What to do?

DudeWheresCorey · 08-13-2009, 07:03 PM

I am getting new tubes for my JCM 800 2203 ressue, I am ordering the high gain option from eurotubes.

When I recieve my tubes will I have to get my amp re-biased, this will be the first time I have re tubed a marshall, I just got it about 4 weeks ago.

anthonyr · 08-13-2009, 07:39 PM

Yes, you'll need to rebias. It's fairly simple if you take you time. I use a digital multi meter to check voltages on pins three and five of the power tubes. You can then plug those numbers into a handy dandy equasion and it will give you your bias voltage at 70% which is a safe but not neccesarily standard number.

Now I know that my explanation was horrible but the point is that you need to do it. There is a biasing video at eurotubes.com that is pretty good and he's even using an 800 to demonstrate on.

anthonyr · 08-13-2009, 07:39 PM

Yes, you'll need to rebias. It's fairly simple if you take you time. I use a digital multi meter to check voltages on pins three and five of the power tubes. You can then plug those numbers into a handy dandy equasion and it will give you your bias voltage at 70% which is a safe but not neccesarily standard number.

Now I know that my explanation was horrible but the point is that you need to do it. There is a biasing video at eurotubes.com that is pretty good and he's even using an 800 to demonstrate on.

Frankie · 08-19-2009, 07:32 AM

You should always rebias when replacing power tubes.

American Viking · 08-20-2009, 01:28 AM

Quote:

Originally Posted by anthonyr

Yes, you'll need to rebias. It's fairly simple if you take you time. I use a digital multi meter to check voltages on pins three and five of the power tubes. You can then plug those numbers into a handy dandy equasion and it will give you your bias voltage at 70% which is a safe but not neccesarily standard number.

Now I know that my explanation was horrible but the point is that you need to do it. There is a biasing video at eurotubes.com that is pretty good and he's even using an 800 to demonstrate on.

What is the point of taking a reading off of pin #5? What is that telling you and how does that fit into the standard bias equation which is

(plate dissipation of tube / plate voltage of amp) x 0.7 = bias

Frankie · 08-20-2009, 11:15 AM

Quote:

Originally Posted by American Viking

What is the point of taking a reading off of pin #5? What is that telling you and how does that fit into the standard bias equation which is

(plate dissipation of tube / plate voltage of amp) x 0.7 = bias

I was confused here too. Also, in your equation that "bias" result is actually maximum recommended bias. You don't have to set it that hot, but in a Marshall I usually go 70%. You can get over setting it lower in Fenders, they don't cross talk as much.

American Viking · 08-20-2009, 01:24 PM

Quote:

Originally Posted by Frankie

I was confused here too. Also, in your equation that "bias" result is actually maximum recommended bias. You don't have to set it that hot, but in a Marshall I usually go 70%. You can get over setting it lower in Fenders, they don't cross talk as much.

In my equation??????????

What do you think that 0.7 is? Multiply any number times 0.7 and that's 70% of that number. This isn't my first time at the dance. Try reading it again.

(plate dissipation of tube / plate voltage of amp) = maximum do not exceed

(plate dissipation of tube / plate voltage of amp) x 0.7 = bias in mA

Frankie · 08-24-2009, 02:51 PM

Quote:

Originally Posted by American Viking

In my equation??????????

What do you think that 0.7 is? Multiply any number times 0.7 and that's 70% of that number. This isn't my first time at the dance. Try reading it again.

(plate dissipation of tube / plate voltage of amp) = maximum do not exceed

(plate dissipation of tube / plate voltage of amp) x 0.7 = bias in mA

Alright joker, read my post again. Your number doesn't = bias, it = maximum suggested bias before you start burning stuff out. You don't have to bias at 70% max wattage. It's the internet though, so do whatever you want.

American Viking · 08-25-2009, 04:41 AM

I'm usually hearing these numbers (like 70% max wattage) being referred to as where crossover distortion ends and not as a precaution to tube life. I consider the maximum I would recommend to be damn near at 100%, that's where I misread you.

It's all good. I see what you mean now.

Frankie · 08-25-2009, 10:13 AM

Quote:

Originally Posted by American Viking

I'm usually hearing these numbers (like 70% max wattage) being referred to as where crossover distortion ends and not as a precaution to tube life. I consider the maximum I would recommend to be damn near at 100%, that's where I misread you.

It's all good. I see what you mean now.

Sorry if I came off a little edgy too. We road trip/gigged all weekend and I posted from my desk at work Monday afternoon after my morning coffee had wore off.

Normally 70% is recommended maximum idle, with idle being the key word. They work harder when you're playing, so if you bias them at full bore 100% idle, well, they're on fire while you're playing. In a Marshall 70% is a safe bet, but usually anywhere between 50% and 70% is a good range to idle tubes. If you like how they sound at 55% max, well then have fun. Idling too cold will also wear the tubes out quicker.

Also, FWIW, cathode biased amps like my Vox AC30 and Epi Valve Jr. have complete disregard for this idea and normally idle at 90%. How many times have you read a post about EL84 amps eating up tubes like tic tacs?

jitees · 08-26-2009, 04:36 PM

OK, so after you get a reading, how do you change the voltages?

08-13-2009, 07:03 PM	#1 (permalink)
DudeWheresCorey Junior Member Join Date: Jul 2009 Location: North Carolina Posts: 13	What to do? I am getting new tubes for my JCM 800 2203 ressue, I am ordering the high gain option from eurotubes. When I recieve my tubes will I have to get my amp re-biased, this will be the first time I have re tubed a marshall, I just got it about 4 weeks ago.

08-13-2009, 07:39 PM	#2 (permalink)
anthonyr Senior Member Join Date: Jan 2009 Location: Clearwater Florida Posts: 126	Re: What to do? Yes, you'll need to rebias. It's fairly simple if you take you time. I use a digital multi meter to check voltages on pins three and five of the power tubes. You can then plug those numbers into a handy dandy equasion and it will give you your bias voltage at 70% which is a safe but not neccesarily standard number. Now I know that my explanation was horrible but the point is that you need to do it. There is a biasing video at eurotubes.com that is pretty good and he's even using an 800 to demonstrate on.

08-13-2009, 07:39 PM	#3 (permalink)
anthonyr Senior Member Join Date: Jan 2009 Location: Clearwater Florida Posts: 126	Re: What to do? Yes, you'll need to rebias. It's fairly simple if you take you time. I use a digital multi meter to check voltages on pins three and five of the power tubes. You can then plug those numbers into a handy dandy equasion and it will give you your bias voltage at 70% which is a safe but not neccesarily standard number. Now I know that my explanation was horrible but the point is that you need to do it. There is a biasing video at eurotubes.com that is pretty good and he's even using an 800 to demonstrate on.

08-19-2009, 07:32 AM	#4 (permalink)
Frankie Senior Member Join Date: Sep 2007 Location: Clearwater, FL Posts: 362	Re: What to do? You should always rebias when replacing power tubes.

08-25-2009, 04:41 AM	#9 (permalink)
American Viking Senior Member Join Date: Jan 2009 Location: RICHMOND, VIRGINIA Posts: 2,025	Re: What to do? I'm usually hearing these numbers (like 70% max wattage) being referred to as where crossover distortion ends and not as a precaution to tube life. I consider the maximum I would recommend to be damn near at 100%, that's where I misread you. It's all good. I see what you mean now. __________________ Transformers inside amplifiers were not originally designed to be in there, and the tube was not created for the guitar amp. ANY recommended idle current is a guideline and in no way a necessary requirement. There is no skeleton key for any tube amp.

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08-26-2009, 04:36 PM	#11 (permalink)
jitees Member Join Date: Jun 2009 Posts: 68	Re: What to do? OK, so after you get a reading, how do you change the voltages?

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