Little tricks for getting what you want from Fender amps

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Little tricks for getting what you want from Fender amps

Post by øøøøøøø » Mon Mar 08, 2010 10:03 am

Someone requested this, so I started thinking about the little things you can do to maximize the sound of Fender amps. Not talking about "mods," per se-- just little things you can do to make sure your amp is giving you its best. I won't talk about "mods" here-- that's way too broad in scope and could go on forever! I'll say I won't add anything that requires changes to the circuit

Feel free to add your own, or perhaps I will remember some more and add them later.
  • Match PI resistors on cathodyne amps. The 56k resistors on the cathodyne phase inverter in amps with that design (like tweed bandmaster, super, pro, blackface or silverface Princeton, etc) should be matched. This is the one that started it, when I mentioned it on another thread. The closer these are in value, the more evenly matched your PI output will be, and the more balanced the signal between your two output tubes will be. For example, if these two resistors are really far off, using a "matched pair" of tubes doesn't do much good. I've noticed that if they're real far off, the general sound of the amp improves in my opinion when they're matched, as a rule.
  • On a brown, blonde, blackface or silverface amp with two channels, remove the preamp tube of the channel you're not using to increase gain on the channel you are using. If you use "normal" channel, remove V2. If you use "vibrato" channel, remove V1. The cathodes are tied together, so removing the unused tube changes the bias point of the tube you are using, giving you a little more gain in that channel.
  • Turn vibrato on on a black or silverface amp without a footswitch by making a shorting plug. Get a cheap RCA cable, cut the plug off leaving about a half inch of wire. Short the center conductor to the shield and solder it. When you plug this into your footswitch jack, your vibrato is "on."
  • Get control over your dwell, mix, and tone for your reverb on a two-channel BF or SF amp, giving you outboard-reverb type control within your amp--without mods. It's simple. Get a cord that has an RCA male connector on one end, and a standard 1/4" male connector on the other end. Plug the RCA end into the output of your reverb tank, and bring the cable over the top of the amp and plug it into the "normal" channel. Now your "normal" channel functions as your reverb recovery! Your "reverb" knob is now your "dwell." Your normal channel "volume" is now your "mix." Finally, the tone controls on the normal channel (bass and treble, or bass, mid, and treble) are your 2- or 3-band "tone" control.
  • Temporarily and totally reversibly sacrifice your reverb on a BF/SF amp in favor of a footswitchable gain boost/"distortion" function without modding the amp-- Get 2 RCA jacks and a 470k resistor. Connect the resistor between the 2 jacks, insulating the leads so they do not short to ground. Now plug this whole contraption in between your "reverb input" and "reverb output"-- it replaces the reverb tank itself. Now your reverb driver just overdrives the reverb recovery tube, and the amount of distortion can be adjusted using the "reverb" control knob, and it can be turned on/off with the reverb footswitch. You may or may not like the sound of this particular distortion-- honestly it's not my favorite-- but it's totally easy to experiment with.
OK, and some easy circuit changes/mods that require only a part change or two
  • disconnect negative feedback-- this is an easy one. There's a wire that usually connects to the "ext speaker" jack that goes back to the board. Disconnect this wire and tape it off, and you've disconnected negative feedback. Expect a rawer, louder, grittier, more midrangey tone. if you don't like it? Solder it back. Or you can do like I did and route it through the now-defunct "ground" switch on a BF or SF Fender to make it switchable. Warning-- doing this on an amp with a 'presence' control will make the presence control inoperable.
  • Move tone capacitors onto the pots themselves on a BF/SF amp. Tweed amps have them located here. It will look uglier, but will allow you to remove feet of wire from the amp, likely opening up the sound and making the amp less prone to stray capacitances. If you look at the layout, you see that long wires run from the pots to the board and the tone caps, and then back to the pots again. The only benefit of this approach is looks and ease of repair.
  • Modify the reverb on a BF or SF amp for more of a 'textbook' reverb sound. If you just want a simple mod to make the reverb less "surfy" or "splashy" and allow the control to have greater usable range, locate the 500 PF cap in the following location: Off of pin 6 of V2, there is a .02µf cap on the tag board. The other end of this cap attaches to a 500pF cap, the other end of which attaches to a 1M resistor. If you replace this cap with a smaller value like 250pF or even 100pF or even smaller if you want, which will reduce the overall amount of reverb. You can also replace this cap with a 2.2M resistor which will give a smoother, less springy response. Finally, you can replace the 100k linear reverb pot with a 100k audio, which will change how quickly the reverb comes in as you turn up the knob. Personally, the weirdness/quirkiness of the old Fender spring sound is part of the Fender character and I'd hate to change it. But some people want more 'ordinary' verb, and this'll do ya.
  • Fix power tube bias on your Champ. On a champ or vibro champ or bronco, the 6V6 tube is biased all fucked up. Replace the 470Ω resistor and its 25µf cap with a 1k resistor and 50µf cap, and your tube will last longer and you'll probably get more headroom, as well.
  • On a Deluxe or Deluxe Reverb, you can replace the 6V6 tubes and 8Ω speaker with 6L6 tubes and a 4Ω speaker. The PT can handle the increased filament current draw, usually. If you're worried about it, use a plug-in solid-state rectifier, too, instead of the 5AR4. The 6L6/4Ω combination uses the same output transformer and might get you a touch more clean headroom. If you use the solid-state rectifier, too, your increase in headroom will be noticeably more. Especially if you select a very efficient 4Ω speaker, you can really turn a DR into a good clean machine at higher levels than stock. The tube swap will require a re-bias, of course.
That's all I can think of for now. Hope that helps.

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Re: Little tricks for getting what you want from Fender amps

Post by luau » Mon Mar 08, 2010 10:17 am

On SF Vibro Champs/Broncos, and maybe others, clip out the oscillation suppressor cap that's connected to the 6V6's screen grid and cathode (pins 5 & 8 IIRC). This opens up the top end a bit.

edit: Credit to øøøøøøø on this one as it arose out of a discussion we had here sometime back.

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Re: Little tricks for getting what you want from Fender amps

Post by øøøøøøø » Mon Mar 08, 2010 10:19 am

luau wrote:On SF Vibro Champs/Broncos, and maybe others, clip out the oscillation suppressor cap that's connected to the 6V6's screen grid and cathode (pins 5 & 8 IIRC). This opens up the top end a bit.
that applies to nearly every silverface amp! But be careful--- most times it's okay, but occasionally it can trigger a parasitic oscillation that will require re-routing of wires.

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Re: Little tricks for getting what you want from Fender amps

Post by panoramic » Mon Mar 08, 2010 10:23 am

when i can't get mine to sound right I just kick it. then I hear a cool clang. Well this was the answer when i was 17 and my vibro champ would sound weird.
i used to be cool

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Re: Little tricks for getting what you want from Fender amps

Post by Stereordinary » Mon Mar 08, 2010 11:57 am

I can get moar head room out of my Champ?! :wtf:

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Re: Little tricks for getting what you want from Fender amps

Post by Jay » Mon Mar 08, 2010 2:40 pm

In a lot of Champs, 1K may be a little too far to go (biased cold) with the cathode resistor. I did some math and came up with about 860Ω IIRC to get the bias just right in the last VC I worked on. So I simply sorted through a bunch of 910Ω resistors until I got one a little low. It all depends one what sorta voltage you're getting inside. I'd also suggest adding a screen grid resistor in the Champ/VC as well. Anywhere from 470Ω up to 1.5k works well and will progressively increase headroom slightly while making for a little tighter OD tone. Another good champ mod is to replace the lone .02 coupling cap with something other then a ceramic (BF and early SF) or ugly poly (later SF). It's not going to change the character of the amp completely but it can make a noticeable difference. You can also bypass the tone stack completely but beware, this makes for some serious gain and requires adding in a coupling cap. Sounds kinda cool though.

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Re: Little tricks for getting what you want from Fender amps

Post by øøøøøøø » Mon Mar 08, 2010 11:10 pm

All good info, Jay!

470Ω is WAY too low, in any case. I usually just suggest 1k because it's easy to find, and is 'closer.' But the actual value will depend on the plate voltage in that particular amp. The cathode bypass cap should also be increased in proportion to keep frequency response roughly the same.

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Re: Little tricks for getting what you want from Fender amps

Post by JazzBlaster » Tue Mar 09, 2010 12:05 am

I think I'm gonna try and build a champ after I get my car paid off.

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Re: Little tricks for getting what you want from Fender amps

Post by tribi9 » Tue Mar 09, 2010 12:44 am

JazzBlaster wrote:I think I'm gonna try and build a champ after I get my car paid off.

With all the info available to us now... :jacked:

There's no reason not to. ;D

Do it, one of the coolest thing a musician can experience is firing up an amp he/she just built it.

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Re: Little tricks for getting what you want from Fender amps

Post by JazzBlaster » Tue Mar 09, 2010 1:01 am

tribi9 wrote:
JazzBlaster wrote:I think I'm gonna try and build a champ after I get my car paid off.

With all the info available to us now... :jacked:

There's no reason not to. ;D

Do it, one of the coolest thing a musician can experience is firing up an amp he/she just built it.
Yeah, I've got more time these days. Soon I'll have a lot more money.

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Re: Little tricks for getting what you want from Fender amps

Post by UlricvonCatalyst » Tue Mar 09, 2010 1:18 am

JazzBlaster wrote:
tribi9 wrote:
JazzBlaster wrote:I think I'm gonna try and build a champ after I get my car paid off.

With all the info available to us now... :jacked:

There's no reason not to. ;D

Do it, one of the coolest thing a musician can experience is firing up an amp he/she just built it.
Yeah, I've got more time these days. Soon I'll have a lot more money.
There's a guy in the UK who sells Epiphone Valve Jrs rebiased to Champ spec.
I'm a lost cause when it comes to electronics, but apparently it's quite an easy (and cheap) mod and could be a good dry run if you don't feel 100% confident about building from scratch.
Worth a look...?

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Re: Little tricks for getting what you want from Fender amps

Post by Orang Goreng » Tue Mar 09, 2010 5:26 pm

øøøøøøø wrote: [*]Temporarily and totally reversibly sacrifice your reverb on a BF/SF amp in favor of a footswitchable gain boost/"distortion" function without modding the amp-- Get 2 RCA jacks and a 470k resistor. Connect the resistor between the 2 jacks, insulating the leads so they do not short to ground. Now plug this whole contraption in between your "reverb input" and "reverb output"-- it replaces the reverb tank itself. Now your reverb driver just overdrives the reverb recovery tube, and the amount of distortion can be adjusted using the "reverb" control knob, and it can be turned on/off with the reverb footswitch. You may or may not like the sound of this particular distortion-- honestly it's not my favorite-- but it's totally easy to experiment with.[/list]
Great post again, Brad :).

Hmm, sounds like this is exactly what was done to my old Vibrosonic. Interesting... I have an old Hammon-Gibbs reverb tank lying around, which looks like the Fender Accutronics unit based on it.. it came from a Carlsbro 100W head. Could I hook that one up to my other (stock) vibro to check if it works, or is that unsafe somehow? Because if it is, it may be very easy to restore it, although indeed I don't know if I want that, as it is quite a nice sound indeed, in no way similar to a regular silverface with the channel gain turned full up. If I play my SG through it, there's no need for an OD pedal to come close to my regular sound, but nicer.
IIRC it also has a 12AT7 swapped out for a 12AX7 somewhere, probably in that same circuit.
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Re: Little tricks for getting what you want from Fender amps

Post by Jay » Tue Mar 09, 2010 7:35 pm

Check the model number on the Gibbs unit. It might be a drop in it might not—it depends on the input/output impedance of both. I imagine your Vibrasonic uses an 8Ω/2250Ω like most BF/SF Fenders but you'd need to find out for sure. Here's the model designations broken down...
PART NUMBERING SPECIFICATIONS FOR TYPE 1 & TYPE 4

EXAMPLE: 4EB2C1B

DIGIT #1 - REVERB TYPE
1 for Type 1 or 4 for Type 4

DIGIT #2 - INPUT IMPEDANCE
A = 8 Ohm
B = 150 Ohm
C = 200 Ohm
D = 250 Ohm
E = 600 Ohm
F = 1475 Ohm

DIGIT #3 - OUTPUT IMPEDANCE
A = 500 Ohm
B = 2250 Ohm
C = 10000 Ohm

DIGIT #4 - DECAY TIME
1 = Short (1.2 to 2.0 sec)
2 = Medium (1.75 to 3.0 sec)
3 = Long (2.75 to 4.0 sec)

DIGIT #5 - CONNECTORS
A = Input Grounded / Output Grounded
B = Input Grounded / Output Insulated
C = Input Insulated / Output Grounded
D = Input Insulated / Output Insulated
E = No Outer Channel

DIGIT #6 - LOCKING DEVICES
1 = No Lock

DIGIT #7 - MOUNTING PLANE
A = Horizontal Open Side Up
B = Horizontal Open Side Down
C = Vertical Connectors Up
D = Vertical Connectors Down
E = On End Input Up
F = On End Output Up

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Re: Little tricks for getting what you want from Fender amps

Post by Orang Goreng » Wed Mar 10, 2010 1:30 am

Hmm, the Gibbs has no models number printed on it, just patent numbers. Poo.
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Re: Little tricks for getting what you want from Fender amps

Post by Jay » Wed Mar 10, 2010 7:56 am

That's odd. I have a late 60's Gibbs tank I pulled from my Deluxe Reverb sitting on my desk, I'll take a look and see if I can find a code on that one.

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