From: Andy Marshall   (andrew@thdelectronics.com)
Date: 4/27/2001 3:26 AM
Subject: Re: Univalve

Steve,

Thanks for the kind words.

Regarding the light bulb and noise reduction, you hit the nail on the head. That is exactly what we are doing. There is no magic or mystery to it at all. I did not invent it; I merely adapted a circuit that was in common use in the 1940s.

- Andy Marshall 

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From: Carl Z   (summit@theramp.net)
Date: 4/27/2001 1:00 PM
Subject: Re: Univalve


If I remember right don't they use a similar circuit for use with early tube compressors?

Carl Z 

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From: steveR
Date: 4/28/2001 1:16 AM
Subject: Re: Univalve


I don't get out much! :) And so I haven't seen one of your amps, but I'm interested in learning more about the lightbulb trick. 

Is there much more to it than having a bulb in series with the OT secondary to the speaker? What kind of rating does the buld need to be? Are we
talking like 6.3 volt indicator lamp or something like an appliance bulb?

Thanks.

steveR 

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From: Andy Marshall   (andrew@thdelectronics.com)
Date: 4/28/2001 2:00 AM
Subject: Re: Univalve


Steve,

The light bulb filament is in parallel with the output transformer secondary. If it were in parallel, it would act as a compressor, not an expander.

I use a #1816 in the UniValve which is the same size as a #47 6.3V Fender-style indicator bulb, but I think it is rated at 13V and 250 mA.

For each application (power vs. impedance) you will have to select a different bulb for the optimal performance.

There are tricks using pairs of bulbs in a bridge configuration with bypass capacitors and series inductors to try to keep the frequency response flat,
but that is getting a bit complex for an application such as this.

There is a good article on it in the last few editions of the Radiotron Designer's Handbook published by RCA of Australia.

Play with it. I am sure you will be pleased with the results.

Cheers!

- Andy 

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From: Steve A.   (steve_ahola@yahoo.com)
Date: 4/29/2001 7:12 AM
Subject: Clarification, please

Andy:

"The light bulb filament is in parallel with the output transformer secondary. If it were in parallel, it would act as a compressor, not an expander." 

    Not to pick nits but I believe that one of your references here to "parallel" is supposed to be "series"... Can you clarify that so we don't end up
compressing instead of expanding?

    I really appreciate someone like yourself sharing one of your tricks here for all of us to mess around with!

--Thanks!

Steve Ahola 

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From:  Ray Ivers   (RAGEray@aol.com)
Date: 4/29/2001 12:12 PM
Subject: Re: Clarification, please

Steve,

Check out the R.D.H., pp 687-688, for light-bulb expander circuits. 

Ray 

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From: Andy Marshall   (andrew@thdelectronics.com)
Date: 4/29/2001 4:09 PM
Subject: Re: Clarification, please

OOPS! Sorry.

Now let me read this as I am typing it....

Having the light bulb filament in PARALLEL with the output transformer secondary gives EXPANSION.

Having it in SERIES gives COMPRESSION.

(read... read... read..., process... process.. process..., drink coffee..., read...)

Yup, that's correct.

Once again, sorry for the confusion.

This is just that, a "trick", not some magical design of mine (though I would not put it past Mesa Boogie to try to patent it and then sue ME). I admit,
there are a few aspects of what I do as a designer that I would not explain to the world, but not many, and this falls firmly into the public viewing
area in my mind.

Have fun!

- Andy 

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From: Steve A.   (steve_ahola@yahoo.com)
Date: 4/29/2001 5:31 PM
Subject: Re: Clarification, please

Andy:

    Bruce Collins of Mission Amps had shared a different trick with light bulbs a few years back— wire one up in series between the PT CT and ground to simulate tube sag when using a ss rectifier. The rest of the details escape me...

    With all of these bulbs wired up, it'd be like having a light show built into your guitar amp...   :D   (To color the light bulbs you can buy glass paint
at a hobby store. They used to sell a high-temp transparent paint that could be used on 40W light bulbs but I haven't seen that for 30+ years...   :(   )

--Thanks!

Steve Ahola