Biasing 6550 tubes in VBA400

thebigclunk · 10-22-2009, 04:01 PM

l have just had to replace the eight 6550 power tubes in my 10 year old Marshall VBA400 and bought a new set from Marshall via my local stockist (l strongly reccomend that Marshall matched sets are used - they are the best bet and not very expensive. lf you must source elsewhere l reccomend a matched set of Svetlana 6550's and not KT88's).
l obtained the instructions from Marshall technical on the correct way to set up the bias.
see the attached biasing instructions - you will need an accurate DVM, an audio signal generator an oscilloscope and a 4 ohm dummy load - DO NOT ATTEMPT THIS WITHOUT THE CORRECT EQUIPMENT OR ELECTRONIC OR ELECTRICAL ENGINEERING EXPERIENCE AND WATCH OUT? THE POWER SUPPLIES OF THESE AMPS ARE LETHAL! 750VDC AT 1AMP IS LETHAL TO MOST PEOPLE.
IF YOU DON'T HAVE THIS EXPERIENCE YOU MUST TAKE THE AMP TO A QUALIFIED TECHNICIAN OR, BETTER STILL TO MARSHALL THEMSELVES WHO WILL BE VERY HAPPY TO DO IT FOR YOU VERY CHEAPLY.

thebigclunk · 10-22-2009, 04:06 PM

Sorry forgot the info!

American Viking · 10-22-2009, 07:04 PM

...that's one way to do it.

or a $10 multimeter, $15 probe and tiny screwdriver.

There's more than one way to skin a cat, and easier ways I might add.

Marshall does not use the greatest tubes, I wouldn't use them and frankly I don't think many other people on here would use them either.

An oscilloscope is a good tool to have in general and when checking crossover distortion but what you can see is not as important as what you can hear. Nice to see you want to share information, that's awesome and welcome to the forum. You should check out some alternatives, I think there is a lot you can learn on here about tone, tubes, biasing, and general maintenance. I'll give you one tip, you should change your tubes way more often than once every 10 years if you play every other day. I change mine every year and I have multiple amplifiers.

thebigclunk · 10-22-2009, 07:12 PM

Yeah l agree with you cos thats all l use myself - as far as l know they still use Svetlana tubes unless you know something about their tubes that l missed - dont think there's much left of my liver though (or my ears!)

American Viking · 10-22-2009, 07:16 PM

I guess Svetlana's aren't that bad. I've had them before and they sounded pretty good, no complaints on Svetlana's or Winged C. I know that their tube choices aren't always the same though, they'll typically pick out whichever ones are cheaper at the moment sometimes.

Wilder Amplification · 10-22-2009, 07:58 PM

As has been said before, on some amps the crossover notch cannot be eliminated without overheating the valves. This has to do with the plate voltage on the power valves. If it's higher than the recommended voltage for the class of operation that the valve is in (not the valve's MAX plate voltage rating, but the designed operating voltage for whatever class of operation your amp runs in), it's not possible to eliminate it without overheating the valves, unless you find a way to drop the plate voltage down to the spec'ed operating voltage for the class of operation the valves run in.

Now some have said that the crossover is part of the Marshall sound. As to whether there's any validity to this statement I have no clue and since everyone's perception of "The Marshall sound" is different, I doubt anyone else does either.

The important thing when biasing is to not underbias them and allow them to draw too much current and overheat. The multimeters will tell you what you're drawing with no signal applied. As long as you don't go over the max current draw for your plate voltage (you want to bias to no higher than 70% of the valve's max rated plate dissipation on a class AB amp, max rated for a 6550 according to a Winged C 6550 datasheet is 36 watts, 70% of this would be 25 watts), you can play with the bias setting and bias it colder than max all you want...use your ears when doing this. Just make sure you watch the meter and don't go ABOVE the max.

10-22-2009, 04:01 PM	#1 (permalink)
thebigclunk Junior Member Join Date: Oct 2009 Location: kettering northants UK Posts: 3	Biasing 6550 tubes in VBA400 l have just had to replace the eight 6550 power tubes in my 10 year old Marshall VBA400 and bought a new set from Marshall via my local stockist (l strongly reccomend that Marshall matched sets are used - they are the best bet and not very expensive. lf you must source elsewhere l reccomend a matched set of Svetlana 6550's and not KT88's). l obtained the instructions from Marshall technical on the correct way to set up the bias. see the attached biasing instructions - you will need an accurate DVM, an audio signal generator an oscilloscope and a 4 ohm dummy load - DO NOT ATTEMPT THIS WITHOUT THE CORRECT EQUIPMENT OR ELECTRONIC OR ELECTRICAL ENGINEERING EXPERIENCE AND WATCH OUT? THE POWER SUPPLIES OF THESE AMPS ARE LETHAL! 750VDC AT 1AMP IS LETHAL TO MOST PEOPLE. IF YOU DON'T HAVE THIS EXPERIENCE YOU MUST TAKE THE AMP TO A QUALIFIED TECHNICIAN OR, BETTER STILL TO MARSHALL THEMSELVES WHO WILL BE VERY HAPPY TO DO IT FOR YOU VERY CHEAPLY.

10-22-2009, 07:04 PM	#3 (permalink)
American Viking Senior Member Join Date: Jan 2009 Location: RICHMOND, VIRGINIA Posts: 1,510	Re: Biasing 6550 tubes in VBA400 ...that's one way to do it. or a $10 multimeter, $15 probe and tiny screwdriver. There's more than one way to skin a cat, and easier ways I might add. Marshall does not use the greatest tubes, I wouldn't use them and frankly I don't think many other people on here would use them either. An oscilloscope is a good tool to have in general and when checking crossover distortion but what you can see is not as important as what you can hear. Nice to see you want to share information, that's awesome and welcome to the forum. You should check out some alternatives, I think there is a lot you can learn on here about tone, tubes, biasing, and general maintenance. I'll give you one tip, you should change your tubes way more often than once every 10 years if you play every other day. I change mine every year and I have multiple amplifiers. __________________ -I'd rather be a good liver than have one...

10-22-2009, 07:12 PM	#4 (permalink)
thebigclunk Junior Member Join Date: Oct 2009 Location: kettering northants UK Posts: 3	Re: Biasing 6550 tubes in VBA400 Yeah l agree with you cos thats all l use myself - as far as l know they still use Svetlana tubes unless you know something about their tubes that l missed - dont think there's much left of my liver though (or my ears!)

10-22-2009, 07:16 PM	#5 (permalink)
American Viking Senior Member Join Date: Jan 2009 Location: RICHMOND, VIRGINIA Posts: 1,510	Re: Biasing 6550 tubes in VBA400 I guess Svetlana's aren't that bad. I've had them before and they sounded pretty good, no complaints on Svetlana's or Winged C. I know that their tube choices aren't always the same though, they'll typically pick out whichever ones are cheaper at the moment sometimes. __________________ -I'd rather be a good liver than have one...

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