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Old 8th April 2009, 02:40 AM   #1
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Default Cascading volumes on a 5C3 amp

Ok...I'm still trying to tweak this amp design, but my limited knowledge is slowing me down. Is it possible to cascade the 2 volume pots in lieu of a MV on this amp? Also, can I remove NGF for more gain? Thanks again in advance....
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Old 8th April 2009, 06:16 PM   #2
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Bereanbill
Is this an orginal Fender or a kit? Do you have a schematic? What is the configuration of the controls now, in other words, how many controls, and of what type? Hook me up with some answers, and I will offer some suggestions. Pictures are very good.
I ask if you have the schematic because I was able to find it in a flash (at schematic heaven), I dident want to post till I reviewed the design. This is a real simple amp and your choices for re-arainging the controls are pretty limited.
Also try to explain what you expect to improve.
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Old 8th April 2009, 08:28 PM   #3
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Smile 5c3

Firechief...I am building this amp from scratch using the Fender 5C3 schematic from Schematic Heaven. The only changes I am making is to cathode bias the first 6SC7 and possibly remove the NFB...if that does not create any internal problems. My goal is to increase gain in the preamp ( by removing NFB ) and to have some crunch at a reasonable level. By cathode biasing the front end it will be safe for me to use a stomp box to heat up the input a bit. Another idea I had was to use a 6SL7 in the VI position to take advantage of the separate cathodes. Then I can use the second triode as an additional gain stage. I like the woody tone of octal preamps...are there any dual triode octals with an AF higher than the 70 of a 6SC7 or 6SL7 ? I am a relative novice ...I have done about 20 rebuilds/restorations, but mods are not my strong suit. Any ideas will be appreciated...Thanks !!



" We invite defeat when we remember what we should forget "
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Old 8th April 2009, 08:52 PM   #4
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OK
Now I see that you have two ideas that are valid. To test the effect of no negetive feedback, simply remove or disconnect the 1M resistor in line from the output transformer. This alone would be intresting to try. I agree that you have 1/2 half of a 6SC7 that could be put to a different use. Instead of using it as a input amp for the second channel it could be put in series and that would give you gain, and overdrive. I dont know the 6SN7 supper well, but a gain of 70 is enough to make your sine waves in square one's and that translates into what is often refered to as "Crunch". A volume control in between the two halfs of the tube is most desireable, so that you can get her tamed when that is called for. I would say it's very doable. This rig has a volume control for each channel, and one tone control, right?
You have just about all of the parts already installed, its just a matter of changing the hook up. Plan a change that will not violate the DC condition of the tube. (hint, when in doubt, couple signal with a high voltage cap).
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Old 8th April 2009, 09:29 PM   #5
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This is the way to add the MV! Adjust the lower resistor below the 500k so you get Gain=1 from the second half of the 6SC7. Its not that critical. My guess is somewhere between 10 and 15k. With 12AX7 as paraphase inverter, Gibson GA-40 used 470k and 7,5k.

I use this method in our Pro Amp. Works like clockwork.

I also used it on an original 5C3 -53 that I used to own. Was a little more radical and used 12AX7 instead of 6SC7.
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Old 8th April 2009, 10:18 PM   #6
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Ok..I'm kinda with you on this...using the second half of the first 6SC7 for a gain stage instead of input for the second volume is what I'd like to try. I am just not 100% sure on how to couple the stages together...this would be usable for other amps I have as well. Can you walk me through it or send me a drawing ?
Attn. Lars...yeah, MV was another option...do you have a schemo you can show me with this in place?
Thanks again for all the help EVERYONE...I'm really beginning to get stoked !!!!
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Old 9th April 2009, 06:39 AM   #7
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You will totally ruin the 5C3 unique tone by adding second gain stage!

If you want more gain, go for 12AX7 in both places. These two cascaded will give you plenty of overdrive.

Another option if you want even more gain is subsitituting the input tube with a 6SJ7/5879/EF86 moving your amp tone more into old Gibson style.

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Old 9th April 2009, 02:08 PM   #8
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Lars has some good points, and I have not worked on this design, so my suggestion should be ballanced with what he has said. I can walk you through the change that I am suggesting. Disconnect the lower of the two 1M volume controls,. Disconnect the wiper and the top of the pot. This will become your first volume control. Connect the .05 600V cap that was going to the wiper and instead connect it to the top of that vol control. Take the free wiper and route it around to the grid that is the other side of the 6sn7. Now the two halfs of the tube are in series. You will have more gain than you probally need. The good thing about this change is that you could go back to the original very easily.
I have done this kind of thing with 12AX7 and 12AU7's only.
Bob
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Old 9th April 2009, 08:34 PM   #9
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Thanks 'chief !!! I expect thet I need to eliminate the second input as well ? Or does it just become a sort of low gain input?
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Old 10th April 2009, 12:02 AM   #10
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Hi again firechief. Looking at the schematic, the top of both volume pots are grounded...you're saying to unground the first pot and connect the .05 that was at the wiper there, then route the wiper to the second grid...that's where the other input was. Am I following you right?
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