Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Lectroid on April 30, 2023, 11:09:21 am

Title: Dumb question on Matchless Spitfire master volume
Post by: Lectroid on April 30, 2023, 11:09:21 am

I was looking at schematics for 6BQ5 p-p amps and I came across the Matchless Spitfire schematic here:

https://el34world.com/charts/Schematics/files/Matchless/Matchless_spitfire.pdf (https://el34world.com/charts/Schematics/files/Matchless/Matchless_spitfire.pdf)

Can anyone explain how the MV works?  I mentally traced through the variable resistor and came up with two extremes, neither of which makes sense to me.  Either:

1. The wiper is all the way up toward the top of the page, (call that "clockwise") where the pot connects the two out of phase signals together.  In that case, wouldn't they cancel each other out.  (?)

OR

2.  The wiper falls somewhere between full clockwise and full counterclockwise, where it puts a variable amount of that 1M resistance between the two 6BQ5 grids.

It's a production amp so I know it must have worked.  What am I missing?   :w2:

Title: Re: Dumb question on Matchless Spitfire master volume
Post by: sluckey on April 30, 2023, 11:30:51 am

You're not missing anything. It works by cancelling the two out of phase signals. If the two outputs of the PI are not perfectly balanced you will still hear a bit of signal even when turned down. BTW, this is called a crossline MV, probably the least desirable of any MV. I'm not a fan. Probably one of the reasons that I never bonded with my Lightning.

Title: Re: Dumb question on Matchless Spitfire master volume
Post by: PRR on April 30, 2023, 12:27:33 pm

Also: V2 can make about 300Vpk signal both-sides. The power stage only needs 23Vpk for full output. So by the time you have dropped the signal to 8% of full it may be gimping like a 3-legged centipede.

"The other brand" had a Master, so Matchless had to have one. It looked good in the store, where you couldn't honestly beat the stuffing out of it.

Title: Re: Dumb question on Matchless Spitfire master volume
Post by: Lectroid on April 30, 2023, 01:52:31 pm

Quote from: sluckey on April 30, 2023, 11:30:51 am

You're not missing anything. It works by cancelling the two out of phase signals.

At least my analysis was half right but what an ugly kludge.  I keep forgetting how happily amp makers cut corners to save a nickel here and there...

Quote from: PRR on April 30, 2023, 12:27:33 pm

...The power stage only needs 23Vpk for full output...it may be gimping like a 3-legged centipede

 :l2: