6550 Question

[Baggy Trousers]

Recently on another MB, there was a question relating to the output of the WAD 6550.

This particular posting claimed that this amplifier ran in Class A. I have a WAD published description which also states Class A, although I have always thought the nominal 40W output was high for this classification.

In a conversation had with Nick Lucas a few years ago, he said that it was A/B with the switch being made at around 30W.

Can someone please enlighten me, assuming the grid bias voltage to be as the standard schematic requires?

Thanks,

Richard.

[Cobblers]

Originally Posted by Baggy Trousers

Recently on another MB, there was a question relating to the output of the WAD 6550.

This particular posting claimed that this amplifier ran in Class A. I have a WAD published description which also states Class A, although I have always thought the nominal 40W output was high for this classification.

In a conversation had with Nick Lucas a few years ago, he said that it was A/B with the switch being made at around 30W.

Can someone please enlighten me, assuming the grid bias voltage to be as the standard schematic requires?

Thanks,

Richard.

Ok then you have a Class AB amp VERY VERY heavily biased into A!

For all intents and purposes that makes it a class A amp (even if strictly speaking it isn't) since 99.999% of the time you'll be listening in Class A.

Apparently most listening is in the 1st 1-3 watts even with average sensitivity speakers.

[Baggy Trousers]

Thanks, Cobblers.

Richard.

[Richard]

Hi Richard

The original circuit was drawn with 500R giving 44v bias = 88mA current. Plate voltage was 416v. The GE data sheet for class A UL p-p suggests 34W using a shared 200R cathode resistor. This is equivalent to 2 separate 400R resistors and, I think, would have been used to flow enough current to hold the stage in class A rather than allow extra power in AB.

(To hold it in class A you’d set bias to flow greater than half the valve’s possible current so it would clip before either valve turned off. To be sure of doing this might need 90+mA for a 6550 and can’t be done with Kit6550 unless the plate voltage is reduced otherwise the plate max dissipation will be exceeded.)

I can’t see a reference to the class of operation in the supplement write-ups, and second-guessing the designer is not good, but it looks like the original circuit was set to get the maximum amount of class A, as 175mA is the valve’s max dc current so 88mA is halfway. Nick Lucas reports that using 470R cathode resistors it actually gave 46W which suggests 88mA may still be too low to hold it fully in class A and it may have given the extra in class AB if the valves were actually able to swing greater than 175mA.

You may have the later cathode value of 680R fitted. This only allows 70mA current and plate voltage is raised to 427v, so, looking at the data, it is most unlikely the stage is held in class A if it’s giving 40W and I feel sure Nick will be correct.

Rich

[Richard]

Apparently most listening is in the 1st 1-3 watts even with average sensitivity speakers.

There was an article in HFN a while back that set out to discover the "truth" and concluded, I think, that 400W was a realistic minimum for the average home set-up to ensure peaks weren't clipped. Anyone remember it?

Rich

[Cobblers]

Originally Posted by Richard

There was an article in HFN a while back that set out to discover the "truth" and concluded, I think, that 400W was a realistic minimum for the average home set-up to ensure peaks weren't clipped. Anyone remember it?

Rich

I've done the hundreds of watts matched to low sensitivity studio monitors with massive magnets, highly damped cones thing (as I guess have many of the forum members), and found this approach wanting for home reproduction.
Great perhaps for studio reproduction at "realistic" volume (for engineers who couldn't give a monkeys about their hearing), but I don't think it's the best way for home hifi.

[Baggy Trousers]

Originally Posted by Richard

Hi Richard


I can’t see a reference to the class of operation in the supplement write-ups . . .

Rich

Richard, thank you for your response - you have confirmed what I suspected.

As for the reference, I have a page from the old site (I think the "Sound" segment of "KiT6550 Valve Integrated Amplifier") which starts "A beautiful high end class A integrated design, providing 40W into a 8 Ohm load . . . ."

Enter Shop!

This is the only reference made to Class A in all the old marketing blurb which is what prompted me to put the question to Nick Lucas in the first place.

Richard.

[Richard]

Originally Posted by Cobblers

I've done the hundreds of watts matched to low sensitivity studio monitors with massive magnets, highly damped cones thing (as I guess have many of the forum members), and found this approach wanting for home reproduction.
Great perhaps for studio reproduction at "realistic" volume (for engineers who couldn't give a monkeys about their hearing), but I don't think it's the best way for home hifi.

The first watt will be the best the amp can give and wouldn't exist without the last watt. If the last one's distorted the sound will be poor. Hence making sure there's enough power whatever the set-up.

Rich

[Richard]

Originally Posted by Baggy Trousers

Richard, thank you for your response - you have confirmed what I suspected.

As for the reference, I have a page from the old site (I think the "Sound" segment of "KiT6550 Valve Integrated Amplifier") which starts "A beautiful high end class A integrated design, providing 40W into a 8 Ohm load . . . ."

Enter Shop!

This is the only reference made to Class A in all the old marketing blurb which is what prompted me to put the question to Nick Lucas in the first place.

Richard.

Hi Richard

I have the later pages from the site, April 2004 my file date, and no mention of class A so perhaps ammended. This would tie up with thoughts about it being intended as 40W class A and later changed when Nick found the glowing plates and re-biased as detailed in his supplement write-up.

Rich

[Baggy Trousers]

Originally Posted by Richard

Hi Richard

I have the later pages from the site, April 2004 my file date, and no mention of class A so perhaps ammended. This would tie up with thoughts about it being intended as 40W class A and later changed when Nick found the glowing plates and re-biased as detailed in his supplement write-up.

Rich

Gottcha!

Can't remember any posts re hot plates on the old BB but my sheet says "Last update: June 6, 2003" so I suspect you have put your finger on it.

In all the years I have lived in the States, I have only twice had food served on hot plates so we are definitely in need of some bias adjustment here - in more ways than one too!

Thanks Rich.

Richard.