R10 is 47k, R8 is 220k.
61V across 47 kOhm means 1,3 mA.
282V across 220k means 1,3 mA.
Check the same voltages on R9 and R7.
I just noticed the original circuit says 2,5mA on the CCS, and that's wrong.
I'd reduce it to half of it and check how it sounds: Change R18 to 1.2k then try 1k.
Thanks @zintolo, here is what I measured:
R9 - 371V, 319V, 30kohm on board, so 1.7ma?
R7 - 318V, 124V, 220kohm on board, 0.9ma
R9 & R10 both call for 47kohm, but the person who populated this board used 30kohm resistors for whatever reason. So R10, at 61V across 30kohm would be 2.0ma, if I understand correctly.
R13 is also incorrect value, 27k instead of 39K. Should I at this point just get stock values and see where it lands me? Besides stock values, can also get 1.2kohm and 1kohm resistors for R18, or let me know if with incorrect values being installed at R9, R10, and R13 I should get any other resistors.
Very much appreciate your input!
R9 - 371V, 319V, 30kohm on board, so 1.7ma?
R7 - 318V, 124V, 220kohm on board, 0.9ma
R9 & R10 both call for 47kohm, but the person who populated this board used 30kohm resistors for whatever reason. So R10, at 61V across 30kohm would be 2.0ma, if I understand correctly.
R13 is also incorrect value, 27k instead of 39K. Should I at this point just get stock values and see where it lands me? Besides stock values, can also get 1.2kohm and 1kohm resistors for R18, or let me know if with incorrect values being installed at R9, R10, and R13 I should get any other resistors.
Very much appreciate your input!
Zintolo,
Today received parts to swap in. It ended up being that R9, R10, & R13 had correct values installed. R9 read 30kohm installed, and 47k removed. I then noticed the marking on the underside that it is a 47kohm, same for R10 and R13.
R39 had incorrect value, so I swapped that out. I also installed 1.2kohm for R18. At this time I am getting desired range for bias adjustment 🙂. Thanks for your help on this.
What I am still having difficulty with is balancing R5.
C2 - 5.1V
C1 - 137V
I don't have LED's installed. Am I correct that short leg of LED is towards front of the amp board (towards R18)?
The LED must be installed because I believe it has a role in the voltage regulation.
LED installed. I used a multimeter to see continuity between the "top" hole for led and R17, the same between bottom and R18. Mounted LED with long leg going into top hole. Lights up. It took bias voltage down for V2 (TP1 - TP3) as well as V3 (TP2 - TP3). I was able to readjust and get it back up to 550mv for V2, same for V3 but it varies between 546mv-562mv.
Still having trouble adjusting R5. Per the manual "It is also possible to check the voltage of these plates by plugin the black probe to the GND (TP3 by example) and the red probe on C1 or C2 leads (range 600 V DC or 200 V DC*), normally you must read between 150 V and 200 V DC depending of your high voltage level."
With red probe on inner leg of C1 (near 12ax7 tube) and black probe on TP3 I am getting 152V.
Doing the same for C2 - red probe on inner leg near tube and other on TP3 I am getting 315-317V.
Probes clipped on either side of C2 give reading of 247V, doing this (clipping probes to either side of C2) makes V3 redplate very quickly!!
Probes clipped on either side of C1 give reading of 156V.
Any thoughts on how to continue troubleshooting?
Still having trouble adjusting R5. Per the manual "It is also possible to check the voltage of these plates by plugin the black probe to the GND (TP3 by example) and the red probe on C1 or C2 leads (range 600 V DC or 200 V DC*), normally you must read between 150 V and 200 V DC depending of your high voltage level."
With red probe on inner leg of C1 (near 12ax7 tube) and black probe on TP3 I am getting 152V.
Doing the same for C2 - red probe on inner leg near tube and other on TP3 I am getting 315-317V.
Probes clipped on either side of C2 give reading of 247V, doing this (clipping probes to either side of C2) makes V3 redplate very quickly!!
Probes clipped on either side of C1 give reading of 156V.
Any thoughts on how to continue troubleshooting?
Quick question. I don't have R22 installed, I see in schematic it shows R6 with value of 47k. If I don't have pot installed at R22 do I install 47k resistor between the two input wires (shorting them), or between input positive and gnd? My board is Rev 2.0.
Many thanks,
Many thanks,
I guess what was throwing me off is R6 is in the schematic, but it is neither in BOM neither is there a place to mount it on the board version I have. Though I see that changed in later revisions. I ended up mounting as pictured, a 47.5kohm resistor (schematic called for 47k, value installed is what I had in hand). Voltages between C1 and ground, and C2 and ground, are now 233V, too high per the build guide, but about the same on either side. With probes on inner legs of C1 & C2, voltage settles around 50mv after about 10 minutes after power up. but fluctuating +/-35mv. V2 & V3 biased around 550mv, fluctuating +/-15mv. Are these fluctuations within an acceptable range? Any thoughts to reducing voltage for C1&C2? Will still go ahead with readings from schematic, that's just a bit intimidating as I'm nervous that I measure incorrectly.
Changing R18 to 1k reduced voltages for C1&C2 to 209V. I did also install R9 same as pictured on my second board, which still has 620ohm resistor for R18, that one measured 131V across C1&C2.
I've collected the readings on the schematic doc, please see attached. I measured voltages across ground, so for example if it says 28&67 it means one leg measured 28V across TP3, and the other measured 67V across TP3. Hope it is clear/appropriate. I did not measure transistors, but happy to do so if helpful. I did also measure resistors once again to verify their values against BOM. All looks good there.
Thank you for taking the time to look this over.
You have 209 & 344 for R7 and 344 & 209 for R8.
Does that mean 209/344 for one channel and 344/209 for the other channel?
If that is the case then the balance of the ECC83 looks to be off. The voltage drop across those resistors will give you an idea of the current through the ECC83, and the current should be the same on both sides. The balance can be adjusted with R5.
If the current on each side should be 1,25mA through the ECC83, then the voltage drop across the 220k resistor should be 275V, which is around 100V on the anodes?
Does that mean 209/344 for one channel and 344/209 for the other channel?
If that is the case then the balance of the ECC83 looks to be off. The voltage drop across those resistors will give you an idea of the current through the ECC83, and the current should be the same on both sides. The balance can be adjusted with R5.
If the current on each side should be 1,25mA through the ECC83, then the voltage drop across the 220k resistor should be 275V, which is around 100V on the anodes?
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1.3 mA per side is IMHO one of the weak points of the scale-up that has been done from EL84 to EL34, together with negative rail regulation and few others.
100V on anodes means cathodes too low and close to grid current. That’s why I vote for one of the following:
I’ve chosen the last, increasing shunt feedback and reducing UL taps. This gives higher power, high damping and more linearity at all frequencies, because the phase inverter can have current and linearity at the same time.
100V on anodes means cathodes too low and close to grid current. That’s why I vote for one of the following:
- reduce the current to raise the anodes voltage to around half B+;
- reduce the load to have good current and good working point;
- raise B+ to have both good current, good working point and good gain.
I’ve chosen the last, increasing shunt feedback and reducing UL taps. This gives higher power, high damping and more linearity at all frequencies, because the phase inverter can have current and linearity at the same time.
Hi OldHector,
I first started measuring with probes attached to either side of resistors to get a single value, but was getting 0V for various resistors. Thought it more useful if I post what it measures with one probe on one leg, and the other to gnd. So 209/344 means one side to ground measures 209V, the other to gnd measures 344. That would mean ~135V across the resistor. Measuring now I am getting 130V with probes clipped on either side of R7, and 132V across R8.
edit: I also have a second board which is identical except R18 is 680ohms per the original BOM. On that board R7 measures 200V across, but C1 to gnd measures 131V. The other board has 1k on R18. Should I try a lower value resistor for R18?
I also wonder about R22, it is supposed to have a pot installed. I first did not think it necessary, then looking at schematic I saw R6. I installed 47k resistor for R6 in place of R22 as pictured a few posts up, wiring it as shown in the schematic. In the schematic R6 is shown as 47K resistor. In the BOM R22 says for GB 1&2 use 20k pot, for GB3 use 47k pot. Could the issue be I have too high a value for R6?
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Hi Zintolo,
I have one board wired per original design, that one I'm getting 131V between inner leads of C1 & C2. That one has 680ohm for R18, per original spec.
The 2nd has 1kohm for R18, getting 233V between C1&C2.
Thanks again for your time and patience.