Soft start shunt update?
- Thread starter ssssly
- Start date
A week ago we ran the amps in for several days with a sweep generator. We just cleaned and overhauled the system yesterday to make sure everything was at 100%. This morning we compared the two setups. In the initial comparison a few weeks ago I heard differences in the midrange and the bass and they were identically good on the top end.. So today we rigged my active crossover system setup with the amps playing midwoofers. Even after the break-in I heard the same differences I heard initially, even when trying of a few different types of 5670s we ran. So I think we are a step closer to getting this done.
The SR45 is not directly coupled, so adding this type of soft start seems needless..
Paul Joppa
Moderator
Once we get the Paramount version out there, I'll take a look at the SR45 - since I want to at least build one. I have a modified design to try out, with DC filaments. I'll also look at other applications. The plan is still to replace all the C4S boards with this new version, eventually. This may take a long time, but it is in the plan.
Paul:
Per your here I was waiting on these boards for my rebuild/upgrade
"The PT-2 can support a shunt reg driver, copied from the Paramount - anyone who wants to do this should probably wait for the new Paramount driver board with time delay and other goodies; the upgrade kit will be available - might as well convert to the 5670 driver at the same time."
Planned configuration was PT-2, TFA-2004 Nickel, BCP-15 plate chokes and EXO-003. CLCRC full wave schottky rectified power supply with the signal circuit alla the upgrade iron variation with the plate voltage reduced to 275 and current increased to 55mA (also per your suggestion). After reading several posts about the great sonic improvement I also was planning on incorporating the shunt board you have been working on.
Would you suggest this new board for my application or the original SR45 shunt configuration?
Per your here I was waiting on these boards for my rebuild/upgrade
"The PT-2 can support a shunt reg driver, copied from the Paramount - anyone who wants to do this should probably wait for the new Paramount driver board with time delay and other goodies; the upgrade kit will be available - might as well convert to the 5670 driver at the same time."
Planned configuration was PT-2, TFA-2004 Nickel, BCP-15 plate chokes and EXO-003. CLCRC full wave schottky rectified power supply with the signal circuit alla the upgrade iron variation with the plate voltage reduced to 275 and current increased to 55mA (also per your suggestion). After reading several posts about the great sonic improvement I also was planning on incorporating the shunt board you have been working on.
Would you suggest this new board for my application or the original SR45 shunt configuration?
Paul Joppa
Moderator
There does seem to be some confusion here.
The PT-2 can deliver 60mA of DC high-voltage current. If you stay with the 50mA 2A3 current used in the Paramour, there is 10mA left, which is enough for a twin-triode shunt regulated driver - we'll have the new board set up for the 5670 in the Paramount so that's a good choice, but others can work.
If you raise the 2A3 current to 55mA, which is more optimal for the 3K impedance, then you have only 5mA left and can't shunt regulate the driver. Also, the BCP-15 cannot support more than 50mA of plate current. So I'd stick with the 50mA/300v of the original Paramour; the increased inductance of the BCP-15 goes a long way to make up for the slightly off-optimum operating point.
The SR45 replaced the 2A3 with a 45, cutting the output power in half. The 45 operates at 250v and needs only 36mA, so there is enough current left over (just barely) to shunt-regulate the entire amp, not just the driver stage. We produced a very limited run of semi-kits a couple years ago for this - semi-kit meaning that there was almost no manual and not much developmental testing. It was intended for advanced experimenters. We never saw enough potential sales to justify the full development effort, but we liked the sound so much we wanted to make something available to at least a few.
I posted a few minutes ago in the other thread, sorry for the confusion and I hope this note helps make sense of my other one!
The PT-2 can deliver 60mA of DC high-voltage current. If you stay with the 50mA 2A3 current used in the Paramour, there is 10mA left, which is enough for a twin-triode shunt regulated driver - we'll have the new board set up for the 5670 in the Paramount so that's a good choice, but others can work.
If you raise the 2A3 current to 55mA, which is more optimal for the 3K impedance, then you have only 5mA left and can't shunt regulate the driver. Also, the BCP-15 cannot support more than 50mA of plate current. So I'd stick with the 50mA/300v of the original Paramour; the increased inductance of the BCP-15 goes a long way to make up for the slightly off-optimum operating point.
The SR45 replaced the 2A3 with a 45, cutting the output power in half. The 45 operates at 250v and needs only 36mA, so there is enough current left over (just barely) to shunt-regulate the entire amp, not just the driver stage. We produced a very limited run of semi-kits a couple years ago for this - semi-kit meaning that there was almost no manual and not much developmental testing. It was intended for advanced experimenters. We never saw enough potential sales to justify the full development effort, but we liked the sound so much we wanted to make something available to at least a few.
I posted a few minutes ago in the other thread, sorry for the confusion and I hope this note helps make sense of my other one!
Paul Joppa
Moderator
The circuit is done. It's just a matter of getting a batch of parts in and writing the manual. Probably won't happen until the Stereomour manual is done, but the "final proof-of-concept" prototypes tested OK this week so that shouldn't take more than a couple-three weeks - assuming Doc B can get downstairs pretty soon!
I'm fairly sure the initial product will be the Paramount replacement board, including conversion to a 5670/2C51/396A driver tube. I haven't talked to Doc B about the detailed process, but I'd expect the Paramount to be revised accordingly - the main issue is revising the manual, which is WAY more work than you might think.
I'm fairly sure the initial product will be the Paramount replacement board, including conversion to a 5670/2C51/396A driver tube. I haven't talked to Doc B about the detailed process, but I'd expect the Paramount to be revised accordingly - the main issue is revising the manual, which is WAY more work than you might think.
Grainger49
New member
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.
That, of course, doesn't address if the power supply can support it.
Paul Joppa
Moderator
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
New member
I'm a Paramour 1. Well my amps are. The jury is still out on me.
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?
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
Moderator
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.
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.
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?
Paul Joppa
Moderator
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.Brillo said:
Should there be any issues after this modification?
