The Fender circuit

[matt h]

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[beasleybodyshop]

Matt, thanks for the concise explanation! It makes a whole lot more sense than the idea I had in my head

[shocki]

Ok. I changed the first channel complety to a plexi (or 5f6) circuit now. I also use an AX7 PI and the feedback with a presence control. The only thing I did not change yet is the tail resistor.

I still have the original blackface channel.

When the amp is not turned up I have to say the channels sound different, but not extremly different. I was suspecting a much bigger difference. The 5f6/Marshall channel has a bit more mids and tighter bass. But it is really not like night and day.

When pushed the Bassman channel has much more gain on tap. I use as 820 Ohm on the first cathode in the Marhall channel but maybe 1.5k would be better here. Since the 820 in the Marshall is split cathode with 820Ohm.

I don't have Marshall right now. This amps sounds really good. I would say as a 5f6 Bassman. But to me it still does not have this really tight crunch sound I am used to from Plexi amps.

I agree the tubes do not make a huge difference cause I heard really good Marshall kind of amps with 6l6s. So what else could it be? The voltages or transformer?

[matt h]

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[Firestorm]

So are you looking for a 5F6A channel, a JTM45 channel, or a Plexi channel? They aren't the same. The early JTM45 was a straight copy of the 5F6A (except for ECC83s), but later models had some tweaks. One problem you might have is that 5F6A and the Marshalls mixed the channels after the first gain stage. On the Marshalls, they eventually had 500pF bright caps across the volume control on the bright channel (which you could do for more upper mids and treble with the volume at less than full) and another 500pF "treble peaker" across the 270K or 470K mix resistor for that channel, which, assuming you're mixing your channels just in front of the PI, you can't exactly do. (This amp started life as a Blackface Bassman, right?).

Are you using a .68uF bypass cap on the first gain stage cathode of the "Marshall" channel? An 820R cathode resistor here would be a Plexi value. Fairly warm bias and not screamingly bright. On mid-1969 and later Marshalls, this became 2K7. Fairly cool bias and brighter. As Matt said, you should experiment with that.

[martin manning]

Basically a straight copy, But there was a ~3x difference in the GNFB between the JTM45 and the 5F6-A.

[Firestorm]

Basically a straight copy, But there was a ~3x difference in the GNFB between the JTM45 and the 5F6-A.

True that. And Ken Bran was damn proud of it, too.

[martin manning]

Yep; best dumb-ass mistake he ever made! ;^)

[shocki]

For the tweed, yes, you'll want to change the 820 to a 1.5k, you may even prefer a 2.7k (which is yet another doubling in value, notice a trend?) for the first stage for the more "plexi" crunch you're looking to hear. Did you include the "treble-peaker"?

The Marshall I actually build and really liked was this one.
http://www.ceriatone.com/images/layoutP ... iatone.jpg

I used the "normal" channel and it sounds perfect. Not muddy at all. So I did not include the "treble-peaker" but I might try it.

When I am using this calc I notice a slight change in bass response. But nothing that would give me a crunchy sound.

http://www.ampbooks.com/home/amplifier- ... capacitor/

.. and another 500pF "treble peaker" across the 270K or 470K mix resistor for that channel, which, assuming you're mixing your channels just in front of the PI, you can't exactly do. (This amp started life as a Blackface Bassman, right?).

I am using 2 mixing resistors. One 470k/470k in the "Marshall" section where one of them goes to ground so the circuit is like the Marshall. And the "real" mixing resistor 220k/220K for mixing the channels later. So I could use this and will try it.

Are you using a .68uF bypass cap on the first gain stage cathode of the "Marshall" channel? An 820R cathode resistor here would be a Plexi value. Fairly warm bias and not screamingly bright. On mid-1969 and later Marshalls, this became 2K7. Fairly cool bias and brighter. As Matt said, you should experiment with that.

I am using 25uf (the Marhall Bass even uses 320uF). Going from 820R to 2k7 means getting less lows but not more treble? Correct? Eventhough it sounds like getting more treble. So this has no effect to the gain?

Actually I can reduce bass with either the cap value or the resistor value? Or does this have a different effect?

[matt h]

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[shocki]

Thanks Matt. I will just play around with the values.

[shocki]

It may be unpopular advice, but stop using the calculators. I've personally found that site and its calculators to be useless. Not that the calculations are wrong, but a lot of the supporting information is so incomplete as to be totally misleading. Sure, they may give you some idea of what's going on, but they fail to show you how that relates to the circuit as a whole. Please also keep in mind what I told you before, "EQ is relative". What you see as a reduction in bass is also a boost in upper-mid and treble... this is the effect of turning the amp up "louder".

Ok. I played around with the resistor values now. And I really think it does exactly what the calculator says. I hear a very slight change in bass responsive. Nothing else.
I added a 250pF cap cross the mixer resistor. This is a big (!) difference now and really gives me much more Marshall flavour. This and the addition of the presence pot did really alot getting the sound. I really like how the amps sounds now. Thanks for all the help.

[matt h]

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[shocki]

As you've just discovered, a treble peaker in your mixer was a big part of the missing magic. Betcha didn't think you needed more high end content before, eh? hehe

Yes. And now I need to add third gain stage. I would really like the Friedman Brown Eye mod But I think it would ruin the "plexi" channel. Even if I make it switchable.

So a better option would be a new amp. With one channel Plexi and the other one "hot rodded". Like this one.
http://friedmanamplification.com/produc ... /smallbox/

But modding the bassman was a great learning experience for me !

[matt h]

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