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Old 29th April 2006, 01:55 PM   #1
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Default Pronouced 300B amp Midrange

Hi

Trying to figure out why my 300B monoblocks have a pronouced midrange. The bass is present but over shadowed by the midrange.

Circuit is 417A LC coupled with a .33uF cap. 417A is running at 10mA with an EX099 (600H) plate choke. There is a grid choke on the 300B grid along with a 1K grid stopper. No 300B grid to ground resistor. 300B is running at 70mA and ~300V.

I started with the Billie kit quite a few years ago and retain the original Billie iron.

Any ideas on why the pronounced midrange?

/.cheers
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Old 29th April 2006, 02:32 PM   #2
Rocky is offline Rocky  Norway
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You are loading a 417A with 600H? Isn't that like hunting hummingbirds with Tomahawk missiles, a bit overkill?
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Old 29th April 2006, 07:35 PM   #3
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I think you'll find most 300B amps have over-the-top midrange.
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Old 30th April 2006, 03:34 AM   #4
arnoldc is offline arnoldc  Philippines
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Hi, my 300B (and 10Y, 50 switchable) amp is close to yours.

417A driver 10mA (now LED biased, orange) 10K plate load, 0.22uF MIT RTX coupled to the 300B with 5,000H grid choke.

In this configuration, the midrange is not pronounced.

My 45 amp is similar with 100H plate choke to the 417A, LED biased (orange) at 10mA, 0.22uF Mundorf Supreme, 5,000H grid choke to the 45.

In my experience having a "weird sounding" midrange can be seen on a scope. Do you have one? I checked both my 45 and 300B amp and both are good in the 1kHz.

I have an experimental amp I fired last night and it has a pronounced midrange. I took it to the scope and saw 1kHz anomaly (square wave no longer square wave).

Might be worth a try to find out where the problem is originating from.
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Old 30th April 2006, 12:04 PM   #5
lndm is offline lndm  Australia
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I have experienced rolled off bass using an optput stage grid choke. The problem being the inductance of the choke. Theoretically, the grid choke should have an inductance comparable with the plate choke (or preferrably larger).

For example, would you feel comfortable using the grid choke as a plate choke (notwithstanding DC current)?
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