High Paramount 45 plate voltage

[Prairie Dog]

Hello Everyone,

I recently converted my Paramounts to run 45's (NOS RCA) in them, with bigger cathode resistors as recommended in the forum (everything else is stock).  I'm a little concerned with tube life, as my voltages are as follows:

A1 214.2
A2 496
A3 156               
A4 211.3
Line is 221.7

I've rolled everything in my 'AT7 arsenal through them , and this is about the best I could do ('59 Dutch Philips).  Any suggestions would be appreciated...

 

[Grainger49]

Did you read this thread?

http://www.bottlehead.com/smf/index.php/topic,920.0.html

That said the voltages look high but the maximum across the plate/cathode doesn't look out of spec.

 

[Prairie Dog]

Hi Grainger,

I read about all I could find on the matter...I thought plate to cathode was 275 max...I'm running about 286.  Is that anything to be worried about?  When I swap in Telefunkens or Mullards, it gets even higher.  Is there an optimum plate voltage one should be aiming for?  I wonder if a bigger plate choke would knock the B+ down enough to help?

Thanks

 

[Grainger49]

I know you mentioned the cathode resistor change but...  Do you have the power supply set for 2A3 operation?  The filament choke set for 2A3 operation?  All the changes from 300B to 2A3 done first?

 

[Prairie Dog]

Yup, I've been running 2a3s for a year or so now, used the old forum: http://www.bottlehead.com/et/adobespc/paramount/conversion/paramount300Bto2A3conversion.htm.

The voltages with the 2A3s were nearly perfect, using the same drivers:

A1 223.9
A2 466
A3 178
A4 221.6

Do you think I may have missed something in the conversion?

 

[Grainger49]

Poster Paully converted his with just the cathode resistors.  The link I posted had his voltages in it and yours are not that close to his.  I can only guess something is wrong about the conversion.  But you did both amps.  Do both measure the same, well, close to the same?

 

[Prairie Dog]

Pretty close, ball park anyhow...

 

[Grainger49]

Paully's numbers:...........Yours:

a1 196V..........................223.9V
a2 454V..........................466V
a3 148V..........................178V
a4 194V..........................221V

I know he was running the Paramounts on a PS Audio P500 at the time and it was set at 120V AC.
There is something wrong either with your readings (meter maybe) or some component.

 

[Prairie Dog]

I have a pretty good meter (Fluke 189), that I use daily at work...  I'll go over everything again in the next day or two,  I tend to get distracted sometimes and may have missed something...It's that 496 volt reading (with the 45's) that has me concerned the most...

 

[Paul Joppa]

You have found the problem with direct coupled designs - it's hard to get the driver plate=power grid voltage to be just right. We've wrestled with this for a long time; ever since the original eXcite/Afterglow amps. Periodically we almost decide to withdraw the product, but it continues to sound too good to kill it.

I have only three things to offer right now:

1) It's very odd that the driver plate voltage went from 178v to 156v when you changed the power tube cathode resistor; there should have been no change at all. Were there any changes to the driver at all (other than tube rolling)?

2) You can increase the driver plate voltage, either by substituting a different LED at the cathode, or by adding a resistor in series with the LED.

3) Eventually we'll get the driver replacement done; this includes the "slow-start" PC board and an adjustable driver bias voltage. It means you have to re-adjust the bias when rolling tubes, which is a pain, but it will let you trim the voltages to match the design points. Unfortunately it will have a hard time biasing a 12AT7 to a low enough voltage, which is one reason this upgrade switches to a 5670 driver.

Incidentally, the power line voltage of 221.7 looks like a typo. If you are in Europe with 230v line, and 240v transformers, your voltages should all be lower that you quoted - so I'm guessing you are in the USA and you meant 121.7v?

 

[Grainger49]

PD,

I believe the Fluke.  There are a lot of RS meters that can give false readings on the board.

 

[Prairie Dog]

Nope, I haven't changed any thing in the driver cct, other than tubes (I'm just a lowly tech, when I start to fiddle with design, it tends to let the magic smoke out)
What would you recommend as a diode or what value resistor? 
Maybe I should leave the p.a.'s wired for 2A3 until the new C4S comes out... I REALLY liked the sound of the 45, but I don't want to burn them up pre-maturely, with the prices they command...and me being on a lowly tech's wages and all.
Is there a safe way to knock the plate voltage down by 30 volts or so without messing with the grid / cathode voltages?
You were right about the typo -- I'm in Canada and the line is a steady 121.7, eh.
Like I said, I'll go through everything again, to make sure I didn't mess up in the conversion, but the 2A3s seem happy, so I dunno...

Many Thanks
-Trevor

 

[Paul Joppa]

Let's try to get the driver plate up to 186v - that's another 30 volts off the 45 and should be very close to ideal. Fortunately this is simple - the gain or mu of the 12AT7 is 60, so raising the bias 0.5 volts will raise the plate 30 volts. The C4S current is 3.8mA, so the added resistance would be 0.5/0.0038, or 131.6 ohms. 130 ohm resistors should be easy to find. Ideally I'd go for metal film, but because the voltage is low, I'm pretty sure carbon film will work very well. Wattage is small so any size resistor is fine. Put it in series with the LED that goes from B3 to ground.

This will increase the driver's plate resistance by a bit (mu times R, or 7800 ohms) but the 45 has a very tiny grid (Miller) capacitance so this will not adversely affect the treble.

 

[Prairie Dog]

Thanks very much for the help.  I went through the amps again, and everything is correct. I'll  get those resistors in and post the results...that way in 6 months I can reference it and remember why I did it in the first place.

-Unless, of course, the release date for the soft start board is near?  (How's that for fishing?)

Again, thanks Paul and Grainger, much appreciated.