Fender Custom Vibrolux (Distortion Problem)

[mididoc]

Hello,

we have a Fender Custom Vibrolux Type CSR 8 Serial Number AB007538.
The device has a terrible distortion in both channels.

We find at TP17 instead of 17.6 V DC about 70 V DC.
This is the cathode of the driver tube, last stage before power tubes.

I have attached the schematics as a pdf.

Custom_Vibrolux_Reverb_0A4.pdf

Could someone please be so nice and give us an idea, what could be the reason?

Thanks in advance

Cheers

[sluckey]

I think the 17.4v shown on the schematic is wrong. You should have 70 to 100v at that point.

[mididoc]

Hi sluckey,

thanks for your reply.

Indeed I'm measuring something over 70 V (about 75).
Also I found no reason for a difference between 17.4 and over 70.

The cap from the anode of the tube before is well,
also the other resistors around have their values.

However,
I'm having terrible distortion when volume pot set to 2 - 3 and higher for both channels,
changing the tubes didn't change anything.

verified also the fixed bias value of the power tubes,
which is ok with about 35 ma for both tubes.

i really get stuck with this amp.

cheers

[stiltamp]

The Voltage of 70V DC is ok , in comparison with other Fender schematics.
Perhaps there are twisted numbers and a value of 71,4V was meant.

[mididoc]

thanks stiltamp,

may be they have really used 17 instad of 71.
ok i keep on searching for the reason for the distortion ...

cheers

[Stevem]

Check that one of the plate load resistors ( 82k and 100k) are not bad as this is very common in these amps, and by the way ,with the power supply voltage that these have on them they need to be 1 watt resistors, not 1/2 watt!

[mididoc]

thanks Stevem,

i will check these.

cheers

[mididoc]

btw,

i have looked in the schematics of 63-vibroverb, which was the original of this amp,
and it tells also 17.4 V for TP17.

i attach it here.

63RI_Vibroverb.pdf

cheers

[mididoc]

hi again,

think i found the reason for the 17.4 V issue.

In the original 63 vibroverb schematics R 37 is a 6.8 k.

They have replaced this in the new model with a 39 k.

If I replace this one with the original used 6.8 k,
i get the 17.4 V at the driver cathode.

So they just replaced the resistor with a larger one and didn't correct the voltage value.

atleast found solution for this issue.

btw it sounds much better, less distorted, too.

cheers

[tictac]

If there's a 39k tail resistor in the PI then you've got a Bruce Zinky era Custom Vibrolux. This amp doesn't have a feedback loop either so it will distort earlier and have more gain than a standard blackface style amp....

This may not be your problem but you may want to take a look at your preamp tubes and possibly do some swapping....

The amp should sound pretty good in it's stock form and by the way it's cousins which have a similar PI setup (39k tail, no feedback) include the Vibroking, Custom Vibrasonic and Dual Professional...



TT