Trying to design a two channel Fender amp -- need some advice

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thetasigma
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Trying to design a two channel Fender amp -- need some advice

Post by thetasigma » Fri Jul 09, 2021 5:33 pm

I know this is the place pretty much for Marshall, and rightfully so, however I have a few questions for the experts here. I have an empty chassis, but I was thinking of designing creating a circuit that's more of a tweed 5F6-A Bassman for the normal channel, and a tweed 5F5-A Pro for the bright channel. I have designed it so far where V1A would be the Bassman input with a 100uf/1k5 cathode and V1B would be the Pro input with 25uf/15 cathode; plate resistors are 100k and 22nf coupling caps top the volume controls. I have tried to design two separate cathode follower 12ax7s for independent tone control. An issue I am running into, is how to combine these channels into the phase inverter, which is a bassman longtail PI with a 22nf input coupling cap. It seems I should utilize some mixer resistors before the PI input cap so as to separate the two tone channels. Can someone give me some advice on this, and I know this is a pretty low level explanation of what I'm trying to do? I don't have schematic, but do have a Visio wiring diagram I mocked up which I can save to a PDF if that would help?

Any advise most appreciated!
-Nathan

"Eagles may soar, but weasels don't get sucked into jet engines."

danman
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Re: Trying to design a two channel Fender amp -- need some advice

Post by danman » Fri Jul 09, 2021 6:59 pm

I couldn't find a 5f5-a schematic but I did find a 5e5 schematic. If you are looking to build a 5e5 channel, this circuit uses a different type of phase inverter than the 5f6a so you won't get the exact sound in that channel. If you don't plan on using NFB in your circuit, you can run each of your preamp channels to each side of the 5f6 phase inverter just like Vox did in the AC 30. If you do plan on keeping the NFB circuit, you will need to mix both channels just before the PI input cap with a couple of 220k resistors. The other side of the PI is being used to inject the NFB so you can not connect to it. The NFB circuit is a big part of the sound in both of these so you will probably want to keep. Each of these amps uses different values in the NFB circuit so you will need to make some compromises and pick a resistor value that sounds best to you.

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thetasigma
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Re: Trying to design a two channel Fender amp -- need some advice

Post by thetasigma » Fri Jul 09, 2021 11:10 pm

Thanks, that is the exact info I needed. I am planning on using the long tail phase inverter of the Bassman, it's really just want the tone stack of the Pro on channel two, I just needed info on how to have both channels be injected into the PI. I have to ask though, do the mixer resistors have to be such high value like 220K into the PI, or can they be of lower value? I ask only because I want to try and maintain the great breakup of a tweed amp, and don't want to make the amp overly clean.
-Nathan

"Eagles may soar, but weasels don't get sucked into jet engines."

danman
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Re: Trying to design a two channel Fender amp -- need some advice

Post by danman » Sat Jul 10, 2021 8:02 pm

Lowering the value of the 220k's will begin to load down the preceding stage and affect the way it works. Post #2 from this link explains it a bit better than I could.
https://el34world.com/Forum/index.php?topic=24890.0

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