D9 will protect the grid in V3 from overvoltage vs cathode.
I see no volume control !
I see many unnecessary large caps C4 comes to my mind.
I see no volume control !
I see many unnecessary large caps C4 comes to my mind.
V1 and V2 with such different cathode values produce almost the same plate voltages and currents ?
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There's no worry for the bias of the V3 6DJ8 as it works as a buffer. It is a self biased circuit.
What is the issue with V3? If you have the protection diode in backwards, that would definitely make some issues!
1. If the diode is not connected from V3 grid to cathode:
If V2 fails open (draws no current),
then 402V goes through 301k to V3 grid, and then through V3 cathode to 33k plus 2.7k =35.7k to ground.
If V3 filament is warm, there will be 301k + 35.7k = 336.7k
402V/336.7k = 1.2mA grid current. No harm to the 6DJ8 grid.
All is OK.
But if V3 filament also is cold, then 402V could arc from grid to cathode.
(suppose the filament transformer died).
Can a 1.2mA arc destroy a cold 6DJ8?
Your guess is as good as mine.
So you might want, or might not want, to put a diode in, as in the schematic.
2. There is a very important thing to be careful of in this circuit.
6DJ8 tube specs for Cathode to Filament voltages are Different for Section 1 versus Section 2.
6DJ8 tubes were designed to be used in Cascode circuits, with Section 2 as the top triode in the DC series connection, it has a higher Cathode to Filament voltage rating.
Maximum ratings are: Section 1 average 50V. Section 2 is average 130v, 150V peak.
But, even in normal operation of this preamp, the cathode average is at 160V.
Seems a bit of a problem, especially because the stereo preamp most certainly only uses one 6DJ8 (V3.1 and V3.2).
You need to Elevate V3.1 and V3.2 filaments to a DC voltage of at least +160V - 50V = +110V. Be conservative, elevate to +130V (after you test where the cathode voltages actually sit, maybe they will not be at 160V).
And be sure to use a Separate 6.3V filament secondary for the 6DJ8.
With +130V on a filament winding that is common to all the tubes, you might be over the maximum ratings for those other tubes Cathode to Filament voltages (in this case Filament to Cathode voltages).
If V2 fails open (draws no current),
then 402V goes through 301k to V3 grid, and then through V3 cathode to 33k plus 2.7k =35.7k to ground.
If V3 filament is warm, there will be 301k + 35.7k = 336.7k
402V/336.7k = 1.2mA grid current. No harm to the 6DJ8 grid.
All is OK.
But if V3 filament also is cold, then 402V could arc from grid to cathode.
(suppose the filament transformer died).
Can a 1.2mA arc destroy a cold 6DJ8?
Your guess is as good as mine.
So you might want, or might not want, to put a diode in, as in the schematic.
2. There is a very important thing to be careful of in this circuit.
6DJ8 tube specs for Cathode to Filament voltages are Different for Section 1 versus Section 2.
6DJ8 tubes were designed to be used in Cascode circuits, with Section 2 as the top triode in the DC series connection, it has a higher Cathode to Filament voltage rating.
Maximum ratings are: Section 1 average 50V. Section 2 is average 130v, 150V peak.
But, even in normal operation of this preamp, the cathode average is at 160V.
Seems a bit of a problem, especially because the stereo preamp most certainly only uses one 6DJ8 (V3.1 and V3.2).
You need to Elevate V3.1 and V3.2 filaments to a DC voltage of at least +160V - 50V = +110V. Be conservative, elevate to +130V (after you test where the cathode voltages actually sit, maybe they will not be at 160V).
And be sure to use a Separate 6.3V filament secondary for the 6DJ8.
With +130V on a filament winding that is common to all the tubes, you might be over the maximum ratings for those other tubes Cathode to Filament voltages (in this case Filament to Cathode voltages).
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By the way, ECC83 grids are even more sensitive to grid current than 6DJ8 grids.
No protection diodes on V5 grids?
It looks like if V4 ever opens (no current), then V5 grids are going to be cooked.
No protection diodes on V5 grids?
It looks like if V4 ever opens (no current), then V5 grids are going to be cooked.
V3 is a cathode follower and it tracks the plate voltage of the tube connected to its grid. V3's output sits 1 grid-to-cathode drop below the plate voltage of V2, and has near unity gain amplification factor.
V3 and V5 filaments is elevated 160 V above the ground. I am using ECC88, it was brand new but I think is faulty now. it is ringing loud and the 33k resistor is very hot. Voltage measured on this resistor is around 340 V.
6DJ8/ECC88 maximum anode voltage is 550V (tube Cold). No problem with that.
But . . .
With a warm tube, 6DJ8/ECC88 maximum anode voltage is only 130V (Plate to Cathode).
Cathode at 160V, + plate at 130V higher = 290V.
Your B+ is 402V, lots more than 290V.
When things heat up, elements might expand, misshape or whatever.
Might cause a tube Plate to Grid short?
But . . .
With a warm tube, 6DJ8/ECC88 maximum anode voltage is only 130V (Plate to Cathode).
Cathode at 160V, + plate at 130V higher = 290V.
Your B+ is 402V, lots more than 290V.
When things heat up, elements might expand, misshape or whatever.
Might cause a tube Plate to Grid short?
Just to get it up and running, rewire all connections for a 12AU7.
Just wire up 1 triode, leave the other triode disconnected.
12AU7 Heater is different than 6DJ8/ECC88, 4&5 to 9, versus 4 to 5 respectively.
If that does not work at all, you need to check two things:
Whether the schematic is completely correct, it looks like 'copy-work', not original.
All of the wiring versus the schematic.
Just wire up 1 triode, leave the other triode disconnected.
12AU7 Heater is different than 6DJ8/ECC88, 4&5 to 9, versus 4 to 5 respectively.
If that does not work at all, you need to check two things:
Whether the schematic is completely correct, it looks like 'copy-work', not original.
All of the wiring versus the schematic.
this is how Audio Research did on SP-8. this is a copy. only power supply is mine i didn't show it here. I made a PCB so it will be difficult to rewire. It worked at the beginning with no issues and when I placed the PCB in the enclosure then became no good.
Some things are both an art and a science.
The Tektronix 545 Oscilloscope was that way.
The US government told 3 other manufacturers to build copies of the Tektronix 545
(in violation of many US Patents that Tektronix owned).
The 3 manufacturer's oscilloscopes never performed up to specifications.
Perhaps there is a lesson here.
I bet Audio Research knew which 6DJ8/ECC88 tubes were reliable for a long lifetime in their preamp.
The Tektronix 545 Oscilloscope was that way.
The US government told 3 other manufacturers to build copies of the Tektronix 545
(in violation of many US Patents that Tektronix owned).
The 3 manufacturer's oscilloscopes never performed up to specifications.
Perhaps there is a lesson here.
I bet Audio Research knew which 6DJ8/ECC88 tubes were reliable for a long lifetime in their preamp.
ops. so I do have a problem! Another thoughts, Audio Research in 80's allowed other services to repair their equipment and still they are in use and serviced not only by ARC so would they be so strict in their design to fail others?
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From "Animal Farm" - George Orwell
"All animals are created equal"
And later in the book "Some animals are more equal than others"
"Some tubes are more equal than others" - Me
I am sorrey, I have no real answer to your question, and doubt that anybody else is sure either.
One more thing to check, your power supply . . . how many volts is the B+ before the tubes warm up?
is your power supply exactly the same as the Audio Research power supply?
"All animals are created equal"
And later in the book "Some animals are more equal than others"
"Some tubes are more equal than others" - Me
I am sorrey, I have no real answer to your question, and doubt that anybody else is sure either.
One more thing to check, your power supply . . . how many volts is the B+ before the tubes warm up?
is your power supply exactly the same as the Audio Research power supply?