Hi,
I have a VOX V15 twin guitar amp that is faulty. The power light comes on but no sound is coming out of the unit. I opened up the unit and three tubes were missing which I have replaced. Everything else in there looks in quite good condition.
Can anyone give me any advice on the steps to find the fault?
Are all the heaters coming on on all of the valves?
It could be the other 2 are faulty.
Be careful if you do not have experience with working on valve amps, when running there will be voltages of 400-600v and high voltages can remain even when the mains power is taken away, which needs to be discharged before you work on it
When I checked yesterday they appeared to be coming on but quite faint. I’ll check them again and see if they need to be replaced.
Thanks for the warning and I’ll ensure that I’m discharging the voltage when working on it.
The method I use is to feed a suitable-level signal into the input (1kHz sinewave or music from your phone if it has an output jack) and trace it through the circuit using the half-split method. When you find where the signal disappears, switch from AC to DC tests to locate the faulty component/connection.
Replace component/clean connection/resolder dry joint as necessary.
A signal tracer is a small amplifier and speaker with a high-voltage (DC-blocking) capacitor in series with the input probe.
Working with the schematic speeds things up radically unless you know the circuit well.
The warning about high voltages in valve amps is crucial. Wear eye protection and work with your left hand in your pocket when the machine is live.
I hope this helps.
First, look online for a schematic snd service manual.
They also get bloody hot when they’re working.
It looks like there’s a schematic included 
Look in your 2nd photo under the valve chassis.
Also, there are schematics online, e.g. Prowess Amplifiers - Vox - Schematics - V15.
One obvious thing to do is to eyeball it very carefully.
After that it’s down to either signal tracing as @niallmcgrath said or testing individual components 
Unfortunately I don’t recall any easy way of testing the valves 
I’ve had a look at the schematics available online for the Vox V15 and some questions arise:
Did you replace the missing valves with known good correct replacements?
Are the output valves EL84s?
Why is the power transformer HT secondary shown to be 280V RMS when that would result in a full-wave rectified +395V DC, well over the +350V DC rating of any of the reservoir capacitors? And well above the maximum anode rating of an EL84, which is +300V DC.
I don’t doubt these things did work, but i am puzzled by that apparent anomaly.
Unhelpfully the available schematics show no test point expected DC voltages that I can see, and some don’t even specify the valve complement.
I found an article that said the speaker used two ECC83, one ECC81 and two EL-84 tubes. I’ve already replaced three of them and have the two ECC83 arriving tomorrow. I’ve used Mullard valves so they should be good.
I’ve done a visual inspection and no components appear to be compromised. I’ve also used contact cleaner in the pots to clean them out.
I’ll fit the last two valves tomorrow and if it still won’t work I’ll signal tracing as @niallmcgrath has suggested.
Thanks for your help so far everyone.
