50 Ohm 5w 10 turn wirewound resistors at a great price
- Thread starter johnsonad
- Start date
johnsonad
New member
Not sure. PJ worked out the current configuration because 10 Ohm pots were available and cheap. I picked up three pair of them, enough for a few future projects. Of note, these are 1960's vintage parts and very well constructed. Also, the price dropped by 25% since I posted this.....
fullheadofnothing
Moderator
The stock pot has a body diameter of approximately 24mm. The pot mentioned in this thread has a body diameter of just over 46mm. If you mount it in the existing hole, it will hit the terminal strip and the plate choke (and you'll need a new indexing pin hole, as this one is further from the shaft). If you make new holes to avoid those obstacles, you'll hit the power switch.
Also, FWIW, the stock pot is 10Ω 5W, not 10Ω 10W as stated earlier in the thread.
Also, FWIW, the stock pot is 10Ω 5W, not 10Ω 10W as stated earlier in the thread.
Grainger49
New member
The pot is flanked by 22 Ohm 5W resistors. The complete circuit is 54 Ohms. So if you replace the pot and both resistors with a 50 Ohm 5W 10 turn pot you still have much higher resolution adjusting the hum circuit.
Grainger49
New member
I used a smaller pot than the one pictured for that item number. I have a pot that size but it wouldn't fit in the Paramour.
This is the size I used, but not this value:
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2111880_-1
This is the size I used, but not this value:
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2111880_-1
2x22 ohm resistors +1x10 ohm adjustable linear means that there is an adjustment ratio of one 270 degree turn for 10 ohms of adjustment. 1 50 ohm ten turn linear pot works out to 5 ohms per 360 degree turn, or 3.75 ohms per 270 degrees of adjustment. So it's roughly three times the resolution.
Paul Joppa
Moderator
You do not need 5 watt dissipation - I chose the pot because it was wirewound, and hence likely to not make noise with current running through it. Five volts across 50 ohms is 0.5 watts; if you use 10 ohms with two series 22 ohm resistors (as in the stock Paramount) the pot dissipates 0.1 watt. I would want a pot rated for 5 times the actual dissipation for reliability reasons, but half watt pots should be easier to find ...
Grainger49
New member
The 1/2W pots I found online at Mouser and Digi-Key were all PCB mount and tiny. If you intend to put the shaft through the top plate like I did with my Paramours you will need to look at larger power ratings. I have a hand full of 10 turn Bourns and Clarostat (sp?) but all in the 5k-20k range. They do not have a power rating printed on them.
I see some online that are 2W rated but they do no go as low as 10 Ohms according to the data sheets. Those were all under $20 each.
I turned over my Paramours and couldn't see the printing to give you a part number. I'll bring them downstairs and get them under the light/magnifying glass later.
I see some online that are 2W rated but they do no go as low as 10 Ohms according to the data sheets. Those were all under $20 each.
I turned over my Paramours and couldn't see the printing to give you a part number. I'll bring them downstairs and get them under the light/magnifying glass later.
Grainger49
New member
John,
I was able to tweak my hum balance pots down to 1mV and 1.8mV when I put in the 10 turn pots. I hadn't been able to get them that low before. You get to a "one knuckle" resolution on the old 3/4 turn pot.
But it wasn't a noise problem for me, I just wanted the hum numbers down.
I was able to tweak my hum balance pots down to 1mV and 1.8mV when I put in the 10 turn pots. I hadn't been able to get them that low before. You get to a "one knuckle" resolution on the old 3/4 turn pot.
But it wasn't a noise problem for me, I just wanted the hum numbers down.
wylymon said:
With Paramounts and wiring for 4 Ohm impedance, you should be able to get the noise floor down into the hundreds of microvolts (less than 0.0008V) with the stock pot.
-PB
Paul Joppa
Moderator
Historical clarification: I think Grainger had the older 50-ohm pots before he replaced them with 10-turn pots; all our present designs (including the Paramount mentioned by PB) use a 10-ohm pot with a series 22-ohm resistor on each side, giving 5 times the resolution. Grainger's experience with the single pot was the same as ours, which is why we made the change.
2wo
Active member
My 300B amps have a bit of low level hum and I suspected it was from the AC heating. I had one on the bench a while back with the thought of changing to DC or other form of supply. But when I hooked up the scope I am able to easily zero out whatever component is contributed by the 300B filament. the bad news is there is still some hum. these were one of my first DHT projects, I may well have some copulating issues or noise from the driver stage. I am using some old stock WW pots that are about 1.5" in diameter, so maybe that buys me something...John
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