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Old 4th June 2009, 03:15 AM   #1
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Question Where to put the MV ?

Ok folks...noob needs advice again. Where would you put a MV in this circut ? I have already built this...only slightly mutated...and put the MV between the 2nd plate of the 12AX7 and the grid of the 6V6, but it's not acting right. When the volume and MV are at certain spots, I get ugly distortion. Any advice?
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Old 4th June 2009, 07:45 AM   #2
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Bereanbill,
The circuit doesn't really lend itself to a master volume.

A quick squiz of the circuit shows that where you added the Master Volume, between the 2nd 12AX7 stage and the 6V6 output tube is "philisophically" the right place (given what Master Volumes are intended to achieve).

BUT

Unfortunately, in this particular example, that is putting the Master Volume inside the feedback loop and when ever you adjust the Master Volume level, the feedback will work against the volume adjustment you made to try to put output signal level back where it was.
The Master Volume is mucking up the open loop gain and changing what the feedback does. I would hazard a guess that the "nasty" sounding Master Volume settings are the lower settings, right? With low open loop gain the driver is trying to restore output signal level lost in the Master Volume and eventually just runs out of signal swing and/or gain. Before it runs out of signal swing it gets much grungier as it swings more voltage.

That means it (the Master Volume) has to be shifted to before the 2nd 12AX7 stage where the feedback is applied. - Oh, that where the Volume control already is.

ONLY advise I have is ditch the Master Volume Control, its not that relevant to a 2 stage preamp anyway.

SORRY I was'nt more help, well at least I've pointed out why its giving problems.

Cheers,
Ian
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Old 5th June 2009, 12:59 AM   #3
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Thanks Ian...I sort of feared that response. If I understand you correctly, removing the NFB could aleviate the problem...in theory. I actually put a switch on the NFB loop when I built it, hoping to create a sort of "boost", but there is no audible difference...switch open or closed??? Also...unless I am misunderstanding the NFB loop, I thoght it was supposed to re-introduce part of the signal back into an earlier stage. How does this loop do that? It takes from the output jack and sends it to ground ???
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Old 5th June 2009, 09:43 AM   #4
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That feedback is 'cathode feedback'.

think like this:

if the speaker gets a big positive voltage pulse, a fraction of it appear across your 47 Ohm resistor. Since it's a positive pulse, it tends to turn OFF the driver valve (it adds to the bias) - reducing the drive to the output valve - negative feedback!

The volume control you have now is in the MV position. If you really want MV on this amp (which would make little sense on a Champ), you must add another 12AX7/ECC83 stage at the input of the amp (1,5K cathode resistor, bypassed with 22uF/25V, 100K anode resistor). couple the anode to a 1M log pot with a 22n/400V polyester cap. The new pot is the GAIN, the old VOLUME is now MV.
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Old 5th June 2009, 09:48 AM   #5
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rather than try to build an MV circuit, if you want more output overdrive at low volume, look into the London Power 'power scaling' kits. These appear to use a big MOSFET to reduce the voltage (B+) to the output stage.
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Old 5th June 2009, 06:01 PM   #6
m6tt is offline m6tt  United States
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What about moving the feedback to the power tube's grid, and installing a MV there? I tend to prefer a shorter global feedback loop in guitar amps for touch response & sound.

Honestly, the preamp isn't much to need a MV, since generally the point of a second volume control is to isolate a complex preamp's distortion from the power tube's.

Personally I'd install a variable feedback control, since that way you can change the overdrive characteristics (much like a MV), the volume, and it can be blended into the circuit quite easily with a variable resistor/pot in place of the feedback resistor. Bypass with cap to taste. You can use a pot with an integrated switch to allow the user to disconnect feedback entirely for extra grunge and smoother overdrive.
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