Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Paramount => Topic started by: Anton on June 28, 2020, 09:23:24 AM
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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
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Do you still have the stock tubes? You could certainly pop them in and give them a shot.
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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?
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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)
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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.
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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.
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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.
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Do you actually hear any hum? How fresh are the batteries in the meter?
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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.
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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.
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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.
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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.
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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.
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Dan
Thanks for your great explanation on hum balance adjustment with and without a load on the amp.
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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.
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As tubes age, some mechanical warping takes place (as I understand it, typically the filament) due to the number of heating/cooling cycles. Since these tubes are well used, it is reasonable to guess that's the reason for the modest change in hum level.
Here's a little background: The design goal was 2mV, which is quiet enough for all but the most efficient all-horn speakers. Without the DC filament supply, I was not able to get hum below 5-8mV. If the hum is a problem, an electrolytic capacitor can be installed at the output of the filament supply to reduce it further. We didn't include that in the stock amp in the interest of avoiding capacitors in the filament supply, which some listeners think are responsible for a "flattening" of the sound. I find it to be a very subtle difference, but YMMV - we all hear a little differently.
In the current Stereomour and Kaiju, we have separated the DC filament supply as upgrade kits, since for most speakers even 5mV is good enough to be inaudible.
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Hi:
This thread motivated me to measure the hum on my Paramounts. Using the same page 64 procedure as above, with and without a load, I can dial in .000v on both amps. Is that possible? The meter is an inexpensive Radio Shack auto range unit but it has worked well over the years. My speaker sensitivity is approx. 95dB and I hear almost no hum with my ear to the speaker drivers.
Gerry
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0.000V tells you it's less than 1mV, which is generally what I have measured on Paramounts (about 600uV).
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Hi:
This thread motivated me to measure the hum on my Paramounts. Using the same page 64 procedure as above, with and without a load, I can dial in .000v on both amps. Is that possible? The meter is an inexpensive Radio Shack auto range unit but it has worked well over the years. My speaker sensitivity is approx. 95dB and I hear almost no hum with my ear to the speaker drivers.
Gerry
Hi Gerry,
Do you have a standard version of the Paramounts or the one with the soft start upgrade?
Thanks
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Hi Gerry,
Do you have a standard version of the Paramounts or the one with the soft start upgrade?
Thanks
Hi Anton:
Soft Start v1.1.
Gerry
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Hi Anton:
Soft Start v1.1.
Gerry
Thank you Gerry, and Happy 4th of July!
I wonder whether soft start upgrade would affect the hum level?
Thanks
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I wonder whether soft start upgrade would affect the hum level?
No.
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No.
Thanks Paul.