Soft start shunt update?

[Grainger49]

For a non-direct coupled 2A3, like my Paramours this isn't a suggested improvement, right?

That, of course, doesn't address if the power supply can support it.

[Paul Joppa]

If the power supply can support it (i.e. Paramour IIs with the PT-2 power transformer) then it is a major upgrade because it would add shunt regulation to the driver tube. We are not planning a manual for that at first.

[Grainger49]

I'm a Paramour 1.  Well my amps are.  The jury is still out on me.

[Paully]

I am not overly concerned about the soft start for the Paramounts or even upgrading the sound, I have a hard time imagining them sounding any better than they do.  But one thing I remember reading is that the new board would drop the plate voltage on the 12at7 from 350 to 300V.  Certainly not a huge deal but anything to extend tube life on my beloved Mullards even by a little.  Do I remember correctly?

[Caucasian Blackplate]

You can drop the plate voltage yourself, if you like.  Replace the two 174k resistors on each green board with 150k resistors (use Vishay PR02 resistors).  I would also put a little clip on heatsink on the transistor Q2 on the B side of each board (the side with less components).  If you have an MJE-350 there now, consider replacing it with an MJE5731A, which will be easier to cool. 

[Paul Joppa]

It is kind of confusing.

The shunt regulated power supply for the Paramount driver is currently 350 volts. It is protected with a string of Zener diodes, until the shunt regulator tube warms up. In the new design there is a change that exposes the plate load transistor to a possibly greater voltage during the start-up transient, so we had to drop the regulated voltage to 300 volts. This is the effect of the resistor change PB describes. The shunt regulator tube now also has 300 volts on it, so it operates a bit more conservatively.

In the current production, the driver uses either an LED or a shunt-regulator chip to provide fixed bias without electrolytic bypass capacitors. However, that makes the range of actual plate voltage wider than would be the case with ordinary cathode resistor bias. That range is typically 140-220 volts, but can vary a lot with different tubes. There are two problems:

1) With the lower regulated voltage, some 12AT7s will run out of voltage headroom (compliance) when driving a 300B

2) The direct-coupled 2A3 is very sensitive to power line voltage variations, and the driver plate voltage should really be optimized for the particular power line voltage in your home. Even at the design value of 120vRMS from the power line, different tubes will have different plate voltages and can be uncomfortably off optimum.

For these reasons the new board has an adjustable bias voltage, so that the driver plate voltage can be set precisely. Normally it would be around 175-200v, same as the current design - only the C4S headroom (compliance) has been reduced, not the actual driver plate voltage. The adjustable bias arrangement we have implemented will not permit a bias below 2.5v, so the 12AT7 has too much gain and we went to the 5670 - which is also more linear and widely available in premium versions. The adjustable bias does make it easier to use a different tube; I plan to change my personal amps over to 6SN7s to reduce the excess gain.

[Caucasian Blackplate]

Sounds like modding a normal Paramount for 300V on the regulator would also require changing the 237 ohm R1 on the driver CCS to more like 250 ohms? 

[Brillo]

I'm eagerly awaiting this upgrade - mostly to give me something to mess with. 

PJ:  Is the 5670/2C51/396A confirmed in the design?  Basically, is it safe for me to source a pair of these tubes in anticipation of the product release?

[Paul Joppa]

PJ:  Is the 5670/2C51/396A confirmed in the design?  Basically, is it safe for me to source a pair of these tubes in anticipation of the product release?

Unless Doc B makes a sudden shift in focus, yes it is the design choice. Even if it weren't, it would be easy to implement this tube.

[johnsonad]

And once up and running, time for EML tubes :)  Should there be any issues after this modification?

[AudioDave]

Doc or PJ,  would you happen to have any new updates on when the soft start shunt update might be available?  I have kind of been holding off on constructing my 2A3 Paramounts.
Thanks!
Dave

[johnsonad]

Another month gone and hopefully the move is making progress.  How's it coming with the boards? :)

[Brillo]

I picked up a nice pair of 5670s and now jonesing for this product release. 

PJ - any updates you can share on release timing or issues?  I think a few of us are keenly interested in learning more... 

[Doc B.]

We are concentrating on getting the Stereomour out right now, which has fallen a little behind due to unforseen circumstances. Once we get the first of those kits out in the next three or so weeks, we will move on to the soft start update and some of the other long awaited projects.

[fvale]

Any update on the schedule for this long waited circuit? Thank you.