Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: RobBozic on November 28, 2010, 05:14:55 am

Title: Built-in attenuator for fixed bias amps ('Carr Mercury' content)
Post by: RobBozic on November 28, 2010, 05:14:55 am

Hi,

Can anyone tell me what kind of attenuator an amp like the Carr 'Mercury' uses?

I've attached a pic of the amp which shows a switch which reduces the power from 8W to 2W to .5W to .1W. The amp is single ended fixed bias. In the pic the switch is the last black chicken head control (closest to the logo).

We all know the kind Carr uses in his cathode biased 'Vincent' amp which takes the power down from 33W to 7W.

But this attenuator is different in that it works between the amp & the speaker, I'm guessing it's just resistors to ground. Can someone please help with the theory to work out how it's actually done & how to calculate the dB reduction etc..

There's a youtube vid where they demo the amp & it sounds great, here' the link  

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Thanks for your help.

Rob

Title: Re: Built-in attenuator for fixed bias amps ('Carr Mercury' content)
Post by: FYL on November 28, 2010, 06:02:48 am

Could be switchable L-pad attenuators, but I don't see any power resistors mounted on the switch or around it.

Title: Re: Built-in attenuator for fixed bias amps ('Carr Mercury' content)
Post by: kagliostro on November 28, 2010, 10:08:55 am

Near the rotary switch I see a very BIG wite edit wire resistor

take one other, more close, photo of that area

Kagliostro

Title: Re: Built-in attenuator for fixed bias amps ('Carr Mercury' content)
Post by: FYL on November 28, 2010, 10:52:56 am

Quote

Near the rotary switch I see a very BIG wite wire resistor

Yup could be a sand R, but you need two per position for an L-pad, ie. eight large resistors. Maybe the white thing is a tapped R and Carr uses smaller shunt Rs, but I can't tell from the low res supplied pic.

Title: Re: Built-in attenuator for fixed bias amps ('Carr Mercury' content)
Post by: RobBozic on November 28, 2010, 04:10:44 pm

Unfortunately I don't have a closer pic of that area. I downloaded that pic from another forum somewhere.

I'm thinking of emailing Carr amps & asking them politely what kind of attenuation system it uses.

Thanks
Rob