No, I haven't compared them directly. Chinese tubes aren't something I'm a fan of. Feel free to buy them if you wish but my experience has always been Chinese preamp tubes aren't anything I'd ever spend my time and money on.
Thank you Thomas and Strat-Mangler for the answer. I own a LM-508IA since begining of 2018, but at the end of that year, I replaced it for a well regarded Class D power amp, just to try. First, and I don't know why, I found the new amp better (cleaner, more detail, better control, etc), so I kept the LM-508 in a box for several months. In fact, I almost sold it for a fraction of it's cost. Im glad I didn't as I wasn't completely satisfied by the Class D amp, even with several upgrades I did. About a week ago, I decided to try again the LM-508, just for fun. The result was that, even with worse cables (power , RCA and speaker), the LM-508 gave me back everything I was missing from my system (body, presence, texture and smoothness) and it didn't gave up on those fields I thought the class D amp was way better (detail, dynamics, 3D image, etc), in fact, it bettered the Class D amp in some of those fields. My godness, how I missed that glorious midrange and that 3D palpability!!! I think I will never go back to Class D. Just to mention, I use the LM-508 as power amp driving it with a Audio-GD Master 1, wich is an incredible preamp for the price. I prefer it that way over the LM-508 as integrated. Regarding valves, I've tried the following: - Stock Psvane 805 vs Psvane 805-T II, being the latest considerably better. I have two pairs of the T II so I don't know for sure if I'm willing to spend on the ACMEs for a while. I'll think about it. - Shuguang WE Replica 300B vs Gold Lion 300B. First I liked the latest more, but I don't have them with me anymore to compare. Nevertheless, I love the sound of the Shuguang and they are just settling down. - Preamp section valves. I've tried Shuguang Black Treasure (not my cup of tea) and several budget NOS (Sylvania, Tungsol, Ken Rad, RCA, etc), being the Tungsols my favorite by far.
I was rather disappointed to find that nobody has a schematic for the amp. I opened mine up yesterday. First stage is half the 6SL7, then comes a 0,1uF 600V REF Multicap PPMFX, the two half of the 6SN7 are working in parallel as the second stage. After another 0,1uF Multicap we get the third stage, the 300B driving the 805 as a cathode follower directly coupled. The Bias voltage is supplied to 300B and 805 simultaneously through the same wire. I toyed with the thought of replacing coupling caps and the elkos used for the cathode bias of the first two stages (220uF 25V). Problem is the temperature, all higher quality metalized film caps are only rated to 85°C, as are Elna Silmic and Cera as well as Audio Note Kaisei. The Multicaps and Elkos used by Line Magnetic are all rated 105°C. It's almost as though they thought about these things
Anyone have experience comparing 518IA and 508IA? I current own 518IA but got a chance to upgrade to the 508IA. I want to make sure it is indeed a upgrade.
I think there are members here who had both. From what I have read the 300B drivers make a big difference. I would definitely have considered the 805 for my Reference system as those speakers could use the power, but it is on the third floor of our house and I could not leave a big tube amp unattended. But with the 518 in my office system, I have never once thought about upgrading. It is so good!
I have a two-month old 230V 508IA purchased from China that has a hum coming from the speakers. The hum is not affected by a change in volume and it is not audible until the power LED stops flashing after powering on the amp. I'm using a good quality 500W step up transformer from ACUPWR to convert to 110V for use in the U.S., but could it be the cause of the hum? Could it be a bad tube(s)? I don't hear any noise coming from the transformers. I can post a YouTube video of the hum if that helps.
I picked up the 508IA. I found one local to me and it was a good deal so I couldn't resist, but I think it was a good decision. Initial reaction compared to the 518IA is that aside from a better controlled bass, with deeper and better definition, the mid range imaging is also more accurate with better depth and holographic effect. This is with stock tubes. After learning the sonic character, I will try to roll in some tubes I have. including some Psvane CV181T2, NOS Mullard CV181, some old KR 300B and WE 300b from the 90s, and report back my findings. I don't have any good 6SL7. I am not sure if I want to pay $200 for the fancier ones, I might give the RCA 5691 a try. Any other non-crazy priced suggestions? Meanwhile, I managed to go through all 51 pages of this thread. Lots of good info!
I just looked at the schematic. Having the diode there should solve the problem. As for the function of this cap, it is the cap that supply the high voltage power for the 6SL7, 6SN7 & 300b. as those are small signal tubes, I would say that it is rather important and would recommend using a good quality one if possible.
It's normal the hum will start when the amp is ready to play. This is because the audio circuit is actually not enabled until after the tubes have warmed up. This was to prevent any popping sound when first powering up the amp. Yes, the SUT could very well be responsible for the problem. Since it is happening in both channels, a tube is unlikely unless it's the 6SL7 which is the only single tube affecting both channels. If you have a spare, you can change it to see if things improve but my guess is it likely won't. Before you start chasing ghosts, ensure nothing whatsoever has changed since before the problem started occurring. For instance, was the wiring setup changed? Are you in a power bar? If not, bypass the power bar. Plug the SUT in the same outlet as the amp. Are you using any lights with a dimmer on the same circuit? If so, turn them off. Put your cell phone in Airplane Mode and unplug your modem temporarily. If you live in a house, turn off all breakers except for the circuit in which the amp and SUT are plugged in. If after all this, you still have a hum, it's likely the SUT. If you live in a house, have an electrician combine two circuits together so you have direct access to a native 240V outlet for that amp. That's what I did. That removes the SUT from the equation.
Congrats on your new amp. Glad you like it. Depends on what kind of sound you enjoy. The RCA 6SL7 and 6SN7 have a velvet-like soft slow sound. Personally, I thought it made the 508IA sound like a vintage integrated. The 508IA's main attraction are the dynamics and detail. Using those tubes, IMHO, removed those attributes so I can't say I feel like they would marry well with the amp. If you like a lot of high-end extension (or a bright detailed sound), try Sylvania 3-holed 6SN7 (AKA Bad Boys). Be careful! Tons of tubes marketed as Bad Boys and very few actually are. Much cheaper are the Sylvania 6SN7 Chrome Tops which will have a sound in a similar vein, albeit not to the same level of sound quality. The Ken-Rad with staggered plates and black glass will give you tons of bass and lots of low-end detail, some textures on the mids but rolled-off highs. Terrific if you have bass-shy bright speakers. Tung-Sol 6SN7 black round plates offer terrific texture on the mids, reasonable high-end extension, and some good thump on the low-end (though nothing will ever beat the Ken-Rad). Most expensive of the bunch and typically the king of the hill to those who've tried it. For the 6SL7, by far the best one is the Mullard ECC35 which isn't cheap at roughly $150-200/tube but you only need one and it makes such a *huge* difference that it's worth it! The preamp tubes will have the most effect on the sound so I strongly suggest you look into those before bothering with the bigger tubes.
You can try to adjust hum balancer. Also if the hum is 60hz, then it could be due to grounding problem. Then you may try isolation transformer to reduce hum. Step up transformer shall have 1KW capacity at least otherwise it will limit dynamics. After changing stock tubes into Nos or expensive new production tubes, hum tend to get lessened by little bit. But it tend to have little bit of hum even after tube rolling if you have your ears on the speakers not like SS amps. If you can not hear the hum from your listening chair, it is better to ignore it. Good luck.
Since you're handy with schematics, I'm assuming you're handy with a soldering iron as well. I would therefore urge you to proactively replace the cap and add the diode as a precaution against potentially suffering through the same ordeal. FYI, the suggestions I made earlier about the hum you experience take into account that you've adjusted the hum screw for each channel already, the tubes are biased at 120 for each channel, and that you've eliminated a potential grounding issue. That is why I suggested plugging the SUT and amp in the same outlet anyway but just wanted to be sure you knew where I was going with that idea.
Here's a video of the humming noise I'm experiencing that has been present since Day 1. My phone was a couple of inches from the speaker during the recording. If the room is completely quiet with the volume all the way down I can hear the hum from the listening position 11 ft. from the speakers. Prior to my post, I had adjusted the hum balance screw (both channels) and the tubes are biased on both channels at 120. No wiring changes and I've always plugged the SUT directly into the wall outlet. No dimmers on the same circuit, switched off all the breakers except for the one the SUT is plugged into; hum is unchanged. Also unplugged the RCA inputs from the amp and the hum was unchanged. I'm currently in a rental home but plan on having an electrician run a 240V line when I move into my new home in 8 months that I will own so I don't have to use the SUT.
it may be the transformer. do you happen to know if the transformer is a auto-transformer or an isolation transformer? I have previous experience with 240V wiring in the US. 240V here is a balanced wiring, as there is no true 240V wires going into the house, what they do is they take the 2 opposite phase of 120V that goes into the house and call that 240V. some amps might take it well, some might not. my previous amp which was a DIY one didn't. I ended up going to isolation stepup transformer which was quieter.
Did you see my thread and post in this thread about the capacitor that should be changed? I did experience a similar hum prior to that capacitor imploding. Any tech can perform the modification in 5 min. Strongly recommend you make the change now. If you're uncomfortable doing so, have a tech perform the modification. It could be tube-related. To check, you could buy the cheapest possible 6SL7 and 6SN7 tubes and swap them just to see if the hum goes away. Another out-of-the-box suggestion would be to get yourself an adapter so you can convert the dryer plug in your home to a regular one (like the one I quickly found listed below - cheaper ones surely exist) in which you could plug your amp into. This would be purely for testing purposes. If you still have the hum, we'd know the SUT isn't to blame. https://www.amazon.com/dp/B07QSDM4QN/
The SUT I'm using is a autotransformer. 500 Tru-Watts™ Step Up Transformer Converter - Use 220 Volts appliances in 110 Volts countries - AU-500 Based on your comment that we don't have a true 240V line entering into US homes, running a 240V line to the amp may not eliminate the hum if the SUT is the culprit, correct? Do you have a recommendation for a good isolation step-up transformer?
my single sample experience with that one amp I had, was that isolation stepup transformer was quieter (in terms of buzz/hum) than a balanced 240V supply. It maybe due to the design of the amp or the nature of the 2x 120V phase. I cannot explain. I have also used it in another pair of SET amps that also didn't introduce any hum. (though I never tried that pair of amp in balanced 240V supply) I have a Hammond 298E. I don't know if they still make them.
looks like they have something similar. Mine had EU sockets and these have NEMA 6-15R. 298ET Hammond Manufacturing | Mouser
I like your idea of using the 240V line for the clothes dryer but unfortunately the rental home I'm in has a gas dryer and I don't feel like lugging my amp and speakers to a friend's house for this test. Purchasing a pair of cheap 6SN7s and a 6SL7 sounds like the next most economical step in trying to isolate the hum. I'll report back on my findings after swapping out the tubes.
If it isn't the tubes, it could be either the SUT or the cap I mentioned. Only way to narrow it down would be to lug it to a friend's place for testing.
