Fender brownface vibrato in a stomp...does it exist? What about with JFETS?

[skiraly017]

There's something about a Fender brownface vibrato that is unreal. My Carr Rambler has a tube driven trem but it's not the same. Is there a pedal out there that captures the Fender sound? What if you subsituted the tube with a JFET as done in a couple of projects? I know RG just posted about using JFETS in Fender amps to free up a tube. Could this be done or am I thinking too far outside the box?

[markm]

I always thought of the EA Trem reminding me of the brownface trem.

[skiraly017]

I always thought of the EA Trem reminding me of the brownface trem.

I have to disagree. I built the EA Trem and have gone back to it several times just to be sure and that "feel" is just not there. FWIW, my basis for comparison is a '61 or '62 brownface Fender Pro. I'm very close to buying the amp just for the vibrato. It's evil, sexy, magical and swampy all at the same time.

[markm]

Well, I have to say it's been a VERY long time since I've been even near a brownface amp that I'm basing my opinion on bad memory! 
Buy it if ya can.....Real Nice amps those are! 

[Ace]

I haven't tried one, but the "Swamp Thing" was supposed to have that "Brown face" sound.

[markm]

I haven't tried one, but the "Swamp Thing" was supposed to have that "Brown face" sound.

Is that a Homebrew stomper?

[Dragonfly]

The vibrato in the brown face pro I used to have always sounded like a tremolo that had some slight "univibe-like" or "phaser=-ike" qualities going on "behind" it. At least thats *my* recollection of it.


AC

[Eb7+9]

I got the Fender Champ-Trem emulated with jFET's but I had to run the circuit at 18volts and use the Vibe style oscillator for getting good range ... haven't tried this signal path yet ...

[KB]

Look at Fender schematics with the 6G code

6G4 or 6Ga "Brown Super"
6G5 or 6G5a "Brown pro"
these two are my favorites. 

I have thought of trying to build a "Brown Super" just for the "Harmonic vibrato" as fender described it.  There was some discussion on Ampage way back when, about what made these amps sound different.  I cannot really remember what was posted and cannot find my notes now.  I seem to remember that they were a

The vibrato used two tubes (3 in the Ga models) and is a very complex design.  Other fender amps from this period used different methods like directly varying the bias, or the now common optosolator.

These amps sound very distinctive... I remember trying 6Ga Super in London eight or nine years ago the vibrato / trem is different from all the pedal type trems I have heard.  I have to agree with skiraly017 it is a very sexy sound... very addictive... especially at slow speed. 

Kevin

[Sir H C]

Those fenders tremolo the low frequencies the opposite polarity of the high frequencies.  So your low notes are out of sync with the highs after the trem.  A really cool complex effect.  I guess you should be able to do it with transistors as that stage is not supposed to be adding distortion and the like.  Also unlike the blackface/silverface amps the trem does not use LDRs, it is all varying the gain on the amplifying stages.

[Serge]

In the layouts gallery there is a schematic for the Blues Pearl Teja trem.  I thought that was supposed to do the Brownface vibe/trem thing

[stm]

Those fenders tremolo the low frequencies the opposite polarity of the high frequencies.  So your low notes are out of sync with the highs after the trem.  A really cool complex effect.  I guess you should be able to do it with transistors as that stage is not supposed to be adding distortion and the like.  Also unlike the blackface/silverface amps the trem does not use LDRs, it is all varying the gain on the amplifying stages.

This is very interesting information, Sir HC.
Where/how did you learn this?  I took a look at the suggested amp schems and it is not trivial to understand what's going on there!
Also, that description could fit in a 2-stage univibe, where a broad notch is swept either attenuating the lows or highs depending on which part of the sweep you are in.

[tcobretti]

So, is the part of the schem below that is outlined in red the entire vibrato part of the amp?

[skiraly017]

In the layouts gallery there is a schematic for the Blues Pearl Teja trem.  I thought that was supposed to do the Brownface vibe/trem thing

Nope. That's a slighly modified EA Trem.

[stm]

I found the following description:

"The Brown/Blond amps also introduced an effect sometimes known as 'harmonic vibrato,' a phase-shifting tremolo system that required two and one half 12AX7/7025 tubes but had a sweet swirl that has been imitated but not improved on in 40 years. Much of this tremolo's character results from the fact that it separates the low and high frequencies and applies the tremolo effect to each separately and out of phase."

Here: http://www.infoweb.co.nz/fender-amplifier-history

P.D.  Here is more on the same subject: http://www.fenderforum.com/forum.html?topic_number=327608

[R.G.]

The subject vibrato does a split of bass and treble. The split is with a simple one-cap, one-resistor filter, so the bass and treble sides overlap a lot. In addition, each side of the filter has a phase shift associated with it, an inescapable part of a filter.

Each side is then amplitude modulated by modifying the bias on a section of a 12AX7. This introduces little distortion because the both the plate voltages and the input compliance voltages on the 12AX7 are so high. The two sides are then amplified and inverted in the 12AX7 sections, giving another 180 degree phase shift, and added back together in a resistive summer.

The addition of varying amounts of the bass/treble split with the phase shifts of the one-time-constant filter splitter produces phase interference and reinforcements, like a phaser, but not as extensive. The frequencies and depths of cancellation/reinforcement move around as the amplitudes change, again somewhat like a phaser but not as focussed.

The design probably started as a way to make the control voltages cancel. The control voltages are out of phase for high and low frequencies, and the bleedthrough would tend to cancel in the resistive adder. I suspect that they tried this and found out it sounded good as well as cancelling bleedthough.

A quick and dirty way to do this would be to set up the following:

Input buffer feeds an inverter and a single pole RC filter.
Inverter feeds the other sex filter (high vs low pass)
The high and low pass filters are mixed into the summing node of an inverting opamp.
The input mixing resistors are in series with a JET, of the typical P90 types.
An LFO with an inverted output feeds the gates of the JFETs.

Setting up the JFETs is about the same as the JFETs on a P90. The only difference is the inverted LFO feeding one of them.

I suspect that this gets 90% of the sound of the original, maybe even close enough not to need the other 10%.

[Steben]

....Each side is then amplitude modulated by modifying the bias on a section of a 12AX7.....


...An LFO with an inverted output feeds the gates of the JFETs....
Setting up the JFETs is about the same as the JFETs on a P90. The only difference is the inverted LFO feeding one of them.
I suspect that this gets 90% of the sound of the original, maybe even close enough not to need the other 10%.

Actually you can use bias modulation in FET's in the fetzer way almost going to the zero bias end, it is, however a little more tricky, since you have to detect Vgs,Idss,...

[stm]

......
A quick and dirty way to do this would be to set up the following:

Input buffer feeds an inverter and a single pole RC filter.
Inverter feeds the other sex filter (high vs low pass)
The high and low pass filters are mixed into the summing node of an inverting opamp.
The input mixing resistors are in series with a JET, of the typical P90 types.
An LFO with an inverted output feeds the gates of the JFETs.
......

RG, I looked in more detail at the 6G5, 6G14, 6G15 shcems available at the FIS, and don't see any phase inversion between the lowpass and highpass filters.  Apparently the extra inverter you mention wouldn't be needed, or if installed would produce a different effect from what we have.

Also, now I understand why such circuit is considered to be "richer" or "fuller" sounding in comparison to an LDR vibrato/tremolo; the operation of the gain varying stages is inherently nonlinear and adds odd-ordered harmonics.

Actually you can use bias modulation in FET's in the fetzer way almost going to the zero bias end, it is, however a little more tricky, since you have to detect Vgs,Idss,...

Steben, yes, now I understand the principle of operation and can clearly "see" how to extend this technique to the Fetzer Valve, and in general, a JFET amp stage.  Of course is highly recommended to fine tune the circuit to the particular Vp and Idss of the JFETs in use, and also a good idea to use a matched pair for best gain and control voltage cancellation.

[R.G.]

RG, I looked in more detail at the 6G5, 6G14, 6G15 shcems available at the FIS, and don't see any phase inversion between the lowpass and highpass filters.  Apparently the extra inverter you mention wouldn't be needed, or if installed would produce a different effect from what we have.

That's correct, there isn't any signal inverter in the stock circuit. I put that in there because I believe that it would produce a more intense effect. In fact, a switch to feed the filters from either just the buffer or buffer plus inverse would be a good idea.

[R.G.]

Wait a minute. I told you wrong. The filtering is not single-pole high and low pass. It's closest to the two halves of a twin-T. That makes for an even kinkier phase and amplitude response.

The result is that the response wiggles between bass heavy to mid-notch to treble heavy and back again.  It seems to wobble a mild notch filter around 300Hz, transforming to highpass or lowpass once it's much past 300 in either direction.

I think the other stuff is correct.