Hi guys, in this circuit a few of the highlighted 150k connected to the kt120 drive tube cathode have blown. Usually for power tubes, the cathode resistor blown normally means a bad tube, however in this case those blown resistors are connected to the cathode resistors of other side, which confuses me. Could it be the tube next to them or something else?
Greatly appreciated anyone can help me understand🤝
Greatly appreciated anyone can help me understand🤝
No, the cathode resistors are connected to the -200v rail, on both KT120s.
(The KT120 is a Chinese replica of the well remown KT88).
There are a lot of non standard, silly value components, in your schematic.
Valves have a wide range of tollerance and to trim a 1 Ohm resistor with a 27.4 Ohm resistor or a 2uF with a 2n4, is frankly absurd!
(The KT120 is a Chinese replica of the well remown KT88).
There are a lot of non standard, silly value components, in your schematic.
Valves have a wide range of tollerance and to trim a 1 Ohm resistor with a 27.4 Ohm resistor or a 2uF with a 2n4, is frankly absurd!
Thanks for the input Jon. Can I put it this way, if we measure the all the power rail around the tube and if any of them doesn’t read what it should, then we should investigate power rail? Or if all power rails measure fine then it’s the tube?
I was wondering if some of the 150k resistors drift to a smaller value then they start to consume more power, maybe causing them to drift more and then they run away? Are they 0.25W, so that 20 would be 5W? Each one is around 0.165W with 1.1mA through each one.
What is the advantage of paralleling so many resistors like that? Are they well spread out so they dissipate heat better?
What is the advantage of paralleling so many resistors like that? Are they well spread out so they dissipate heat better?
All in parallel so either all blow or all stay.
You are just repeating something you read somewhere.
It might apply here or not, you must analyze the circuit
Not related, not the way to solve a problem.
Not sure what your actual problem Is, what does that circuit apply to, etc
Never seen that.
I suspect simulator "designed" circuit by somebody who has no clue but just "threw some values in"
So if simulator answer was, say, "2.0024 uF" he obediently paralleled 2uF cap with a 0.0024 uF one"
Or something to that effect.
Fine in Simulator World, unrealistic in the Real World where "parts tolerance" is ... ummm .... "Reality".
Thanks for in input JM appreciated.
This is a mono power amplifier, one night we turned it on then follows by burnt smell, it took these resistors in the picture and the main power fuse. I’m thinking to seek some info on here regarding the cause of the problem before sending it to the technician, in case if it’s a simple fix of replacing the resistors and tube then will save the hassle.
Only a few of them look burnt but I haven’t pull the rest out to test
As I’m a newbie, understand that I could be wrong and we should analyse the circuit, the reason i said this is I used to have a similar power amplifier like this but lower spec, every time if I found any resistors of the cathode, plate or screen associated with a certain tube blown, usually a arcing tube, replace the tube and blown resistors, checked the voltages on the tube socket, always fix it.
Oops my bad😁
This is a mono power amplifier, one night we turned it on then follows by burnt smell, it took these resistors in the picture and the main power fuse. I’m thinking to seek some info on here regarding the cause of the problem before sending it to the technician, in case if it’s a simple fix of replacing the resistors and tube then will save the hassle.
High voltage across high value resistors and them failing is one of the most common faults seen in general servicing. The disipation may seem well within limits but is the voltage rating of the resistor suitable? Resistors have a voltage spec which is often overlooked.
I would say either a 'fashion fad' or simply poo
Imagine that for a millisecond a tube fail giving 420 volt to cathode
With -200 volt you have 620 volt
So each 150 kohm has 4 mA of current that means 2,4 watt of dissipation
+ the max voltage that one of them can handle;
with 1 watt is difficult to get mre than 400 Vdc on ends.
Walter
With -200 volt you have 620 volt
So each 150 kohm has 4 mA of current that means 2,4 watt of dissipation
+ the max voltage that one of them can handle;
with 1 watt is difficult to get mre than 400 Vdc on ends.
Walter
I suggest you to put a 3 watt good resistors with max Vdc at 600 V
You need 15 kohm of valure , with 47kohm x 3 in parallel you can get the same results
Walter
You need 15 kohm of valure , with 47kohm x 3 in parallel you can get the same results
Walter
The KT120's I have are all marked as made in Russia by Tung-Sol.
Not heard that china also made this tube.
On the circuit diagram R41 and R40 is specified as 1meg. This is a bit worrisome and seems a little high as the data sheet specifies 0.24meg max with self bias.
True this is a cathode follower however I would still have concerns for the long term stability of the bias level of the output tubes.
If the 150K resistors have burnt the only source of current is through the connected driver Kt120 cathode .
So that connected Kt120 could have to have drawn excessive current and destroyed the 150K resistors.
Perhaps if that driver tube has become a bit leaky and with a 1Meg grid resistor it may have run away. If so the connected outputs will possibly have been eaten as well.
As the cathode follower is boot strapped and we have only part of the circuit diagram it is not possible to estimate the maximum voltage the 150K resistors may be subject to and so if failure was due to over voltage or over power.
Perhaps consider to post the whole circuit diagram and the connected power supply.
True this is a cathode follower however I would still have concerns for the long term stability of the bias level of the output tubes.
If the 150K resistors have burnt the only source of current is through the connected driver Kt120 cathode .
So that connected Kt120 could have to have drawn excessive current and destroyed the 150K resistors.
Perhaps if that driver tube has become a bit leaky and with a 1Meg grid resistor it may have run away. If so the connected outputs will possibly have been eaten as well.
As the cathode follower is boot strapped and we have only part of the circuit diagram it is not possible to estimate the maximum voltage the 150K resistors may be subject to and so if failure was due to over voltage or over power.
Perhaps consider to post the whole circuit diagram and the connected power supply.
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I've had 10r cathode resistors go high in value or failed open circuit. These were decent Vishay 2w resistors, I had similar issue's with other good quality resistors. This was in 120w PP EL34 amp, the valves were good. In the end I used Welwyn wirewound W22 resistors which solved the issue.
The failure in your amp looks like either a massive current surge or a high voltage arc between traces, the amount of blackening is indicative. Did any fuses blow, that is if the power supply has them? I agree it looks like a badly designed and laid out circuit. Before you replace any components I'd recommend you have a closer look at the power supply design, any protection circuits & the amp design in general.
Andy.
The failure in your amp looks like either a massive current surge or a high voltage arc between traces, the amount of blackening is indicative. Did any fuses blow, that is if the power supply has them? I agree it looks like a badly designed and laid out circuit. Before you replace any components I'd recommend you have a closer look at the power supply design, any protection circuits & the amp design in general.
Andy.
Thanks for the inputs, this is the whole diagram I have and that’s all, this channel has 6 power tubes but on the diagram there’re only 4 shown, so some part of this diagram is missing.
Ok, it's a bit of a nightmare, can't figure out the front end, IE the input etc, but what you have is two KT120's configured as cathode followers to drive the OP valves. The cathode followers are V8 & V?, these are the valves where your cathode resistors have died. These as well as driving the OP stage, bias the OP valves. It looks like the amp is running in Class AB2: what's the OP power BTW?
I make the actual resistance of 6 150k R's in parallel as 25k, so you could try a a 25k 5 to 7w cathode resistor instead of all those 150k's, but the voltage rating at power on is a bit tricky. I'd fit a reversed bias diode, something like a 1N4007 from grid to cathode of V8 & it's opposite. This is standard protection for cathode followers. On power on, until the valves warm up, he diode keeps the grid & cathode within 0.6v of each other. After the valves have warmed up, it's switched out effectively. See - http://www.valvewizard.co.uk/dccf.html the end of the article.
Andy.
I make the actual resistance of 6 150k R's in parallel as 25k, so you could try a a 25k 5 to 7w cathode resistor instead of all those 150k's, but the voltage rating at power on is a bit tricky. I'd fit a reversed bias diode, something like a 1N4007 from grid to cathode of V8 & it's opposite. This is standard protection for cathode followers. On power on, until the valves warm up, he diode keeps the grid & cathode within 0.6v of each other. After the valves have warmed up, it's switched out effectively. See - http://www.valvewizard.co.uk/dccf.html the end of the article.
Andy.
I agree. This amp isn't for beginners. Like all queries from those asking for help it's hard for us to gauge their skill level, test gear they have etc. If the OP is a novice I'd recommend taking the amp to an experienced tech, however experienced tech's are thin on the ground or non existent depending on where you live.
With a failure like this before power is applied the whole amp wants going through with a fine tooth comb. Check every fet for shorts, check for damage else where then if you do power up, do so using a lamp limiter and variac if possible without valves in. If no variac use a LL and just two OP valves in. TBH this amp looks like a blo*dy nightmare to work on.
Andy.
With a failure like this before power is applied the whole amp wants going through with a fine tooth comb. Check every fet for shorts, check for damage else where then if you do power up, do so using a lamp limiter and variac if possible without valves in. If no variac use a LL and just two OP valves in. TBH this amp looks like a blo*dy nightmare to work on.
Andy.