Paramour 2 with 5670?

[Mike F1]

Hello all,

I'm currently running a Paramour 2 with MQ upgrade - 12AT7/2A3.

I was wondering if there would be any sonic benefit in removing the 12AT7 and replacing it with a 5670?
Can a 1/2 section of the 5670 tube drive the 2A3 adequately in this topology?

I'm currently using a Soul Sister (5670) preamp so I feel I have enough gain from there.

Thanks

Mike

[Paul Birkeland]

The real trick would be to buy the Paramount Soft-Start kit and put it in the Paramour II.  The issue will be the available voltage, which is quite a bit lower.

The Soft-Start kit regulates to 300V, and you have 320V of B+ available.  This may work just fine right out of the box, but if you happen to have significantly less than 310V of B+, the 149K resistors can be changed to 143K to give yourself more compliance.

This upgrade would give you the sonic benefits of the driver stage available in the Paramount.

If you want to rewire the socket for half a 5670, this is also possible.  You can leave the C4S board set as-is, but replace the LED at the cathode with a 1.1K resistor.  Do be aware that the 5670 has a very odd pinout (it actually looks beautiful and straightforward on the datasheet), so you'll want a fresh socket.

[Mike F1]

Thank you Paul.

I did the conversion last night and I can hear a noticeable difference. It's hard to describe but it appears to be smoother, less noise and slightly less gain (which was to be expected)

The 1.1K Cathode resistor gave me too high of voltage on the 5670 Plate (246VDC)
I ended up installing a 1k Cathode for a Plate voltage of 208VDC
With the 1k Cathode restore the 5670 is running ~4ma.
Do those specs seem reasonable?

FWIW, leaving the LED in brought the Plate voltage down to 120VDC.

Still sounds great!  The 5670 will be staying in!

[Paul Birkeland]

Hmm, are these new old stock 5670's? 

3.5mA of plate current across a 1100 Ohm cathode resistor gives 3.85V of bias.  3.85V of bias and 3.5mA of plate current should put you right at 175V, but it would be higher with a worn tube, a flaky connection, or if your R1 C4S resistor is less than 237 Ohms.

What R1 resistance do you measure on the board?

Alternatively, what voltage do you measure across the 1K resistor you have installed?

[Mike F1]

Yes, I also thought the voltages were a bit off.

I put another NOS 5670 tube in (which I suspect is good since it was in my Preamp)
All the solder joints look good.

The CCS R1 is 239ohms.
5670 Plate 205V
Rk - 1000ohms with 4.01VDC across it.

Thoughts?

 

[Paul Joppa]

This looks very good to me.

The Paramount 5670 driver has an adjustable bias voltage, and you have discovered the reason - there is significant variation from tube to tube. In the Paramount, with its lower regulated voltage and the higher voltage swing demanded by the 300B, we needed to make it trimable.

In Paramour or Stereomour, the 2A3 makes less severe demands you have a greater range of acceptable plate voltages. A plate voltage of 110 to 260 will work, though I would advise 130 to 240 for best performance. But 200v is "practically perfect in every way"   :^)

Incidentally, it might drop a bit if the tube were really new old stock. Since the tube was in use previously, it's safe to assume it has stabilized.

[Mike F1]

Thanks for the reply Paul. I really appreciate the comments and thoughts.

I would assume that ~4ma on the 5670 is the target zone for this application?

I've been running this set up for a few days now and I'm pretty happy with the way it sounds.
It would appear to me that the 5670 tube is a nicer sounding tube than the 12AT7.

Regards

Mike