Vox Cambridge 15 and Cambridge 30 reliability upgrades, mods, and tweaks

[R.G.]

On the two burned resistors: the cheap zener shunt regulator strikes again.

This seems to be the standard way to provide for opamps in an amp like this, and it quite often provides for charring of the PCB and sometimes outright failure.

The simplest way to fix it to better than new is to put in higher wattage resistors and space them up off the PCB. Same-value resistors will dissipate the same amount of heat, but the bigger bodies of a higher-wattage resistor will spread it out so the surface temperature is lower. Spacing it up off the PCB reduces the surface temp at the PCB by the inverse-square law for the radiated heat.

A technically more elegant way is to replace the resistor/zener with some way to get a series regulator in there. I've satisfied myself that the reason they didn't use a three terminal regulator in there is that the input voltage is too high, and zeners are cheap.

[Tone Meister]

RG, I believe the original resistors were 3 watt, and I replaced them with 5 watt wire wounds and spaced them about 3/8" off the board. The board was heated pretty good, but the PCB is fine and I replaced those resistors as a preventative measure.

[corlay]

Hello.

I opened-up my Cambridge 15 to remove the LED clipping diodes, and replace the 12ax7 tube, and also see some heat-scorching in the area of Resistors R66 and R67.

this thread is a bit older.

has a schematic ever surfaced for this combo amp? has anyone ever learned the official Vox spec for these two resistors?

thank you.

[g1]

also see some heat-scorching in the area of Resistors R66 and R67.

this thread is a bit older.

has a schematic ever surfaced for this combo amp? has anyone ever learned the official Vox spec for these two resistors?

They are for the heaters of the tube and will normally run very hot. That is why they are so much larger than all the others. The discoloration is normal with age. They are 82 ohm 3W resistors. If they do not measure a higher resistance, I would just resolder their connections.