5F4 - FAT mod

[bluesfendermanblues]

Dear all,

I'm new to this forum - this is my first post, but I have already read a lot of the 'old' posts.

I have recently build a 5F4 Tweed super clone - very close to stock, using hammond transformers and a chassis from TAD.
Speakers are a pair of Weber 10A125, (which I used to use in a RI Bassman 59 - so they are well played in)

As you probably all know, a 5F4 is a pretty simple and straight forward amp, and it worked flawless at first power up.

However, a 5F4 is one of the more bright tweed amps and is often referred to (on the clone companies websites) as the perfect humbucker amp. I'm a strat player, so what do a guy do?

I searched a lot of forums - this one included - in order to get some tricks to fatten up the tone. Have tried all tricks in the book:

After a lot of experimentation I have arrived at the following fat switch - (BTW I didnt 'invent' this feature, but am only posting to share my personal experience that this is a usefull to mod the tone stack in a 5F4):

Fat switch:
Installed an on-off-on switch with a 500p Mica cap and a 1000p Mica cap - between V1 and V2 below the chassis - that can put none of one of the caps in parallel with the 250p treble cap. It provides you with the following sound options:

Pos 1 (adding a 1000p): sounds like a big fat Tweed Twin 5E8a
Pos 2 (no extra cap): stock 5F4 circuit
Pos 3: (adding a 500p): sounds like a 5E5 pro

Cathode Bias Switch
I have also used the 'ground' switch for cathode bias with a 250 ohm parallel by a 50uF cap - this option sounds very close the sotck fixed bias option.

Paul C mod
Have also tried the Paul C mod, but IMO it didn't do anything for the tone of this amp (same goes for a new build 5E3). Paul C used this mod on a Princeton Reverb, but according the himself never on a tweed: http://www.firebottle.com/fireforum/...g=dlx&enter=go

Hope this info is usefull to some of the 5F4 cloners out there!

[Chuck H]

Thing is, that mod doesn't make the amp less bright, it just increases mids. Though this is also good for single coils used in a rock setting. If you do need to tame highs there are a couple of good ways to do it. One is to be sure you have 250k pots in your guitar. You really don't lose anything with this value on a single coil pickup except any harsh top end. You can install a small value cap (4.7pf or 6.8pf) on the first triode stage tube socket pins between the plate and grid. This seems to preserve the basic tone of the amp but kill beamy/spikey highs since in this circuit the high end attenuation is proportionate to how hard the triode is working. You can use a not so bright preamp tube, like the Sovtek 12ax7WA/WB. Perfectly good tube and very low microphonics, but not much top end. The difference is instantly heard. Shunt filters are also an option. A 2kV cap in series with a 20watt 6.8k resistor across the power tube plates. The cap value chooses the knee frequency. you can experiment with values from 1500pf to .02uf and choose what sounds right. And as a last resort there are shunt resistors to ground. Pick a spot in the signal path and place a capacitor from there to ground to shunt highs out of the circuit. Different points in the signal path have a slightly different effect and the value would vary between 100pf and .01uf.

[bluesfendermanblues]

You're absolutely right, the mod doesn't make the amp less bright as such.

My observation is that the stock 5F4 circuit has a lot of its preamp energy/gain around 2-3 kHz, which is determined by the 250p treble cap..

Taming/reducing the highs merely takes energy/gain out of the amp - I wanted keep the enegy, but move the 'spotlight' frequencies from the "icepick area" down to where they benefit a strat with vintage type pickups - similar to a parametric boost. IME Humbuckers work fine with the stock circuit.

You can achieve similar results with a mellow 12ax7 tube.

[bluesfendermanblues]

Chuck H, I tried a more frequency balanced tube. Much better stock sound.

[Chuck H]

+++
Because "vintage" style amps usually don't have top end bleeder circuits I find that such amps modded for extra gain can sometimes sound brittle and/or harsh. I usually try a Sovtek 12ax7wa as a first course since it is usually effective and doesn't require circuit modification.