Hey again guy's, been workign on my laney PA amp and have a problem...
Basically there was a resistor burnt up inside it when i got it, it feeds the preamp tubes. the resistor is next to another resistor of a diffrent value which also feeds power to diffrent pins on the preamp tubes.
I got in touch with someone that has the same amp and he had a look inside his and came back that there should be one resistor that is either 47K or 4K7 (he couldnt make out if one band was orange or red) and the other should be 22K or 2K2.
The one that i have in my amp that isnt burned out is 47K so i guess that the other one should be a 22K and i replace it with a 22K 2W resistor which then burns out, replace it with another 22K 2W resistor this time connecting my multimeter in series with it and find out that with no valves installed the amp draws 300mA (too much for a 22K resistor me thinks)
I cant find any faults in the circuit so im guessing that the value of the resistor is wrong?? or am i missing something?? i can try and draw a schematic if it helps and if someone can direct me to a site where i can get a free schematic cad type package from..
many thanks,
Owen
Basically there was a resistor burnt up inside it when i got it, it feeds the preamp tubes. the resistor is next to another resistor of a diffrent value which also feeds power to diffrent pins on the preamp tubes.
I got in touch with someone that has the same amp and he had a look inside his and came back that there should be one resistor that is either 47K or 4K7 (he couldnt make out if one band was orange or red) and the other should be 22K or 2K2.
The one that i have in my amp that isnt burned out is 47K so i guess that the other one should be a 22K and i replace it with a 22K 2W resistor which then burns out, replace it with another 22K 2W resistor this time connecting my multimeter in series with it and find out that with no valves installed the amp draws 300mA (too much for a 22K resistor me thinks)
I cant find any faults in the circuit so im guessing that the value of the resistor is wrong?? or am i missing something?? i can try and draw a schematic if it helps and if someone can direct me to a site where i can get a free schematic cad type package from..
many thanks,
Owen
ok, done a quick schematic in paint, hope it makes sense, its for ONE preamp valve, there are 6 in total. the valve is an ECC83 and this piccie is from the solder side of the socket not the valve side. i have marked the resistors with their colour codes and the resistor that i think should be a 22k i marked 22k??
sorry for the rubbish quality.
Thanks,
Owen
sorry for the rubbish quality.
Thanks,
Owen
Attachments
You probably have some shorted cap to ground, that's the cause of high current drawing from the B+.
Carefully check ALL the caps and ALL the resistors for shorted ones, drifted resistors, bad solder joints etc etc.
Also check your preamp tubes (but I haven't seen a shorted preamp tube yet).
Hope this helps!
Carefully check ALL the caps and ALL the resistors for shorted ones, drifted resistors, bad solder joints etc etc.
Also check your preamp tubes (but I haven't seen a shorted preamp tube yet).
Hope this helps!
Hey,
The model is LA100PA Super.... it's a 100Watt 6Channell PA head.
The 300 mA current was without ANY valves installed ( removed them to check incase one of the valves had shorted). How do i test the capacitor? there is one that goes from the resistor to ground that i think if any are faulty it is probably that one, would it be good enough to just dissconect it and measure the current again?
There are also quite a lot of resistors that COULD be faulty but they all LOOK good (havent measured them yet) but i will measure them to see if anythign is going wrong on any of them.
Reason i didnt check the capacitor is that for the few short moments the amp was living it sounded totally awsome! so humm's or hiss's atall.. and then the volume just faded and there was a smell of burning and i turned it off and had a look and the resistor had burnt out again.
Have resoldered all the solder joints already as there were a couple that looked bad but that didnt seem to help.
Will see what the values of the capacitors are and replace all the power supply capacitors i guess as they are probably due to be renewed on such an old amp.
Someone has been in the amp and done repair work in the past before i got it aswell as one of the power tube sockets have been replaced and the power socket has been replaced with a piece of cheap electrical wire with a plug on the end ( which will be returned to standard when i get home again)
Thanks for your help,
Owen
The model is LA100PA Super.... it's a 100Watt 6Channell PA head.
The 300 mA current was without ANY valves installed ( removed them to check incase one of the valves had shorted). How do i test the capacitor? there is one that goes from the resistor to ground that i think if any are faulty it is probably that one, would it be good enough to just dissconect it and measure the current again?
There are also quite a lot of resistors that COULD be faulty but they all LOOK good (havent measured them yet) but i will measure them to see if anythign is going wrong on any of them.
Reason i didnt check the capacitor is that for the few short moments the amp was living it sounded totally awsome! so humm's or hiss's atall.. and then the volume just faded and there was a smell of burning and i turned it off and had a look and the resistor had burnt out again.
Have resoldered all the solder joints already as there were a couple that looked bad but that didnt seem to help.
Will see what the values of the capacitors are and replace all the power supply capacitors i guess as they are probably due to be renewed on such an old amp.
Someone has been in the amp and done repair work in the past before i got it aswell as one of the power tube sockets have been replaced and the power socket has been replaced with a piece of cheap electrical wire with a plug on the end ( which will be returned to standard when i get home again)
Thanks for your help,
Owen
I haven't got a PDF of that one, but I do have a JPG, and I've attached it. It sounds like one of the decoupling capacitors is S/C (or at least VERY leaky), you should be able to find it with your meter on the ohms range. In the JPG the suspect ones are shown as the two 16uF either side of the 10K.
Oops - JPG too large, try it at My Website
Oops - JPG too large, try it at My Website
that schematic looks slightly diffrent to my amp but i know the two capacitors you mean, they are on the power supply section of the circuit but on mine the 10K is a 47K and it is connected to the other side of the 22K resistor (which is the one that keeps burning out and the caspacitors in question go from the 22k to ground and the 47k to ground. I will check it when i get home anyway and check and see if the capacitors are shorted. if they appear shorted on my meter would it be acceptable to dissconect them to remeasure the current draw or would it damage other components if i did this?
many thanks,
Owen
many thanks,
Owen
ok, i tested the two capacitors (15 uF 400V in my amp) and tested them with my meter and sure enough the one conected to the resistor that kept burning out was short circuited... COMPLETELY!..
Removed the capacitor and the amp works well.. slightly unstable at low frequencies and a little mains hiss but the resistor isnt burning out anymore.. am going to look on ebay for some replacement capacitors and replace the two 15uF capacitors and see what it is like then and mabey get some new power tubes for it as it only has 2 in there at the moment as one was broken when i got the amp (had a crack in it
)
thanks for all your help,
Owen
Removed the capacitor and the amp works well.. slightly unstable at low frequencies and a little mains hiss but the resistor isnt burning out anymore.. am going to look on ebay for some replacement capacitors and replace the two 15uF capacitors and see what it is like then and mabey get some new power tubes for it as it only has 2 in there at the moment as one was broken when i got the amp (had a crack in it
)thanks for all your help,
Owen
i'm going to be useing it for bass guitar mainly. have a solid state PA for vocals, etc..
I plan to mod it a bit so that the diffrent chennels give diffrent sounds, useing diffrent valves and mabey try and set up a couple of the channels for higher gain.
first things first though, have to get it working properly as standard. am i right in thinking that because it uses solid state rectification i can substitute the capacitors with higher value capacitors or would this not work? just i have some left over from my preamp that i built.. nice 22Mfd 500V ones..
Owen
have a solid state PA for vocals, etc..I plan to mod it a bit so that the diffrent chennels give diffrent sounds, useing diffrent valves and mabey try and set up a couple of the channels for higher gain.
first things first though, have to get it working properly as standard. am i right in thinking that because it uses solid state rectification i can substitute the capacitors with higher value capacitors or would this not work? just i have some left over from my preamp that i built.. nice 22Mfd 500V ones..
Owen
DoomPixie said:i'm going to be useing it for bass guitar mainly.
I plan to mod it a bit so that the diffrent chennels give diffrent sounds, useing diffrent valves and mabey try and set up a couple of the channels for higher gain.
I prefer the clean sound of transistors for bass as well, but there are many players who like the lower quality valve sound - and it does give you the advantage of being louder, as you can over drive it without the distortion becoming unusable.
I bought my daughter a Behringer BX1200 120W bass combo, I'm VERY pleased with it - it sounds great, it's loud enough to play with a LOUD drummer in a rock band, and it's fairly small - it is heavy though?. Although comparing weight with a valve amp, it's probably not heavy at all
first things first though, have to get it working properly as standard. am i right in thinking that because it uses solid state rectification i can substitute the capacitors with higher value capacitors or would this not work? just i have some left over from my preamp that i built.. nice 22Mfd 500V ones..
Owen
They would be fine, for any type of rectifier, they are only low level decoupling capacitors - you might also try reading the tolerance specs on the original ones! - they may have been 22uF already (often -50% +100% tolerance).
yeh, i wasnt too sure about useing it for bass but i spoke to a guy that has one he uses for bassa nd he said he likes it, and from the short test i ran i like the sound of it aswell, it is very tight with an ecc82 phase splitter instead of the ecc83 phase splitter bass amp's are always heavy. before i got this running i was useing an old H&H 100Watt solid state PA head with a single 15" and horn PA speaker, it is a good amp and not quite as heavy as this valve amp but it still weighs a fair bit. still i prefer the laney pa head.. even with just the two output valves its deffinatley louder and has an overall better sound.
Distorted audio sounds considerably louder than clean audio, this makes valve amps sound louder than transistor ones - simply because the odd harmonic distortion from a clipping transistor amp is so horrible you can't use it!. The even harmonic distortion from a clipping valve amp is less objectionable, and guitarists seem to like it.
In general (with guitar/bass) a 25/30W valve amp will easily keep pace with a 100W transistor amp - but then try them with vocals!, where distortion matters
Personally I like bass to be clean, and really the bass amp should only be for on-stage monitoring?, the main front of house should be through the PA.
In general (with guitar/bass) a 25/30W valve amp will easily keep pace with a 100W transistor amp - but then try them with vocals!, where distortion matters
Personally I like bass to be clean, and really the bass amp should only be for on-stage monitoring?, the main front of house should be through the PA.
true for larger gigs i spose but most of the stuff we do is at home practicing or in small pub's so the PA is usually only used for vocals and sometimes to mike up an acoustic or bongo's.. etc..
The PA we use for most of our gigs is either our H&H 100 watt PA head or our warfdale 260Watt PA head and a couple of 12" + horn PA speakers but as it's mostly used just for vocals it works well.. I have a larger PA system i bought in specially for a open air gig we did. anyway, g/f shouting me to check pizza...
cheers,
owen
The PA we use for most of our gigs is either our H&H 100 watt PA head or our warfdale 260Watt PA head and a couple of 12" + horn PA speakers but as it's mostly used just for vocals it works well.. I have a larger PA system i bought in specially for a open air gig we did. anyway, g/f shouting me to check pizza...
cheers,
owen
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