VOX Essex Bass "Death Cap" rewire

[cjlectronics]

I am working on a VOX Essex Bass with a loud hum. First thing I did was check the chassis to earth ground using a volt meter and I measured 120VAC. The death cap is alive and well in this thing.

I want to rewire the primary side of the amplifier and remove the line reverse switch and cap, then add a three prong power cord. I think I've got the unit wired correctly but was hoping I could get a confirmation from others that I have wired the amp correctly using the 3 prong plug. I've never done this on an old amp like this so I'm a little apprehensive to flip the power switch on without someone else giving me the reassurance that it is wired right. I traced the primary and secondary side of the amp and drew it in ORCAD. Please give me a thumbs up if you agree with my rewire.

Just to provide a little more info, the electrolytic caps in this things are all leaking. I have replaced all of them in the power supply section.

Thanks!!

[Jazz P Bass]

Looks good from here.
The only nitpicking that I see is the -24V cap is drawn backwards.

[R.G.]

One thing - you don't show the fuse, but I'm sure it has one. Be certain that the fuse is in the line/hot side with the power switch. A fused neutral/cold side can make the amp very dangerous if the fuse blows. It's still live, but the indicator light won't come on. People make the connection to neutral then with their bodies.

Also think about the consequences of wiring up that accessory outlet. I would leave it disconnected. It can't carry safety ground without hogging out a bigger hole for a three-prong outlet.

[jmaf]

Where are the rectifiers?

[cjlectronics]

Yep, the -24v caps is backwards and the rectifier diodes have been left off...(because I was in a hurry and forgot to check the drawing ). Yes, I was thinking about leaving the accessory outlet disconnected. Great idea, it will not be wired up.

Thanks everyone for your input!!

CJ

[soundguruman]

What you want to do, when converting an amp to grounded (earthed) chassis...
Make sure that your heaters are not grounded. Lift any heater (filament) connection off of ground, correct the wiring of the circuit as required.

On many older amps, the filaments were tied to the chassis on one side of the heater winding. Change this so the heaters float, before grounding the chassis to a 3 prong power connector.
In many older amps, the heater grounded on one side of the winding, several other components may use the heater wire as a ground.
Separate these components, and ground them directly, lifting them from the heater buss wiring. (the heater buss will no longer be grounded.)

Balance the heater winding as usual, with the center tap of the winding grounded,
or install ballast resistors, as in a fender amp, two 100 ohm resistors to ground,
or a hum balance control.

[cjlectronics]

Just wanted to update everyone on the progress made on this amp.

I removed the Line select switch and death cap wiring. It no longer is active and wired a 3 prong power cord to the primary of the Xformer with the GND wire going to the chassis.
I replaced 3 DC filter caps after the secondary of the main power transformer... two 2200uf/50v and one 470uf/35v
Repaired a broken lead on a .47uf cap after the first stage transistor amplifier (this is in the signal path)

At this point the amp is functional and I can play a guitar through it. When holding the strings of the guitar I can hear slight hum with the amp cranked to 8. I'm sure it is a bad capacitor somewhere. If it was my amp I'd replace every electrolytic cap and change the resistors to metal film or 5% or better tolerance resistors. I will have to ask the owner what they want to do.

Thanks again for all the assistance. This forum is awesome and I just donated money to keep it going. I hope everyone else considers doing the same.

CJ

[jmaf]

When holding the strings of the guitar I can hear slight hum with the amp cranked to 8. I'm sure it is a bad capacitor somewhere. If it was my amp I'd replace every electrolytic cap and change the resistors to metal film or 5% or better tolerance resistors.

If you turn down the guitar volume all the way, does the slight hum disappear?

[soundguruman]

Just wanted to update everyone on the progress made on this amp.

I removed the Line select switch and death cap wiring. It no longer is active and wired a 3 prong power cord to the primary of the Xformer with the GND wire going to the chassis.
I replaced 3 DC filter caps after the secondary of the main power transformer... two 2200uf/50v and one 470uf/35v
Repaired a broken lead on a .47uf cap after the first stage transistor amplifier (this is in the signal path)

At this point the amp is functional and I can play a guitar through it. When holding the strings of the guitar I can hear slight hum with the amp cranked to 8. I'm sure it is a bad capacitor somewhere. If it was my amp I'd replace every electrolytic cap and change the resistors to metal film or 5% or better tolerance resistors. I will have to ask the owner what they want to do.

Thanks again for all the assistance. This forum is awesome and I just donated money to keep it going. I hope everyone else considers doing the same.

CJ

Very often in an old vox, a leaky transistor or the output transistors out of match will cause a slight constant hum.