New Tubes in my DARED MP 2A3C :(

M

MexicoMiike

Active Member
Today I installed a set of electroHarmonix 2A3 Gold tubes in my Dared. Instant 60HZ hum which does not vary with the volume control setting. The amp is totally silent with the Shuguang 2A3Cs I had installed many months ago. Looks like the EH's were a total waste of money… :(
 
Doesn't make technical sense with direct lit tubes like these, other than out of spec current draw.

Sent from my SM-N900V using Tapatalk
 
I've had lots of good luck with Shuggys in my various Dared products. I think I've had one loose base in the bunch but even it sounded pretty good and quiet.
I've tried EH on signals but there was just something I didn't like. Dunno what but something was missing with them for me.
Good to see Mikey back! = )
Are you still a BLH convert?
 
Could be a filament - grid short. It can happen if one of the tubes was treated a bit rough. I'd send them back where they came from for an exchange.
 
dosmalo said:
Are you still a BLH convert?
Click to expand...

Yep, Still a BLH fan! :)

"Who recommended the EH over the Shuguangs?
Your Dared was voiced for the Shuguang 2a3c's"

Re trying the EH tubes - it was just an attempt at some tube rolling. The Shu tubes that I installed a while back were a decided improvement over what came with the amp (I posted about that). I was hoping the EH might do something "interesting," but 60HZ hum wasn't what I had in mind! :)
 
Hmmm…actually, I think there is. There is no info about them in the dared instructions and I have never found a schematic for the amp but I seem to recall reading somewhere that there are controls (factory adjusted) for that. I'll pull the bottom off and take a look.
 
MexicoMiike said:
... I seem to recall reading somewhere that there are controls (factory adjusted) for that. ...
Click to expand...

Be sure to use a Sharpie marker or such to mark the position of the control before you turn it, so if the turning doesn't help matters, you can restore it to its original position. I'd make a mark crossing where the control shaft is next to a stationary portion of the control.
 
MexicoMiike said:
Yep, Still a BLH fan! :)

"Who recommended the EH over the Shuguangs?
Your Dared was voiced for the Shuguang 2a3c's"

Re trying the EH tubes - it was just an attempt at some tube rolling. The Shu tubes that I installed a while back were a decided improvement over what came with the amp (I posted about that). I was hoping the EH might do something "interesting," but 60HZ hum wasn't what I had in mind! :)
Click to expand...

What about the small preamp tubes ( either 6n3 or 12ax7 depending on your model ) did you replace them? Those made all the difference in the world compared to what came in my Dared MP-2a3c.
 
All the tubes in the amp were replaced last year and I posted that I was quite happy with the results. I bought a complete set of EH tubes but initially the only ones I installed were the power tubes. Later I was going to install them all but one of the 12Ax7s was DOA - the getter flash was totally white out of the box, despite the box having an inspection sticker/test label for each tube. Makes me think that that labeling is more about marketing than an ACTUAL inspection/test.:(

I haven't yet pulled bottom cover…now thinking I won't even bother. I am happy with the sound of the amp with the ShuGuang 2A3Cs and the Tung Sol (Russian) small tubes. So I think I'll just write off the new tubes as one of those learning experiments! :)
 
Assuming 1) the amp has AC filaments on the power tubes (which is almost certainly the case) and 2) that it has a hum adjustment pot (rheostat) across the 2A3 filament... it should be adjusted for minimum hum any time an output tube is changed IME. That is what they are there for. While it's certainly not a bad idea to mark the current setting (especially if they're not hum pots!) it is (again IME) pretty darned easy to set them for minimum hum by ear. The action of the hum pots is typically pretty easy to hear (yet again, IME).


EDIT: Of course, if the hum is loud, it may well be an issue with the tube itself -- those rather wispy filament/cathodes in a 2A3 could certainly sag/warp and touch the grid or some other internal structure of the tube and wreak all sorts of havoc. There are multiple reasons that DHTs fell out of favor for general-purpose use (except as HV rectifiers) about seven (if not eight) decades ago :p
 
Is the hum present in only one channel or both? If only one, does the hum follow the tube if you switch it to the other channel?

FWIW those Shuguang 2A3Cs are pretty hard to beat. I run them in my bottleheads and have pretty much stopped looking for anything better.
 
The hum stays the same regardless of switching tubes and, as I said earlier, it is not affected by the volume control. It is not particularly loud but it is noticeable with no signal. With the Shuguangs there is absolutely no audible hum under the same conditions.
 
That sounds like a job for the hum pots -- if so equipped.

I haven't found a schematic for this amplifier. There are clearly a couple of pots located below decks -- they could be for adjustment hum levels on the outputs, but if the amp is fixed bias, I suppose they could be for bias trim... or maybe something else; who knows?

DSC_0964.JPG


source: http://www.excelstereo.com/Upgrades/Dared2A3.htm

EDIT: A couple of web links indicate that it is self-biasing... making it more likely that the two pots above are in fact for hum level adjustment (I presume).
 
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Hum pots. Almost always have to adjust them on my 2A3 to tune-out hum (to ear) when changing outputs tubes.

Note where they are set first just in case but try adjusting them (and be careful poking around under there!).
 
Billfort said:
Hum pots. Almost always have to adjust them on my 2A3 to tune-out hum (to ear) when changing outputs tubes.

Note where they are set first just in case but try adjusting them (and be careful poking around under there!).
Click to expand...

That's the part I don't like about adjustin' 'em :p

The Redboy/JE Labs/Angela (RJA?) SE 2A3 that lives upstairs now has 'em thoughtfully installed atop the chassis. Easy-peasy.

IMG_6585.jpg


image source: http://www.audiokarma.org/forums/showthread.php?t=112767&highlight=simple
 
I've read that a 2a3 with a little hum is preferred over one that's silent. As a dead quiet amp can also be dead sounding.
 
I wouldn't think so.
At any rate, it's a matter of degree.
I cannot hear any hum from the above-mentioned amp on low-90s sensitivity loudspeakers; on the Duplexes,things are about 10 dB different.
 
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