[cidiel]

I'm just finally setting up my little office system and delving into the world of tube amps. so the starter was the miniwatt N3 using a E-MU 0404 DAC for and martin logan motion 4's for speakers. my intention was to run it 4ohms from the N3 and see where it goes. everything hooked up, and lone behold only sound comes out of the right speaker.. (sounds great) but only one side.

couple questions..
1. i ordered replacement tubes from tube depot and found the exact tube, Sovtek EL84/6BQ5s.. now if i replace one tube do i have to replace both so the dates match?? i.e. what is this 'matched pair' wording that everyone makes a big deal about.

2. the left tube glows much more orange when the amp is running, (remember sound out of right side only) does that imply that the tube is trying to output the power somewhere but the connections is lost along the way?

3. along all that if i ever get this thing to work properly, can you replace/upgrade the speaker tubes without upgrading your power tube, or are we working on a balancing issue again?

-pretty new to this stuff so any help would be much appreciated!  right now i'm at a loss on what to do next!!

[Ericus Rex]

Your first step should be to swap the L & R channel tubes and turn amp on again.  If sound changes to other channel then a tube is bad.  If sound still only comes out of R channel then the problem is not with tubes but something in amp itself (or possibly the 12AX7 which drives both channels).

General answer to your questions:  1.) Since your amp only uses 1 power tube per channel matching is not so much an issue.  You should stick with same brand otherwise you'll get sonic differences between the two channels.  Matching is far more important when the signal is split and shared between 2 or more tubes in each channel.  Not the case for your amp.  2.)  Tube can be tricky.  One can look a little brighter than the other due to orientation of internal components.  This doesn't necessarily mean there is a problem.  3.)  You can change the tubes as much as you wish, just keep the two EL84s from the same company.

Swap the L & R EL84s and get back to us.

[cidiel]

ok, have done all almost everything i can think of.. switching tubes switching speakers etc etc.. have been very careful to make sure tubes are cool and everything is off before messing around with all this. but alas same problems so i guess tubes aren't the EL84's aren't the issue. i don't have a replacement for the 12AX7 handy so i guess i'll order that next and see if that solves the issue. didn't know you could blow out the one side of the triode on that thing. sounds incredible out of 1, can't wait to hear 2 speakers.

[rlee8394]

cidiel,

The 12AX7 is the driver tube, the EL84/6BQ5 are output power tubes. There is no speaker tube per se. In any event, when you swap the two EL84/6BQ5 tubes, does, the sound still come from only the right channel? Does the left tube, that was in the right channel before swap, still glow brightly? If this is the case, then there is a problem with the left channel of the amp. Probably with the bias supply. Is there a way to adjust the bias on this amp? If so, turn down the bias and see if that helps. If the glow is not from the filament wires, but instead from the plate of the tube, the large grey piece of metal, then it is definitely a bias issue. Shouldn't be a difficult repair for any competent tech. If still under warranty, contact The Legato Group for return authorization. If not under warranty, someone here can probably recommend a good repair tech.

Hope this helps,
Ron

[cidiel]

the glowing bright problem was very temporary, it went away when i replace the 2 EL84s with a pair that i got from tube depot that had matching dates. i'm assuming the bias issue was the fact that i had 2 EL84s that were unmatched for just a second in there when i tried to just replace one of the EL84s instead of both. Also I have flip flopped the LR tubes and it hasn't impacted the problem. what i still don't understand is the 'matched pair' concept and how do you know if you are running 'matched pairs'? for example i ordered 4 EL84s from tube depot and they are supposed to be matched pairs. but does that mean all 4 are matched pairs? next step is to replace the 12AX7 and see where that goes. will keep it posted.

[Ericus Rex]

The 12AX7 is a twin-triode which means it has basically two tubes within one glass housing.  While it is possible that one channel of the 12AX7 was damaged during shipping I think it's highly unlikely.  I think your problem is another part of the amp (or cables, have you switched those as well?).

I don't think your amp has user-adjustable bias so don't worry about that; repair would have to be done by a tech or the factory if you are having bias problems.

'Matched tubes' basically means that each tube operates roughly the same given the same set of parameters (voltages, bias, current, etc).  There is a good amount of variance, even within batches of the same tube type, due to the manufacturing process.  Tube makers and vendors have specialized equipment which subjects tubes to 'real world' (hopefully) conditions and then takes readings and then pairs- (or quads-) up like-responding tubes.

If you order two matched pairs that's probably what you got.  There is probably some variation between the pairs.  'Matched quads' should be four similarly testing tubes.

If switching cables (interconnects and speaker cables) still gives you sound only in the R speaker then your amp needs to be repaired.

[cidiel]

well after much reading the mini-watt is a self adjusting bias amp so it was meant to mix match the tubes as long as the power tubes are a matched pair... so i went and got a mesa 12AX7 and plugged it in as a final test try and still no L speaker. I have tried everything i can possibly think of including switching the speaker wires, (gives me no R).. guess i'll be sending this one back and hopefully warranty and all covers this thing appropriately. this was a brand new out of box amp.

[Ericus Rex]

Best of luck and keep us informed on what was the problem.

[rlee8394]

cidiel,

OK, if the amp is "self-biasing" then it has a single cathode resistor connected for each channel. My guess is that the resistor went bad. Since a single resistor is shared between the output tubes for each channel, it's best to have "matched" output tubes so that there is even current draw from each tube. In addition to the resistor, there is a capacitor in parallel with the resistor. So, with the unit off, remove the EL84/6BQ5 tubes. On the good channel, measure from pin 3 on the tube socket to ground and note the resistance. Then do the same for the bad channel. Are the readings the same? I'm guessing that you will measure somewhere between 200 - 500 ohms for the good channel. The bad channel will probably measure very high or even open. I'll bet that hat tube that glowed brightly had failed and ended up blowing out the cathode resistor. So no matter how many sets of good tubes you install, it won't work. Of course the resistor may have had a fuse in-line to protect the resistor in case of such a situation, but unlikely. Open up the unit after t has been powered off for several hours if you feel comfortable doing so, and you may be able to spot the problem. Take a digital photo if possible and post. I'll help out as much as I can.

Ron

[cidiel]

good news on this one, alo audio took it back no problemo and ran into the same issue as myself with no obvious explanation. got the new one back and it sounds amazing with the martin logan motion 4s.. although i think higher efficiency speakers would have been a little better just to be able to really crank it up.. although the clarity/size of the martin logans is incredible. haven't seen much better in the way of size/efficiency.

[Ericus Rex]

ROCK ON!