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Old 1st February 2011, 02:48 PM   #1
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Default '57 twin question

Hello,
I was looking at the schematic for a '57 Twin. Thinking about building one because it looks like a fairly simple schematic. If I can find the right tranasformers I probably will. However I was trying to discern the difference between the "normal" and "bright" channels. They look like they are exactly the same on the schematic. Can somebody give me a hand or at least confirm that I'm not crazy? I've been looking at this schematic below. Thanks.


http://support.fender.com/schematics..._schematic.pdf

Also If anybody has any suggestions about getting more headroom out of this amp then I's much appreciate it as that would be a further away goal. Thanks again!
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Old 1st February 2011, 03:25 PM   #2
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There is one 100 pf cap C4 in the bright channel. I would be tempted to leave off one of the pre-amp tubes and build just a bright channel. Then you could always put in a switch to remove that cap from the circut and. Antique Electronic Supply may be able to help with the transformers.
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Old 1st February 2011, 04:46 PM   #3
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AHA! that would be it. thanks man. I don't know how I missed that cap. I must have looked through that circuit like 30 times. I will check out Antique Electronic Supply. Thanks. Now I feel stupid haha.
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Old 1st February 2011, 07:33 PM   #4
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Most guitarists I know prefer playing through the "Brite" channel. Bassists, on the other hand, like the normal channel. Most of these amps, especially the early ones, were built for both guitarists and bassists in mind, that's why there were both channels. Depends on what you want to do....
Daniel
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Old 1st February 2011, 07:40 PM   #5
RTF671 is offline RTF671  United States
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Yeah, the twin has a rather crude mixing buffer considering the parts it adds. Going with only two inputs with a bright switch (a la marshall style) will save you a whole valve. On the same stage, the cathode bypass cap (C1 220u 16v) can be reduced to 22u or so. 22uF handles all frequencies down to 10 or so Hz. 220u goes down to 1Hz, which is not necessary because most OTs and speakers aren't suited to handle anything under 20Hz.

Are you going to keep the 5U4s? If so, be sure to check the secondary resistances of you PT and do not exceed the minimum limiting resistance for the valves.

For more headroom, the power stage can be altered to run about 50w peak power (More power means more clean headroom because you will not be pushing it to it's limit) without any more 6L6s.

A good mod to reduce heater hum is to remove both 100R resistors to ground and replace with a 330R or 470R trim pot with each leg connected to the heater lines and the wiper to ground. That way the lowest noise floor can be dailed and set manually.

A quick protection element that does not affect tone is to add a power diode along with a 10k resistor in series from the grid to the cathode (before the cathode resistor) of V3B. What that does is protects the cathode from stripping while warming up. Once warm the diode biases off completely and stays that way.

Do you still plan on using the 5U4s? Both Weber and Heyboer produce replacement transformers for fender twins although I'm not certain if they have 5v lines and if they do are they big enough. Each one uses 3A on a 5v line so you would need 6A@5v which is quite a lot of heater current (30 watts! more than the rest of the amp combined). If you want the 'sag' from a valve recto it can be recreated by a sag resistor with a solid state one.


Are there any mods you had in mind?
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Old 2nd February 2011, 11:38 AM   #6
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thanks for all the info RT. Have you built or modded this amp before? This project is pretty new in my head I haven't even begun to acquire parts yet so I haven't really any ideas on what i really want to mod. I have heard this amp before and I do love it. But I would not want to always be using an amp that i would have to turn down to get rid of the natural distortion in order to replace it with one from a pedal. So thats why I was asking about a way to give it more headroom. or rather a way that I can switch between its normal amount and more headroom. But I wouldn't actually begin to mod it until I build it the way it is and index my favorite tones once I start to match it with my guitar. Then I would have a better idea on what exactly I wanted out of it if I wanted anything out of it at all.

Any other mods you know of however will absolutely be considered. your heater hum trim pot mod is a great idea.
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Old 2nd February 2011, 03:06 PM   #7
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Quote:
Originally Posted by tokyoburns View Post
thanks for all the info RT. Have you built or modded this amp before? This project is pretty new in my head I haven't even begun to acquire parts yet so I haven't really any ideas on what i really want to mod. I have heard this amp before and I do love it. But I would not want to always be using an amp that i would have to turn down to get rid of the natural distortion in order to replace it with one from a pedal. So thats why I was asking about a way to give it more headroom. or rather a way that I can switch between its normal amount and more headroom. But I wouldn't actually begin to mod it until I build it the way it is and index my favorite tones once I start to match it with my guitar. Then I would have a better idea on what exactly I wanted out of it if I wanted anything out of it at all.

Any other mods you know of however will absolutely be considered. your heater hum trim pot mod is a great idea.
Sounds like you want a master volume? I guess a 500 K log pot between C12 and R35? (Not dead sure about the value )
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Old 2nd February 2011, 06:02 PM   #8
RTF671 is offline RTF671  United States
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Quote:
Originally Posted by Miniwatt View Post
Sounds like you want a master volume? I guess a 500 K log pot between C12 and R35? (Not dead sure about the value )
Not quite, replace R35 outright with a 1M log pot (aka audio taper). It will perform the same function and also give you a Master Volume which controls how loud the output stage will be.


I have not built this amp before but I'm a toob geetar amp lover
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Old 6th February 2011, 12:57 PM   #9
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Awesome that is a great suggestion. thanks.
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