i have a jcm 800 lead series which was fine up until recently it now decides to scream at me,and not in a good way,any time i turn on the my distortion pedal.ive bought new leads,tried different pedals,got my guitar checked over,messed around with the settings lowered the gainetc.the next thing im gonna do is leave the head in for a service but can anyone offer any help on why its doing this.
i second the thought it's a pre-amp valve.
it's something you could replace yourself, but be careful to touch capacitors and stuff as they hold killer voltages for a long long time.
it's likely a 12ax7 (aka ecc83) like stevie j said, but you might want to consider an ecc82 or another 12ax7 substitute. the ecc82 has less gain than an ecc83, whereas other substitutes can have more.
i would go for the jj brand. it's reasonably priced and decent quality. the likes of sovtek/eh and the chinese makes don't always get good reviews. the like of grove tubes are actually just rebrands (maybe better quality control than some rebrands, but most of them seem to be chinese-made).
again, like stevie j says, try bairds. they'll probably have a better idea than other shops in the town, and certainly give you more support than buying via mail-order/internet.
JJ`s are class - buy some spares and while you are at it order a set of matched powervalves to have - if yours are more than 2 years old and the amp has been heavily used you will need to consider changing them - dotn do it unless you know what you are doing as very dangerous.
Watford and Hotrox sell very high quality valves.
Hmmm... Could be a parasitic oscillation...
If the amp is just on the verge of oscillating all by itself, the extra gain from the pedal could put it over the edge. Yes, most likely is a pre-amp valve - they are all ECC83s / 12AX7s (Change them yourself, pre-amp valves don't have to be biased by a technician. Output valves do). The first one nearest the input socket would be the most likely. Failing that, other oscillations can be awkward to stop. Lead dress, bad earthing and component drift are all possiblities. Or are you using an unreasonable amount of gain from that pedal? I have actually had to modify a couple of amps to cure it in the past.
i didnt think there was such a thing as unreasonable gain.
ive got some preamp valves so well see what happens.
[quote:efbf210358="Thedevilsfavouriteson"]i didnt think there was such a thing as unreasonable gain.
i used to think like that. nothing wrong with it either, as sound/tone is a personal thing. my clean sound used to be mild overdrive, with so much gain/distortion for my heavier sound that there was sometimes more likely to be a volume drop when i kicked in another pedal. in the end, i actually added an octave pedal after my distortion boxes as it somehow seemed to make the difference (i guess it was down to eq'ing or something, and an eq pedal would actually have been more beneficial than adding distortion on top of distortion).
nowadays, i've taken a step back. most of the sounds of my influences are actually a lot cleaner than i thought they were. i still get a distored-guitar-hardon though.
mr thecomeons i was only kidding there but i do like my guitar to be quite 'harsh',as some sound men have described it.no gain no pain.