Little tricks for getting what you want from Fender amps

[luau]

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.

[øøøøøøø]

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.

[panoramic]

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.

[Stereordinary]

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

[Jay]

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.

[øøøøøøø]

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.

[JazzBlaster]

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

[Fiddy]

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...

There's no reason not to.

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

[JazzBlaster]

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...

There's no reason not to.

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.

[UlricvonCatalyst]

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...

There's no reason not to.

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...?

[Orang Goreng]

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.

[Jay]

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

[Orang Goreng]

Hmm, the Gibbs has no models number printed on it, just patent numbers. Poo.

[Jay]

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.

[Jay]

Thought of another Fender mod... on the Deluxe Reverb (and probably any of the 'ab763' amps) the Rev/Vib channel uses a .02 coupling cap right after the tone recovery stage whereas the Normal channel has a .047. If you prefer the slightly beefier sound of the normal channel, replacing the .02 with a .047 makes the frequency response of both channels pretty much identical. This also could work backwards... so if you prefer the slightly thinner verb tone, then replace the .047 with a .02.