Hi,
I noticed a quiet hum from one of my speakers yesterday evening. I suspected that the bias needed adjusting so I checked it on all four output valves. Three out of four were reading around 350mV, where they are supposed to be, but one was almost 600mV. I adjusted it down to 350mV but the hum increased and, on switching off, the speaker produced a series of pops. Measuring and adjusting again, it seemed like the valve was underbiased at 350mV and was actually biased better up around 600mV. Looking inside, it turns out that the 10ohm cathode resistor on that valve has burnt and now has a resistance of 18ohms thus giving a higher voltage reading when biased at the correct 35mA.
The other three cathode resistors are also showing signs of burning, with small patches of brown on them. Clearly, I need to replace the offending resistor, and the other three as a precaution. The question is, what caused this? Could it simply be the resistor's fault? Given that all four show signs of overheating, perhaps they are under-rated.
They appear to be 2W or 3W metal oxide resistors so I am intending to replace them with 3W metal oxide. Is there anything else I should be checking?
Thanks,
James
I noticed a quiet hum from one of my speakers yesterday evening. I suspected that the bias needed adjusting so I checked it on all four output valves. Three out of four were reading around 350mV, where they are supposed to be, but one was almost 600mV. I adjusted it down to 350mV but the hum increased and, on switching off, the speaker produced a series of pops. Measuring and adjusting again, it seemed like the valve was underbiased at 350mV and was actually biased better up around 600mV. Looking inside, it turns out that the 10ohm cathode resistor on that valve has burnt and now has a resistance of 18ohms thus giving a higher voltage reading when biased at the correct 35mA.
The other three cathode resistors are also showing signs of burning, with small patches of brown on them. Clearly, I need to replace the offending resistor, and the other three as a precaution. The question is, what caused this? Could it simply be the resistor's fault? Given that all four show signs of overheating, perhaps they are under-rated.
They appear to be 2W or 3W metal oxide resistors so I am intending to replace them with 3W metal oxide. Is there anything else I should be checking?
Thanks,
James
You could swap tubes and make sure that the tube with the 600 mV reading will show 350 mV when placed in another socket. If it does, the tube seems healthy.
Regards, Gerrit
Regards, Gerrit
Hi,
Thanks for the quick reply! I have checked this as at first I suspected the valve. i swapped the pairs over and the valve did indeed read 350mV in a different position.
Thanks,
James
Thanks for the quick reply! I have checked this as at first I suspected the valve. i swapped the pairs over and the valve did indeed read 350mV in a different position.
Thanks,
James
Yep, that's a good idea. I do have some 7W wirewounds but they're a bit too big. I'll order some 5W. Do you reckon it's just a case of them being underspec'd then? The amp is 13 years old.
Hi James,
Your tubes look OK. Just get 4 decent wirewound resistors. 5 Watt versions will do.
Regards, Gerrit
Your tubes look OK. Just get 4 decent wirewound resistors. 5 Watt versions will do.
Regards, Gerrit
JMO, 5% tolerance parts in a bias set position is not good enough. This 1% tolerance part will do the job well.
BTW, metal oxide resistors have a reputation for bad sonics.
BTW, metal oxide resistors have a reputation for bad sonics.
Of course, it’s allways better to use resistors with lower tolerances. Perhaps buy 20 and select the best matching 4 resistors. Or find a good source for 1% tolerance parts.
Regards, Gerrit
Regards, Gerrit
Ohms Law.
0.6V across 18 Ohms is 0.02 Watts.
Mental cross-check: less than a volt across 10 ohms or more is less than a tenth watt.
Those resistors are not burning at idle.
At full roar of a pair of hi-fi EL34 we might expect 0.324 Watts. Unless your resistors are wee things (and you rock-out all night), they are not music-smoked.
So I would be think a shorted tube. Maybe a loose bit inside. I have seen an EL34 burn a power transformer.
These resistors should be 1/2w film resistors. The current at normal
use will not exceed 500mA. At flashing-over tubes the resistors should
vaporize fast and protect your output transformer.
A tubes that has flashed is damaged ans shoudl be junked.
In your case, replace at least the pair where the damage occured with
a new matched pair. Replace the resistors with 10 ohm 1/2 w
Do not use higher powered resistors, it's the equivalent of replacing
the fuze with a metal insert.
What is the DC resistance of the output tubes g1 to ground?
Often, when cathode resistors are smoked/browned, it indicates that the tubes are going into thermal run-away.
Just using a higher wattage rating cathode resistor will not fix the problem,
More tubes may go bad, the output transformer may be damaged, the power supply may be damaged.
There are many commercial amps out there that violate the rules when it comes to maximum tube ratings.
They work for a while, often many years, and then . . .
Some Caps in RC coupling have voltage ratings that are OK for a warmed up amp, but become leaky from age, or because the B+ is too high at power up before the tubes warm up and conduct. A leaky cap can cause the tube to draw too much current.
Apparently, this amp model has many versions of modifications. Some modifications trade off reliability for some other improvement.
Please post an accurate schematic.
Often, when cathode resistors are smoked/browned, it indicates that the tubes are going into thermal run-away.
Just using a higher wattage rating cathode resistor will not fix the problem,
More tubes may go bad, the output transformer may be damaged, the power supply may be damaged.
There are many commercial amps out there that violate the rules when it comes to maximum tube ratings.
They work for a while, often many years, and then . . .
Some Caps in RC coupling have voltage ratings that are OK for a warmed up amp, but become leaky from age, or because the B+ is too high at power up before the tubes warm up and conduct. A leaky cap can cause the tube to draw too much current.
Apparently, this amp model has many versions of modifications. Some modifications trade off reliability for some other improvement.
Please post an accurate schematic.
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All of the vr70x schematics use 100k to the bias supply.
ANother question is : what tubes are used ?
Hi,
The amp matches the schematic you have kindly uploaded. The only difference is that R31 and R33 are 5.6k instead of 10k. The DC resistance between G1 and ground is measured to be just over 100k on each valve. The amplifier has not had any modifications made to it. The valves are no-name Chinese EL34s, there are no markings on them apart from the bias voltages written on top in felt pen.
Clearly, there is not enough power dissipated at idle to warrant a higher rated resistor. I was assuming that it could be due to power dissipated in use, without doing any calculations. It seems that this is incorrect though. I should confirm that the resistor is not badly charred and only has a small brown burn mark on it. The fact that all four cathode resistors show the same to some degree is what is making me think it is not just down to that valve failing. WRT power ratings, they appear to be either 2W or 3W in there at the moment, I can't tell exactly.
Thanks,
James
The amp matches the schematic you have kindly uploaded. The only difference is that R31 and R33 are 5.6k instead of 10k. The DC resistance between G1 and ground is measured to be just over 100k on each valve. The amplifier has not had any modifications made to it. The valves are no-name Chinese EL34s, there are no markings on them apart from the bias voltages written on top in felt pen.
Clearly, there is not enough power dissipated at idle to warrant a higher rated resistor. I was assuming that it could be due to power dissipated in use, without doing any calculations. It seems that this is incorrect though. I should confirm that the resistor is not badly charred and only has a small brown burn mark on it. The fact that all four cathode resistors show the same to some degree is what is making me think it is not just down to that valve failing. WRT power ratings, they appear to be either 2W or 3W in there at the moment, I can't tell exactly.
Thanks,
James
Sane tubes in a sane amp will not draw more then, say, 100mA in any
conditions. The fact that the resistors are "burned" tells us that under
some conditions the tubes draws huge currents. This might damage the
tubes.
Change to 1/2w resistors might give a clue about when this happens.
It could be that the preamp sends a huge pulse at power off, it could be
energy in very low or very high frequencys.
Or make it 1/4w resistors to really detect when it happens. Use carbon
or metal film as they evaporate faster without sputtering too much.
I'd be interested to hear the argument for using a higher wattage resistor, since this was recommended by several. I can absolutely see the argument for using the minimum required rating as a mechanism to protect the output transformer. There are no fuses on the secondary on this amplifier so it would rely on the mains fuse blowing. I have read about people fusing the cathode but others argue that correctly rated resistors would actually blow faster than a fuse.
I initially ordered some 1% 5W wirewounds and I have 1% 1/4W metal films. I don't have any 1/2W so I'd have to order them.
As far as I can tell, it's going to be quite difficult to diagnose the cause. The only apparent problem is the damaged resistor and it hasn't failed spectacularly. The amplifier is functioning fine otherwise and moving the valves around did not cause a fault on the other side. Any suggestions for further investigations?
Thanks,
James
I initially ordered some 1% 5W wirewounds and I have 1% 1/4W metal films. I don't have any 1/2W so I'd have to order them.
As far as I can tell, it's going to be quite difficult to diagnose the cause. The only apparent problem is the damaged resistor and it hasn't failed spectacularly. The amplifier is functioning fine otherwise and moving the valves around did not cause a fault on the other side. Any suggestions for further investigations?
Thanks,
James
1/4w resistor will do .
Install them and use the amp. If it fails you will find blown resistor and can continue
with replaceing resistor and powertubes.
Install them and use the amp. If it fails you will find blown resistor and can continue
with replaceing resistor and powertubes.
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