Hello all,
As noted in the main thread, I've decided to go with 2A3 in my TSE-II initially and leave the option to go for 45 later. I will be moving to Japan eventually, so I am planning to go with a 300-series Hammond "universal" PT. I considered the options that George discusses on his website, but just today I was sifting through the whole catalogue of 300-series and noticed the 370KX. It appears to be an ideal transformer for this project:
The HV Secondary is quite "stiff" at 322mA, but I used PSUD-II to model the TSE-II power supply with this transformer and all looks to be in order, resulting in a B+ of 295v. This should be an "easy" B+ load for this transformer and I think (correct me if I am wrong) this will give me a bit of breathing room while running the 6.3V winding at full tilt for those 2A3 filaments. Please check my work below and comment if anything is incorrect.
The only thing I am not sure about regarding my model setup is the load value. I know the 2A3s will be drawing about 53mA each (at this B+ voltage) and I just tacked on another 40 or so mA for the 5842 tubes, losses, etc. adding up to an estimated 150mA load (as seen in the schematic). This was a complete guess and I would like a more accurate estimate of the total B+ current draw. If anyone can help, that would be great!
Here is how it shapes up in SE Amp CAD. Plate voltage ends up very close to the "ideal", at 273v.
Note that SE Amp CAD has an incorrect value of "250 v" for the Vmax for 2A3. Every spec sheet I have checked lists it as 300 v.
As noted in the main thread, I've decided to go with 2A3 in my TSE-II initially and leave the option to go for 45 later. I will be moving to Japan eventually, so I am planning to go with a 300-series Hammond "universal" PT. I considered the options that George discusses on his website, but just today I was sifting through the whole catalogue of 300-series and noticed the 370KX. It appears to be an ideal transformer for this project:
| Shipping Weight | 2.3 lbs (1.04 kg) |
|---|---|
| UNSPSC | 39121301 |
| UPC | 623980050072 |
| REACH Compliant | Yes (more info...) |
| RoHS Compliant | Yes (more info...) |
| Power Rating (VA) | 229 |
| Primary VAC | Universal - 100, 110, 120, 200, 220,230, 240 VAC 50/60 Hz |
| A.C. High Voltage Secondary RMS | 500V C.T. @ 322ma. |
| Bias Tap on High Voltage Secondary w/ C.T. Grounded (VAC) | 50 |
| Filament #1 (VAC) | 5V C.T. @ 6A |
| Filament #2 (VAC) | 6.3V C.T. @ 6A |
| Mounting Style | Vertical ("X") |
The HV Secondary is quite "stiff" at 322mA, but I used PSUD-II to model the TSE-II power supply with this transformer and all looks to be in order, resulting in a B+ of 295v. This should be an "easy" B+ load for this transformer and I think (correct me if I am wrong) this will give me a bit of breathing room while running the 6.3V winding at full tilt for those 2A3 filaments. Please check my work below and comment if anything is incorrect.
The only thing I am not sure about regarding my model setup is the load value. I know the 2A3s will be drawing about 53mA each (at this B+ voltage) and I just tacked on another 40 or so mA for the 5842 tubes, losses, etc. adding up to an estimated 150mA load (as seen in the schematic). This was a complete guess and I would like a more accurate estimate of the total B+ current draw. If anyone can help, that would be great!
Here is how it shapes up in SE Amp CAD. Plate voltage ends up very close to the "ideal", at 273v.
Note that SE Amp CAD has an incorrect value of "250 v" for the Vmax for 2A3. Every spec sheet I have checked lists it as 300 v.
No takers? Hmmm.
In the meantime, I have a related question.
If I decide to splurge on a dedicated 6.3v filament transformer (to feed the 2A3 tubes) will the 100v mains voltage in Japan matter?
In other words, if I run the Hammond 266S6 (which is 117v/234v 50Hz/60Hz primary to 6.3v CT 10A secondary) on a 100v mains supply, the secondary voltage will drop to ~5.4v CT.
100 / 117 = 0.8547008547 * 6.3 = 5.38461538462
Since the circuit will be configured for for 2.5v filament and the Microchip regulator only needs 3.5v to work effectively (v-IN >= v-OUT + 1 in data sheet), it seems the 5.4v should be just fine. Is this correct?
In the meantime, I have a related question.
If I decide to splurge on a dedicated 6.3v filament transformer (to feed the 2A3 tubes) will the 100v mains voltage in Japan matter?
In other words, if I run the Hammond 266S6 (which is 117v/234v 50Hz/60Hz primary to 6.3v CT 10A secondary) on a 100v mains supply, the secondary voltage will drop to ~5.4v CT.
100 / 117 = 0.8547008547 * 6.3 = 5.38461538462
Since the circuit will be configured for for 2.5v filament and the Microchip regulator only needs 3.5v to work effectively (v-IN >= v-OUT + 1 in data sheet), it seems the 5.4v should be just fine. Is this correct?
Update to post #2 - Of course I have just realized that the 5842s will also draw from this dedicated transformer (since there is only one 6.3v input on the board) and they will want the full 6.3 volts. Ideally I would use the 6.3v winding on the main PT for the 5842s and the dedicated 6.3v PT for the 2A3 tubes, but it seems that would require some magic that I am not quite capable of.
I think you misunderstood my statement. Sorry I wasn't clearer.
The Microchip regulator only needs to be fed a minimum of 3.5v in order to output the required regulated 2.5v for the power tube filaments.
v-IN >= v-OUT + 1 in other words, voltage input into the regulator needs to be at least 1v greater than the desired output voltage.
But it won't work anyway, because the 5842s will not be happy with 5.4v. I think I might abandon this idea of a separate, dedicated 6.3v transformer for the 2A3 tubes.
If you choose to use a dedicated 6.3 volt transformer, get a large one, say 8 to 10 amps with a CT and use it for the only 6.3 volt source. The board has the magic built in. The Microchip part will get about 3.5 volts DC and the 5842's will get 6.3 volts DC.
The issue will be attempting to run this kind of setup on 100 volts. The "magic" in the board is a full wave bridge across the 6.3 volt winding with the CT grounded to create a plus and minus supply of about 3.5 volts. The positive supply uses Schottky diodes for the low voltage drop to barely make 3.5 volts to feed the regulator when loaded by a pair of 2A3's and the 5842's. The negative supply only feeds the 5842's which are running on both positive and negative supplies. A dropping resistor is required to get 6.3 volts on the 5842's.
The board will draw about 50 mA (5842's, mosfets, and voltage divider resistors) plus the current drawn by the output tubes.
I am using a Hammond 372HX transformer in my 2A3 amp. Yes, it pounds out 370 to 380 volts of B+ on my 125 to 129 volt line voltage. The pole transformer is right next to my house.
I'm running some cheap Chinese 2A3's that are at least 10 years old. I run them at about 70 mA each without issue. The 372HX and the 370KX both have a 6.3 volt 6 amp winding, so that should work well for you as well.
I can change the jumpers and run 300B's in the same amp, but one must be real careful not to put 2.5 volt tubes into the amp without checking the jumpers first. was stupid enough to put a filament voltage switch on an older amp and I vaporized the filaments in a pair of 45's by feeding them 5 volts.
I get about 4 watts from the 2A3's and about 6 from the 300B's. both are working into 3000 ohm Transcendar OPT's.
Thanks for the explanation of the 6.3v power section, George. I will put the idea of a dedicated 6.3v transformer on the back-burner for now. It complicates things and, based on your extensive experience, it isn't necessary.
You've also got me re-considering this option to also run 300Bs. Your warning about adding a filament voltage switch is loud and clear, but I am thinking maybe a very well marked top-side switch would suffice. "45/2A3 <-> 300B" label under the tubes, with a line drawn to a clearly marked "45/2A3 <-> 300B" switch.
I also don't want to push my 45/2A3 plate voltage as hard as you are, so I would like to figure out a switchable solution for that. You mention adjusting C4 value to manipulate B+. Could this be made switchable with two caps on a switch?
For example, with the 372HX configured in PSUD2, a C4 value of 3.3uF results in a B+ of 292v (for 45/2A3) and a C4 value of 47uF results in 352v B+, sufficient for 300B. Possible to install a switch to flip between 3.3uF and 47uF caps (like people do with cathode bias resistors in SSE)?
I am imagining the caps in parallel, with a switch to enable/disable the 47uF cap. Ideally, it would be one switch that provides both functions (switches filament voltage and capacitor at the same time).
You've also got me re-considering this option to also run 300Bs. Your warning about adding a filament voltage switch is loud and clear, but I am thinking maybe a very well marked top-side switch would suffice. "45/2A3 <-> 300B" label under the tubes, with a line drawn to a clearly marked "45/2A3 <-> 300B" switch.
I also don't want to push my 45/2A3 plate voltage as hard as you are, so I would like to figure out a switchable solution for that. You mention adjusting C4 value to manipulate B+. Could this be made switchable with two caps on a switch?
For example, with the 372HX configured in PSUD2, a C4 value of 3.3uF results in a B+ of 292v (for 45/2A3) and a C4 value of 47uF results in 352v B+, sufficient for 300B. Possible to install a switch to flip between 3.3uF and 47uF caps (like people do with cathode bias resistors in SSE)?
I am imagining the caps in parallel, with a switch to enable/disable the 47uF cap. Ideally, it would be one switch that provides both functions (switches filament voltage and capacitor at the same time).
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Another option (probably safer, for both the tubes and the humans) would be to install miniature PCB-mount binding posts for C4 and for PADs 1,3,4. When I want to run 300B, I can simply open up the amp, swap a bigger cap into C4 position, change the jumper, close up the amp. I can do it all without getting the soldering iron out.
I did this for the coupling caps and cathode resistors in my SSE and it has been very handy!
I did this for the coupling caps and cathode resistors in my SSE and it has been very handy!
Back to transformers. I am now looking at the 373EX. What a monster! I like the look of the 6.3v winding on this one:
So, no problem at all running 300Bs with this, obviously, and it'll be better capable of powering the 2A3 filaments for sure.
Here's what PSUD2 looks like with this transformer, 47uF C4 and 230mA load (2x 300B @90mA + "board load" @50mA):
And here is what PSUD2 looks like with the same transformer, 2.2uF C4, and 156mA load (2 x 2A3 @53mA + "board load" @50mA):
So, what's not to like about the 373EX (other than the price)?
| Primary VAC | Universal - 100, 110, 120, 200, 220,230, 240 VAC 50/60 Hz |
|---|---|
| A.C. High Voltage Secondary RMS | 650V C.T. @ 345ma. |
| Bias Tap on High Voltage Secondary w/ C.T. Grounded (VAC) | 50 |
| Filament #1 (VAC) | 5V C.T. @ 6A |
| Filament #2 (VAC) | 6.3V C.T. @ 9A |
| Mounting Style | Vertical ("X") |
So, no problem at all running 300Bs with this, obviously, and it'll be better capable of powering the 2A3 filaments for sure.
Here's what PSUD2 looks like with this transformer, 47uF C4 and 230mA load (2x 300B @90mA + "board load" @50mA):
And here is what PSUD2 looks like with the same transformer, 2.2uF C4, and 156mA load (2 x 2A3 @53mA + "board load" @50mA):
So, what's not to like about the 373EX (other than the price)?
Not as designed. I suspect it could be reconfigured to do so, since it is providing the driver tubes with 6.3v. However, the board is designed to accept 4-pin sockets, so octals would have to be mounted off-board and jumper wires used. Also, this would completely eliminate the option to run 300B, 45, etc.
If I only want to go for 2A3, then there is no risk to them at all. I will simply make the right choices for 2A3. It only gets a bit complicated because I would like to maintain the option of running 300B as well.
Which two worlds would I be getting the best of with your suggestion? I am curious.
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Ahhh, I see. Hmmm. I sort of feel like I am on the verge of complicating things too much already, so I think I'll stick with my plan (which has changed 2 times already).
Currently I am thinking of going with a power transformer that will be adequate for 300B, 2A3 and 45 (including the 2A3 filament amperage requirement). Very likely it will be the Hammond 373EX. I will begin with a 2A3 configuration, and use a choke and small value C4 to tame the B+. To switch to 45 will require a simple bias adjustment. To switch to 300B will be more involved, but I can make it relatively simple:
1) Replace C4 with 47uF, to bring B+ up - miniature binding posts on back of board will make this simple
2) Change PAD 1,3,4 jumper - PADs to be labelled and jumpers installed on back of board
3) Increase resistance to R2 - To facilitate easy change between 100ohm (for 2A3/45) and 300ohm (300B), a 300ohm resistor will be soldered in place, with a 150ohm resistor "piggybacked" in parallel for 100ohms (2A3/45) - on the back of the board, of course. To switch to 300ohm for 300B, simply remove 150ohm resistor.
4) Adjust bias
This will require me to heat up a soldering iron, but I won't have to remove the PCB from the chassis to do it. This process will prevent me from making the mistake of running 5v into 2A3 or 45 filaments. Truth be told, I may never even run 300Bs, but I do own a pair so I'd like to have the option.
On the way: 373EX, C-14X, various low-value C4 caps, mini binding posts, 200V Zener (D6) (backorder), TO-220 mounting kits, CL-90, CL-140.
I've figured out a way to deal with IC3 heat (for running 2A3), as well. It'll be a supplemental heatsink configured similarly to George's example on the website, but re-arranged due to top-plate mounted board configuration. Vent holes in the top-plate, and heatsink aligned in such a way that heat-driven air will draft through the fins and up through the holes. Adequate (but safe) air entry to be provided on chassis sides and/or bottom.
I've got sound PSUD2 models to make the transformer work well for 45, 2A3 and 300B. I can always throw in a 5U4GB as an extra option to tame B+, in conjuction with C4, for 45/2A3 if I want, and the CL-90 (mains) and CL-140 (HV center-tap) will tame the 5U4GB inrush.
P.S. People may wonder why I chose the 373EX. The reason is because I somewhat regret choosing the Allied 6K7VG for my SSE, which I feel is underpowered for the purpose. This transformer works fine in the SSE with a 620ohm cathode bias resistor, running a B+ of 430v and 53mA plate current (running EL34, for example, at about 22W dissipation). In theory, I should be able to change the CBR to 560ohm and get 63mA plate current at 430v B+, for slight inclrease in power and decrease in distortion. In reality, when I install 560ohm CBR the B+ sags to 420v and plate current can only maintain about 55mA. On top of this, the 6K7VG makes an obvious mechanical hum when configured this way. This isn't a "problem". The amp sounds great either way (I run it with the 620ohm CBRs). I simply don't like the fact that the power transformer is the bottleneck in this amp - I'd like to be able to push the tubes a bit more to see how the sound changes, and whether I like it or not. This is especially true for 6L6GC. It is a real shame because they are good for 30W of plate dissipation and I can only push about 22-23W. The obvious solution is to replace 6K7VG, which I plan to do anyway due to my future move to Japan. I just wish I had gone with a Hammond 374BX right from the start. Considerably more stout and includes a 100v primary. Would have been perfect. So, for the TSE-II, I'm buying lots of headroom. I don't want the PT to be the bottleneck.
I've figured out a way to deal with IC3 heat (for running 2A3), as well. It'll be a supplemental heatsink configured similarly to George's example on the website, but re-arranged due to top-plate mounted board configuration. Vent holes in the top-plate, and heatsink aligned in such a way that heat-driven air will draft through the fins and up through the holes. Adequate (but safe) air entry to be provided on chassis sides and/or bottom.
I've got sound PSUD2 models to make the transformer work well for 45, 2A3 and 300B. I can always throw in a 5U4GB as an extra option to tame B+, in conjuction with C4, for 45/2A3 if I want, and the CL-90 (mains) and CL-140 (HV center-tap) will tame the 5U4GB inrush.
P.S. People may wonder why I chose the 373EX. The reason is because I somewhat regret choosing the Allied 6K7VG for my SSE, which I feel is underpowered for the purpose. This transformer works fine in the SSE with a 620ohm cathode bias resistor, running a B+ of 430v and 53mA plate current (running EL34, for example, at about 22W dissipation). In theory, I should be able to change the CBR to 560ohm and get 63mA plate current at 430v B+, for slight inclrease in power and decrease in distortion. In reality, when I install 560ohm CBR the B+ sags to 420v and plate current can only maintain about 55mA. On top of this, the 6K7VG makes an obvious mechanical hum when configured this way. This isn't a "problem". The amp sounds great either way (I run it with the 620ohm CBRs). I simply don't like the fact that the power transformer is the bottleneck in this amp - I'd like to be able to push the tubes a bit more to see how the sound changes, and whether I like it or not. This is especially true for 6L6GC. It is a real shame because they are good for 30W of plate dissipation and I can only push about 22-23W. The obvious solution is to replace 6K7VG, which I plan to do anyway due to my future move to Japan. I just wish I had gone with a Hammond 374BX right from the start. Considerably more stout and includes a 100v primary. Would have been perfect. So, for the TSE-II, I'm buying lots of headroom. I don't want the PT to be the bottleneck.
I used to recommend the Allied 6K7VG for SSE's as my SSE can run KT88's at 100 mA each and still make 435 volts or more depending on line voltage with a 5AR4 and 465 volts with diodes. I sold the old SSE last year at the Dayton hamfest, but the 6K7VG was from the same batch as others that I'm still using with 10/12/2004 dates on them. Obviously there have been some cost cutting changes made since mine were made nearly 20 years ago.
Indeed. That sounds like the most likely explanation. What is the rated amperage of the HV winding on yours (on the sticker), George? Mine states 150mA, so I certainly can't be too surprised that it struggles so much.
My Mouser order arrived yesterday. HOLY **** the 373EX is MASSIVE!
Why not just build for 300B, make no compromise, and nail the implementation?
I've done the game of musical-component-chairs to swap a 45 in where there was a 2A3 and so on and ultimately, while there are audible differences, in the end I've found that:
I've got one room for listening, one system, and one set of speakers. No subs or active subs.
Obviously people can do whatever they want with their time and money, but how much difference do you REALLY think exists between these tubes? If you want to have different listening experiences, why don't you get VERY different amps instead of swapping one lower-powered DHT out for another with more power?
Full disclosure: I used to believe all the hype around 45 vs 2A3 vs 300B but after going around the block a few times, it's not at the level of "I need a 2A3 on Tuesday for this album, but a 45 for this album on Wednesday."
I've done the game of musical-component-chairs to swap a 45 in where there was a 2A3 and so on and ultimately, while there are audible differences, in the end I've found that:
- 45 sounds nice
- 2A3 sounds nice, but with better bass
- JJ 2A3-40 with increased B+ sounds even better, and has better bass
I've got one room for listening, one system, and one set of speakers. No subs or active subs.
Obviously people can do whatever they want with their time and money, but how much difference do you REALLY think exists between these tubes? If you want to have different listening experiences, why don't you get VERY different amps instead of swapping one lower-powered DHT out for another with more power?
Full disclosure: I used to believe all the hype around 45 vs 2A3 vs 300B but after going around the block a few times, it's not at the level of "I need a 2A3 on Tuesday for this album, but a 45 for this album on Wednesday."
I appreciate your perspective @Thekak (truly)! You make a lot of sense.
It is true that I am targeting 2A3 for the initial build, but I don't feel like I am compromising the 300B option. In fact, I think the 300B configuration option is more optimal than either the 2A3 or 45 configurations because the 2A3/45 configurations will have higher B+ ripple due to small value C4. See the second screenshot in post #10, which is the 300B configuration ... 370v B+ with 45mV of ripple. If there is something sub-optimal that I am not realizing, please let me know.
Really what it comes down to is I own some nice 1960 RCA 2A3s and some new Linlai 300Bs, and I know I won't be able to resist trying both.
It is true that I am targeting 2A3 for the initial build, but I don't feel like I am compromising the 300B option. In fact, I think the 300B configuration option is more optimal than either the 2A3 or 45 configurations because the 2A3/45 configurations will have higher B+ ripple due to small value C4. See the second screenshot in post #10, which is the 300B configuration ... 370v B+ with 45mV of ripple. If there is something sub-optimal that I am not realizing, please let me know.
Really what it comes down to is I own some nice 1960 RCA 2A3s and some new Linlai 300Bs, and I know I won't be able to resist trying both.
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"Really what it comes down to is I own some nice 1960 RCA 2A3s and some new Linlai 300Bs, and I know I won't be able to resist trying both."
It does come down to this, doesn't it?
I don't have experience with George's stuff (would love to hear any of it!) so am not commenting on or reviewing the proposals above. Just throwing those 2 cents out there.
Good luck! Have fun!
It does come down to this, doesn't it?
I don't have experience with George's stuff (would love to hear any of it!) so am not commenting on or reviewing the proposals above. Just throwing those 2 cents out there.
Good luck! Have fun!
