Luckiestmanalive Posted June 17, 2022 Posted June 17, 2022 I finally have my first valve amplifier and it is a beauty! I have imported a lovely vintage Luxman A3600 power amplifier, which originally came with 4x8045G tubes but has at some time been converted to run 4xKT88s instead (the original output valves haven't been manufactured for decades). In original form it was capable of putting out 50W per channel but I think it currently puts out about 31W a side. It was an import from Japan so I'm running a Tortech 500W step down transformer. Does anyone have an old Luxman power amplifier and can offer advice about their operation and maintenance? The photocopy of part of the manual is in Kanji so I'm on my own. I've hooked it up to the pre-outs from my Plinius 9200 and have the L and R gain(?) knobs turned all the way up. Is that OK or should I turn them down a bit? I warm up the amplifier for about an hour before I start sending music to it. The music sounds wonderful out of my old Audio Physic Spark IIs (about 87db efficient) and it is plenty loud enough for my medium-sized room. 10 2
muon* Posted June 17, 2022 Posted June 17, 2022 (edited) I'd be inclined to use a 750 watt to 1000 watt step down for it. In saying the above I'll add that the 500 watt one shouldn't run into problems. Nice amp. Edited June 17, 2022 by muon*
xlr8or Posted June 17, 2022 Posted June 17, 2022 (edited) I have the same amp with the same KT88/6550 mod. What specific info are you after? You can use KT120's as a substitute for the 8045 tube, but you need to add a resistor to the first bias circuit before it hits the left and right channels, which is separate to the second bias circuit used for adjusting the PP pair. The value that needs to be added is 8.20kohm. The Luxkit A3600 is equivalent to the Luxman MQ3600. The former amp was only sold for the Japanese domestic market, as a DIY kit. The latter amp was the export version, in completed form, for the European market. Sir Tim de Paravicini designed this unit for Luxman in the mid 1970's. Here is what you're after: https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Luxman-LX-33-Service-Manual.pdf One of my posts is also here: Edited June 17, 2022 by xlr8or 3
Luckiestmanalive Posted June 18, 2022 Author Posted June 18, 2022 (edited) Thanks, guys! In particular, thanks, X - I've read some threads online from owners who have installed KT120s without adding that resistor - just upping the bias to 80ma! And thanks for the link to that set of instructions. I'm not going to make any changes to mine until I understand what changes have been made already. I have to get some piggyback tube adaptors like yours so I can take readings easily. (I don't fancy having it on its side while I try to take measurements). I've found the values of some components are different to the instructions for converting the circuit to take KT88s/6550s in either ultralinear or triode mode. For example, if I'm looking at the right components and deciphering the rings correctly, R306 (the white rectangular object near the top of the second pic below) is unchanged at 270 ohm 10w, while R305 (the blue resistor just above R306) been changed from 820 to 6.8K ohms? Also, R135 and R136 (see blue resistors on the right side of PCB in first pic) appear to be 32 rather than 22 or 11 ohms? Edited June 18, 2022 by Luckiestmanalive
xlr8or Posted June 18, 2022 Posted June 18, 2022 (edited) 1 hour ago, Luckiestmanalive said: I have to get some piggyback tube adaptors like yours so I can take readings easily. (I don't fancy having it on its side while I try to take measurements). You don't need them. Also, the unit needs to be mounted on its side with a DMM to adjust the bias. I use test probes fitted with parrot clips to be hands free whilst using a flat-head screw driver to adjust the bias across the 10ohm cathode resistor connected across pins 1 and 8. The main bias circuit to L/R channels uses 25kohm pots. The second bias circuit uses 10kohm bias pots to adjust each PP pair. There isn't enough resistance range to bias more than 60mA. Also, the bias pots are very sensitive in any adjustment. The 8.2kohm resistor allows one to bias higher. I fitted mine just before the first 25-kohm pot. I will see if I can dig out a few photos of the inside of my unit and post here as an edit. Edit: photos added. Edited June 18, 2022 by xlr8or 2
Luckiestmanalive Posted June 19, 2022 Author Posted June 19, 2022 Brilliant - thanks, Kirk! I'll have to get some parrot probes then and get comfortable with turning it on its side, then. Then I'll be able to take some measurements. You've shown me the little resistors between pins 1 and 8 for measuring current on output tubes but are the points on the circuit board good for taking measurements, too? There is B2 to B4, F1 to F4, G1 and G2, H1 and H2, and O1 to O4. They have the advantage of being accessible from the top, too! I notice that the circuit board in your pic doesn't have any extra resistors on it while mine does. In particular, it appears R135 and R136 have had an extra 32-ohm resistor added in parallel, halving the resistance to 16 ohm?
Luckiestmanalive Posted June 19, 2022 Author Posted June 19, 2022 Also, I forgot to ask. I've been told the output tubes are KT88s but there are no markings on any of them that confirms that. Can you tell by looking at them (see first pic at the beginning of this thread)?
xlr8or Posted June 19, 2022 Posted June 19, 2022 (edited) 2 hours ago, Luckiestmanalive said: You've shown me the little resistors between pins 1 and 8 for measuring current on output tubes but are the points on the circuit board good for taking measurements, too? There is B2 to B4, F1 to F4, G1 and G2, H1 and H2, and O1 to O4. They have the advantage of being accessible from the top, too! I would advise against it, as a simple slip of the test probe tip can be disastrous. If you want to play it safe you can try 4 octal tube socket bias adapters hooked up to 4 el cheapo DMMs of the same brand/model type. 2 hours ago, Luckiestmanalive said: notice that the circuit board in your pic doesn't have any extra resistors on it while mine does. In particular, it appears R135 and R136 have had an extra 32-ohm resistor added in parallel, halving the resistance to 16 ohm? Just leave those resistors for the moment and see how it sounds. My unit also has some anomalies, which I admit I haven't bothered to follow up as it sounds really good with the KT120. 2 hours ago, Luckiestmanalive said: Also, I forgot to ask. I've been told the output tubes are KT88s but there are no markings on any of them that confirms that. Can you tell by looking at them (see first pic at the beginning of this thread)? Sure. Share some close up photos of the internal construction features of the tubes and others and I will try to confirm if they are KT88's and who is the likely manufacturer. Edited June 19, 2022 by xlr8or 1
Luckiestmanalive Posted June 19, 2022 Author Posted June 19, 2022 Thanks, mate! I tell you, it sounds amazing once it's warmed up! The presence it lends to music, musicians and instruments, is spell binding... 2
Luckiestmanalive Posted June 20, 2022 Author Posted June 20, 2022 I ordered this: https://www.ebay.com.au/itm/124336932247 It should allow me to take readings from each of them, one at a time, from the same multimeter! 1
Luckiestmanalive Posted June 20, 2022 Author Posted June 20, 2022 Also, would there be lower distortion on the speaker outputs if I turn down the level controls (L+R) a few notches from maximum? Or might it be the other way around?
Luckiestmanalive Posted June 20, 2022 Author Posted June 20, 2022 On 18/06/2022 at 5:22 PM, xlr8or said: The 8.2kohm resistor allows one to bias higher. I fitted mine just before the first 25-kohm pot. Also, the pic with the circle in it is where this resistor should be added if I bought me some shiny new KT120s?
Luckiestmanalive Posted June 20, 2022 Author Posted June 20, 2022 (edited) I took the output tubes out of their sockets to inspect them more closely and take pics to confirm their pedigree. I now see there are very faint markings (etched?) onto the bottles - they are Svetlana KT88s and I can see figures on one - 03 01 - maybe month and year? It's extremely difficult to make out on the others... Edited June 20, 2022 by Luckiestmanalive
muon* Posted June 20, 2022 Posted June 20, 2022 (edited) Possibly 2001 manufacturing which makes them Re-issues from New Sensor, rather than original true Svetlana made in the St Petersburg plant. Edit: although there was an early period then where true svetlanas were still being branded as such, before the legal case brought by New Sensor over branding name rights pushed JSC Svetlana to switch to the SED and Winged C branding that they retained the legal rights to. JSC Svetlana is a Russian company that owns and operates the Svetlana factory in St. Petersburg, Russia, which has been making vacuum tubes for almost a century. These tubes have been known in the United States as Svetlana brand tubes and have had the "S" logo. In 1999 or 2000 American New Sensor Company secured the rights to use a bunch of brand names including Svetlana for USA and Canada. OK, if they have round holes they are New Sensor re-issues, likely year 2003, and that is what they appear to be. If they have the rectangular holes like these they are original Svetlanas. Edited June 20, 2022 by muon* 1
xlr8or Posted June 20, 2022 Posted June 20, 2022 2 hours ago, Luckiestmanalive said: Also, the pic with the circle in it is where this resistor should be added if I bought me some shiny new KT120s? Yes.
Luckiestmanalive Posted June 20, 2022 Author Posted June 20, 2022 (edited) Thanks, M and X! I didn't know anything about the history of Svetlana tube production. I've taken a closer look at the markings. It has a thick stylised S like the one on your boxes, followed by Svetlana Electron Devices Inc, KT88, Made in Russia, 0302. Edited June 20, 2022 by Luckiestmanalive
xlr8or Posted June 20, 2022 Posted June 20, 2022 (edited) 5 hours ago, Luckiestmanalive said: Also, would there be lower distortion on the speaker outputs if I turn down the level controls (L+R) a few notches from maximum? Or might it be the other way around? Always have those 2 rear pots set to maximum output unless you choose to use them as attenuators for a fixed source input. They also can be used to adjust left/right balance. Edited June 20, 2022 by xlr8or
xlr8or Posted June 20, 2022 Posted June 20, 2022 2 hours ago, Luckiestmanalive said: It has a thick stylised S like the one on your boxes, followed by Svetlana Electron Devices Inc, KT88, Made in Russia, 0302 As @muon* has indicated they are New Sensor KT88 reissus made in the 2nd week of 2003. 1
Luckiestmanalive Posted June 21, 2022 Author Posted June 21, 2022 Thanks, very much, X! I'll leave you be a while and enjoy going back through my music collection and come back when I've done measurements of output tubes. Delivery of the doo-dad is due early July. 1
trobbins Posted June 22, 2022 Posted June 22, 2022 (edited) Well worthwhile ensuring adequate access to cool ambient air for those output tubes. KT88's have a wider glass envelope than the 8045G, and there is noticeably less separation with KT88 or other similar tubes, making it more difficult to keep glass temperature sufficiently low. Luckily the photo in the initial post shows that KT88 anode wings point to free ambient, rather than to each other, and that the KT88 heater dissipates 6W less than the 8045G. Edited June 22, 2022 by trobbins
Luckiestmanalive Posted June 23, 2022 Author Posted June 23, 2022 Thanks,T! Yeah, I've noticed how close the spacing is with KT88s and how much heat they give off. It is a practical reason why its on the top shelf - it's not just to show it off! I'm getting a little hum that I think is coming from the power transformer. Is that OK? It isn't coming through the speakers and it is barely noticeable from my seating position.
trobbins Posted June 23, 2022 Posted June 23, 2022 There could be many reasons for some remnant hum, and chasing hum can be hit or miss when making changes. Most chasing and changes can get quite technical - so not something to suggest unless tools and experience are at hand. Even before starting any chase there needs to be an awareness of whether the amp originally had any such noticeable hum, and if it didn't then what has since changed. 1
Luckiestmanalive Posted June 23, 2022 Author Posted June 23, 2022 OK, thanks, T - very useful advice. I will go back to enjoying the music, worry free!
xlr8or Posted June 24, 2022 Posted June 24, 2022 (edited) I suspect the hum you're experiencing is coming from the 2-prong AC mains (cheetah) power cable and the Tortech step-down transformer, as my A3600 does the same. My Japanese 845 amp also exhibits the same hum for the same reason. My other Japanese tube amps fitted with a 3-prong male IEC-320 mains socket and earth connection don't exhibit the hum. You could try fitting a Furutech IEC-320 socket to the rear with earth connection. Alternatively, try decoupling the amp and stepdown transformer feet with some dampening materials, including isolation feet, to mitigate the hum level. Edited June 24, 2022 by xlr8or
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