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How do you tell if an amp is cathode-biased?

5/22/2005 10:21 PM
GlennW How do you tell if an amp is cathode-biased?
Lame question, I know...but I've never done this before. I've spent a lot of time reading different articles about setting the bias, and it seems like "the cathode-resistor method" is the best and safest way for someone new to try it. One article says;  
"Note that these instructions assume that your amplifier is biased by applying a negative voltage to the control grids; cathode-biased amplifiers cannot be adjusted other than by changing the value of the cathode resistor(s) so this method does no apply to them."  
Anything in particular that I should look for on the chassis or schematic?  
It's a Univox U-155R with a pair of 6AQ5 output tubes, if that helps.  
5/23/2005 2:09 AM
Greg Simon
It seems to me that you may have some confusion here about the safest method of setting the bias, and the type of bias method? If not, then ignore that part of this post!  
When they're talking about having a 1 ohm resistor from cathode to ground, and you using your meter to measure a voltage across this resistor, this is not the same as cathode bias! The 1 ohm resistor thing is independant of the bias method, and it is a way to check the bias and have it be reasonably accurate and repeatable, and safe. If you need more help on how to actually do this to set the bias then I can write more about that, or there are other things online regarding that.  
Cathode bias just uses a large resistor between the cathode and ground. If you haven't read it yet, go to Aiken's site and read the article called "the last word on biasing" in the advanced section. The site address is below. Here is a short blurb from his site about cathode biasing.  
Do cathode-biased amplifiers need to be biased? The short answer is yes. The cathode biasing method is self-regulating, to an extent, because increases in cathode current create a larger voltage drop across the cathode resistor, which in turn, creates a larger negative grid-to-cathode voltage, which counteracts the increase in current. The tube will reach a stable point of equilibrium and stay there. However, just as different tubes from different manufacturers will draw varying amounts of current in a fixed-bias amplifier, the same is true of a cathode-biased amplifier. For this reason, the bias should always be checked, even with cathode-biased amplifiers.  
With cathode bias, you can just measure the voltage across the cathode bias resistor with your meter, and divide your reading by the resistance of the resistor, and that will be your cathode current. With fixed bias, a negative voltage is injected into the grid of each power tube to set the bias. The key with either method of setting the bias is that the grid has to be more negative than the cathode in order for the tube to work. If the grid goes more positive than the cathode, the tube shuts off because the electrons coming from the cathode to the plate will go to the grid instead of the plate.  
In cathode bias, the resistor is there to raise the cathode above the grid voltage level, and keep it there. The cathode resistor is basically self adjusting the bias back and forth. With fixed bias, the negative voltage injected into the grid keeps the grid voltage level below the cathode, which will be at zero. The negative voltage is usually chosen so that the tube will be able to operate at idle between 60% to 70% of its rated dissipation level. With cathode bias, the resistor value is chosen to keep the tube in the same 60 to 70 range, although because it is self correcting, you can sometimes run cathode bias past the 70% level with no issues.  
In looking at an amp or schematic, you can usually tell if an amp is fixed bias or cathode bias because in fixed bias, the cathodes will be grounded, or there will be a 1 ohm resistor from each cathode to ground. In cathode bias, there will always be a large value resistor like a 470 ohm or something between the cathode and ground. Sometimes there will be one resistor common to multiple tubes, or each tube will have it's own resistor on the cathode to ground.  
To set the bias with fixed bias, there is usally (but not always) a pot that you can adjust to change the negative voltage injected into the grid, and you measure across some 1 ohm cathode resistors. This measures the cathode current AND the screen current, but the screen current is usually so small it doesn't make a difference. If you get one of the products available like the Bias King, or Swamp Probe, they interrupt the cathode connection to the tube, so it measures the cathode current only. These are a socket that goes into the power tube socket, and then your tube goes into it, and then you turn the amp on and measure away. THESE would actually be safer than measuring across a 1 ohm resistor, but they aren't available for every tube tpye, and certainly as far I know, they aren't available for 6AQ5 sockets. Some fixed bias amps are non adjustable, which means they have a preset resistor in teh circuit that puts a particular voltage to the grids of the power tubes. If you want to set the bias on these, you have to change the resistor value or put a pot in its place. Mesa Boogie amps are usually set like this.  
To set the bias with cathode bias, you must change the cathode resistor value if your tube is not idling where you want it. You can't use a pot here because there is so much current a pot can't handle it. A cathode resistor in a cathode biased amp is usually a 10 watt or 20 watt resistor or more. Pots are usually 1/2 w or 1 w.  
Well I hope that helped you a little bit...if you need more help, I can qualify things or provide more details. Read the link at Aiken's site and see if that helps too.  
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5/23/2005 4:33 PM
Hey Greg, thanks a whole lot for the thorough answer. Here's what I had as far as the Aiken site - it came up on a search, but I didn't go into the site any deeper. I printed the part I thought would apply.  
The link you posted has a lot more info. I printed out your reply, and will also go to the link and print some more stuff.  
Was also looking at another thing by Lord Valve on the Duncan site, and reading Torres' book (mainly geared towards Fender stuff).  
Thanks again.
5/23/2005 5:08 PM
Back your post it says, "In cathode bias, there will always be a large value resistor like a 470 ohm or something between the cathode and ground. Sometimes there will be one resistor common to multiple tubes, or each tube will have its own resistor on the cathode to ground."  
On the schematic it shows what I think are the two cathodes from the two separate 6AQ5's wired together, and then to a 250 ohm 5 watt resistor and a 30 ? (the symbol looks little "u" with the left leg hanging down like a "p") cap in parallel, and then to ground. That stuff is isolated from everything else. Would they be the bias resistor and cap? It also says "25v" above the cap symbol. Would that be the figure to shoot for?  
I've only had it on once for a short while after letting it warm up for a couple of hours with a current limiter (light bulb), and one of the output tubes looked real hot - the plate was almost all red. I'd like to try to solve the overheating problem before putting in new tubes.  
5/23/2005 5:48 PM

GlennW said: "...(the symbol looks little "u" with the left leg hanging down like a "p")..."  
The symbol you're talking about is this: "µ"  
It's the greek letter "my" and means "micro"  
Your cap was a 30µF/25volts type - an electrolytic cap.
5/23/2005 6:12 PM
Don Symes

... and in the part of the world without Greek characters on keyboards, a 'u' is used (uF).  
Some OLD schematics will use mmF as in milli-milli-Farad (1/1000 of 1/1000) of a Farad.  
5/23/2005 6:26 PM
Greg Simon
Hi Glenn. The Aiken site has TONS of useful info on it. Just go to and click on each section and inside there will be lots of interesting articles that you can print out.  
On your amp, the 250 ohm resistor is the cathode bias resistor, and the 30uf 25v cap is the cathode bypass cap. You should unsolder that 250 ohm resistor and make sure it measures 250 ohms onyour meter. If it does, solder it back in. If not, go buy another one and put it in. It should be a sandbox type wirewound 5watt resistor, although a 10w would work fine too if you can't find a 5w one. Anything within 5% of the 250 ohm value is fine.  
If that cap is an electrolytic type, which it probably is, then you should replace it with a new one. Look at the cap and see if it has a + symbol on one end. If it does, it is electrolytic. These go bad with age and non use and need to be replaced in most older amps. The cap may also have some arrows pointing to the negative end instead of a + symbol. When you replace it, make sure the new one is electrolytic also, and make sure it is hooked up correctly with the + end going to the tube and the negative end going to ground!  
If the plates on one of your output tubes were glowing red or orange, there is definitly something wrong. You'll want to figure out what the problem is before you turn the amp on again. You should also get another set of 6AQ5 tubes for that to use as known good tubes. If you find the problem and fix it, and the amp works fine with the existing tubes, then you can use the new ones as back ups, or stick them in and use the old ones as backups. My guess is that the cathode cap and/or cathode resistor are bad or out of spec and are causing the bias to be incorrect for the tubes.  
You could take the tubes out and then turn the amp on and measure some voltages at the cathodes, the plates, and the grids. Knowing what the voltages are would help others to figure out what is wrong. You shouldn't turn the amp on with the tubes in until you fix whatever the problem is.  
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