Have this Hasimoto built amp and it's been working fine for years. Yesterday when I fired it up I noticed that the bias started increasing beyond its 50ma mark. Have a meter on the front. Quickly turned it off and took it to my work bench. Tried to readjust the bias with just the tubes in one channel and got it down to 50ma but now the voltage across the 10ohm cathode resistor measures only 20mv. Measured every other voltage on the amp and they are all according to the schematic. Even switched to another multimeter just to be sure. So now I`m absolutely confused. Never experienced this before.
You say you have a meter on the front. There's 50 mA of cathode current per KT88 flowing. So how does this meter fit into the schematic?
Actually you should first verify the actual tube current. Measure Rk and the voltage across it, then you know the current. After all, your meter circuit could also be flaky.
Jan
Jan
Like one would have measured over the cathode resistor with a multimeter. A 5 position rotary switch to choose which tube to measure.
I know. But when I measured the voltage across the cathode resistor, before I tried to re-adjust, the voltage was 1,5volt.
Since you tried the amplifier with the tubes in only one channel and since the one meter can measure all four KT88's, I conclude that your amplifier is a stereo amplifier (so not two monoblocks).
With the tubes of only one channel in place, you measured all other voltages as being according to the voltages in the schematic. I assume than that your amplifier is a double monoblock, so each channel has it's own power supply (else you would expect much higher measured voltages since only one channel draws current from the shared power supply).
So what is the other channel doing? Does that channel still works and measures fine?
In the one channel with tubes you measured 20 mV over the 10 Ohm resistors. That would mean a cathode current of only 2 mA which seems unlikely. When looking at the schematic, I find it hard to see how you can even bias the KT88's so negative that they would only pass 2 mA each.
1.5 V over the 10 Ohm resistor would mean a cathode current of 150 mA, so a dissipation of 78 Watt per KT88 which also seems unlikely.
You do know that the bias potentiometer is for balancing? If you adjust the control grid of one of the KT88's more negative, than the control grid of the other KT88 gets less negative, and viceverca.
How did you do your measurements? Did you monitor both cathode voltages at the same time?
With the tubes of only one channel in place, you measured all other voltages as being according to the voltages in the schematic. I assume than that your amplifier is a double monoblock, so each channel has it's own power supply (else you would expect much higher measured voltages since only one channel draws current from the shared power supply).
So what is the other channel doing? Does that channel still works and measures fine?
In the one channel with tubes you measured 20 mV over the 10 Ohm resistors. That would mean a cathode current of only 2 mA which seems unlikely. When looking at the schematic, I find it hard to see how you can even bias the KT88's so negative that they would only pass 2 mA each.
1.5 V over the 10 Ohm resistor would mean a cathode current of 150 mA, so a dissipation of 78 Watt per KT88 which also seems unlikely.
You do know that the bias potentiometer is for balancing? If you adjust the control grid of one of the KT88's more negative, than the control grid of the other KT88 gets less negative, and viceverca.
How did you do your measurements? Did you monitor both cathode voltages at the same time?
Swap the tubes around. Does the problem follow the tube?
Check the screen. A tube with a bad screen won't conduct much current at all...
Replace the 10R resistor with 600R/5W or 10W and ground the grid (cathode bias) - You should now have about 50V across the resistor. If not, the tube is bitched.
Check the screen. A tube with a bad screen won't conduct much current at all...
Replace the 10R resistor with 600R/5W or 10W and ground the grid (cathode bias) - You should now have about 50V across the resistor. If not, the tube is bitched.
What about coupling Cs?
try to unsolder 2 coupling C from negative grid voltage.
And insert 100 ohm cca at G2 and OT ultralinear tap.
This way You can check all currents just for output stage.
And on the open end (usoldered end) of coupling Cs 2 x 0.047uF/630V on the schematics, should not be any +DC voltage present.
IF there are some voltage trough the Cs replace them.
Anyway change the value of coupling Cs Preferably 0.47uF/630V. Because present value are cutting Bass maybe too much. With 0.047uF and 100Kohm -3db cutt off is @ 34Hz 🙁
Put some R load in the loudspealer conn. say 6.8-8.2 ohms / 10-20W. before turning on.
try to unsolder 2 coupling C from negative grid voltage.
And insert 100 ohm cca at G2 and OT ultralinear tap.
This way You can check all currents just for output stage.
And on the open end (usoldered end) of coupling Cs 2 x 0.047uF/630V on the schematics, should not be any +DC voltage present.
IF there are some voltage trough the Cs replace them.
Anyway change the value of coupling Cs Preferably 0.47uF/630V. Because present value are cutting Bass maybe too much. With 0.047uF and 100Kohm -3db cutt off is @ 34Hz 🙁
Put some R load in the loudspealer conn. say 6.8-8.2 ohms / 10-20W. before turning on.
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A normal stereo amp. All that is mension happens to all four tubes. Tried other tubes. Same thing happens. Uses two multimeters, one pr channel. 8 ohm 50w resistors over the output. Have to be quick and wont leave it on for too long so I don't destroy the tubes.
Attachments
All four coupling caps dont go bananas at once, do they? Anyway, I replaced all of them since I have a handful laying around. 0,22uF 800V Vishay MKP 1839 HQ Polypropylene.
Again, how likely is it that all four tubes suddenly have a screen failure? I tried other tubes and its the same problem.
Don't get me wrong, I really appreciate all the suggestions from you guys but it's so strange that this occurs to all four tubes at the same time. Never ever experienced this thing before that is so X-file like.
Excellent point! If all 4 at the same time 'die' it's extremely unlikely it's the tubes.
What is in common? The adjust and measurement system.
That's why I said: what is the actual current, not what the meter says.
Jan
I`m not sure but what is the safest way to measure it with the scope? I forgot I have one. 🙄
Ya I missed that it happened to all of the tubes at the same time. My bad.
As Jan said, the problem probably lies in the bias system.
As Jan said, the problem probably lies in the bias system.