i´ve got a russian amp, Natural A7, that behaves strange. Starting up it works well for about 30 minutes then one output tube, el34, starts to glow intensely and the amp starts to distort heavily. It´s a triode conected pp with 12ax7 input and ecc88 phase splitter. The problem is not tube related but position. All tubes have the same bias -28v, seems a bit wrong in triode connection? Both channels distort but the only common part is the 12ax7 and that´s ok, and psu. I can´t find anything defektive among the passives and everything seems to measure identacal between the tubes. Gratefull for any tips on this.
Regards
Regards
did you check all the voltages after the amp had warmed up and the fault had occurd? mabey theres a component that is failing to short when hot or similar..
Owen
Owen
Sounds like a bad coupling capacitor to me.. The grid bias may be on the low side as well depending on the supply voltage - if it is under 400V it is probably ok..
Thank´s for input, turned the amp uppsidedown and monitored, what happens is: bias of brightly glowing tube drops, backwards so to speak to about -14V while the rest remains at about -25V.
Cathode of bright tube goes to 2V, cathode resistor is 10 ohms so cathode is just some mV above gnd. Will replace cathode resistor (measures 10ohm off power) but still can´t understand why both channels go into Neil Young distortion teritory (good as it is) when just one tube seems to fail.
Cathode of bright tube goes to 2V, cathode resistor is 10 ohms so cathode is just some mV above gnd. Will replace cathode resistor (measures 10ohm off power) but still can´t understand why both channels go into Neil Young distortion teritory (good as it is) when just one tube seems to fail.
If Kevinkr is right about the coupling cap failure (and I think he is) then the leakage might be affecting the bias supply.
Replaced cathode resistor, did´nt make any difference, so i´ll
try replacing coupling caps. It´s a "pure" triode connection, no resistor between plate and grid, could the problem be related to excessive grid current? As i´m planning to make it switchable to pentode mode maybe it shoulb be better to introduce some 50 ohms between plate and grid for triode mode?
try replacing coupling caps. It´s a "pure" triode connection, no resistor between plate and grid, could the problem be related to excessive grid current? As i´m planning to make it switchable to pentode mode maybe it shoulb be better to introduce some 50 ohms between plate and grid for triode mode?
According to the schematic, the designer did not find it necessary with the present layout. However if you plan to extend the screen wiring for switching, it might be a sensible precaution against possible parasitic oscillations.
Will do that, after i get it to work properly as is, then start the modifications. It´s nicely built, hardwired and with pretty good or very decent parts. Before problem sounded good but a little bit low on power, very good bottom end so opt´s are probably good.
The problem is simple, you measured a decrease in bias voltage on the affected tube to -14V and a resulting increase in the voltage across the cathode resistor to 2V. Two volts across a 10 ohm cathode resistor is 200mA.
The situation described above IS the classic symptom of a leaky coupling capacitor, replace it and all will be well.. I recommend you replace all of them with ones of identical value in the next higher voltage rating or 600V which ever is higher..
Incidentally this one tube is drawing as much power by itself as the entire amplifier should at idle, it is seriously dragging down the B+ and that is what is causing the distortion on the other channel.
Note that you should not run it this way for a variety of reasons, one the excessive current in the output transformer primary may cause heating and damage the insulation, and two, the affected EL34 will be ruined by excessive dissipation and the resultant outgassing if it does not simply fail on the spot.
Kevin
www.kta-hifi.net
The situation described above IS the classic symptom of a leaky coupling capacitor, replace it and all will be well.. I recommend you replace all of them with ones of identical value in the next higher voltage rating or 600V which ever is higher..
Incidentally this one tube is drawing as much power by itself as the entire amplifier should at idle, it is seriously dragging down the B+ and that is what is causing the distortion on the other channel.
Note that you should not run it this way for a variety of reasons, one the excessive current in the output transformer primary may cause heating and damage the insulation, and two, the affected EL34 will be ruined by excessive dissipation and the resultant outgassing if it does not simply fail on the spot.
Kevin
www.kta-hifi.net
Bias--
Worth replacing while you are in there, are the resistors that feed the bias voltage to the grids, These could be a little off, which may make the tube go into a runaway scenario. They are only cheap so definately worth doing.........
Worth replacing while you are in there, are the resistors that feed the bias voltage to the grids, These could be a little off, which may make the tube go into a runaway scenario. They are only cheap so definately worth doing.........
just by reading the past posts I learned something new... that too much voltage is not the only cause for glowing tubes, too much current is as much the same then.
😎
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I agree with Alastair E on the bias resistor thing, at the minimum they should be checked.
Kevin
Kevin
Have you double checked the relevant neg bias preset isn't duff or has a dry joint ? I've had sim happen......Depending on preset value for example I wire a 100K resistor from each wiper tap to outer neg line....should track go o/c then at least the tube will go into cut off rather than stew.
I'm a bit suprised that if one tube is pulling down the B+, to distort the other channel. It isn't a good psu to operate in fixed bias mode. Unless it's a guitar amp.
richj
I'm a bit suprised that if one tube is pulling down the B+, to distort the other channel. It isn't a good psu to operate in fixed bias mode. Unless it's a guitar amp.
richj
So - i´ve replaced coupling caps, btv the 4,3k resistors are not there so the schematic is not 100% consistent with this amp, different version or.... i do not read russian that well. The problem remains but whithout the hysterically glowing tube. 20 minutes then all distortion. The psu seems to be well up to the job, mains transformer, torodial, is somewhere in the 500va range, rectifiers are ss and they either work or not, I´ve added 800uF of caps/channel long time ago but nothing seems to be wrong there as plate supply seems to stay up, only measured with dvm, it´s heavy so i have´nt hauled it to the workshop for scope test yet( scope even heavier). I suppose that defects in psu also would result in hum or noice at output but there is none, defects in opt´s would, i presume, blow the fuse in B+line (it is there as in schematic). So what to do- throw it out?- Make a litterbox for the cat out of it?-Rip it apart and use the (realy good looking) chassis for a diy project? Sell it to a well-to-do friend and claim it worked yesterday? Creative suggestions appreciated.
Possibly more bad (leaky) caps elsewhere in the circuit. If the cap on pin 7 of VL2 and or VL3 are as defective as the bad one on that output that glowed red. that could screw up the biasing on that tube and cause distortion. It's likely all the caps were made by the same vendor and same age, and will fail in the same mannor. I'd replace all the caps.
To rescue the dilema we need a little more info....From the schematic I can't make out the B+ value.....but if the grid bias is around -25V them B+ will be approx 400V.
From your schematic the power supply B+ is up before the tubes warm up. Check this instantaneous value, but be careful (there's alot of joules) With this type of psu using low Z mains transformers, the instant switch-on B+ shouldn't be more than 5-10% above the normal B+ after everything has warmed up.
Generally I'm against these types, as I have an amp that squeals on warm up due the front end warming up sooner than the other tubes.
Note/mark the suspect output tube; Do the AB swap check with the other channel and see if sim happens.
About suspect leaky interstage caps....... since the instant B+ appears on switch-on, the coupling caps from phasesplitter anodes to the output stage will see true B+ to - grid bias values. What is the voltage value stated on the caps ?
Do NOT dispose of the B+ fuse. Transformers and fires are expensive.
richj
From your schematic the power supply B+ is up before the tubes warm up. Check this instantaneous value, but be careful (there's alot of joules) With this type of psu using low Z mains transformers, the instant switch-on B+ shouldn't be more than 5-10% above the normal B+ after everything has warmed up.
Generally I'm against these types, as I have an amp that squeals on warm up due the front end warming up sooner than the other tubes.
Note/mark the suspect output tube; Do the AB swap check with the other channel and see if sim happens.
About suspect leaky interstage caps....... since the instant B+ appears on switch-on, the coupling caps from phasesplitter anodes to the output stage will see true B+ to - grid bias values. What is the voltage value stated on the caps ?
Do NOT dispose of the B+ fuse. Transformers and fires are expensive.
richj
Another consideration is that one tube is now destroyed, so you may have fixed the original fault, but now have a new one.
Cat preferes ss litterbox
So, decided not to throw it out, cat doesnt like high tension devices- makes havoc with her fur, so i´ll go for a mix of repair and redesign. Will replace all passives, there are no decoupling caps on cathodes, neither input tube nor outputs, should´nt there be? At startup B+ rises slowly to 402 then drops and settles to 366V, no ripple. Grid bias, warmed up, is -26V, cathode is some 500mV wich doesn´t feel right, should be higher? Phase splitter/driver is ECC88, 280V on plate seems to high on this tube? Will connect for pentode mode only, maybe replace drivers with ECC83?
So, decided not to throw it out, cat doesnt like high tension devices- makes havoc with her fur, so i´ll go for a mix of repair and redesign. Will replace all passives, there are no decoupling caps on cathodes, neither input tube nor outputs, should´nt there be? At startup B+ rises slowly to 402 then drops and settles to 366V, no ripple. Grid bias, warmed up, is -26V, cathode is some 500mV wich doesn´t feel right, should be higher? Phase splitter/driver is ECC88, 280V on plate seems to high on this tube? Will connect for pentode mode only, maybe replace drivers with ECC83?
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