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Old 29th March 2009, 01:37 AM   #1
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Question Adding a master volume to a Fender 5C3 Deluxe

Hi again ...Thanks to everyone who helped me with my other threads.
Next question: I am building a replica of a Fender 5C3 Deluxe amplifier and would like to add a master volume. I have heard that post phase inverter MV sounds the best, so I was thinking of putting a double 1 meg pot between the phase inverter and the grids of the outputs...that should work, no ? If not, wher else would be good ? Schematic for the 5C3 is available at Schematic Heaven.
Again, thanks in advance...love this site !!!
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Old 29th March 2009, 08:08 AM   #2
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Hi,

Post-PI master volume is the only option for this amp as it has only one gainstage before a PI As you can see from the schematic, existing volume controls are already placed right before the PI so it wouldn't make any sense to add a second pot there, without any gain stages in between them. In addition, the PI is quite peculiar in this circuit and additional controls could disturb its operation. If you have a working amp already, I'd suggest to try power scaling for power tubes instead. If you don't want any distortion at all, you can just use existing gain pots to cut down the signal.

If you want a master volume, just build a different circuit, because you won't get any distortion from this circuit at low volumes (which is only reason to have a master volume that I know of). Classic Bassman 5F6-A would be a good amp to begin with if you want little (but not much) preamp distortion, and you could add master to this amp in a way you described. Post-PI master volume does not always sound "better" though.. to some extent it is also a matter of taste.
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Old 29th March 2009, 12:08 PM   #3
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Thanks Moonshine,
I kinda figured as much, but being fairly new to this I like to as others with more knowledge.
So I am looking for a bit more gain in this amp...I already have the board built aside from the electrolytics and the grid leak resistors on the first preamp tube. I notice that the second 6SC7 is cathode biased...would changing the first one help at all ? Would I benefit from changing the preamp to accomodate 6SL7s for the independant cathodes? Thanks again...
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Old 29th March 2009, 01:14 PM   #4
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If you want more gain, put the two halves of the first tube in series and cathode bias (at least) the second one. 1,5k cathode resistor should work fine, and you can optionally bypass it with a 2,2uF (or larger, but try to avoid using an electrolytic) capacitor for more gain and distortion. Then put master volume after the second stage and gain pot between first and second stage. The result should be a nice clean preamp that can do crunch-tones as well if desired. Without modifications the preamp will be very clean, probably nice for jazz. Just remember that amps always tend to sound best when master is dimed

I'm too tired at the moment to tell how PI works in this amp, never seen such a circuit before IIRC.. Therefore I recommend tweaking preamp and leaving the rest as it is, unless someone else steps forward to help. Preamp modification should be really simple, check out the Bassman circuit if you need an example schematic.

Edit: yes, you would need a tube with separate cathodes if you modify the first stage as outlined above.
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Old 29th March 2009, 01:20 PM   #5
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Thanks again Moonshine,
Get some rest, my friend...
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Old 29th March 2009, 05:00 PM   #6
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"If you want more gain, put the two halves of the first tube in series and cathode bias (at least) the second one. 1,5k cathode resistor should work fine, and you can optionally bypass it with a 2,2uF (or larger, but try to avoid using an electrolytic) capacitor for more gain and distortion. Then put master volume after the second stage and gain pot between first and second stage. The result should be a nice clean preamp that can do crunch-tones as well if desired. Without modifications the preamp will be very clean, probably nice for jazz. "

"Just remember that amps always tend to sound best when master is dimed "

That's what I'm talkin about! With a tweed Deluxe you can do that
without breaking windows.

I did this mod to my old 5D3 back in 1975 when I was in high school.
Among other mods, I put the 2 sections of the 12AX7 (changed
from 5C3 to 5D3) in series and added a master volume. 1.5K to 2.2K
is a good Rk value as above. I copied a lot from the AB-763 in my
mods to that amp. Probably because I had a beloved AB763 some
years earlier.. ripped off... my modded 5D3 I left in the care of a
good friend... Hmmm Nowadays it would be a sin to hack up an old
5D3. I wonder would it be just as bad for me to hack up my current
HR Deluxe? I never liked the tone of that amp, but it seems to
have a following...

Cheers!

Michael

PS 6SL7 would be sweet. I'd go 6SL7 if I were to do it again. All
Octal... I was partial to 12AY7 back then in the 2 stage. I'm still
partial to 12AY7 come to think of it.
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Old 29th March 2009, 06:30 PM   #7
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Hi...Thanks again to all...
Yeah, Michael, I'd like to stay all octal which is why I chose the 5C3. I love the thick, woody tone of an all-octal amp. I'm probably just going to build this guy with the 6SC7s since the board is 99% finished, and just change the first 6SC7 to cathode bias. That gives me 2 advantages over the original design: I can tinker with the values of the cathode resistor and cap, and I can safely push the front end with a pedal.
BTW...why use a non-electrolytic as a bypass cap? I was under the impression that you should do the opposite...all the amps I have ever seen were bypassed with"lytics. Was that because of cost/availability of a mylar/poly cap of that size ???

This is really beginning to be FUN !!!!!
P.S. Go ahead and hack up the HRD...it can't sound any worse!! Big ole balls of mud to my ear...except when you open 'em up. Or you could use it for a boat anchor....
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