Low tremolo output

[Paul P]

I've been trying to find out why the output of the tremolo/vibrato channel in
my amp (brownface clone) is so much lower than the clean channel when
I've heard from two separate sources now that it's supposed to be the other
way around. All my voltages are very close to the ones on Fender's schematics.
The sound of the tremolo circuit seems fine in every way except for the low
volume.

Here is the output part of the tremolo circuit with voltages. Red is Fender
and my DC voltages and blue is my AC voltages for a 150mv sine wave input
at 2 Khz :



The bottom triode is low because of the 2 khz input. This triode handles
the bass part of the tremolo effect. However, even at 800 hz it's still no
higher than 269 mv ac. I don't think this is a problem because sound-wise
the frequencies of the effect are nicely balanced.

I've tried playing with the 4.7k cathode resistor. Raising to 6.8K had little
effect on the output other than causing more clipping of the signal. This
resistor raised the bias voltage a bit to 3.8 vdc and raised the plate
voltages to around 345 vdc.

I tried a 1500 ohm cathode resistor. This increased the output to a max of
9 vac but the tremolo effect was diminished. Bias voltage went down to
2.6v and the plates dropped to 282 vdc.

I tried replacing the 470k resistors with 270k ones. This caused an imbalance
in the plate voltages (317,340), produced a max output of 8v, but was
clipped more.

I measure about 250K ohms to ground at the junction of the two 470K resistors
but this value goes up quickly, must be the filter capacitors.

I tried increasing the output of the preamp to the tremolo circuit, easy to
do since there's a voltage divider at the output of the preamp on this
channel, but that just overloaded the tremolo circuit.

On the other hand the clean channel preamp puts out a max of around 75 vac
before going through a 220K mixing resistor (the tremolo channel channel
doesn't have one) which brings things down to around 16v or so (I don't
have the exact figure because I switched the output of the clean channel
to go to the reverb circuit).

It seems that the two 470K both work well to keep the two sides separate
but also really cut down the signal level. Is it normal that they cut it down
so much ?

What is the normal AC voltage range of a signal in a brownface amp right
before the .001 PI coupling capacitor ?

Any help would be much appreciated. I have a signal generator and a scope
to help see what's going on.

Paul P

,

[Paul P]

I went over and over this circuit and came to the conclusion that
there wasn't anything wrong with it. It just wouldn't go as loud as
I'd like without bad things happening. So I cashed in my extra triode
coupon and added a gain stage after the circuit.

I now have volume coming out of my ears . I have to throw most
of it away but my amp will go as loud as I want, way louder than
the normal channel. I'm starting to think about adding a master volume...

Paul P