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Re: O'Connor Preamp

4/7/1998 2:12 AM
Jack Orman Re: O'Connor Preamp
Forgot to mention... the ground point of your 2 x 100 ohms resistors on the heater supply should also connect directly to the star ground as mentioned in RG's reply.  
4/7/1998 7:42 AM

Like Jack said, the Re'an plastic body jacks  
do a fine job of isolating the input from the  
chassis. Probably not as "roadworthy" as metal  
jacks however, if that's a concern.  
Your stipulation "... without getting a  
special jack" would require what I would call  
sleeved nylon washers. These are washers with  
a stepped sleeve about as deep as the chassis  
is thick. You can find 'em in Mouser's catalog  
for sure.  
In a pinch, a little hefty shrink tubing  
around the jack threads, and some homemade  
plastic washers will isolate the jack.  
Or creative use of a big straw etc.  
With all of the above methods, you will  
need to enlarge the chassis hole a bit to  
As for your filament voltage, if that's the  
voltage you read when everything's up and  
running, then it's about 1 volt too high.  
This could be causing hum, and it's definately  
shortening the life of the tubes.  
Quick fix for that is to put a 3A/1000V  
diode inline with the positive feed to the  
filaments, which will drop 'em into a happier  
6.5 volt range. Or a 10W resistor in the  
.1 - .5 Ohm range.  
Don't worry about temporarily using those  
1/4 W ref. resistors... worst that can happen  
is that they burn out.  
Best of luck...  
... Whit
4/7/1998 9:38 AM
Would a .47ohm 5W resistor do in place of the .1-.5ohm 10W? I hate to place such a small order to Mouser and the only close resource for parts is (sadly) RS. They don't stock 1N5408 (3A/1000v) diodes, but I can order them from them, if that would be better.  
It sucks when you really don't know what you're doing. I just keep reading and reading... I might try backing over it w/ my truck and see what happens.....
4/7/1998 9:44 AM
cheer up greg, and remember that the definition of an expert is  
someone who's made every possible mistake at least once!
4/7/1998 10:16 AM

Hi Greg,  
First off is this 7.28vac the filament voltage while under load  
(connected to all of the filaments) or with the trannie running under no load?  
Before I would say OK to the 5 watt .5ohm resistor, I would like to know how many tubes and which ones are using the 6.3vac filament line.  
The reason being, if it is too big a load on the .5 ohms then the 5 watt resistor will end up being a very big heat soruce.  
A rough example in a near perfect world is...  
2 6L6s at 1.9amps and 3 12AX7s at .9 amps for a total of around 3 amps (to make this easy on me).  
so if you were pulling 3 amps through a .5 ohm resistor, it will drop 1.5volts. that will take your 7.28 volts down to 5.78vac.  
That would be a little low but I it might be OK.  
However, how much power does the resistor consume dropping 1.5 volts at 3 amps?  
Ohms law again: 1.5v x 3 amps = 4.5watts....that's too close to a 5 watt rated resistor to be safe in my opinion.  
Now work this backwards...  
You really only wanted to loose 1 volt from the 7.28 line.  
so, 1v/3a = .33ohms.  
So a single .33 ohm resistor will drop 1 volt at 3amps and consume  
about 3 watts. That's better but for 50 cents more.....  
How about this: a pair of .15 ohm 5/watters... two .15ohm /5 watt resistors, but one in each leg of the filament supply line.  
The entire filament voltage will be divided by the three resistances.  
The filaments themselves willl have the least resistance at take the lions share of the voltage drop and power but what is left over will be divided into the two fixed resistors.  
The tube filamets will drop 6.28volts and the two resistors will share the remaining 1 volt at a half volt each but they all will be subjected to the same current.  
So the voltage drop across the .15 ohm resistors now is only 1/2 volt each and 3 amps x .5v is 1.5 watts each.  
That should last forever with a 5 watt reisistor.  
4/7/1998 11:16 AM
Yep, 7.28-7.30vac w/ everything hooked up. This is just a preamp, so it only has two 12AX7's. What if I jump up and down on it?
4/7/1998 9:41 AM

The 7.28vac filament voltage may not be contributing to the hum problem, but your tubes won't last long under such conditions.  
Is the transformer you used reclaimed from an early piece of gear, built back when the line voltage rating was 110 or 115 volts? Or maybe the transformer is rated to supply a lot more filament current than your little preamp is able to draw, subsequently not pulling the voltage down closer to 6.3v.
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