Fender and Marshall Amp Mods

Fender and Marshall Amp Mods

Cautionary Statement:

Amplifiers contain voltages high enough to Kill you even after the amp has been powered Off !!! If you don't have enough electronic experience to identify the components which contain these deadly voltages, take your amp to a qualified tech. If you don't know a qualified tech in your area (look in the yellow pages under musical instrument or amplifier service or repair), post a request in the alt.guitar.amps newsgroup, the folks there can usually help you find one. If you have a question that you feel needs a personal answer, send me an email. The following mods are not for people who don't know what they're doing, and in fact, some of them are pretty complicated.

Fender amp mods

Silverface to Blackface Conversion Mod-
Starting in 1968, Fender began the move into the Silverface Era under CBS owned Fender. "Improvements" were made that were supposed to make the Blackface Era amps Cleaner and Louder. It's generally accepted that these changes tonally degraded the Blackface designs. This lead to many requests for Blackface conversion mods and kits.

There's good news and bad news for people who want to convert their silverface amps. The good news is that if you have an early silverface (68 to 76), you'll be able to regain much of the Blackface tone. It won't be exactly like a blackface due to things that can't be changed like wiring composition and lead dress. But it will greatly improve the tone of your amp.

The bad news is that, (generally speaking), if your amp was made after 76, the blackface conversion will do little to help your amp. This is when Fender started using more powerful transformers to increase their amp's volumes. To even begin to change this type of amp you'd need to replace the transformers which really isn't worth the expense.

The Parasitic Supressor Caps:

1. Remove the .002uf (or sometimes 1200pf) capacitors from the grids of the power tubes (pin 5). This gives you back the high end sparkle of the blackface era amps.

The Phase Inverter Differences:

1. Change the .001 Coupling Cap increased from back to the blackface era value .01 cap. This "redefines" the bass response that became overbearing during the silverface era.

2. Change the 330k Cathode Resistors on the grids of the Phase Inverter back to 1 meg. This lowers gain to the blackface era spec.

3. Change the 47k Plate Load Resistors back to either 82k or 100k. This also lowers gain to the blackface era spec.

The Reverb Differences: (*Note - Many "Prefer" Silverface Era Reverb)

4. Change the Reverb Driver (1.5k, 820, or 680) ohm unbypassed resistors back to the blackface era spec 2.2k resistors bypassed with a 25uf-25v electrolytic cap.

5. Remove the Reverb Driver Plate to Cathode Caps.

6. Remove the Cap to ground after the Reverb Tank.

The Bias Supply Differences:

1. Change the two 70uf Bias Supply Filter Caps back to a single 100uf 100v. This gives you back the ability to set the bias level.

This covers the major differences between the Blackface and Silverfacer era amps.

6L6 to EL34 Mod-
6L6 power tubes can be a little too clean for the tastes of many. One option is to use 5881 tubes. These are direct replacements for the 6L6 which require nothing more than a rebias after installation. They give a little less headroom, break up a little sooner, and add a little more grind to the amp.

Another option is to swap your 6L6's with EL34's. EL34's have a darker tone, the break up alot earlier than the 6L6, and their overdriven sound is radically different than a 6L6. If you're looking for a big change in tone, then this may be just what you're looking for. But EL34's ARE NOT direct replacements for the 6L6. You'll need to make a slight modification to your amp so it can use them (Some current Fender amps now have a simple switch on the back that allows you to bias between EL34 and 6L6 tubes).

1. Replace the 470 ohm resistor that's between pins 4 and 6 of each output tube socket with a 1000 ohm, 2 watt resistor.

2. Disconnect pin 1 only of the 1500ohm resistor that's between pins 1 and 5 of each output tube socket.

3. Disconnect the wires on pin 1 of the output tube socket.

4. Reconnect the wires that were on pin 1, to the end of the 1500-ohm resisors that are now standing free on each output tube socket.

5. Connect pin 1 to pin 8 of each output tube socket with a solid wire.

6. Increase the value of the resistor that's in series with the bias diode to reduce the bias voltage until you obtain proper bias. This will vary from amp to amp.

The good news is that after this mod is in place, you can use either EL34's or 6L6's just by adjusting the bias.

Harmonic Distortion Mod-
In small amounts Harmonic Distortion can actually be a Desirable addition to your amp's tone. This mod allows you to add or remove harmonic distortion at will, as well as allowing you to easily bias unmatched tubes.

1. Remove the single bias adjustment pot in your amp.

2. Install small cathode resistors and separate bias adjustment pots for each output tube.

Now you can slightly "Unmatch" the power tubes via the new separate adjustment pots to add harmonic distortion to your amp. And even if harmonic distortion isn't your thing, the extra adjustment pots allow for more exact and easier biasing of matched or unmatched power tubes.

Hum Reduction Mod-
Before you attempt this Mod... Make sure the hum you're trying to correct isn't the fault of worn out components. Filter Caps are the number one culprit for this malady. Transformers can also be the cause. If this isn't the case in your particular circumstance, you can try the following mod to help your hum issues.

1. Add a 270K/27K resistor divider from B+ to raise the filament windings above ground. This helps to reduce hum by keeping filament electrons from hitting the plate.

Design changes such as the above are rarely necessary to correct hum with mainstream designs from vendors like Fender, Marshall, etc... If you have a funky clone design from a vendor like Silvertone, National, etc... this may be needed. Otherwise, stick with verifying that the original components aren't going bad.

Fender Warm Up Mod-
Is your Fender amp just a little too bright for your tastes? Well, you can warm up the amp by making a change in a few component compositions and values.

1. Replace the 250 pf disc cap in the preamp section with a Mica cap of the same value.

2. Replace the .047 ceramic cap in the preamp section with a .022 Polypropylene cap.

3. Replace the .1 ceramic cap in the preamp section with a .022 Polypropylene cap.

You can try this mod in stages and see how you like the warm up as it progresses.

Fender Gain Mod-
If, like some people, you only use the Vibrato/Reverb Channel of your Dual Channel Fender amp. The following solder free mod will increase the Gain characteristics of your amp.

1. Simply remove the first preamp tube on the right (as you look into the back of the amp).

Tubes in a dual channel amp share a common power supply. The more tubes you have, the more current is drawn, and the more the voltage drops in the power supply. Removing the one tube drops the amount of current being drawn, the voltage rises in the power supply, and this in turn causes the remaining pre-amp tubes to run a little hotter, which effectively increases the overall gain.

Fender Midrange Mod-
This mod will increase the Midrange characteristics of your amp. The sonic difference is a fatter, fuller, more Marshall-esque tone. As always, let your own ears be your guide.

1. Replace the stock Ceramic Coupling Cap at the Phase Inverter's Input with a 0.01uF Polypropylene, Polystyrene, or Mylar cap. Let your ears be the judge as to which composition you like best.

An increase in midrange creates the sonic perception of a larger amp.

Fender Brightness Mods-
You may know changing component values in your amp can radically alter tone, but changing component composition can also change your amp's tone.

1. Replace the stock Ceramic 0.02uF Coupling Cap in the Vibrato channel's second gain stage with a Polypropylene or Polystyrene cap of the same value.

2. Replace the stock Ceramic Coupling Cap at the Phase Inverter's Input with a 0.001uF Polypropylene, Polystyrene, or Mylar cap. Let your ears be the judge as to which composition you like best.

3. Replace the 10pF Ceramic Cap that parallels the reverb/dry mix 3.3M resistor in the output of the Vibrato preamp's third gain stage, with a Silver Mica Cap.

4. Add a second 820 ohm resistor to the existing 820 ohm power amp feedback loop resistor to increase the power amp's gain. This smooths the transition from clean to overdriven tones.

These mods will increase the overall brightness and perceived clarity of your Fender amp. Do them one at a time and judge the difference for yourself at each stage.

Hotrod Champ Mod-
I'm not a big fan of hot-rodding vintage amps, but this mod is completely reversible with very little effort. Even so, I would still only recommend doing this on Silverface era Champs. Even though only the "cosmetics" changed between the blackface and silverface Champs, the collectible blackface amp prices are still much higher. So leave the blackfaces stock, eh?

1. Replace the tube rectifier with a "plug in" solid state rectifier. The tube rectifier, (over a solid state one) in a champ doesn't make much of a difference sonically other than reducing your overall volume. This is because a single 6v6 isn't enough to cause any noticeable sag in the 5Y3 rectifier. 1N4007 diodes can be used to build your own SS rectifier which will push the amp to its max capacity, (a little over 6 Watts). But be aware that old or dying caps will fail under the increase caused by your new SS rectifier. But the standard value caps (if they're in good condition) will handle it with no problem (basically, if they blow, they probably should have been replaced anyway).

2. Replace the 6v6 power tube with a 5881 for a little more headroom. The higher voltage of an SS rectifier also tends to wear out 6v6's much faster. With the 5881 this will never be an issue.

3. Increase the value of the Cathode Bypass Capacitor to 100v. This allows your champ to run "safely" under the increased load of the SS rectifier. Don't change the value of the cap, just the voltage rating.

The good thing about all the above tweaks is that they're all basically "pluggable" mods, with the exception of the bypass cap. (That's more of an upgrade to a better part than an actual mod anyway). If you don't like the way it sounds, you just put your 5Y3 rectifier and your 6v6 power tube back in and your back to a stone stock champ with no change in the original tone.

Hotrod Princeton Mod-
As I said before, I'm not a big fan of hot-rodding vintage amps, but most of this mod is more of an overall upgrade than a hotrod mod. Many of the amps Fender considered to be in its "student model" line (which included the Princeton) were built with the cheapest components possible to save on production costs. This mod simply upgrades those cheaper components to high quality replacements. Still, I would recommend this only being done on silverface era Princetons. The collectibility and value of the blackface era amps are usually good reasons to leave them bone stock.

1. Optional; Remove the original speaker (Princetons came with terribly inefficient 10" Oxfords). Try a Jensen or Celestion (or a JBL if you want, although some people like them, some people don't. It's up to you). Make sure the impedance/ohm value matches that of the speaker you removed.

2. Add two 470 ohm resistors to the screen grids of the 6V6 power tubes (this was a customary practice in all of the higher priced big Fender amps).

3. Using the available schematics, go through the entire power section and replace all of the components using the highest quality parts you can find. Make sure they match the original values exactly.

4. Finally, a slight inherent "mismatch" exists in the Phase Inverter section of the Princeton design. Matching this up will complete the mod.

Now your little Princeton will be virtually twice as loud and the overall tone will be much fuller and fatter.

Marshal amp mods

Marshall Cream Switch-
The following mod will give your Solid State Rectifier Marshall the "creamy" sag of a rectifier tube as well as that wonderful "compressed" sound of cranked/overdriven power tubes.

1. Wire a .022 25 volt ceramic disk cap on to a mini toggle switch (on-off).

2. Ground the switch and run the hot wire of the switch to the "In" (or "wiper") of the Presence pot.

NOTE: When this mod is "on" or "active" it will render the presence knob inactive. When the mod is off the Presence knob is restored to it's original operating specs.

Marshall JCM800 2203/2204 Mod(s)-
1. Locate the 68K input resistor (R3) and place a 330pf 1kv ceramic cap across it. This will beef up the mids and lows and make the entire amp feel more "meaty" and "thick".

2. Locate the 10K cathode resistor (R6 found on V1B) and replace it with a 4.7K resistor. This will beef up the gain. This seems to be the "sweet-spot" on these amps. If you want insane gain use a 1.5K.

3. Locate the 820ohm cathode resistor (R9 found on V2A) and place a .68uf cap across it. This will increase the gain a little, and give you a nice "Plexi" midrange boost.

4. Locate the 33K "slope resistor" which is in the EQ section (R15) and replace it with a 47K resistor. This bumps the amp's entire midrange frequencies up a notch. The result is a very "fat", "thick" or "beefy" tone that is really awsome.

5. Locate the 22K Midrange knob (VR4) and replace it with a 50K Liner or Log pot. This is for those who are seeking more flexibilty out of the midrange knob.

6. Locate the .68uf cap (C1) which is found on V1A and replace it with a .47uf cap. This alters the midrange response and adds a "heavier" tonal quality to the amp.

Note: All resistors are 1/2watt unless otherwise specified and caps/resistors are marked according to the Marshall schematics.

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