The design isn't complex for any competent tech. Only thing simpler is a guitar amp, really. Any good tech should be able to work on this. Most parts are soldered onto a PCB and the caps, resistors, and most other parts are easy to access and switch as none are of the esoteric kind. Not exactly. If a cap in line with the 6SL7 tube were defective, it'd affect both channels.
What I mean was the left channel still plays louder than the right channel when NFB set to 2. I also hear a slight distortion sometime when rotating the NFB knob from one position to another. The distortion goes away when the knob locks into the new position. Not sure if it’s normal. Do you guys hear similar things when switching NFB ?
Yeah, I too suspect something is broken during shipping. When I received the Amp, I noticed both the 300B was out of the socket and laying inside the tube cage. The 300B tube wobbles when pressed slightly. The seller claimed that it is normal for the 300B but that seems weird that only the 300B would do this. Have you made this a permanent fix ? Try a HumX ?
Speaking of NFB, is 1 the lowest NFB and 4 the highest or is it the reverse ? My speaker due to its design works better with zero or low negative feedback. Hence I am curious....
1 - low, 4 is high feedback. Based on my tests I would not recommend to use 3 and 4. It is better to connect speaker to 4 ohms terminals instead, which also changes sound.
300 tubes wobbles a bit when pushed, but if it loose - that can be fixed by by adjusting the socket. But for that you need to open the bottom cover. For the hum elimination I use special adapter attached to power cord plug.
@Devg what I find strange is that you say the channel imbalance only happens on NFB2. Unclear to me from schematic how that would even be possible. Sorry you are having so many troubles. Aside from a bad stock 805 tube that got replaced immediately, my LM805ia has worked flawlessly from day 1. You said you like NFB2 the most - that seems to be the sweet spot for most users. But you say you like low/zero feedback - that would be NFB1. And there are some users who prefer NFB1 - its' certainly a more lively and might work better with some speakers. p.s just added a pair of ECC32's - they are awesome and a step up from even TS BGRP 6SN7's ...
NFB1 seems to have more gain than others. It also has more details which sometimes can sound a bit harsh and forward in my setup. Moreover, the tube rush/hiss for NFB1 (and NFB4) is lot more compared to 2 and 3. I am not sure if this is normal or a bad tube or something broken inside. I think I will open it up and see if I can find anything obvious.
I have no idea how that is possible. Maybe the NFB switch itself gone bad or not making proper contact ?
can you elaborate why do you think 3 and 4 are not recommended ? Also are you saying that the 4Ohm terminal in general changes the sound or is there a relation to the NFB ? My speaker has a nominal impedance of 8Ohm I think and I am using the 8Ohm tap currently. I do plan to experiment with other taps.
Main role of feedback in this amplifier is to reduce output impedance. When feedback is increased, output impedance and distortions at low power drop. But there is a tradeoff: higher feedback causes ultrasonic peak to show up (and associated ringing in response on sharp signal change), and increase of distortion at high power. When feedback is set to 3 and 4, ringing becomes very pronounced and amplifier practically clips at 30W into 8 ohms load. With feedback at 1 and 2 there is little ringing and usable power is at least 40W. Connecting speaker to 4 ohms terminal also decreases effective output impedance, but has less effect on distortions. If speakers are rated as "8 ohms", that does not mean that they work best with 8 ohms output. You need to try and decide for yourself. In my case 4 ohms terminals and feedback at 2 gives the most smooth sound.
In the meantime, I ordered a 3rd Mullard ECC32 to use in the 6SL7 slot but it's been 2 weeks and it's *still* has yet to left the building the box was dropped to! Brutal. Once I get it, I'll be sure to submit a tone report.
Thanks for the explanation. The Cube Audio Nenuphar, due to its single driver design, need higher output impedance so that the damping factor can be as low as possible. It also works better with zero or very low negative feedback. So ideally with this speaker, NFB 1 theoretically would work the best but at this setting there is an increased noise in my setup. I am still running stock tube, except the 6SN7 pair. Will need to experiment this with different tubes and speaker taps.
another question that strikes me is how often do you folks check bias on this amp and find it needs adjustment ? I adjusted the bias 2 days back at preciously 120ma and today I see it fall very slightly (between 115-120) after the amp being powered up for an hr.
If FB1 is too noisy, try to replace 6sl7 tube. That one makes most of noise. Also replacing bypass capacitor in cathodes of low signal tubes may help.
Power line voltage has big influence on bias. When I set to 120, it varies from 110 to 130 based on time of day and weather outside (which influences power consumption in the neighborhood).
I check the bias every couple of months but I've never had to adjust it unless I've swapped out the tubes. I do run my LM805ia through a P12 regenerator that maintains a steady 120V.
I have a RCA 5691 red base, which many seems to like on this Amp, on order. Will find out when they arrive. You mean the 4 1uF caps ? IMG-7483
110% on this. My speakers are rated 8 ohm and while they are somewhat sensitive at 90db they do dip well below 8 ohms in the bass unit. The 4 ohm was a far better match and brought much improved bass drive and thus tonal balance. Indeed some argue that most 8 ohm rated speakers sound better on 4 ohm taps.
Yes, increased feedback naturally reduces output and so NFB1 will have highest power. Definitely forward and lively, though don't recall particularly more noise per se. I assume you know this, but these amps hold huge amounts of voltage that you should let drain out before opening up ...
RCA 5691 is a pretty quiet tube, but for $10 + socket adaptor you can do better with an older version of the Sylvania 7F7. It is dead quiet and sounds fantastic in the 6SL7 slot. Get the one with the mica's oriented in a plane, not the perpendicular ones. I know some will argue that the Mullard ECC35 is a better sounding tube, and in and of itself it might be, but in this 1st stage slot in the LM805 it is also a pretty noisy tube. ime.
