Vibrolux No Balls

[jaster55]

I had a 1965 Vibrolux Reverb come into my shop recently. Client loved the way it sounded. He bought it a few years ago and wanted to know if it was original. First thing I noticed is it did not have the balls that a vintage fender tube amp should have. I had to turn it up to 5 to get what most fender amps that size give you at 2.

The amp was original except for a ground plug and filter caps dated 1991. There was a pair of Sovtek 5881 Power Tubes in it. I know they are lower watt tubes than the 6L6’s.

I found the 100 ohm resistor between the negative feedback resistor and ground disconnected. When I connected it back up the lack of power (Fender Balls) came back.

My Question Is… Is that a Mod to reduce the amps power because it appears to have been done on purpose. Maybe to use the 5881's???

[thetragichero]

so the 100 ohm resistor just wasn't there at all, leaving the cathodes in the phase inverter not connected to ground? doesn't seem like that would be an intentional mod (i could just be reading this wrong)

[dwinstonwood]

Or, would the PI cathodes have been grounded at a higher impedance through the extension speaker jack, some how?

[pdf64]

With the 100ohm open / removed, the LTP’s DC 0V return goes via the 820ohm series feedback resistor then the OT secondary (the speaker voicecoil is a parallel path to the latter, but higher resistance).

It’s common for that 100ohm resistor to fracture and fail open circuit, perhaps intermittently, due to that comedically awful board material warping / developing a hillock in that area.

…I had to turn it up to 5 to get what most fender amps that size give you at 2.
…

Of course I get what you mean, but just to note that Fender 1-10 knobs start, er, at 1, and don’t leave the 0V region until after 2. With modern crappy CTS pots, no sound till nearly 3, as for a few decades, their 0V region has been enlarged to accommodate pots with a built in rotary switch. 

[PRR]

...100 ohm resistor between the negative feedback resistor and ground disconnected. When I connected it back up the lack of power .....

Lack of gain. You have to pluck almost 8 times harder to get the full 45 Watts.

[jaster55]

Noted...