Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Paramount => Topic started by: Nutube on May 30, 2010, 03:10:55 AM
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My Paramounts 300B using TJ have the pot way to the left till it stops as my lowest noise. I still can only get 4.2 on right channel and 5.1
I sure feel that I can get lower hum is I can move the hum dial further left.
What can I do to get a Hum pot so that there is more leeway? Change to a new Hum pot ? Change a resistor somewhere?
I will try the EH tube and report back
Thanks
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The EH tube one faired very well lowest noise of 2.4 mV The other was way too hummy 27.5 mV. I switched sides still hummy.
In terms of the dial the good Eh provided more range .
Just wondering if there is a hum pot that can handle alot more range. I have looked at Mouser most are 10 turns.
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Jumper across the two white wirewound resistors coming off the pot. This will increase the adjustment range on the pot, but at the same time make the adjustment very sensitive.
I'd recommend jumpering across the resistors so you can take them out later if you find a different set of tubes that you like.
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It has been awhile since I adjusted my Hum pot using the TJ. which drives my Bass Bin. I bottom out on adjustability going to the left and it was 4.6mV or so
the othe side was in the 5.0 plust also bottoming out towards the left side.
So I decided to try my pair of EH. One was Waay loud - so I swapped sides and the loud hum followed the tube. The good EH was in the 2.5 range
Upon reading both your posts I decided to revist and Bam ! I saw only 2 out of 4 filaments lighted on the bad tube. I tried reheating the pins and added new solder but no thing works.
So - I replaced the TJ and the bottoming out to the left gave me same results BUT! I decided to turn the dial of the Hum pot way left and then way right (moderate speed by faster than I normally would in adjustment). I still measured 4.5 plus but this time I turn to the right and noticed the number started to go down again!!
Short story is It is now 2.1 mV on one and about 2.6 mV on the other . Sadly discovered the one EH is a goner. What I learned was that when it bottom out way to the left- I should slowly turn back and it will begin to start going down again.
CB I will try that jumper across. That is good to know! but for now I need to have an appointment with with Billie H. she is waiting for me on 45 rpm... : )
Question...when not turned on I read 1.8mV and as I mentioned above . Should I subtract the 1.8 from whatever reading to derive my final figures ?
Thanks again you guys
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Acute observation that half the filament was dead on one tube! That would indeed create asymmetric performance of the tube and make it harder to balance any hum.
"..when not turned on I read 1.8mV..."
You may have a magnetic field that is interfering with measurements, or that the output transformer is picking up, or both.
First, read the AC voltage with the meter leads shorted to each other. If this is not near zero (less than 0.2mV, I would say) then twist the meter leads together and try again. If the twisting helps, then there is a big magnetic field which is intercepted by the wire loop made up of the meter leads - the twisting reduces the area enclosed by the loop. Leave them twisted, and maybe even tape them together every foot or so.
Once you are able to measure near zero AC voltage with the leads shorted, then try the amp's outputs again. If there is still 1 or 2 mV at the output with the amp turned off, then the output transformer is picking up a field. The usual and most likely source is the power transformer on some other piece of gear, but old knob-and-tube wiring in the house can also generate large fields, or appliances, certain lighting fixtures, etc. You may find that moving or re-orienting the amps will help.
Once you can measure near zero output from the amp when it is off, then you can assess what noise is being added by the tubes and power supply.
Sorry that this seems to be getting complicated.
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No not complicated in the way you wrote it ...
I will try to measure again with the leads twisted and re orientation....I do notice now that you mentioned it this amp is right near my JBL D130 which has a hefty magnet. The other amp near it would be the Paraglow
Its way past "sipping time" for me and I am not gonna do testing with too many sips already. Tomorrow I will do that
Thanks Paul. Good night