Turn the normal channel of you old Fender into a Plexi

[Kent S.]

Re: Turn the normal channel of you old Fender into a Plexi

Lew also spoke of changing out the 68k grid resistors, something to bear in mind is the when plugged into the high (1) input it has a 1meg to ground impedane and the signal travles thru both 68k resistors (see the way the two jacks are connected together) in parallel for a total of 34k on the grid;when plugged into the pad input (2) the signal goes thru one 68k and shunts off thru the other 68k to ground ... a 1R/2R ratio, which halves your voltage
( the 1meg to ground cancells itself out).
The down side is that when in the low input your input Z is 68k, not 1 meg, loading down your signal,just like a low valued pot would (explains why most people don't like using it even if they have to). Lew's suggestion is cool ... if you don't use the pad input. If you do, (say with hot HB, it's reasonable, although since it's your lead channel it kinda seems dumb that you'd be trying to get a pristine clean out of it) then what you wind up with is an input Z of 34k to ground, even more high end eaten up. So although his will give you a bit of a boost in the high side, your tone would be degraded even more than normal in the low side. Something to consider.

[Kent S.]

Re: Turn the normal channel of you old Fender into a Plexi

A lot of this Lew beat me to ... however, one thing that people still miss is that I've seen people change out the tone stack component values with Marshall's, (for trying to get a marshall clean, not dirt in this case) and forget that the cathode follower set up, that Lew mentioned, involves a very different source resistance (which shifts the frquencies of the tone stack).
The Marshall CF is like 1k3 compared to the Fender CC which is like 38k.
If you are happy with the gain of the common cathode (Fender), but wanted to mimic the tone stack of the marshall, you'd want to change the caps, and slope resistor ... but you'd have to change them in relation to 38k, not 1k3 ... see what I mean. Also the marshall hooks up the bass and mid controls a bit different than most fenders did, some tweeds have their tone stacks wired like the marshalls though, and they use a CC as well, although they used 12AY7 (no big there). On the typical fender the the CCW lug of the bass pot hooks to the CW lug of the mid pot;the mid cap connects there along with the mid pot's wiper, and the CCW end goes to ground. On a typical marshall the CCW lug of the bass pot conects to the CW lug of the mid pot; the mid cap hooks to the wiper (of the mid pot), and the CCW lug goes to ground. You'll have to see the schematic to see what I'm talking about.
Another thing you might want to consider is to look at the bassman, tweed, and BF/SF preamp schematics, look at there tone stacks, the twins has 10k pots for mid controls, the bassmans, tweeds had 25k (more mids when turned up),the bassman and tweeds have 1M pots for the bass control, the twin 250k( a bit more low end). Also the slope resistors on a lot of fenders were 100k, the bassman had a 56k,the brownface pre is a neat little circuit, it's a bit odd as to how it's set up the mid cap is a .02, but what becomes the bass cap is actually before the slope resistor (.05) and the mid resistor is only engaged at the bass controls maximum setting, which nullifies the mid cap pre se' ... look it up on fender's gearhead site, it's really neat what they are doing there.
The small mod I talked about earlier, as in running the switch end of the bright cap over to the slope R side of the treble cap. That's kinda a take off on the marshall idea (fender cap 250pF,marshall cap 470pF) ... with the treble control at 10 both caps are in parallel (120pf {bright cap} + 250pF {treble cap}) for a total of 370pf (which is brighter with upper mids a bit louder), the marshall cap is 470pF ... but the difference is that as the treble control is turned down the 120pF cap still bypasses it sending a brightness boost to the volume control, you're treble still gets turned down, but it gets some of the upper mids out of your face without sacrificing total brightness.
One thing on swinging the slope resistor though, you not only have hook a pot there, but you have to provide for a minimum amount ... the Marshall jcm 800 had one that has a 100k (linear pot) wired as a variable resistor, but it was in series with a 10k resistor; the maximum slope R was then 110k, the minimum, 10k; the standard value being 33k. If you wanted to do that with the standard 100k fender, then a 27k resistor and a 350k(linear) would get about the same swing (a tad more). The idea has special merit because if you don't go to install a mid pot in place of the mid attenuation resisitor (they are either 6k8 or 4k7), then you can still tune the split between the bass and treble controls (remember the treble control is acting as a balance control between your bass and treble, the bass control trims and shifts the bass shelf, that slope resistor tunes the entire crossover Fc of the two). Short version is that it would be very valueble in your current set up.

[Jeff Seal]

Re: Turn the normal channel of you old Fender into a Plexi

I had no idea there were so many here with rather extensive understanding's of thermioninc emission.....I guess it's addictive!!
Ultra-cool!!!!!
Jeff Seal

[Scott_F]

Re: Turn the normal channel of you old Fender into a Plexi

yeah, it should. I am going to go back and do it the way Lew was talking about. weber writes it up the same way in one of his books that I have.

[That90'sGuy]

Re: Turn the normal channel of you old Fender into a Plexi

Would this mod work on my silverface Vibrolux Reverb?

[Indie P Bass]

Re: Turn the normal channel of you old Fender into a Plexi

Like Lew said earlier...It takes more to making a Plexi Marshall preamp vibe than just changing the midrange set resistor on a BF preamp...The cathode follower and where the tone controls are located in both circuits makes the 2 preamps sound and act differently...if you compare the 2 schematics,you'd see that the BF's tone controls come very early in the preamp...The Marshall's tone stack comes up very late in the preamp circuit...Marshalls don't give you as much tonal adjustment because of where the tone stack is compared to the BF Fender circuit..Also,the BF Fender circuit is designed with "warm,clean tone in mind and a different tone and midrange shaping..The Marshall has a pretty decent clean tone,but it has more gain and grit and snarl because of it's differences compared to the BF Fender circuit...

John

not to mention, I did it to a 20 year old solid state squier bass amp....
it was cool that I morphed the tone like that, but.. it was horrendous, I'll keep that amp a clean amp, thanks.

[STRATDELUXER97]

Re: Turn the normal channel of you old Fender into a Plexi

I tried the whole, "Plexi" thing on my Champ15b... it didn't work, when I clipped the resistor off the chip board, it just went, "FUZZZAHZHZAHZAH," when I played through it... so... I soldered it back on and it's back to it's normal self.
I'm happy with my squier's tone, anyway. Probably the best sounding squier amp ever built.

Like Lew said earlier...It takes more to making a Plexi Marshall preamp vibe than just changing the midrange set resistor on a BF preamp...The cathode follower and where the tone controls are located in both circuits makes the 2 preamps sound and act differently...if you compare the 2 schematics,you'd see that the BF's tone controls come very early in the preamp...The Marshall's tone stack comes up very late in the preamp circuit...Marshalls don't give you as much tonal adjustment because of where the tone stack is compared to the BF Fender circuit..Also,the BF Fender circuit is designed with "warm,clean tone in mind and a different tone and midrange shaping..The Marshall has a pretty decent clean tone,but it has more gain and grit and snarl because of it's differences compared to the BF Fender circuit...It is very common to see a guy take his hand and run it across all controls on the Plexi circuit and set all controls to 10...The Plexi's tone controls are very subtle in how they effect your tone..

John

[Indie P Bass]

Re: Turn the normal channel of you old Fender into a Plexi

I tried the whole, "Plexi" thing on my Champ15b... it didn't work, when I clipped the resistor off the chip board, it just went, "FUZZZAHZHZAHZAH," when I played through it... so... I soldered it back on and it's back to it's normal self.
I'm happy with my squier's tone, anyway. Probably the best sounding squier amp ever built.

[STRATDELUXER97]

Re: Turn the normal channel of you old Fender into a Plexi

Cool Scott...My 66 PR has reverb on the normal channel,but it was done by me the "correct" way buy using 220k isolataion resistors between the 2 channels...Torres used to show people how to have reverb on both channels,but his original method was just pure stupid... Try changing your .1 and .047 tone stack caps to both .02...The only other mods I did to that channel was changing the cathode cap and resistor values,but the outcome was similar to yours...The channel is crisp,more open sounding,has reverb,and grows some teeth sooner on the volume increase...If you want a drawing from me on how to get your reverb the right way on the normal channel,let me know and I'll scan it to you buddy...Enjoy your amp...One of Fender's best combos.I've had mine for many,many years..Would never part with mine! feel free to bounce some stuff off me anytime..
John

Scott...I don't use the vibrato and added a midrange pot and a presence pot to mine...No extra drilled holes! A 10k pot replaces the 6K8 resistor...

[STRATDELUXER97]

Re: Turn the normal channel of you old Fender into a Plexi

This one is so freaking easy and totally reversible.... I can't take credit for it, but it works.

On the normal channel of my Pro Reverb, there is a 6800k resistor connected from one wiper on the bass pot to ground. Just snip it on one end, then move it away out of the circuit and your normal channel becomes a screaming gain channel.

I'm going to go back and run a ground line in place of that resistor when I get back in town. What I did worked, but I think it can get better.

With and ABY switch, I now have a nice lead gain channel. Try it. It works.

Cool Scott...My 66 PR has reverb on the normal channel,but it was done by me the "correct" way buy using 220k isolation resistors between the 2 channels...Torres used to show people how to have reverb on both channels,but his original method was just pure stupid... Try changing your .1 and .047 tone stack caps to both .02...The only other mods I did to that channel was changing the cathode cap and resistor values,but the outcome was similar to yours...The channel is crisp,more open sounding,has reverb,and grows some teeth sooner on the volume increase...If you want a drawing from me on how to get your reverb the right way on the normal channel,let me know and I'll scan it to you buddy...Enjoy your amp...One of Fender's best combos.I've had mine for many,many years..Would never part with mine! feel free to bounce some stuff off me anytime..
John

[Scott_F]

Re: Turn the normal channel of you old Fender into a Plexi

Yeah I'd buy it in a heartbeat just for the transformers alone.

I spent about an hour in Half Price Books and I scoreD! with a capital D.

I found two excellent old tube reference books, one with nothing but circuits, all sorts of circuits. Unimaginable how big this book is, like a dictionary but bigger and it cost me a whopping 20 bucks. Then I found another couple that were very cool, but not as comprehensive and paid 2.50, 4.00 and 9.99 for them. All of them were printed back in the 60's (last printing to date).

That was my first time in there. What a cool store.

[Curly]

Re: Turn the normal channel of you old Fender into a Plexi

would you guys buy a blackface 66 bassman for $200.

that would be a very good deal, IMO

[flank]

Re: Turn the normal channel of you old Fender into a Plexi

would you guys buy a blackface 66 bassman for $200....... AB165 circuit.....??? ive got a twin but im thinkin the bassman might be cool for some warmer jazz stuff....

[Curly]

Re: Turn the normal channel of you old Fender into a Plexi

haha ... I've read all the amp books, except Torres. I just don't quite have Scott's "can do" attitude.

I've read critical comments about Gerald Weber's books, saying he was just wrong electronically about some of his ideas. But, you know what? I found his book one of the easier to understand, and it seems more common sense to me _ especially the section on "how to make your tube amp sound better".

I'm still not mod crazy, though. I'd say get those BF amps running well, take care of any noise or filtering problems, replace any worn tubes, bias correctly, get the right speaker in there, and play it for a while first.

Re: Turn the normal channel of you old Fender into a Plexi

Thanks fellas! I can't wait to get back home. My first thing will be the negative feedback thing. I'm thinking of using that ground switch to engage the "normal" value, then when I flip it, I go to a larger cap to provide less NFB.

Lew, great suggestions buddy. YOu know I'm a newbie on this stuff and I love the mods!

Whatever I do, I'm taking pictures so I can put it back.


LEW! Why did they put that cap on my reverb jack? Was that a fender thing or do you think some tech did it while back?

Thanks Scott, I'm just a hack compared to my brother.

That must be a silverface Fender. The caps are to shunt high frequencies to ground and eliminate oscillation that could have been avoided with neater wiring.

That's one thing about those silverface Fenders...the wiring gets pretty sloppy and some use alot of excess wire, mostly from the preamp tubes to the circuit board.

I usually clip those caps off and remove them...but if you notice oscillation, you either need to correct the sloppy/excess wiring issues or put the caps back on again.

Gerald Weber says he's removed a foot or two of unshielded wire from the preamps of silverface Fenders, just by unsoldering one end and clipping off the excess and then resoldering. Just all the slop running from the preamp and reverb tubes back and forth to the circuit board...especially to the grid/input of the preamp tubes.

I've done it, and it does seem to tighten up the tone and quiet things down.

Did you do a blackface mod to the bias circuit yet? You should if the bias adjustment is currently a bias balance control rather than actually allowing th bias voltage to be adjusted.

I found the Gerald Weber books, the Dave Funk book and even Dan Torre's book, Inside Tube Amps, to all be a big help in getting a handle on this stuff and coming up with a personal understanding of how to picture how everything works.

I'm a real intuitive "tech"...I don't really understand electronic theory. But I know what all the parts do and how changing them one way or the other affects the tone.

Lew