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PostPosted: 02 Oct 2015, 15:40 
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Joined: 13 Sep 2015, 08:34
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If coupling caps are responsible for shaping the tonal characteristics of an amplifier, how would one cut the bass frequency?
I am new to this, and have acquired a Magnavox 9304-20. I have added the balance resistors to replace the balance pot, and have just replaced the factory coupling caps with Soviet K40Y9's...brought the amp up and it has TREMENDOUS bass output...
I use a David Hafler DH-110 preamp, and currently the bass is completely to the negative side. the amp (with tone adjustment) is beautifully rich with clear impact of both bass guitar and kick drum, I would LOVE this sound if it is possible to achieve at neutral tone....

HOW DO YOU GUYS DO THIS????

Currently it has the replaced factory values of .047 and .0015 coupling caps....

I am not the sharpest tool in the shed about these tube amps, but I am reading....

Help the new guy?

Thanks,
Izatso


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PostPosted: 03 Oct 2015, 11:28 
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Location: Vänersborg, Sweden
Using coupling caps to fix the frequency characteristics is not an idea that calls for my attention. IF you have to much low notes, the problem is not the coupling caps.
Normally I run all my amps w/o tone controls and I haven't so far experienced the need of them I guess I have just been lucky with my speakers to have them pretty much linear from DC to 100kHz (well, almost ...).

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PostPosted: 03 Oct 2015, 12:24 
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In the simpleast sense, when chosing a coupling cap, it depends on the following stage's resistance. Example.

A power output tube has a grid leak resistor of 150K. I want the F3 to be 10 Hz. So we look for a capacitor whose reactance equals 150K @ 10Hz.

http://www.electronics2000.co.uk/calc/r ... ulator.php

Turns out that the common 100nF coupling cap is up to the job.

Now for good measure, we want some head room. So personally I would use a 220nF capacitoe instead of 100nF.

Cheers!

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PostPosted: 03 Oct 2015, 13:12 
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So then....
By the calculator, should I want the roll off to begin around 45 Hz, it would take a capacitor of appx. 3.5 uF???

I knew there had to be a formula....will this in any way effect the amplifier output wattage?


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PostPosted: 03 Oct 2015, 16:25 
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If that is the route you want to go then the value would be 3.5 nano farads ( xxx nF) not micro farads (xxx uF). Only a 1000 to 1 difference.

My suspicion is that if you have too much bass then a number of other things may be causing it. A serious impedance mismatch from the source device to the preamp can do this. For example taking the sound from the speaker terminals of a device and feeding it into the preamp. Another possibility is that your speaker system has too much bass. A third possibility is that there is something wrong with the amplifier that is causing the problem.

I would hunt for the source and try to fix that and not alter the response of the amp.

Good listening
Bruce

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PostPosted: 05 Oct 2015, 08:55 
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I have recently finished a Bob Latino ST-70 clone, which I have to say sounds AWESOME!, I had that as a reference.

The pre-amp and input sources are working properly through the ST-70, so I will assume they are fine.

I have already made plans to replace all resistors and capacitors for the amp, but I was thinking about the pre tubes as well. currently one is a Tungsol, the other a Magnavox. I know that they are not matched, and can only guess their condition. I am in rural Mississippi and have no one close to check these tubes to see...

The quest continues!
This little amp has an amazing sound, the bass, although IMHO much too pronounced, is when dialed down, tremendously clear and clean, capable of a wonderful impact....mids and highs are rich and wonderful.....are there any other 9304 Magnavox owners out there, is this just the sound that is inherent to this particular amp?


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PostPosted: 05 Oct 2015, 09:43 
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Joined: 02 Sep 2008, 14:34
Posts: 527
Location: Rochester, NY
izatso wrote:
....are there any other 9304 Magnavox owners out there, is this just the sound that is inherent to this particular amp?

There was at one time a Magnavox Forum on Yahoo. You may want to look there.

EDIT
Found it - Magnavox Freinds

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