Hi, i'm new around here, looking for some advice.
Recently my twin reverb starts to make a "cut noise" that sounds a little like a broken speaker. I checked and the speakers seems to be ok, but one of the power tubes emits a blue glow with at the same time the noise is happening. I flipped that tube with the next one and the glows dissapeared but the noise continues.
https://soundcloud.com/brauliopanseco/tube-noise
any thoughts of what can be the cause of the problem?
thanks!
Recently my twin reverb starts to make a "cut noise" that sounds a little like a broken speaker. I checked and the speakers seems to be ok, but one of the power tubes emits a blue glow with at the same time the noise is happening. I flipped that tube with the next one and the glows dissapeared but the noise continues.
https://soundcloud.com/brauliopanseco/tube-noise
any thoughts of what can be the cause of the problem?
thanks!
Sounds like serious crossover distortion. Probably a duff 6L6. I would replace the pair and re bias them at about 40mA per envelope.
A blue glow is not abnormal.
A blue glow is not abnormal.
Ok but i'm using TAD tubes with 35 ma PC marked on the tube and i called the distributor asking for a 35ma tube to save money for the bias adjustment.
For what you say, may i assume that if i don't change the bias value it could happend again?
Thanks a lot for answering.
For what you say, may i assume that if i don't change the bias value it could happend again?
Thanks a lot for answering.
Not at all. Any 6L6GC will fit as matched pairs. There is a bias adjust pot inside the chassis to set the bias current. The value written by the seller, on the valve, is meaningless as if the bias pot is moved, so is the quiescent current. It is a way of charging more, nothing less.
It is false economy to change just one valve. Twin reverbs have four 6L6GCs, if one has failed, change the lot. If you carefully measure the current across the open contacts of the standby switch and power up the amp. You are looking for a reading of about 160mA and you will be somewhere near the Fender rich sound. If you adjust for say 40mA then your Fender will sound like a Marshall.
The maximum current for a 6L6GC is around 45mA per valve anymore and you have exceeded Class A and it will overdrive the valves and cause 'red plating' or a glowing anode.
If you don't feel competent to set up the bias, find someone who can.
IT IS A DANGEROUS PROCEDURE!
It is false economy to change just one valve. Twin reverbs have four 6L6GCs, if one has failed, change the lot. If you carefully measure the current across the open contacts of the standby switch and power up the amp. You are looking for a reading of about 160mA and you will be somewhere near the Fender rich sound. If you adjust for say 40mA then your Fender will sound like a Marshall.
The maximum current for a 6L6GC is around 45mA per valve anymore and you have exceeded Class A and it will overdrive the valves and cause 'red plating' or a glowing anode.
If you don't feel competent to set up the bias, find someone who can.
IT IS A DANGEROUS PROCEDURE!
You can just, sell the amp to me?
All joking aside (well kinda) you may be able to just pull two output tubes - my Peavey has four output tubes, but the heaters are wired in series and you cannot just pull the two tubes...dont know about your twin...a quick check of schematics may help determine if you can...
All joking aside (well kinda) you may be able to just pull two output tubes - my Peavey has four output tubes, but the heaters are wired in series and you cannot just pull the two tubes...dont know about your twin...a quick check of schematics may help determine if you can...
Fender never wires tube heaters in series, you can pull any tube in the Twin and the remaining ones still function.
Thanks a lot for all the information here. The thing is that the bias of my amp is adjusted to this set of valves, as i said marks 35PC so if i replace them for another set of valves marked 35PC aswell i should not need any bias adjustment. Is that information correct?
ok i replaced the four 6L6GC and the noises stills.
Since its been producing with the amp vibration could it be something loose inside?
any thoughs?
thanks
Since its been producing with the amp vibration could it be something loose inside?
any thoughs?
thanks
Maybe an oscillation, that's the second thing that came to my mind, but that can hardly happen because the Fender factory built amps that very rarely had any oscillation, oscillation is more common in Marshall amps then Fender, because of its layout... Third idea is something has worn over the years, cold solder joint, maybe bad bias cap (they can usually be 25v 25uF). Anyways, if you have a multimeter, make sure to check the caps and continuity.
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