Question on hum balance

[jamesp]

First I want to congratulate the Bottlehead team on an excellent manual; I've written tech manuals before (not electronic) and I know how easy it can be to know exactly what you are talking about without being able to conveying it to others; such is not the case here.  The manual was very easy to understand and follow for an electronic know-nothing such as I.

That is right up to page 76 where somehow I lost track.  I understand connecting the meter leads to the speaker binder posts but I cannot get my head around shorting the RCA jack.....is that every RCA jack at the same time, one at a time, left side then right side......?

Thanks in advance for any clarification!

My build was fun; all resistance checks were very good as were the voltage checks.  I adjusted hum as best I knew how without shorting any RCA jacks but the end result has a bit of hum in it (almost not noticable but there).  Otherwise the sound is very good.  While I am an electronic neophite I am far from a tube amplifier beginner having had (or strill have) ST-70's, Sherwood tube equipment, Fisher tube equipment, Marantz tube equipment, McIntosh tube equipment.  The Stereomour is switching out a MC-240 which frankly needs some attention.

One other question (maybe I should start a new thread but here goes); I get a fair bit of hum at initial power-up, which goes away after about five to seven seconds.  Is this indicative of a problem or is it somewhat expected; I've seen similiar with other tube gear before but did not expect it here.

Thank you

JamesP.

[Paul Birkeland]

Hello James,

Shorting the input jacks kills one source of noise that can't be nulled by the humpot, but you can get the same result by turning the volume control all the way down.

Were you able to get an AC voltage number on the speaker terminals while adjusting?  You can post this number as a bit of a benchmark in terms of noise.

The hum during startup is very normal, no need to worry there.

-PB

[jimiclow]

When I adjusted my hum pot, I used a sensitive headphone (Grado SR60i). You can fine tune it to the last "click". Now that I have portable o'scope, I get the same reading as "ear" tuning. I get 1mv on both channels, using 45 tubes.

[Paul Birkeland]

1mV is about right.  Doc B and I went through his wall of 300B Paramounts the other day (18 total) with the scope to check the hum balance positions.  We found a noise range between several hundred microvolts to just over 1 millivolt (the amps are wired for varying impedances, so in actuality this is very consistent), so your measurement is spot on.

-PB

[jimiclow]

Thanks Paul. The scope I'm using is just a cheapo Velleman HPS140i. While at it, I also measured the power output and I got 1.5 watts before clipping (at 4 ohms load), 7.7 watts plate dissipation. Still plenty of power for my 97db speakers.

[Paul Joppa]

1mV RMS is very good; I thing 2mV is more typical with 2A3s but old stock 45 designs had (I believe) some built-in hum reduction - anyhow they are often a bit quieter. The Stereomour has AC on the filaments, Paramount has DC with some ripple (hence it still has a hum pot) and if I recall correctly we usually found about 1mV pk-pk, maybe 0.3mVRMS when it was first produced.

Incidentally, the classic push-pull tube amps use indirectly-heated cathodes and have substantial feedback. All three of these differences make it easier to keep hum low. And all three contribute to the sound of SET amps.

[Paul Birkeland]

Ah, yes, didn't see that the OP was using a Steremour.

It is interesting how quiet a 45 can be with AC on the filaments, I had my SR-45's on 102db sensitive speakers with zero audible hum!

[2wo]

I agree, there have been a number of post, where folks have proposed DC, or other filament supply's for 45's, specifically for the SR-45. I don't think it is needed...John       

[jamesp]

Have not had a chance to get back to the amp and this site for a few days......cranked the volume down to nil and reworked the hum balance; now reading right at 1mv and can very much hear the difference; significantly quieter.

I also swapped an early Telefunken for the US made 12at7 that came with the amp (looks like maybe GE but I am not sure, no graphics on it).  Not sure yet as I only have a few hrs on the german tube but I think it is crisper; I very much prefered the telefunken 12ax7s on my mc-240 (1st shielded slot) but really not experienced with the 12at7, we shall see....I have a Mullard waiting in the wings once I have a week on the telefunken.  Having fun and enjoying the music!

Thanks for all the help, for a good product, for a good build experience!!!

James p.

[jamesp]

Just an update on my experience with minor tube rolling.  I tried the Telefunken 12at7 and did not like it; the sound was decidedly 'boardy' and clipped.  Installed the 'Bugle Boy' and wow, what a difference.  This 12at7 is open, lively, and very nice to listen to.  Have had that tube in for about three weeks now.

Very pleased with results.

Stereomour plumbed into Marantz preamp and Klipschorns.

James P