Stereomour complete - Where's the bass?

[blue360cuda]

Hey everyone! Just finished up my Stereomour II build last night. Resistance and voltage checks all within spec and she fired up perfectly. I played around with the hum pots a bit and even with my over 100db speakers its pretty darn quiet.  Have the shunt regulator and DC upgrade sitting on the bench so those will come soon.

The amp sounds really nice but I'm honestly surprised how thin the bass is? My SEX3.0 is going considerably deeper. It's like someone just cranked the bass knob all the way down - Mids are beautiful and highs are very nice but everything below 100hz or so sounds very rolled off. The only non stock part I did was change the 0.1uf 630 poly cap to a Mundorf 0.1uf 1000v silver/oil. The output is stock but will swap that with a 3.3uf Mundorf soon. There were a couple resistors that had substitutions in the kit - the dale 3650F was some other metal film version but everything measured to spec before install. I'm currently wired for 8 ohms into 8 ohm altec speakers - maybe I'll try changing to 4 ohm taps?

I have the stock supplied tubes as well as a pair of chinese NOS Guiguang 2A3s that received high marks from other BH members. Feeding the amp is an Eros2.  Haven't tried any other sources yet.


Any thoughts? 

[Paul Birkeland]

Just for the sake of double checking, maybe post a picture of the underside.  There are minor errors that can be made that will suck out all the bass but won't present themselves as DC voltage issues. 

[blue360cuda]

Dang PB!  You're fast!!    Appreciate the help!

Let me know if any other specific views/angles would be better . . .

[blue360cuda]

You'll see one "woopsie" I had and trimmed one of the 130oh resistors too short - 2 of them go into 44L so I wrapped the lead of one around like the rectifier diodes and did a short extension of some bare buss wire so it'll look like only one 130 going into 44L  but both are there.

[Paul Birkeland]

I don't see anything super obvious, so I'd leave it playing for a while to let it break in.

The SEX amp does have a bit of a low frequency resonance that bumps up the bass a few dB, so that may be what you're hearing.  I'd definitely double check that the 249K resistors are the correct value as well.

[Doc B.]

A few thoughts -

It's difficult for us to comment on how an amp should sound when parts have substituted from the get-go. Try the stock caps and see if anything changes. You may have simply discovered how bright silver caps can be in some cases.

Let the amp break in. Give it 50-100 hours of run time before doing any critical listening.

Going to a 4 ohm tap will not add bass, but it might seem to take some small amount of bass away.

All that said the SEX amp does present a little more bass than the Stereomour. But it shouldn't be as dramatic difference as you sem to be experiencing.

[blue360cuda]

Thanks Doc and PB !

Both 249k resistors are spot on, just double checked.  Lack of bass was similar between the stock tubes and my other set of 2A3s.

Maybe this is just the sound characteristic of the tube? Admittedly, I've never heard a 2A3 or 300B in person but from what I've read they're supposed to have a bit more fullness down low - not as much as a KT120 PP amp but this surprised me. It sounds exactly like a receiver that had the bass cranked down, mids all way up and treble in the middle.  The mids this thing has are quite spectacular!

I have the same 0.1uf silver/oils as well as the 1.5uf in the output position in my SEX3.0 and thought they made a nice improvement in that amp so I'm somewhat familiar with their sound but every amp is different and this is my first big DHT.

Is there a cap value I could play with to try and help the bass out a bit?

Will swap for the stock 0.1uf and see what changes.   Isn't this hobby fun?

Thank you both!

[Paul Joppa]

Definitely try another source - your description of the sound makes me think of an Eros problem. Just sayin'  :^)

The S3X and stereomour use the same plate choke and output transformer, and should have very similar frequency responses. (They operate at different impedance levels, but the parafeed capacitance and choke inductance differences compensates for that.)