Ways to lower a high B+ because of high powerline voltage

[saildoctor]

So uh - my B+ is running a little high still:  ~495V.  The amps are set up direct coupled as per 2a3's but with a 6k cathode resistance using 45's.  My powerline voltage is always very high as in 123-125V!  I have some chokes which I was going to put into the high V power supply to replace the resistor.  I got to thinking if it would be a good idea to instead add them (they measure 270ohm) onto the end of the of the PS to make it a CRCLC and lower the voltage a bit?  If that should work out I'm going to get some of those Claritycap TC's to put into the end.  Any ideas on what a safe minimum cap value I should shoot for?  There are popular available values for those in 33uf, 45uf, and 110uf.

 

[Paul Birkeland]

I'd expect 480+V in your implementation.  123-125V on the line is no big deal.  Out of curiosity, what's the grid voltage of the 45, and the cathode voltage (at the top of the 6K cathode resistor)?

 

[saildoctor]

Right now it's just the stock CRC filter.  I figure between the driver and 45 I'm drawing about 47ma - so adding the choke should knock it down about 12-13V.  I know that i may need to play with where to put it to avoid picking up other transformer's fields.  I'll measure what's going on in there and report back later.  My main question is how much capacitance is prudent in the last in the filter cap position?

 

[Paul Birkeland]

You could drop that voltage by removing the 270 Ohm 5W resistors and putting in 500 Ohm 10W resistors.

If you want a choke, the Hammond 157L would be a decent drop in to have a CLC filter that will drop some extra voltage.

 

[saildoctor]

Right now I'm using some NOS Tung-Sol's that only have about 10-15 hours on them.  They seem to work a little under max dissipation, my older pair of Philco's I could only bias down to be just over.

A1 - 225.7 V
A2 - 496.5 V
A3 - 170.2 V
A4 - 223 V
On top of 6k cathode resistor - 224 V

I already have the Triad 270ohm chokes so I may get some of those film Claritycap TC's and tacking them onto the end of the filter network because I'm interested in seeing if it can lower the noise floor.

 

[Paul Birkeland]

So, you have 270V P-K, 37mA of current, and 55V of bias (10W dissipation).  This is pretty much the text book operating point, but a 4-5K load is called for (the SEX/Steremour iron will work really well here).

Are you looking to slide down to ~230V/35mA operating point to work better with the 3K iron?

 

[saildoctor]

It sounds like the high voltage is not much of an issue then.

Now there's a thought.  That could be achieved by bumping up the cathode resistor and then fiddling with the driver voltage yes?  For the 250V operating point that uses 3900ohms load I'd need around 7.25 kohm on the cathode?  Although I may be more interested in getting some exo-45 / bcp-15's.    Is there a way to figure how much power is being lost because of the 3k opt?

 

[Paul Birkeland]

You may actually be getting a tiny bit of extra power from the 3K transformer, but at the expense of distortion.

 

[VoltSecond]

You can use a Variac.  I'd get one that is rated for about 1.5 to twice the line fuse rating of what you are driving. If you can find a 50Hz rated Variac, consider that a bonus if your line frequency is 60Hz.

http://www.parts-express.com/pe/showdetl.cfm?partnumber=120-842 One option of many.

Watch the filament voltage. You'll want that in spec too.

 

[jrebman]

Kerry,

Are you saying that the noise floor is a problem with this amp?

-- Jim

 

[Paul Birkeland]

Kerry,

Are you saying that the noise floor is a problem with this amp?

-- Jim

I think he was concerned with adding a choke and coupling magnetic fields.

We have used Paramounts many times on headphones with fantastic results!

 

[jrebman]

PB,

I understand and that's why I was concerned and wasn't sure if it had something to do with the 45 implementation. Understand now. ;-)

I actually have the same issue as Kerry -- high line voltage -- 125v most of the time and am consideringg a 45 version of the paramounts.

-- Jim

 

[saildoctor]

I do have a small amount of hiss going on - it could very well be my solder joints.  I will try out installing the choke and cap (because I have both already!)  just to see how it works out voltage wise and to see if I hear a diffeence.  I'm in San Francisco sailboat racing for the next week and a half and won't have time for a few weeks to try it out.