From other builders I heard LUs tolerate it down to 6K PtP with a little THD increase.
Jelle, another alternative candidate would be 38HE7, same 12-pin envelope. You keep the original sockets and first 2 stages intact and just use the power pentode section, could be up to 15 Wa as well.
AFAIR smoking-amp did some torturing on these, he might share the details on best use.
Now, this is maybe a lame idea, but they have the power diode in each so you could make it a separate half-way rectifier for each channel fed from same single trafo, center-tapped.
Oh, someone already did:
http://www.diyaudio.com/forums/tubes-valves/106599-compactron-output-stage-power-supply.html
Jelle, another alternative candidate would be 38HE7, same 12-pin envelope. You keep the original sockets and first 2 stages intact and just use the power pentode section, could be up to 15 Wa as well.
AFAIR smoking-amp did some torturing on these, he might share the details on best use.
Now, this is maybe a lame idea, but they have the power diode in each so you could make it a separate half-way rectifier for each channel fed from same single trafo, center-tapped.
Oh, someone already did:
http://www.diyaudio.com/forums/tubes-valves/106599-compactron-output-stage-power-supply.html
Thanks for that. I have been doing DC coupled 12ax7 followers and IRF820 followers for AB2 and they work well. I will certainly look into the IRFBC20, thanks for that. 🙂
The local NFB loops, if the sound gets too lively, I will add them. That was something I was considering. I kind of like the sound of open loop triodes, so we will see. Pentodes...different story. Is pentodes with local NFB what you guys call Schade over here?
Hi, Good to hear that you like the KT88. I looked at the MQ88u, does it have the same OY15-6K output transformers that the MQ60C has? What is the plate voltage?
And have you tried the 6550? I can fit the 6550, but the KT88 would require moving the tube sockets a bit, like was done on the KT88 modded MQ60C that is now for sale on Ebay. I'd rather not do that.
I'm taking a shot at answering your questions here instead of your PM, since I get asked similar questions a lot.
I know nothing about your Luxman amp since I rarely dig into commercial HiFi stuff, and haven't fixed or modded any guitar amps in about 30 years. It is not in original condition, so you are willing to modify it as needed to make what you want, so let's see what can be done.
You say you like the sound of open loop triodes. You want about 30 WPC. You have 400 VDC of B+ to work with and 6K ohm OPT's. A typical pair of triode wired audio tubes should provide about 30 watts under these conditions. 35 watts is attainable with AB2 operation. A good TV sweep tube can provide 35 watts in pentode AB1, but some feedback will be needed.
I don't know anything about your speakers except their stated efficiency of 94db. The graph of actual input impedance VS frequency is important. Serious dips particularly in the bass region require an amp with good damping (low output impedance). This may be attainable with some open loop triodes, but not with open loop pentodes, or screen driven sweep tubes. A conventional pentode can be tamed with some feedback but this increases the drive requirements. A screen driven sweep tube already needs hundreds of volts of drive, adding feedback requirements on top of that would require some serious experimenting with the driver circuitry given the relatively low B+. I don't know how many available tube sockets you chassis has, but I'm assuming two octal sized output tubes, where straight sided 6550's fit, but "coke bottle" KT88's do not, and two 9 pin miniature sockets for the driver circuitry, where one is now a 6CG7.
I can think of two ways to get there from here that I have considerable experience with. Almost 10 years ago I co-designed an amplifier using 6L6GC's or a dozen or so other possible tube choices. This was a cooperative design done on opposite sides of the earth resulting in a nice amplifier in Australia, and an amp breadboard here that I still tinker with today. It is a rather long thread with ample learning opportunity. My current version uses 6CG7's for both tubes in the driver with a mosfet follower feeding the output tube grids to allow AB2 operation on high power peaks. I have run dozens of different output tubes from DHP's at 20 watts and TV sweep tubes to big fat transmitter tubes at power levels over 200 watts. For your power levels good 6L6GC's or EH 6550's would be my tube choice. My driver schematic is in post #638. The long thread is here:
http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp.html?highlight=6L6GC+AB2
Another choice is Pete Millett's Engineers amp. Any TV sweep tube would get you 35 watts, and any of several small signal pentodes would work as drivers. 6EJ7's come to mind.
http://www.diyaudio.com/forums/tube...-new-p-p-power-amp-design.html?highlight=6HJ5
I know nothing about your Luxman amp since I rarely dig into commercial HiFi stuff, and haven't fixed or modded any guitar amps in about 30 years. It is not in original condition, so you are willing to modify it as needed to make what you want, so let's see what can be done.
You say you like the sound of open loop triodes. You want about 30 WPC. You have 400 VDC of B+ to work with and 6K ohm OPT's. A typical pair of triode wired audio tubes should provide about 30 watts under these conditions. 35 watts is attainable with AB2 operation. A good TV sweep tube can provide 35 watts in pentode AB1, but some feedback will be needed.
I don't know anything about your speakers except their stated efficiency of 94db. The graph of actual input impedance VS frequency is important. Serious dips particularly in the bass region require an amp with good damping (low output impedance). This may be attainable with some open loop triodes, but not with open loop pentodes, or screen driven sweep tubes. A conventional pentode can be tamed with some feedback but this increases the drive requirements. A screen driven sweep tube already needs hundreds of volts of drive, adding feedback requirements on top of that would require some serious experimenting with the driver circuitry given the relatively low B+. I don't know how many available tube sockets you chassis has, but I'm assuming two octal sized output tubes, where straight sided 6550's fit, but "coke bottle" KT88's do not, and two 9 pin miniature sockets for the driver circuitry, where one is now a 6CG7.
I can think of two ways to get there from here that I have considerable experience with. Almost 10 years ago I co-designed an amplifier using 6L6GC's or a dozen or so other possible tube choices. This was a cooperative design done on opposite sides of the earth resulting in a nice amplifier in Australia, and an amp breadboard here that I still tinker with today. It is a rather long thread with ample learning opportunity. My current version uses 6CG7's for both tubes in the driver with a mosfet follower feeding the output tube grids to allow AB2 operation on high power peaks. I have run dozens of different output tubes from DHP's at 20 watts and TV sweep tubes to big fat transmitter tubes at power levels over 200 watts. For your power levels good 6L6GC's or EH 6550's would be my tube choice. My driver schematic is in post #638. The long thread is here:
http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp.html?highlight=6L6GC+AB2
Another choice is Pete Millett's Engineers amp. Any TV sweep tube would get you 35 watts, and any of several small signal pentodes would work as drivers. 6EJ7's come to mind.
http://www.diyaudio.com/forums/tube...-new-p-p-power-amp-design.html?highlight=6HJ5
Yes, KT-88s sound fairly good, JJ-new-makes and relabeled "Luxman", look very accurate and solid comparing to cheapo Chinese counterparts, I swapped them between pairs just out of curiosity - seem being a matched quad. Putting in some old 6550 does not get me excited.
Originally the idle was set to 40 mA making the Mains trafo hot. After the PSU improvements I tried it down to 20mA – still no crossover but of course less dynamics, so I put it back to 35 and now Mains stays reasonably warm. I cant imagine how hot you will push it with 4 x 100 mA.
I learned even though the PSU part of schematics is implemented all right, two-points earthed etc, it’s overall quality is just sufficient and rather mediocre from the modern Hi-Fi stand of view.
First, the regular rectifiers with snubber caps went replaced with carbide HV ultrafasts.
It seems the 15-3.5 OPTs are good only for AlNiCo but a too low PtP for my speakers’ LF impedance dive. This was a major reason for thinking about modding it away of KTs.
After some tryings it resolved by lowering the HV from 480 to approx. 400-410 with adding the MOSFET choke + re-grouping the filter caps + one extra, now an individual el cap + a film cap soldered to each OPT’s HV input. Such the PSU re-do took sound to the level I can like. I also stuffed the first stages with British Mullards.
With lower HV and lower idle the KT-88s are way under-run, but after all I listen it 90% on about 2-3 Wa level (on 10 it gets unbearably loud) so they certainly will last. .. maybe longer than me ))
when i built my first 6LU8 pp amp, an oldtimer kept coming back to the house asking to hear it, on the third day, he picked up the ampa and took it home...
i also made a 12EN6 pp amp,
Attachments
It should be rather here:
Indeed, the "Introduction" (page # 1 rel.) very confirms my personal experience, the KT-88 seems destined for and really shines in UL.
As it pointed out, I tried the UL OPT and its THD very little depended on 4-6-8E load impedance, I counted it as a strange observation but now I see it was by design.
kodabmx, thanks for sharing, interesting reading, didn't come across this before.
Indeed, the "Introduction" (page # 1 rel.) very confirms my personal experience, the KT-88 seems destined for and really shines in UL.
As it pointed out, I tried the UL OPT and its THD very little depended on 4-6-8E load impedance, I counted it as a strange observation but now I see it was by design.
I am picking up a Luxman lx-38. Supposedly the 50CA10 tubes are OK. If I have to replace the tubes, I don't need much power, only great sounding 10-15W output power for 100 dB sensitivity speakers. Any thoughts on easy tube substitutions while keeping the original transformers?
I am not a tube amplifier designer or repairer. Just a music lover who has a few different pieces now of 1970s - 1980s equipment. I am getting this amplifier at a reasonable price and just trying to prepare for the future. It appears to be a beautifully-designed amplifier with attractive (to me) vintage looks.
I am not a tube amplifier designer or repairer. Just a music lover who has a few different pieces now of 1970s - 1980s equipment. I am getting this amplifier at a reasonable price and just trying to prepare for the future. It appears to be a beautifully-designed amplifier with attractive (to me) vintage looks.