Paramount 300B hum balance adjustment

[Anton]

Hello,
I have Paramount 300B monoblocks I built back in 2010 and currently use with Sophia Electric Princess 300B Mesh Plate tubes.
Recently I decided to check my hum balance, as per page 64 of the manual. The manual says the reading should be 0.8 - 2.0 mV - but I couldn't get mine below 2.5 - 3.0 mV.  Should this be a concern and what can I do to fix it?   I am using these 300B tubes since 2011 - is this time to change them?
Thank you

[Paul Birkeland]

Do you still have the stock tubes?  You could certainly pop them in and give them a shot.

[Anton]

Hi Paul,
Thanks. Yes - I still have stock EH 300B tubes. I actually tried to pop them in, but the reading is still around the same range 2.9 - 3.9 mV.
So - I guess this is not a tube issue then?

[Paul Birkeland]

Do you get the same reading with no 9 pin tube installed? 

What meter are you using to measure the hum voltage?  Is there a load on the amp? (A speaker connected)

[Anton]

Paul,
I have the same readings without 9 pin tube (12AT7EH) installed.
The meter I am using is "Fluke 115 True RMS Multimeter" - I set it to AC millivolt scale.
During the hum voltage measurement, I don't have a load on the amp - a speaker is not connected. 

[Doc B.]

Hum measurements at the output are taken with a load on the amp. Hook up your speakers and take the measurement again. That will give you a reading that corresponds to the way our hum spec is measured.

[Anton]

Hello Doc,
Thank you very much.
I went ahead and hooked up a speaker to my Paramount and re-check the measurement. It still reads in the same range 3 - 4 mV.
It doesn't stay steady, but rather changing constantly in that range.

The speakers I am using are Cain&Cain Abby with a sensitivity of 95 dB and a nominal impedance of 8 ohms.

[Paul Birkeland]

Do you actually hear any hum?  How fresh are the batteries in the meter?

[Adrian]

Hum measurements at the output are taken with a load on the amp. Hook up your speakers and take the measurement again. That will give you a reading that corresponds to the way our hum spec is measured.

My Paramount manual does not mention the need to connect the speakers to the binding posts along with the meter leads when adjusting the Hum Balance.
I don't have the 300b Kaiju.  However, is this also recommended for adjustment on the Kaiju?
Just curious.

[caffeinator]

Make sure to do this test with the input shorted. I remember checking the hum balance on my Paramounts - a looooooong time ago - and it seemed high to me. Shorting the input brought that back in line with the nominal specs in the manual.

I keep meaning to get myself a set of RCA shorting plugs...one of those things one needs only once in a great while, though it's easy to MacGyver something on the spot.

[Anton]

Do you actually hear any hum?  How fresh are the batteries in the meter?

Paul,
Yes - I hear some hum from the speaker when connected to the paramount. Not very loud - you have to stand near the speaker to hear it. But still - it is louder, than when connected to my S.E.X. integrated amp.

I just changed the battery in the meter and re-took the measurements. it reads the same - I guess the battery was fine.

Also - I did short the input when taking the measurements. 

[Doc B.]

Just to clarify, you don't need a speaker load to adjust the hum balance. In fact it might be easier to find the lowest hum level without a load, as the hum level will be higher without a load and it might be easier read the changes on your meter as you twist the balance pot looking for the low point. However you do need a speaker load on the output to determine a useful number for hum magnitude, as it will be lower than with no load. And that is the standard used for hum measurement.

[Paul Birkeland]

Of course I also forgot to ask if you have the amp wired for 16 ohms, which will increase the hum voltage seen at the output.

[Adrian]

Dan
Thanks for your great explanation on hum balance adjustment with and without a load on the amp.

[Anton]

Of course I also forgot to ask if you have the amp wired for 16 ohms, which will increase the hum voltage seen at the output.

Paul,
I think my Paramounts are wired for 8 ohm, just standard version.