Fender vibrolux (90's) blocking distortion

[Swampy]

Yo,

I've scoped this sucker and it seems the problem lies in the phase inverter section.

Relevant measurements are:

tp 16 - 5.2 V pk to pk
tp17 - 70 V (schem reads 17.4 v??)
tp18 - 266 V and 67 Vrms (schem reads 33.5 Vrms??)
tp19 - 267 V

Tube swapping doesn't change anything. What could be causing the excess gain in this stage?

Schem is up here:

support.fender.com/schematics/guitar_amplifiers/Custom_Vibrolux_Reverb_schematic.pdf

Cheers,
Matt

[g1]

TP17 DC voltage appears to be a typo, probably should be 77V. Each of the splitter plate resistors have about 100V drop across them (supply point Y approx. 360V, each plate approx. 260V). At approx. 100K for the plate resistors, this means 1mA current per side (I=E/R). So 2mA combined through a cathode resistance approx. 40K is around 80V (E=IR).
For TP18, 33.5Vrms would be about 95Vp-p, you are seeing 190Vp-p ?
What are you getting at the speaker jack with 4 ohm load? (TP21)
What is the problem or symptom with the amp?

[Swampy]

I suspected that was a typo on the schem.

Disregard my measurements from yesterday, I discovered my oscilloscope was playing up.

The problem is the amp has massive amount of gain and compression, a quiet swirling fuzz and when played hard breaks up into loud cracks and rips that sound as if they are tearing their way through the sound into the room.

Here are my measurements from this mornings investigation. I'm certain the problem is located between V4 and V6 somewhere...

[

TP 9	339 mV rms
TP 10	10.6 V rms / 207 VDC
TP 15	15 V pk - pk = 5.3 V rms (must be another typo)
TP 16	14 V pk - pk = 4.94 V rms (must be another typo)
TP 17	78.6 VDC
TP 20	438 VDC (the 200 v rms is beyond my scopes measurement capacity)
TP 21	17.6 V rms into 8 ohm load

What say you??

[g1]

This is great, now I don't trust the drawing or your scope.
Your input signal is 1Khz sinewave?
What is TP18 rms voltage? and TP19 also.
If TP15 & 16 are not typos, it would explain your extra gain. R36 & C18 are worth looking into.
With 8 ohm load you should be down on power, yet you have an increase. Is it clipping heavily at the load? I take it you don't have a 4 ohm load resistor to try?
As far as the crack & rip sounds, have you tried another speaker?

[Swampy]

This is great, now I don't trust the drawing or your scope.
Your input signal is 1Khz sinewave?
What is TP18 rms voltage? and TP19 also.
If TP15 & 16 are not typos, it would explain your extra gain. R36 & C18 are worth looking into.
With 8 ohm load you should be down on power, yet you have an increase. Is it clipping heavily at the load? I take it you don't have a 4 ohm load resistor to try?
As far as the crack & rip sounds, have you tried another speaker?

Yes, 1Khz sinewave.

with 335 v rms at TP 9


TP 10 10.6 V rms
TP 15 5.3 V rms (must be a typo as I had already replaced R 36 and C 18)
TP 18 38.8 V rms
TP 19 41 V rms

And a clipped waveform at TP 18 / 19

TP21 17.6 V rms into a 8 ohm load...new dummy load is in the post.

Chur

[g1]

A bit tough trying to guess what may or may not be typos. Are you using x10 scope probe?
Suggest trying this, 1khz into normal input, vol. treb. bass set to 12 o'clock. Adjust signal generator output to get 5.2Vp-p at TP16.
List rms levels at TP1,2,4,5,8,9,10,18,19,21

[Billy R aka DynaFreak]

your amp is a "Custom Vibrolux" correct? for some insight into the "typo" issue you might want to look at the 63 Vibroverb schematic. very similar amp, with some component changes, and test point values that might have transferred from one schematic to the other. most problems reduce gain. if you think you truly have a too much gain problem then double check component values and especially ground points like where the voltage divider before V4B grounds thru the reverb pot. if that pot goes bad or has a bad connection you no longer have a divider and the input voltage to that tube bumps up.

[g1]

Thanks Billy, that is very similar, and it looks like part of the drawings are copied. As far as the typo, either the TP17 or R37 value is shown wrong. The 63 Vibroverb shows R37 (phase splitter cathode resistor) as 6.8K, with cathode voltage shown as 17VDC. This makes sense, as cathode current should be around 2mA.
However, the Custom vibrolux drawing shows R37 as 39K yet gives the same cathode voltage (17VDC). So either the resistor value is not as shown, or the cathode voltage is actually much higher.

[Swampy]

Thanks Billy, that is very similar, and it looks like part of the drawings are copied. As far as the typo, either the TP17 or R37 value is shown wrong. The 63 Vibroverb shows R37 (phase splitter cathode resistor) as 6.8K, with cathode voltage shown as 17VDC. This makes sense, as cathode current should be around 2mA.
However, the Custom vibrolux drawing shows R37 as 39K yet gives the same cathode voltage (17VDC). So either the resistor value is not as shown, or the cathode voltage is actually much higher.

Cheers guys, after a bit of tube swapping and general tinkering I got this thing to behave as it should.

Matt