Best Fender Tweaks

[Gibsonman63]

Now my TW Express clone is complete, I have a much better idea of what a great sounding amplifier sounds like.

A little background.

I have a 1972 Super Reverb. I played it for years stock and then it sat around for about 15 years after I got my first Marshall amp.

Two or three years ago, I converted it to the AB763 circuit, everything but the reverb. It was my first "major" amp project and I was still getting feet wet. I did the usual overhaul and replaced all the electrolytic capacitors, installed metal film resistors on the power tube sockets, cleaned all the pots, switches and tube sockets. I also replaced many of the resistors with metal film and swapped (but kept) the Mallory caps with the orange drop caps. Transformers, speakers and everything else is stock other than cosmetics.

It is a decent sounding amp, but nothing compared to my TW clone. It is also dead quiet when you are not playing and has tons of power.

If I adjust everything at a single volume, I can get a pretty decent tone out of the amp, but not as clear as I would like. When I push the amp, the tone doesn't stay balanced and starts to sound shrill. I would like the amp to be more touch sensative and chimey at least on the reverb channel. I would also like to add more drive to the normal channel, so I can switch it with an A/B box and get my lead sound.

Off the top of my head, I am thinking of trying my SS rectifier tube replacement thingy, playing with the values of the tail resistors in the PI (although I could really use a starting point) and copying the V1 values from a Marshall schematic for my normal channel.

If you gents can point me in the right direction or post your favorite Fender mods, I will happily spend the day tomorrow swapping out componants and then report back. I have some mustard caps on order. For $30, I want to see what all the hype is about.

[billyz]

If you want more chime change the NFB to a 20:1 ratio instead , or the closest you can get. like a 2K instead of the 820 ohm feedback resistor. For a little more drive use a 25K middle pot instead of the 10k pot. If you just want to mod the normal ch, changed the fixed 6.8k middle resistor to 10-15K. You could do a marshall style circuit on the normal channel . Since you have already changed out the resistors to MF( I would not have) and changed the Caps to OD's ( I would not have) might as well change the treble cap to a Polystyrene 250pf or 330pf( makes the treble control a little fatter). Make sure your OD's are polyester not Polypropylene. The PP caps are too sterile and can make the amp a bit shrill when cranked. I would have used CC resistors throughout, I find they give a more woody flavor than the MF resistors, the CC's seem to have a better sounding distortion tone.

[selloutrr]

you should check out the Hip Desk Top Reference to Vintage Amps there is a section dedicated to the super reverb. alond with some nice Ken Fisher Mod's and silver face to black face conversions.

the Post PI Master Volume mod is awesome! really unlocks the amp.

[Gibsonman63]

Thanks for the responses guys. Now that I have a little experience under my belt, I am thinking that I did not make the best componant selection the last time around so it is not big to chalk it up to a lesson learned and change out a few resistor and capacitor types.

I have that book and I looked at GW's recommendations last night. I split the cathode resistor/capacitors on the first two pre-amp tubes. If it made a difference, it is subtle. I am not so sure about his other recomendations and I didn't see anything that helps to improve the chime or the touch sensitivity.

[billyz]

Thanks for the responses guys. Now that I have a little experience under my belt, I am thinking that I did not make the best componant selection the last time around so it is not big to chalk it up to a lesson learned and change out a few resistor and capacitor types.

I have that book and I looked at GW's recommendations last night. I split the cathode resistor/capacitors on the first two pre-amp tubes. If it made a difference, it is subtle. I am not so sure about his other recomendations and I didn't see anything that helps to improve the chime or the touch sensitivity.

Try the NFB mod I suggested before doing anything else. If I read you right It may be what your looking for. some use a control pot mounted to the rear instead of a fixed resistor. Try a 5K with the 820 fixed in line.

[Alexo]

Steal one of the 12AT7 stages from the reverb driver and use it to insert a cathhode follower between V1A and the tone stack.

[selloutrr]

add a grounded power cord and use the Ground switch hole for the master volume mod.

If you need capacitors check out Sozo they just reissed the "Blues"

Another book you might like is "Great Tube Amp and Guitar Mods" by Mike Thompson. I have to say if given the chance browse it first. some of his Mod's are not correct, others are useless, and his better ones are ripped off from KF. Ok nevermind forget that book...

I've had great luck with the GW book if you follow pages 139-143 you should have a very rocking amp! Any additional voicing I would suggest tubes.

Just take you time double check any modifications before you power up and enjoy your great amp!

[Gibsonman63]

I had a few minutes at lunch, so I put a 10K pot in series with the FB resistor. It's all I had on hand but good enough for a test. It is still subtle, but it takes the harshness off of the top. Nice elegant mod and super easy to do. I will order a 5K pot over the weekend. I plugged in my solid state rectifier and got a little more improvement. Also, I forgot that I left the original tone caps and resistors on the normal channel. It definately sounds better than the reverb side, especially with the chassis out of the amp and both sides being run dry.

This amp is a 72, so the grounded power cord was original equipment. I did remove the death cap a while back, though.

I'll post more findings tomorrow.

[Gibsonman63]

I wired the normal channel input and my PI to match my express. Not much difference really. I am still looking for the magic.

[guitarmike2107]

what is the magic.... try and explain what you are after in terms that translate into circuit design.

What type of sound are you after, tight cleans, great dirty, great on the verge of breakup.. .loose and vintagey.. tight and modern.
extended high frequency response(chimey) big lows, tight lows, scooped mids, strong high mids, strong low mids...

The options are almost endless.

For touch sensitivity you want to tailer the gain from one stage to the next, you generally want the last stage distorting first, the second last distorting straight afterwards etc etc. that give the most control from your guitar.
you can set this using pots between each stage.

if you have aTWreck and a Marshall dont turn that amp into another one of those, give it its own heartbeat.

I think you are also finding that some changes make very subtle differences... thats the way it is..

Mike

[Gibsonman63]

I am trying to find the voice of this amp. People talk about amps sounding "Fendery", but to me there really is no one Fender sound. My 65 Vibrolux reverb sounds clean and and chimey and as you push it, the midrange comes out more. My Twin Reverb is different animal. Even cleaner, more chime, but really tight bottom end. My BF Bandmaster, different still. Not much chime but clean rich harmonics. My SE Fenders, still different, but all good.

For now I am going to back up and get rid of the MF resistors and put some decent tone capacitors (which are backordered of course) in it and start from there.

Any tube recommendations?

[billyz]

One of the ingredients to any amps tone is the transformers. A 65 Vibrolux runs lower Voltages than a Bandmaster or Twinn etc. I usually prefer the lower/browner voltage. And then there is the actual construction of both the OT and PT. Core material etc. There is something to be said for a vintage OT. by 72 Fender had changed Transformer designs/suppliers and it a part of the overall sound. Not that they can't sound good but different to be sure.

A good friend blew his 66 vibrolux ot, we replaced it with one from Triode ( being cheap). It worked but the amp lost a lot of Mojo tone. That was all that was changed, the OT. now he wants to spend some bucks to get the tone back, so we either need an original OT or maybe one from MM.

What are your Voltages? By 72 I believe the super had a 5u4. If you run a gz34 your going to get 40+ more volts. It will change the feel of the amp. If you run Diodes you'll get even higher voltage. I would be looking to get around 440v +- on the plates.

If it has the master vol get rid of it or use a better Post PI type( I would lose it altogether) to get the classic fender tone.

And then there are the speakers. and the cabinet and the baffle.

good luck in your quest for the elusive .

[Gibsonman63]

Thanks much BillyZ. Fortunately, 72 was before the master volume sillyness. I will check the voltages against the schematic once I get back into it. Now I have some direction, I know I can play with the rectifier to get closer to BF voltages. I have the CTS speakers with the square backs on them, but really prefer the goldback Jensens in my other Fenders, so that is another potential area for improvement. I had to replace the speaker baffle during my restoration and I copied my Twin and Vibrolux baffle mounting and floated it. I have restored a couple of SF Fenders now where the baffel was glued in a routed slot in the cabinet. The original looked like someone fell into the amp, but on the bright side I got a pretty cool amp cheaply back when needed a better amp, but had little money to do it with.

There is still some wierd wiring in the amp. I got rid of the snubbing capacitors on the power tubes with no oscillation problem. The wire bundle between the board and the pots has a thin wire coiled around it that is grounded on one end for shielding I presume. I have also noticed that there seems to be a lot of extra wire in this amp. When my parts come in, I will double check everything against the AB763 schematic and layout and use my Vibrolux Reverb layout to get closer to the BF layout and then go from there on voicing.

[selloutrr]

If you are after the touch response I still suggest a post phase inverter master volume. It'll give you the ability to make the amp sing and then control the gain from the guitar volume pot. another option would be a powersoak/ power brake between the amp and speakers.

one thing you have to keep in mind with any amp. Just make it the best it can be but it's not going to be hugely different then the design of the amp. I guess you could rebuild the circuit but the iron has the signature tones. you might want to convert the output tubes to El34's or 5881. have you tried increasing and lowering gain in the preamp tubes to better suit your response and drive?

GAIN
12ax7=100
12au7=80
12at7=70

[billyz]

If you are after the touch response I still suggest a post phase inverter master volume. It'll give you the ability to make the amp sing and then control the gain from the guitar volume pot. another option would be a powersoak/ power brake between the amp and speakers.

one thing you have to keep in mind with any amp. Just make it the best it can be but it's not going to be hugely different then the design of the amp. I guess you could rebuild the circuit but the iron has the signature tones. you might want to convert the output tubes to El34's or 5881. have you tried increasing and lowering gain in the preamp tubes to better suit your response and drive?

GAIN
12ax7=100
12au7=80
12at7=70

i do like the sound of el34's in a fender circuit. but they do draw more heater current and will make the PT run hotter. A 5u4 also draws more current.

the layout on these later fenders is pretty bad, and can be much improved, even beyond a Blackface era amp layout, ie put the tone caps right on the pots.

the black/gold jensens are my favorite in a super reverb too.

Look over the Blackface conversion carefully, there are a few things that can get missed.

BTW a 12au7 = amplification factor of 20 not 80. less than a 6201. I am sure that is a typo.

And then there are the tubes. It never ends.

And then you die.

Z