Paramount Plate Voltages for 2a3 and 45

[Paully]

I am trying to get a grip on voltage values I will see when I change over to 45 and looking at the voltages on the 2a3 something isn't making sense.  On the 2a3 in the Paramount the voltage on the Plate is 454V where the maximum plate voltage for a 2a3 is 300V.  So that looks 50% over maximum.  But given my level of understanding, I am positive I am missing something here, we wouldn't be running tubes that hot would we?  So what am I missing?  Understanding might help me see what should be there for the 45.

And can anyone help me with the voltages I should see at the pin sockets and any terminal strips that would change to be sure I am running the 45 conservatively in spec once I change over?

 

[Paul Joppa]

The rated maximum plate voltage in the specifications is relative to the cathode, i.e. filament. Since these are direct coupled, the grid is at the driver voltage and teh cathode is greater than that (by the bias voltage value). Here are the nominal values for a Paramount using 45 tubes:

Power supply voltage at top of plate choke: 490v

Plate voltage: 475v
  (There is a 15v drop through the DC resistance of the plate choke.)

Cathode voltage: 225v
  (Plate to cathode voltage is 250v, 10% less than the rated maximum.)

grid voltage: 175v
  (Bias is -50v.)

Plate current is 225v/6000 ohms, or 37.5mA.

Note that the operating point is set by the driver plate voltage which is identical to the 45's grid voltage since they are direct-coupled. The new driver board will have an adjustment to set that voltage more precisely; meanwhile check the voltage now before you swap out the 431 for an LED. You may want to fiddle with the driver bias until the new board becomes available, and it is safest to do this while still configured as a 300B amp (if I recall correctly that's the current arrangement?). You might for instance want a small resistor in series with an LED to get the driver plate voltage closer to optimum, or it's possible the voltage is close enough with the 431. Once optimized, don't change the driver tube - it is quite tube-dependent.

 

[Paully]

I already swapped it to 2a3.  But let me digest these numbers and get a better understanding with Grainger (it is so nice having an audio guru when one is a Latin teacher) and we can move on from here.  Thanks for the response, hopefully we won't have too many follow up questions.

 

[Paully]

I wanted to post some voltages, if I need to go back and check some more I will but this is what I got so far:

a2 405v
a3 148v
a2 to a3 306v

 

[Paul Joppa]

...
a2 405v
a3 148v
a2 to a3 306v

First, check your meter - or your calculator. 405v - 148v equals 257v A2 to A3, not 306v! I can't tell which voltages to believe.

Here are the nominal voltages for 2A3 version:
Power supply voltage at top of plate choke: 470v

Plate voltage: 452v (A2)
  (There is an 18v drop through the DC resistance of the plate choke.)

Cathode voltage: 220v
  (Plate to cathode voltage is 250v, 10% less than the rated maximum.)

grid voltage: 175v (A3)
  (Bias is -45v.)

Plate current is 220v/4000 ohms, or 55mA.

When we are sure what the voltages are, we can analyze them relative to the design target and see if any adjustments would be desirable or not.

 

[Paully]

Boy, you have the patience of a saint.  If it makes you feel any better, I find dealing with me frustrating too.

I rechecked and I had a2 written down wrong.  So let's try again:

a1 196v
a2 454v
a3 148v
a4 194v

a2-a3 306v

So, assuming my target is 250v to be 10% under 275 ohm (maximum for a 45 I believe) than I should up the 6000 ohms to around 7000 ohms or there abouts. 

My options are I can clip in one of the 2000 ohm resistors in series with the two 3000 and see where that puts me as I have those lying around.  Zero it in from there and maybe buy one 7500 ohm resistor to replace the two already there as that is a standard value.  I haven't been able to find a 1K 25 watt resistor that I could easily solder in between the two 3000 ohm resistors to replace the wire that puts them in series and up the resistance that way.  Anyway, I hope I am explaining myself well enough.

 

[Grainger49]

I spoke to Paul this morning.  I think he has found the sound of one hand clapping.  The 45s are installed and now he knows why so many guys like them.

 

[Paul Joppa]

...
I rechecked and I had a2 written down wrong.  So let's try again:

a1 196v
a2 454v
a3 148v
a4 194v

a2-a3 306v
...

Excellent! Thanks for the quick measurements; we're in business now!

The rated 275v maximum is plate to cathode, i.e. plate (A2) to filament (A1/A4) - you are at 259v which is fine. The cathode is a bit low, so the cathode current is also low - don't increase the cathode resistor which would further reduce the current.

The best adjustment would be to raise the 148v driver plate/output grid voltage a bit - but not all the way to 175v, since your high voltage is a hair low (power line voltage is probably a bit under the nominal 120v). Maybe 165v or so. But you are quite close now; I doubt the change would be very audible.

My recommendation is to leave it as is, especially if you are planning to eventually get the not-yet-released new driver setup with 5670 tube and new board which has adjustable driver bias.

 

[Paully]

I am planning on getting the to be released driver board.  So I will just leave it as is and listen since I am obviously not going to be burning up tubes at the level I am running them at and that is what I was concerned about.  And I have control of the line voltage through my PS Audio P500.  I have it set for 117.  I can bump it to 120 easily.

I appreciate all of the help!  I am not sure I fundamentally understand everything that is going on, maybe I will spend some time rereading and getting a better grasp of what is actually happening behind the scenes.  Its tough but I am curious to understand things better.  But once again thanks for setting me straight, it is very appreciated!

 

[Paully]

I probably have 10 hours on the NOS 45 tubes now that I picked up.  Also picked up a pair of used 45 tubes from ebay.  Everything I have heard thus far makes me think I will probably be going back to 300B.  They have a detail to them that is superb, it is like someone gave the treble a boost.  But due to some problems with my ears that isn't necessarily a good thing.  And I think I miss the ambiance and palpability of the 300B.  Anyway, I am going to give the 45 tubes a full 100 hour burn in then decide.  The beauty is if I think I am wrong I can always switch back again!