Luxman SQ38u - help identifying valves.
The inside on my Luxman contains the following compliment of valves.
The EL34s are obviously the power section but can anyone help me identify the purpose of the remaining valves? They're shared between the preamp and the phono stage but there's no markings on the boards to indicate which valves serve which function (or even which type of valve belong in each posiition).
Anyone got any knowledge of this before I start removing valves to see what stops working?
I can make some guesses looking at the layout, but I'll wait a bit, as someone who actually knows the answer will probably be along soon.
Ok, here goes! Bottom left, 1xECC83- phase splitter for power amp section; 2xECC82- drivers for EL34s (same layout as my Velleman power amp).
Upper right, 2xECC83s in shielding cans- RIAA section for phono amp (same as my Yaqin MS-12B). Lower right, ECC83 & ECC82 in cans- preamp line section.
As the amp can be split into pre and power amps, the screened ECC82 is probably a buffer.
If someone else knows this amp, go with their explanation, I am guessing, based on my own amps, but maybe it will help.
I can identify that the ECC82 1st on the left is specific to the Left channel and the ECC82 3rd from the left is specific to the Right channel.
The ECC82 4th from the left is running significantly hotter than the other tubes.
So much so that I'm concerned that it may be the wrong type of valve for that position.
Thanks for the input Dick, the reply is appreciated.
I indulged in a little trial and error today with the following results.
Pulling either the first or third (from bottom left in the pic) results in the loss of one channel, which is what I'd expect.
Pulling any of the other valves (82 or 83) results in complete silence, which is not what I was expecting. I expected pulling some to kill just the phono stage and the rest to kill the pre.
Basically I'm looking to replace the valves and am trying to identify where the best bang for the buck might be if I wanted to splurge a little on something a little higher up the food chain than the standard valves.
Anyway, pulling all the valves and cleaning the pins with servisol got rid of an occasional problem I was having with one channel so the effort wasn't entirely wasted.
Can you tell that tube based circuits are "before my time".
If you google your amp, the Stereophile review lists 4 ECC83s and 3 ECC82s, so I think they are ok. The 83 has much more gain than the 82, so if they were misplaced I think you would notice.
Run them with the cans off, and see if that one glows brighter. I suppose the heater could be over voltage, or the valve faulty. You really need some one who knows more than me. Why not look through various threads, find someone who seems to know valves, (I can think of several) and PM them very politely?
I'm surprised no-one else has chipped in yet!
- - - Updated - - -
Swap the 82s around and see if the problem is with the valve or the position. Hey, maybe it's supposed to run hotter.
Good thinking Batman, I'll give that a try tomorrow.
Originally Posted by awkwardbydesign
Yay, found a schematic
Layout is as follows...
I've just seen this thread. Do be a bit careful running the amp with valves pulled out. Depending on the design the anode current that the valves draw may also be being used to lower the HT voltage to a safe level. When a valve's not in place it will draw no anode current obviously and the HT voltage on other valves may then rise to an unsafe level. Likewise if any of the interstage capacitors are a bit fragile they may not appreciate having a much larger voltage applied across them than they're used to. And if the amp has any direct-coupled stages then pulling the associated valve will seriously disturb the operating point of its neighbours. Having said all this, this amp doesn't appear to have any valve rectifiers, so all these high voltage problems may be non-existent here since the circuit may have to withstand full HT before the valves have warmed up. Unless the designers built in a soft-starter of course ... And one last thing worth mentioning is that valve heaters may sometimes be wired in series. So pulling one valve will cause another one (or maybe even more than one) to cease working.
Can you show the schematic, or post a link? Then we can have a look at the heater circuit.
BTW, let us know how swapping valves goes.
Thanks for the reply VB, I'll bear that in mind in future! I appear to have got away with it this time.
I think I need to buy a copy of Morgan Jones and learn how these things really work once and for all.
Swapping over the ECC82s in the power section has made for a noticable improvement.
Is it likely that the circuit only uses one side of each valve in that position and swapping them over means I'm now using the unused sides of those valves?
For peace of mind I'm inclining towards just replacing all the valves with new for a known good starting point and keeping the existing valves as spares.
Last edited by Howlindawg; 01-08-2012 at 11:00 AM.
It's just a block diagram so not much use for chasing the heater circuits but here ya go.
No, the circuit diagram (see below) shows that they use both halves of all the valves, which is almost always the case. It's possible that components, including valves, have drifted differently and that your latest combination of off-spec components happens coincidentally to work better than the earlier combination. But frankly I'd be surprised if that was the case. If you're convinced that there's a real difference then it's more likely that you've disturbed a slightly grubby/corroded contact in one of the valve bases and improved things that way. There is something to be said for unplugging and replugging things once in a while just to keep the contacts good. Don't overdo it though. It's possible to wear/snap things too if they're flexed too often.
Originally Posted by Howlindawg
Actually it's quite informative. Heaters are represented by little loops or V-shaped sections in the wire. I've ringed them here
Originally Posted by Howlindawg
The seven ringed in green are almost certainly the power section heaters. They are all in parallel so removing any one of them won't affect the others. Their supply is 50Hz AC which can be a little noisy. But this won't matter because they are at the output (= low sensitivity) end of the circuit. The four heaters ringed in red are almost certainly the phono and pre ones. These are high-sensitivity parts of the circuit so it is worth running these heaters off DC and that's what they've done. It's a little easier to build a higher-voltage lower-current DC circuit than the other way round. So they've put these four heaters in series (totalling 25.2V, 300mA). This means that if you pull any one of these four valves out the other three will go off too, which I guess is what you found.
Yes, series heaters would do that. BTW, did the hot valve still run hot when changed around? And if you buy new valves, it may be worth changing the small signal valves one at a time. That is if you are still having a problem, of course. If not, there will be no difference to hear!
Last edited by awkwardbydesign; 01-08-2012 at 12:20 PM.
Thanks for that last post VB, it all makes sense now and I've learnt something new.
I cleaned the pins as I removed each valve and everythings working perfectly now so that's all good.
There must have been a bad connection to one of the pins caused by corrosion or just a badly seated valve.
I know the PO was using some older valves and just swapped the originals back in before shipping the amp to me so possibly a bit of debris or a loose valve.
I didn't swap the ECC82 that was running hot yet, too many people around the house to do it safely at the moment.
Glad it's working ok now. Mind you, as it's valves, it's bound to go wrong again sometime. But the more you learn about it, the easier it is fix next time.
I am the second hand owner of Luxman SQ-38U for few months and recently I hear some thumbing noise from my phono stage when the voulume is at 11 o'clock without any phono cables attached to back of this unit from my TT. When I switch to CD or other source, no noise can be heard. Does that mean my phono stage tubes need to be replace? Please advise.
Probably not. The phono stage has a higher gain than the line stage and that is probably the cause of the hum. Especially if you have no i/p connected.
Hello to everybody. A few weeks ago I bought myself a second hand Luxman SQ-38U and I was trying to find information in the Internet on how tubes are shared. Accidentally I ran across this thread that was indeed so helpful!
But I have a couple of questions...
So... Why are there, in the pre section two different types of tubes and not two of a kind? One is for both channels and one is for voltage or whatever?
And could I tweak sonic results? By changing pre valves or by changing the small valves that power the EL34s?
Thank you in advance!