Vox AC30/6 TB probs

[pippa]

Earlier this year I bought a vintage (1972) Vox AC30/6 TB. After playing with my new toy for a few weeks, I sent it away for a good once-over by a Service Engineer. It's been back for a couple of months, and a fault has recently occurred which has stumped the pair of us !! I'm reluctant to send it back if it's something simple. (Also, "Would it withstand the rigours of another courier service?", I ask myself) The tone of the Brilliant Channel is no longer affected by the Treble and Bass controls. The Cut control works fine on all channels, but the sound from the brilliant channel can't be altered, apart from volume and Cut. The fact that the Cut works properly indicates that the tone valve is OK, and there's plenty of gain, vibrato etc. I've accessed the excellent collection of circuit diagrams on the Vox amp website, http://www.voxamps.co.uk/support/circuits.asp, although I've not examined the circuitry to see if there's anything obvious, like a wire off, but got any clues guys? UK based. www.gemini3.eu

[Daver]

The cut control only affects the phase inverter tube not the tube driving the tone controls. The ground for the tone stack may have disconnected. That would make it not work. I'm not familiar enough with how your specific amp is wired, but both the Bass pot and 10K resistor from the tone stack need to be grounded. Start there. Good luck!
Dave

[pippa]

Thanks Dave,
More sent by direct email........

[R.G.]

A single broken wire from the bottom side of the treble control to the rest of the tone stack, or from the bass control and it's paralleled resistor to ground would cause the observed symptom.

[pippa]

Many thanks, RG.

Together with Daver's contribution, it looks as if I could be staring a solution in the face very soon. I'll let you know how we get on.

Thanks guys, much appreciated.

David JM east Kent, UK

[pippa]

Problem Solved !!:

Hi Guys.

I had passed on your thoughts on the problem to my chosen repairer, who replied that he would pay full attention to them when he received the amp.

He took on board your very welcome suggestions and headed straight for the components on the "tone stack" (as you US guys would call it). Despite the fact that everything looked fine, he systematically checked out the soldered joints, especially the earth (ground) connections and replaced virtually all the caps and resisitors. The ECC83 valve (sorry - tube) was OK, so all that was left were the treble and bass pots. He'd replaced these with new ones early in October, having found that the factory originals had been changed at some time during its thirty years, but with the wrong values. He therefore assumed that his replacements were OK.

However, having "nowhere else to go" he changed the treble pot - no difference.

He then checked the bass pot, only to find that the track had gone open circuit. Fitted a new one - Hey presto !!

I'm now awaiting its return, and we should be up and running again very soon.

It just goes to show that new components do suffer from early failure at times - as most Skoda owner/drivers will tell us.

So. Thanks guys. I'm not 100% up to speed with amp design and circuitry, but I've had a look at this ( http://www.voxamps.co.uk/downloads/c...s/ac30dals.jpg ), and it looks to me as if one end of the bass pot goes to ground, so you were both right. If so, pat on the back, and have a metaphorical beer on me next time you're anywhere near one. Or two. Or three. Or.......

David JM.

[Daver]

David,
Glad everything worked out. I'll have a Newcastle, thank you very much. Or two or three.....
Dave