First Attempt at Biasing....Are these values OK?

[beamer23]

Hello, I finally did it. I bought an Amp-Head bias probe and did my first bias. The first one I tried it on was my JTM30 combo. It has JJ 6L6 tubes and I got 54mA for the cathode current and 437V for the plate voltage. Isn't that really high for the bias? I thought it should be around 50mA for 437V. But inside there doesn't appear to be a bias pot. Any suggestions?

My second bias reading on my JCM900 MKIII was 36mA @ 450V which seemed pretty good, but PLEASE correct me if I'm wrong.

Anyways, I know the JTM30 originally came with 5881's but when I got mine used it had the 6L6's in it. I've read that many have done this tube swap.

Any help would be appreciated. Am I in danger of ruining the tubes??

Thank you

 

[RobS]

5881 - 6l6 - same valve, just different naming convention

 

[wkcchampion]

5881 - 6l6 - same valve, just different naming convention

Exactly, like ECC83 / 12AX7. European vs USA naming

 

[jcmjmp]

It has JJ 6L6 tubes and I got 54mA for the cathode current and 437V for the plate voltage. Isn't that really high for the bias? I thought it should be around 50mA for 437V. But inside there doesn't appear to be a bias pot. Any suggestions?

54ma is very hot, even when using the cathode resistor method. I assume that its a 50w amp or something in that range? I wouldn't turn the amp on with a bias set that hot.

My second bias reading on my JCM900 MKIII was 36mA @ 450V which seemed pretty good, but PLEASE correct me if I'm wrong.

If the JCM900 has 6L6 tubes in it, I would say that you might be a little hot. I would back that off. My max value, using the cathode resistor method would be 31ma max with the amp having run for at least 30minutes.

 

[big dooley]

i agree, even for a cathode biased amp that's very hot... somehow i think they calculated this at 70% of a 30watt plate dissipation, and even then it's pretty hot
(JJ rates their 6L6gc at 30watt)

you can measure the voltage drop on the screen resistors and devide that number with the impedance of that resistor so you know how much current the screen is drawing
and count that off the cathode current so you know how much plate current you're drawing... i did it like this with my JVM, so i knew what the actual plate dissipation was.. the testpoints on a JVM are cathode based

 

[cudamax2343]

Use this formula that I was given from this site for any amp.
You take your plate voltage readings with the tubes in speaker load hooked up and with all vol & tone controls on "0"
You use 30% for the 6l6GC's and maybe a few others and everything else 25% and divide that number by your plate voltage. So in your case 30 divided by 437 is 0.069 that would be your highest value then times 70% for your lowest value 0.48
So your range with your JTM-30 is .069 at is hottest & .048 at it's coldest
I think?
I had a JTM-60 and it had an adjustment underside for the Bias. I bet-cha yours has one too

 

[jcmjmp]

Use this formula that I was given from this site for any amp.
You take your plate voltage readings with the tubes in speaker load hooked up and with all vol & tone controls on "0"
You use 30% for the 6l6GC's and maybe a few others and everything else 25% and divide that number by your plate voltage. So in your case 30 divided by 437 is 0.069 that would be your highest value then times 70% for your lowest value 0.48
So your range with your JTM-30 is .069 at is hottest & .048 at it's coldest
I think?
I had a JTM-60 and it had an adjustment underside for the Bias. I bet-cha yours has one too

That's wrong. If the amp is rated for 50w, you can't use a different formula just because the tube can handle more power. The bias point does not dictate the amount of power the amp produces. The tube may be able to produce up to 30w of power, but the amp is designed for 25w. Use the 25w value as a reference.

Also, your numbers are wrong. The 70% dissipation value for a class AB amp is typically the maximum you would want to set the bias point at, not the minimum.

 

[Jonathan Wilder]

That's wrong. If the amp is rated for 50w, you can't use a different formula just because the tube can handle more power. The bias point does not dictate the amount of power the amp produces. The tube may be able to produce up to 30w of power, but the amp is designed for 25w. Use the 25w value as a reference.

Also, your numbers are wrong. The 70% dissipation value for a class AB amp is typically the maximum you would want to set the bias point at, not the minimum.

Don't you mean that the valve can DISSIPATE up to 30 watts?

Again...common misconception is that if the amp puts out 50 watts that the valves actually see that 50 watts. Not so. All they do is control load current. Load current drops a voltage across the load. Output power is the product of load current and the voltage it drops across the load (i.e. load voltage). Plate current IS in fact equal to load current, but since the valves do not see the load voltage drop, they have no way of seeing load power.

Unlike load voltage (load voltage increases as load current increases), the voltage dropped across the valve itself is DECREASING while plate/load current increase. As such, plate dissipation only rises to max dissipation until it hits the Class A/Class B transition (right at 1/2 swing). Once the signal passes that transition, plate dissipation DECREASES until the signal hits max swing.

It's the load that sees the output power, not the valves. The valves just control the load current and should never exceed their max dissipation rating at any point in the swing while controlling this load current. Max dissipation in a Class AB amp occurs mid swing at the Class A to Class B transition, and if the load is optimized for the valves being used at the plate voltage they're running at, and the bias is set to the proper operating point, the average power that the valves see should never exceed the valve's max dissipation rating.

That being said...this whole "bias figure for any amp/any valve" doesn't exist unless you know the load in which the valves will be working into. It's not just 70% of plate dissipation that comes into play...the load that the valves are working into also comes into play in figuring out where your max bias figure will be. Believe it or not, while plotting the load lines for an EL34 running at 500 volts (with 250 on the screen) into a 10K plate-plate load, the Class A load line crossed 500 volts right at 35mA, which equals 17.5 watts at idle, which is in fact 70% of the max 25 watt dissipation.

Of course this assumes a 100% regulated power supply and fixed screen/dual rail operation. Most guitar amps use "sliding screen" operation and a single rail unregulated supply so they can get away with running a much lower load at 500 volts...more than likely in the neighborhood of 6.8K for two valves/50 watter would be ideal, but all Marshalls ran the same load while their plate voltages varied widely all over the place.

Here's a picture of that plot...the dot on the graph is the idle bias point. You can see where Class A and Class B cross...which represents roughly mid-swing...right at the max dissipation curve on the graph -

 

[beamer23]

Thanks everyone for the information. I did forget to mention that my JCM900 had EL34s. I guess the worst part of this is that I took my JTM30 into my tech not long ago and he said that the bias was "OK". I guess it pays to be informed. If anyone has experience with setting the bias on a JTM30, I would welcome the information. When I researched it seems that it is a fixed bias amp with no bias adjustment pot. And from what I gather, that means replacing resistors to achieve the correct bias. This amp does have a fizzy sound to it. Could this be due to the bias being so high?

Thanks again you guys,,,,

 

[jcmjmp]

Don't you mean that the valve can DISSIPATE up to 30 watts?

Wilder, I'm trying to dumb it down. If you read the post I was responding to, you'd see that it appears as though he was assuming that with a 6L6GC, his 50W amp was now a 60w. That's what I was trying to refute. As you know, it just doesn't work that way as you have to account to plate load, plate voltage and screen voltage. As I said in the post, the bias point does not dictate how much power the amp will deliver.

The valve does not dissipate the power, the load does. If the valve were to dissipate the power, the would be nothing left for the speakers.

I think we agree. You have to understand that not everyone understands your lengthy posts and a lot of people will tune out when you start to get too technical so its as if you didn't post anything at all.

 

[Jonathan Wilder]

If the valve were to dissipate the power, the would be nothing left for the speakers.

But...but...I thought power was an endless source and that it came for free.

 

[mymarshallrocks]

Don't ask techs to dumb things down..we don't learn that during our training, though it would make fixing things a lot easier

 

[jcmjmp]

When you go through design reviews, have to explain technical issues to management, do product demos and teach, dumbing it down and simplifying things is part of the job. You have to know your audience.