I think one should take a step back and don't make any changes just yet.
The measurements seem strange, but i suspect the DMM used to load the circuits too much,
giving false readings. An VTVM or "classical oscilloscope" ( not one of the usb , that
also has too low impedance to be used on tubeamps).
A VTVM ( vacuum tube voltmeter) can be found quite cheap on ebay, same with
a classical oscilloscope. A scope is invaluable if one intend to continue working
with tubeamps.
If however the interest is not to continue working with tubeamps i recommend to
leave the amp to a technitian that has the right tools.
The measurements seem strange, but i suspect the DMM used to load the circuits too much,
giving false readings. An VTVM or "classical oscilloscope" ( not one of the usb , that
also has too low impedance to be used on tubeamps).
A VTVM ( vacuum tube voltmeter) can be found quite cheap on ebay, same with
a classical oscilloscope. A scope is invaluable if one intend to continue working
with tubeamps.
If however the interest is not to continue working with tubeamps i recommend to
leave the amp to a technitian that has the right tools.
i use a fluke 87V will have very high impedance...around 10Meg....http://www.transcat.com/media/pdf/80vUserManual.pdf
i use this to test resistors in circuit, be suspicious if readings stray off the nominal value,
i lift one side and confirm....
Yes they are already replaced.
Yes i will try to do this
Thanks i will give you an update on this.
Several matters remain contradictory. (The coupling caps of 0,3µF are of secondary importance, a different matter to the 6AN8 questions.)
The low voltages on pins 2 and 3 of the triode section conforms, but is contradictory to a low voltage at pin 1 as well - that should be higher than 170V, not lower. (I note something wrong in the diagram. There is no dot (thus no connection?) at the crossing of the leads to the 150K and 15K resistors. Can one take it that as a schematic error?
Then: Is there +1,3V at pin 8 (pentode) irrespective of the position of the 500K input pot? Even when at the earthy side of said pot?
Important now, that you short pin 8 to earth and then give voltage readings again please, as asked earlier. This before any alterations.
And Junm, keep courage!
When strange voltages show up it is usually easy to locate the fault(s). It is when things do not work and no apparent discrepancy is noticable that one suffers!
The low voltages on pins 2 and 3 of the triode section conforms, but is contradictory to a low voltage at pin 1 as well - that should be higher than 170V, not lower. (I note something wrong in the diagram. There is no dot (thus no connection?) at the crossing of the leads to the 150K and 15K resistors. Can one take it that as a schematic error?
Then: Is there +1,3V at pin 8 (pentode) irrespective of the position of the 500K input pot? Even when at the earthy side of said pot?
Important now, that you short pin 8 to earth and then give voltage readings again please, as asked earlier. This before any alterations.
And Junm, keep courage!
When strange voltages show up it is usually easy to locate the fault(s). It is when things do not work and no apparent discrepancy is noticable that one suffers!
Sorry!
Missed something. Junm, in your intro post you said that the readings you found were common to both channels? The same values?? This then poses a further question, as your supply voltages do not differ by all that much.
This then takes us back to the problem of the +1,3V at pin(s) 8. That is also the same for both channels?
What am I missing?
Missed something. Junm, in your intro post you said that the readings you found were common to both channels? The same values?? This then poses a further question, as your supply voltages do not differ by all that much.
This then takes us back to the problem of the +1,3V at pin(s) 8. That is also the same for both channels?
What am I missing?
yes, that is to be considered....
i am not so familiar with the sansui 1000 receiver, but these go for big bucks as far as i can remember...
in a pioneer sa-81 tube amp i repaired, there were resistors that looked normal, i.e. no signs of burning or damage, but they test open in my dmm, and replacing just one that had opened, a 100k plate load resistor, restored the operation...
JunM is about over 100 klicks from where i live...
he will figure it out eventually....
i am not so familiar with the sansui 1000 receiver, but these go for big bucks as far as i can remember...
in a pioneer sa-81 tube amp i repaired, there were resistors that looked normal, i.e. no signs of burning or damage, but they test open in my dmm, and replacing just one that had opened, a 100k plate load resistor, restored the operation...
JunM is about over 100 klicks from where i live...
he will figure it out eventually....
completely agree..
yes it is a schematic error, i once had a similar case where one channel had more gain than the other, and trouble was traced to a leaky 150pf cap..
that styrol cap became a low ohms resistor....
the amp was plugged into 220 volts when the setting was at 110 volts..
there should be zero volts there as the negative bias is developed at the pentode's cathode resistors...
and as a matter of practice, i put a 1meg resistor from the grid socket pin to ground as protection in case the wiper of the pot losses contacts...
The schematic is indeed dont have s dot but on the other channel it is dotted and connected. 1.3v at pentode grid 1 is the culprit i think . I had already put a 680k resistor from the grid to ground but that voltage did not go low or zero. I see the full schematic
and see that pot is already isolated with a coupling cap before the pot.
and see that pot is already isolated with a coupling cap before the pot.
680k is far too big resistance. It is important now that you connect the g1 to ground and see if the voltages go to normal.
The volume potentiometer is isolated - of course - but the the anode capacitor at the revious stage is leaking, this may cause the positive voltage to g1 of the pentode.
You mentioned that you replaced the coupling caps but what about the other caps? The Japanese gear from this period used a lot of small grey caps that are notorious for their poor reliability and failure rates. I'd strongly suggest that you replace ALL of them before you go any further. I would then make sure all of the resistors are in spec.
Because they are direct coupled, the operating point of the pentode sets the operating point on the triode. After you replace the grey caps and check the resistors, play with the value of the screen (G2) resistor on the pentode section. It's listed at 600k on the schematic. Adjust the value of that resistor so that you get the correct plate voltage on the pentode section. The value you end up with will vary from tube to tube so repeat the procedure for each tube. I've seen where some people suggest installing a 1 meg pot in place of the G2 resistor to find the value needed to produce the correct voltage on the plate. I suppose you could either leave the pot in (so that it could be adjusted as the tubes age) or just use it to determine the value of the resistor. I believe these pentode/triode tubes (7199, 6AN8, 6GH8, 6U8, etc) are similar in this and that the desired voltage on the plate of the pentode is usually around 70-75v. You will then have the similar voltage on the cathode of the triode section. If higher voltages are used the risk of heater-cathode leakage increases.
Because they are direct coupled, the operating point of the pentode sets the operating point on the triode. After you replace the grey caps and check the resistors, play with the value of the screen (G2) resistor on the pentode section. It's listed at 600k on the schematic. Adjust the value of that resistor so that you get the correct plate voltage on the pentode section. The value you end up with will vary from tube to tube so repeat the procedure for each tube. I've seen where some people suggest installing a 1 meg pot in place of the G2 resistor to find the value needed to produce the correct voltage on the plate. I suppose you could either leave the pot in (so that it could be adjusted as the tubes age) or just use it to determine the value of the resistor. I believe these pentode/triode tubes (7199, 6AN8, 6GH8, 6U8, etc) are similar in this and that the desired voltage on the plate of the pentode is usually around 70-75v. You will then have the similar voltage on the cathode of the triode section. If higher voltages are used the risk of heater-cathode leakage increases.
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With due respect to all concerned, my modest opinion is that one should try to remove fault conditions before trying to set voltages by changing component values. (I think one might assume that the voltages indicated were found with the components indicated, allowing for tolerances.)
Fact remains that a positive (or any voltage) at pin 8 of the 6AN8 is wrong, irrespective of whatever else - full stop. (Just for the record, +1,3V at pin 8 would completely upset electrode voltages in a pentode.) After all the requests, let us trust Junm to provide a new set of electrode voltages after pin 8 has been shorted to earth.
(At this stage, Junm, I would suggest a dead short, not via any resistor.)
*PM: Sorry, Junm, we posted simultaneously!
Fact remains that a positive (or any voltage) at pin 8 of the 6AN8 is wrong, irrespective of whatever else - full stop. (Just for the record, +1,3V at pin 8 would completely upset electrode voltages in a pentode.) After all the requests, let us trust Junm to provide a new set of electrode voltages after pin 8 has been shorted to earth.
(At this stage, Junm, I would suggest a dead short, not via any resistor.)
*PM: Sorry, Junm, we posted simultaneously!
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The hole thing doesn't make much sense.If you take the voltages in black, the pentode has allmost no negative grid voltage leading to grid current.As for the concertina, with the measured red voltage, cathode current 25V/50k=½mA and anode current (215V-124V)/50k=1.8mA.Where is the 1.8-0.5=1.3mA gone ? Mona
Good call - getting the DC voltage off the pentode's grid should be the first step, either by grounding it or eliminating it using a cap on the input.
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