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It could be a microphonic or faulty preamp valve. My AD15VT is quite hissy on a couple of the settings, and it's related to using the valve in a particular mode. (Boutique Clean, AC15, AC30 and Boutique OD - I know the VT40 doesn't have quite the same settings.)
I can't remember having opened up a VT40, but if it's anything like the AD15 it will quite a pain to get into and change the valve...
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Yes, I just played around with it a bit more. By turning down the power value and turning up the volume the hiss reduces quite a lot (to nothing in some cases). Turning the treble down reduces it even further.
The amp settings where the hiss was really noticeable were - US Blues; AC£); UK Metal; US Highgain; BTQ Metal - this was before turning down the power and upping the volume. This was with all other effects, reverb, delay etc off. Gain was set to around 10 o'clock for all of them so it was only the power level and volume that I was changing.
So is this normal or is the valve faulty in some way? Any other tests I should do? Should it make a difference if I leave it for the valve to get warmer or doesn't it work like that?
Cheers
The modelling circuit does actually change the valve bias and whether one or both halves of the valve are used, it's not just a token 'valve in the circuit'. It's slightly annoying because the noisiest setting on mine is the AC30, which is - possibly not by coincidence - the best sound in the amp. (And worse, the AD15 does not have a power control, being one of the smaller amps in the range - so I can't fix it like that.)
The valve will be fully warmed up after only about thirty seconds to a minute, after that it won't make any difference.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
A friend of mine simultaneously owned two ToneLab type amplifiers. Peering through their rear ventilation grilles, it was obvious that the valve in one of them lacked a healthy glow. Almost every screw on the outside of the amplifier needed undoing to access the valve. It was also necessary/advisable to disengage the block connector on a flying lead to avoid the possibility of damaging it.
Fortunately, my friend has since sold both VOX amplifiers. If he had ever asked for my assistance a second time, I think that I would have resorted to something like this.
Changing the valves in these amps is actually relatively easy although it's rarely needed because they should last the life of the amp particularly as they only have low voltages going through them as I said. It's the blue series that are awkward because you can only see the valve tips sticking through the chassis and in the blues they do have much higher voltages going through the tubes. But duct tape wrapped around the tube end and gently wiggling it lets you get to these without having to undo the chassis.
The valves do glow in the VT series, and there is no LED. Although there is no high voltage applied to the *plates*, the filaments are operated normally.
Getting to the valve requires taking the chassis out completely, and getting it out of its holder without taking the PCB out is possible but tricky - there isn’t enough room to reach it easily by hand, you need to use something like a flat screwdriver to lever it out, and a couple of bits of wood to grab it. Guiding the new one in and pressing it into the socket is a similar faff...
This should show why!
http://www.voxshowroom.com/ct/amp/uk/AD_series/ad15vt_tube.jpg
http://www.voxshowroom.com/uk/amp/ad15_hood.html
(The description at the bottom of how to change the valve is wrong, by the way - you need to remove, not loosen, the eight chassis mounting screws, and also disconnect the speaker to get the chassis out.)
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I'm not 100% sure if the layout of the VT40+ is the same inside, but I think it's likely to be at least similar.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
On a separate note, I've been thinking about a valve lunchbox head as I have a spare speaker. Will start a new thread for that.
It's all part of the "try to prove @ICBM wrong on something for a change" challenge.
It's a glorious feeling when you do. Apparently..
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein