Ceriatone Forum

Hello there, for some reason, i can't bias each 6l6 to more than 25mA. It sounds fine and they are chucking tremendus amounts of heat so i wouldn't was to bias them any hotter, but its a bit lower than the max of 40 odd mA that the valves can take if i were to want to get them hotter. Now i haven't acctually taken the plate voltage yet, i assumed that it would be around 450 and i will take a reading later on but still, i think i should be able to get a hotter bias from the pot. Is this normal?

thanks.

What type of 6L6 are you using- most should cope with around 35ma-42ma bias?

Its a JJ 6L6GC, and i am sure the valve can hhandle the extra current, its just the amp doesnt provide that extra current, the bias pot is maxed!

There are two resistors that alter the range of the bias adjustment. Your bias voltage is too high; lowering it will make your tubes run hotter.

There may be a 1.2K 1W resistor on the rectifier board that can be raised (3.3K is the Dumble value here). There is also a resistor that bleeds the bias voltage to ground. There may be a 15K resistor there. The Dumble value for that one would be 27k. Not sure why he changed the bias circuit, but it may have to do with giving people the range to run EL34s.

 ???So, should i lower the current or raise it? Is it running to hot at the moment or not?

Your bias voltage is a negative voltage and it 'slows down' your tubes bias current. If I understand correctly, you are getting too much bias voltage. Raising the 1.2k resistor will lower your range of bias voltage and allow you to make your tubes run in the usual range 30-35ma or so. If the highest you can get your bias current is 25ma then your tubes are running cold.

ah right i see, yes. Ok, a future mod for when i am next in the city. Thanks racer!

I am sure that ic-racer knows more about this stuff than I do, but...
My amp tech (who is very sharp) insists that a lower bias works fine and gives the tubes a longer life. He biases my Bassman & Super Reverb at between 25-30mA. Both sound very good and are running at a plate voltage around 475 VDC, which is the other part of the equation you need to know to set your bias. Both the Bassman & SR sound exactly the same at 35mA, which I believe is kind of the safe bet when you are biasing 6L6s with out knowing th e plate voltage. I have not checked, but the OTS plate voltage probably runs about the same as the Bassman/SR . You know you are too cold with your bias when your tubes get very hot and may have a red glow. This will shorten the tube life drastically and may cause permanent damage to the amp components if this is done over a longer period of time.

Having said all that, if you cannot bias above 25mA there is something wrong and it is almost sure to be the resistor used to set the bias. If it were me I would have an amp tech check it out. Some of the guys on this forum could talk you through some troubleshooting if you wanted to do it yourself. Nik may actually be the best place to start. Good Luck!

I'm having the same problem... Just finished my overtone special, and everything works fine and it sounds great especially the cleans...

But when my bias pot is up full, it only shows 28mA... this is when measured from the point between the two 6L6 tubes and to one of the two other points...


René

I recently spent some time with 4 sets of 6L6 tubes in my amp and I found there was a little 'grainy' character with the bias low (25ma) and this got better for each tube up to about 35ma. One set even sounded best at 42ma, but another old, old set arced (internal spark) at 42ma. The sound at 25ma was still very acceptable for all the tubes, though, just depends on how critical you are. It is a trade-off between smoothness of tone and heat. I wound up liking from 35ma to 38ma, but this is a home recording amp. For gigging, less heat and more reliability might dictate around 30ma.

In terms of changing your bias circuit, either raise that 1.2k resistor with experimentation, or e-mail Nik to see what he recommends.

I've emailed Nik, and he told me that i have to measure from the red point to the black point, with my meter set at DC

Then i have to regulate it till it says 30-40 mV....

I did that and when measuring the from the same point with my meter set at mA, it only reads 11,6 mA... But according to Nik this is fine

I thought you meaSUred the bias current, not voltage.

Since the overtone is build with bias measure points, which have 1 ohm resistors, the voltage(mV) is the same as the current(mA). (Just a Ohm's law calculation)
So measured in mV is the same as the actual current in mA

oh yeah, duh...

I don't understand this at all.  If mA and mV should be reading the same and Nik said bias should be 30-40 mV, why would a reading of 11.6 mA be fine?? ???  Am I missing something?

Nik wrote this to me

 " I dont think the standard meter will give an accurate reading. The resistance inside the meter screws the reading, I would think."

The whole idea of build-in 1 ohm bias measure points is that you measure the dc-voltage!!!!
If you start measuring the current, you got wrong numbers because that's not the way bias measure points are build for. It is simple standard electronics but a bit too much for this thread.
If you're really interested in the the explaination; google: aiken amps, he has lots of info on tube tech related matters.
Or visit your local amp tech for a proper bias :P.

Jap, The 1 ohm idea is fine in theory.....

If I measure from one of the testpoints (the tipjacks) to to the center testpoint between the output tubes, I get 1,2 ohms, which means I have to deduct 20% from my mV readings.

Hereof, a measurement om 25mA (mV) actually means 21 mA - a pretty severe error margin don't ya think??

Whats the moral?? use your freaakin' ears to bias an amp

Here we go....

The additional 0.2 ohm resistance of your test leads would only come into play if these leads were in SERIES with the one ohm resistor. This is  not the case. When you take a VOLTAGE reading with the multimeter, the total resistance across the leads, INCLUDING THE BOX, is probably on the order of a thousand ohms. So the additional 0.2 ohms of the leads would make it one thousand point 2 ohms. This will  not change your readings.

What you do need to be aware of, is not all the tubes current comes through the cathode. Some leaks through the grid, and this is what you will NOT be measuring with the cathode bias method. The leak is usually small.
So when you measure 32mv you have 32ma coming through the cathode. The total tube plate current (including that going through the grid) would be like 35ma or so. Its OK to make this estimation, because actually measuring the plate current can be dangerous, but is easy to do if you are so inclined ;)

I am in 100% agreement to use your ears, but I was getting a smoother sounding tone up past 42ma on one set of tubes. That's fine, but one arced on me when they were that high. So, you want the best sounding bias setting that is up to or below the recommended heat dissipation of the tube in question.

Ok, point taken.

So set bias to 32-35 mA and then use your ears... ;D

Exactly!

i just measure the bias again using the DC 200mV setting and i am now getting a voltage of 96mV! seems a bit far off the 26mA i was getting. Something wrong perhaps?

Fred,
I would suggest not using the current measurement at all. If you have a 1 ohm shunt resistor installed, as I believe there is in the OTS, the mV reading in directly proportional to the actual current. Most multimeters have an internal fuse that can blow  without the user knowing. The voltage reading is much more reliable. If you are reading 26mV that is probably what it is w/in the accuracy of the 1 ohm resistor (1.2 ohms is really unacceptable for this purpose, the resistor for bias measurement should have a 1% tolerance). If you are really in to using this bias method and the resistor is really 1.2 I would replace it.
Also try another meter that is known to be reliable.

Good Luck

You do want to use a 'voltage' setting on your meter. You want the voltage potential difference on each end of the 1 ohm resistor. Then you calculate the amperage by multiplying by one. (even I can do it in my head :) ).

At 96mV, maybe you are measuring both tubes? A single tube with that actual reading would likely be red!

thats what i thought. I have the red probe in one of the red sockets and the black probe in the centre black socket, which should measure just the one.

Have you turned the bias pot fully clockwise, when you have the 96mV... Because when i turned mine up to 26-28 mA (at its fullest) I had around the same readings in mV, and the 6l6 got extremely hot... try to bias the amp with DC reading between black and red jack. And bias it so it says something between 30-40mV

Kind regards

René

I also had 25-26mV when I checked my bias. I increased it to 30 then 35 mA. There was no noticeable change in amp tone for me. It is possible that the tubes will run a little cooler, but my tubes were not running abnormally hot at the original bias. I am sure Nik has his reasons for this bias setting.

And...Although the bias check feature on the OTS seems to work fine. I use an AMP-HEAD Dual bias tester on my other amps. It has dummy sockets that plug in the tube sockets and then you plug in your tubes. It is pretty convenient for a quick bias check. AMP-HEAD also makes a device that measures plate voltage that will piggy back with the Bias tester. You don't really need this stuff if you change tubes infrequently. But if you change vacuum tubes frequently they are nice.