but it seems strange.

FWIW, in amps where I interchange 6L6s and EL34s -- which is all of the ones I've built -- I incorporate a bias voltage range of about -55 to -38 VDC to accommodate every single tube I've ever come across. To achieve that, when using a Fender style PT, I use a 2.2K on the "front end" a 15K tail, if I recall correctly.

Cheers,

Gil

Look at the bias board if you used a 50 watt power transformer 3k3 is too high

Lower it to a 2k2-1k8 on the bias board, 50 watt bias 50 volts AC. 100 watt bias 60 volts AC. Fender PT.

A 10k pot 22k-27k tail resistor should be fine. If you look at a Fender 1969 Super reverb you will see a 1k8 is used.

Steve.

[Structo]

My 2x 6L6 D'lite has a grid voltage of -48v (Winged =C= 6L6)

That is with a plate voltage of 435v and a bias current of 36ma

120vac wall voltage

But I have dual bias pots on that so I can get the idle close on mismatched tubes.

[telentubes]

Hammond 290EX PT
http://angela.com/hammondpowertransformer290ex.aspx

Prosonic OT
http://angela.com/fenderprosonicoutputtransformer.aspx

Hammond 4H @ 90mA choke

I used the Talbany layout and the schematic from the 3rd generation 50 Watt stickey