2000 Vibrolux Reverb

[J Luth]

Looking at this as a favor, I prefer not working on circuit board amps.

He says it’s from around year 2000. it has not been serviced, still has all the original caps in it.

His complaint is it doesn’t sound right. Well that is not all that descriptive.

When playing through it, it sounds weak and thin between 1 ½ and around 3 ½ on both channels.

Turning the knob up to 4 and above and it gets very distorted on both channels.

Put the scope on it and it shows a good trace until 3 ½ on the volume, keep turning it up and it starts to clip at 4, as the volume is turned up it distorts more and more the more it turned on both channels

Tried changing some tubes but no different.

Chassis is out, did a visual, checked a few things nothing found yet.

[Helmholtz]

Schematic? Supply and tube (cathode and plate) voltages? Input signal level? Output clipping level?

[Jon Snell]

Check the 100k anode load resistors.

[J Luth]

I'm still looking for a schematic I can upload that will be readable. The one I have here is too big for me to scan.
I am checking voltages and will make a list of the test point voltages.

Right now the ones I have checked so far appear not to far out of line except test point 17 which is the voltage from the 820r resistor to the cathodes of the PI. The schematic calls for +17.4 V I am getting +72.5 V at the cathodes. The resistor R38 1M, R39 820R, R40 1M values check out OK.

I'll see if I can find a schematic later today

[Helmholtz]

I'm still looking for a schematic I can upload that will be readable. The one I have here is too big for me to scan.
I am checking voltages and will make a list of the test point voltages.

Right now the ones I have checked so far appear not to far out of line except test point 17 which is the voltage from the 820r resistor to the cathodes of the PI. The schematic calls for +17.4 V I am getting +72.5 V at the cathodes. The resistor R38 1M, R39 820R, R40 1M values check out OK.

I'll see if I can find a schematic later today

17.4V at TP17 seems to be a typo (they probably meant 74V). Makes no sense, much too low. Your 72.5V look good. What total resistance do you measure between PI cathodes and ground?
And please post plate voltages (preferably for all tubes).

[Chuck H]

I think that voltage for R39 must be measured across the resistor itself rather than to ground? Or a typo? I've always seen a similar voltage to what you're reading there. In other words, I don't think your measurement is indicative of a problem.

[Helmholtz]

I think that voltage for R39 must be measured across the resistor itself rather than to ground?.

That would be a factor 10 too much for a useful bias.

[Chuck H]

That would be a factor 10 too much for a useful bias.

Good point. It's a typo then.

[Helmholtz]

Good point. It's a typo then.

See post #5. The schematic for my Vibroverb RI has the same TP17 typo.

Your link doesn't work.

[Jazz P Bass]

TP 17/ PI cathode should be measured/ referenced to the grid.

Be aware that this is the "Custom" Vibrolux.
It is meant to 'breakup' early.

[J Luth]

Link to a schematic

Has the same voltage for TP17

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Chuck H]

Your link doesn't work.

Huh? It's right there on my screen. I think there are some limitations depending on ISP and browser specifics. Just use Jazz's link then.

[Helmholtz]

TP 17/ PI cathode should be measured/ referenced to the grid.

As written above, that wouldn't make sense either. A grid bias of -17.4V would put the triodes in total cut-off, while it would require 22mA (11mA per tube) to produce that voltage drop across 820R. Typical PI plate current is around 1mA per triode for 12AX7 with ~100k plate resistors.
That TP17 voltage is simply wrong.
(And schematic's notes say all voltages measured wrt ground.)

Your pdf doesn't work for me either.

[vintagekiki]

https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Custom-Vibrolux-Reverb-Reissue-Schematic.pdf

1)
https://www.yumpu.com/en/document/read/34362263/fender-custom-vibrolux-service-manualpdf

[Helmholtz]

https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Custom-Vibrolux-Reverb-Reissue-Schematic.pdf

1)
https://www.yumpu.com/en/document/read/34362263/fender-custom-vibrolux-service-manualpdf

Thanks, kiki.
Very helpful as usual. Your links as well as the link in post #11 work for me.