Luckiestmanalive Posted July 2, 2022 Author Posted July 2, 2022 OK, that's done - the pot is indeed at the end of its range... 1
xlr8or Posted July 2, 2022 Posted July 2, 2022 (edited) On 02/07/2022 at 12:24 AM, Luckiestmanalive said: Yeah, that's what I thought. So this morning I repeated the procedure with the quad bias tester but with the bottom off and the amp angled backward using a couple of laptop ramps so I could get to the pots. It's a real beggar that the two pots to adjust the bias are set at the back (making them hard to get to) and the front two only balance the two pairs of output tubes. I managed to reduce the output of V6 and V7 down to the 50s fine but I couldn't turn the other pot for V4 and V5 any lower than 70mA. So I turned the other one up to match, made sure the output tube pairs were balanced and turned it off. It looks like I will need to do more troubleshooting or find someone who knows what they are doing to look at it. The fact that one pot is fine and the other isn't suggests the two sides of the V1 (6AQ5) and/or V2 and V3 (6240Gs) are unbalanced? And the bias circuit has an incorrect or failing component? Expand A few things to consider now that you have shared this update is that the quad set of KT88's is not matched. I would suggest splitting up the 70mA pair and mixing each tube with the 50mA pair. Also, running the KT88 that high will eventually cause the KT88 to red plate and you are then at high risk of taking out an OPT. I would suggest also installing KT120's as a priority to allow for the correct biasing range. The 6240G and 6AQ8 are unrelated to the ascribed problem. Edited July 2, 2022 by xlr8or
Luckiestmanalive Posted July 2, 2022 Author Posted July 2, 2022 (edited) Ah, OK. I get it - thanks, Kirk! So I swapped the V5 and V7 tubes and went through the whole measuring and adjusting process again. It gets easier with practice (!) so now I have got all the tubes balanced around 62mA. I can't get it any lower because one of the bias pots is at its lowest setting and the other is set to match. I know 62mA is still too high so I will use the amp sparingly, if at all, while I save up for a matched quad of KT120s. If you know where I can get a good price (ie better than I can do by google) then please let me know. I'm just glad I got the bias tester so quickly and did those measurements! I can't imagine how long the tubes and OPTs could have survived at the original bias settings... Edited July 2, 2022 by Luckiestmanalive 1
muon* Posted July 2, 2022 Posted July 2, 2022 Audiophonics in France have quads for 259 euro, that's around $400AUD plus shipping, no stock till September. https://www.audiophonics.fr/en/tubes-and-lamps-tung-sol/tung-sol-kt120-power-tube-high-quality-matched-quad-p-13633.html Cheapest I could find. 2
xlr8or Posted July 2, 2022 Posted July 2, 2022 (edited) How confident are you at using a DMM? Do you think you can take a measurement of B+ DC volts relative to cathode, if you remove one of the tubes and measure between pins 3 and 8 from the top? It would be the reverse configuration to what you see from underneath. Edited July 2, 2022 by xlr8or
xlr8or Posted July 2, 2022 Posted July 2, 2022 On 02/07/2022 at 6:01 AM, Luckiestmanalive said: If you know where I can get a good price (ie better than I can do by google) then please let me know. Expand I have a back up quad set that I don't want to let go. The best thing to do is post in the WTB section. Even 2 pairs will be fine. 1
Luckiestmanalive Posted July 2, 2022 Author Posted July 2, 2022 (edited) On 02/07/2022 at 10:11 AM, xlr8or said: How confident are you at using a DMM? Do you think you can take a measurement of B+ DC volts relative to cathode, if you remove one of the tubes and measure between pins 3 and 8 from the top? It would be the reverse configuration to what you see from underneath. Expand So it is OK to remove one tube only, insert my probes in pins 3 and 8, switch the DMM to DC volts (500V scale?), switch the amp on (no input), and take a reading? Yip, I can do that! Looking at the tube socket from the front and top, it looks like these are pins 3 and 8 (key is to the left). Edited July 2, 2022 by Luckiestmanalive Correct pin diagram
xlr8or Posted July 2, 2022 Posted July 2, 2022 (edited) Yes. That's right. Here is an image from underneath. So it's the inverse looking from the top relative to the guide key. Edit: This image may also help. Edited July 2, 2022 by xlr8or 3
Luckiestmanalive Posted July 2, 2022 Author Posted July 2, 2022 Thanks, K - I've corrected my diagram in the last post so there is no confusion. Two more questions before I go ahead: 1. Do I need the speakers connected to the amp? 2. What reading should I expect to see? My cheap multimeter has a 250V fuse... Also, I have a copy of the original owner (builder) of the amplifier with their calculations of the changes from the circuit conversion to 6550/KT88. I can make out some of the calculations but not others. Is it worth posting these?
xlr8or Posted July 3, 2022 Posted July 3, 2022 On 02/07/2022 at 11:09 PM, Luckiestmanalive said: Thanks, K - I've corrected my diagram in the last post so there is no confusion. Two more questions before I go ahead: 1. Do I need the speakers connected to the amp? 2. What reading should I expect to see? My cheap multimeter has a 250V fuse... Also, I have a copy of the original owner (builder) of the amplifier with their calculations of the changes from the circuit conversion to 6550/KT88. I can make out some of the calculations but not others. Is it worth posting these? Expand Yes, speakers connected with volume control on zero and no signal passing through. The voltage should be circa 500v DC. No problem with the fuse on the DMM, it's unrelated.
Luckiestmanalive Posted July 3, 2022 Author Posted July 3, 2022 OK, done it - I got a reading of 520V.
xlr8or Posted July 3, 2022 Posted July 3, 2022 (edited) On 03/07/2022 at 3:27 AM, Luckiestmanalive said: OK, done it - I got a reading of 520V. Expand OK - that's not good. I was kinda hoping it would be 450v or less. So the KT88's are running way too hot. It would be best to trace the circuit that I provided, and my own circuit, back from the 2 centre pots to the power transformer to see if all resistance values are correct. The resistance needs to drop. I'm assuming you have the same pot values as mine (25kohm). Edited July 3, 2022 by xlr8or 2
Luckiestmanalive Posted July 3, 2022 Author Posted July 3, 2022 (edited) OK, I'm checking the photo you provided with mine... so far, I can see a resistor in yours that is 820 ohm (Grey Red Brown - R305 has this value). That resistor in my amp is 8.2K ohm (Grey Red Red) - it is listed on the conversion instructions as one to be changed to the value in my amp. Other than that, there are only three others I can't make out very well in my photos for comparison with yours (I'll take it apart again to check them later). One of them is an extra resistor on the left power cap - I can't make out the bands to determine its value (you can see it in my pic on the first page). I was concerned about how high the first reading was so I repeated it, but was more steady and patient. The voltage readings jumped up to 530V after switching the power on before settling back within a minute to 500V before I turned it off. So, still high but not as bad. Edited July 3, 2022 by Luckiestmanalive 1
xlr8or Posted July 3, 2022 Posted July 3, 2022 (edited) Where is the 4kohm resistor positioned in the control grid circuit coming to the 2 centre pots? I see that value highlighted in the 2 schematics that have been provided. Is it that additional resistor installed on the second cap? Edited July 3, 2022 by xlr8or
Luckiestmanalive Posted July 3, 2022 Author Posted July 3, 2022 (edited) On the three resistors I said I'd have to check close up: 1. The extra resistor on my amp connected to the left power cap is a 33Kohm resistor (orange orange orange). 2. There is a difference between a resistor connected to the left power cap - on your amp it is a 150Kohm resistor (Brown Green Yellow - R307?) and on mine it is 27Kohms (Red Violet Orange). 3. The other one is a 240Kohm resistor (Red Yellow Yellow - could be R303) on both our amps, which matches the value of the circuit diagram supplied with the conversion instructions but its value is 330Kohms on the stock MQ3600 and A3600 circuit diagrams. Oh, and there is a 10Kohm resistor (Brown Black Orange) in the preamp socket that I can't compare with yours. Edited July 3, 2022 by Luckiestmanalive
Luckiestmanalive Posted July 3, 2022 Author Posted July 3, 2022 On 03/07/2022 at 11:01 AM, xlr8or said: Where is the 4kohm resistor installed in the control grid circuit coming to the 2 centre taps? I see that value highlighted in the 2 schematics that have been provided. Is it that thr additional resistor installed on the second cap? Expand What number is it in the schematic?
xlr8or Posted July 3, 2022 Posted July 3, 2022 (edited) It's best to take it to Chris at Kimil Electronics in Croydon and have the circuit properly corrected. I'm afraid by exchanging what resistors should be where we aren't going to be able to fix it. The grid bias circuit needs to be traced back from the tube sockets (pin connection 5, the control grid) all the way back to the power transformer via both pot sets. The person who altered the circuit to take the KT88 or 6550A has added a 4kohm resistor somewhere in that path based on the original 8045G design. If that 4kohm resistor can be removed then the KT88 should bias nicely. Edit: Here is what I can see from the circuit diagram mods that have been applied. Edited July 3, 2022 by xlr8or 1
Luckiestmanalive Posted July 3, 2022 Author Posted July 3, 2022 Ah.... OK - thanks, so much, Kirk! I'll let you know how I get on. 1
Luckiestmanalive Posted July 4, 2022 Author Posted July 4, 2022 I cannot find a 4.7K resistor in the circuit that shouldn't be there. However, perhaps there is another culprit? I've gone back over the list of instructions to convert the amp to run KT88s in ultralinear mode. I've checked each of the 8 steps and there is only one that I cannot verify and could be a likely suspect: (6) Change a resistor in the bias circuit from 10K-ohm 2W to 20K-ohm 2W for change of bias voltage. There is a 10K-ohm 2W resistor in the preamp plug socket that has no nomenclature but is shown in the schematic, drawn and simply noted as 'plug'.
xlr8or Posted July 4, 2022 Posted July 4, 2022 That plug is for connecting a separate Luxkit tube preamp directly to the Luxkit A3600 to power it up. It's not related to the bias circuit. When I bought my Luxkit A3600 I also bought the Luxkit A3300 tube preamp. Since then I have acquired the separate A33 power unit for the A3300 tube preamp. 1
Luckiestmanalive Posted July 4, 2022 Author Posted July 4, 2022 (edited) OK, bugger. Then how about this? The pic below from the instructions shows a resistor that on my amp is 33Kohms (Orange orange orange) and should be 20Kohms 2W. Edited July 4, 2022 by Luckiestmanalive
xlr8or Posted July 4, 2022 Posted July 4, 2022 Check for continuity with your DMM from the end of that 33kohm resistor to the first centre pot tag. If there is continuity then that is likely the culprit. 1
Luckiestmanalive Posted July 4, 2022 Author Posted July 4, 2022 On 04/07/2022 at 9:41 AM, xlr8or said: Check for continuity with your DMM from the end of that 33kohm resistor to the first centre pot tag. If there is continuity then that is likely the culprit. Expand Sorry, to be clear, the one circled in red below?
xlr8or Posted July 4, 2022 Posted July 4, 2022 On 04/07/2022 at 9:52 AM, Luckiestmanalive said: Sorry, to be clear, the one circled in red below? Expand Yes and no. Yes to the ends of the 33kohm resistor. No to the tag on the pot. Please look at my yellow wire that goes to that big cap. 1
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