All,
Im looking to settle on my ST70 design (Im thinking of dual rectifiers in parallel), and have a few quick questions:
#1 - For those running KT88's - what current draw are you sucking down ?
#2 - I know the PA-060S is rated @ 300mA, so I guess this is the main limiting factor of #1....
#3 - What is the C354 rated at for throughput ? If it is less than 300mA (which I have a feeling it might be ?), any suggestions on what else can be crammed under the chassis ?
The Edcor trannies are rated @ 60w; so I have some room to play. I was really wanting to get some cool 816's in there; but due to the low cap input value; I won't be able to get the B+ voltage that I would need.
So I think maybe just some nice ST glass 'normal' rectifiers will work fine. I do need to see how much room I have though; as I was intending on putting the 2nd rectifier socket where the cap can goes, and using an SDS board. I believe however that the SDS board goes directly under this; so I don't know if I can fit a socket in the mix as well.
Thanks !
Steve
C354 Current Handling
13 posts
• Page 1 of 1
C354 Current Handling
by SteveH » Thu Dec 20, 2007 10:14 am
- SteveH
- KT88
- Posts: 422
- Joined: Thu Dec 14, 2006 5:38 pm
- Location: Delaware
by EWBrown » Thu Dec 20, 2007 11:24 am
C354 is rated 1.5 Henries, 200 mA and has a DC resistance of 56 ohms.
The Hammond 156R is similar, but for this purpose, I'd suggest the C354 or its equivalent replacements from Triode, etc.
KT88s should be biased at least 50 mA, and better yet, 70 ma each, to get the full sonic benefits.
In the typical ST70 application, the SDS cap board fits underneath one of the output trannies, so the original can cap hole is "free", and the C354 fits under the other output trannie, The second C354 could sit in the area occupied by the original ST70 boas circiutry if you use that on the SDS or driver board, and it would still need its own capacitor(s) downstream. This will be like trying to cram 40 pounds of (stuff) into a 20 pound box :o Yellow_Light_Colorz_PDT_08 Just about every square inch of chassis space, on top and underneath, is occupied, and the chassis depth is only 1.5 inches which precludes putting most chokes other than the C354 / 156R underneath.
The alternate solution, which will be in the "KT88 custom chassis amp" that I recently bought from Roy Mottram, is set up for two tube rectifiers (5AR4 / GZ34) and two separate C354 chokes, so that each tube and choke feeds one channel and carries approximately 140-150 mA (IIRC, the driver board is around 11-13 mA). The main DC HV, from the cathodes of teh 5AR4, feeds its first cap, then immediately branches into two separate chokes and their following caps, to deliver two (mostly) separate B+ lines.
To implement this in an original style ST70 chassis and using an SDS cap board would be difficult, if not outright impossible.
This uses the Trafomatic "ST-70" Toroid Transformer set, which are very conservatively rated and seriously overbuilt. I could easily pull 50WPC out of this combination. The driver board is based on the classic grounded cathode VA and Schmidt LTPI design, and uses three 12AT7 / 6201s. THe chassis is already punched and drilled for this board, so all the hard the "dog work" is already done for me
THis chassis plate is a LOT larger than the standard ST70 chassis, being 17 X 10 inches, and it sits atop an inverted 17 X 10 X 3 inch Hammond chassis (and I'll add a wooden base for aesthetic reasons). Stuffing all of this into and onto a standard sized ST70 chassis would be a daunting, if not altogether impossible, task.
HTH
/ed B in NH
The Hammond 156R is similar, but for this purpose, I'd suggest the C354 or its equivalent replacements from Triode, etc.
KT88s should be biased at least 50 mA, and better yet, 70 ma each, to get the full sonic benefits.
In the typical ST70 application, the SDS cap board fits underneath one of the output trannies, so the original can cap hole is "free", and the C354 fits under the other output trannie, The second C354 could sit in the area occupied by the original ST70 boas circiutry if you use that on the SDS or driver board, and it would still need its own capacitor(s) downstream. This will be like trying to cram 40 pounds of (stuff) into a 20 pound box :o Yellow_Light_Colorz_PDT_08 Just about every square inch of chassis space, on top and underneath, is occupied, and the chassis depth is only 1.5 inches which precludes putting most chokes other than the C354 / 156R underneath.
The alternate solution, which will be in the "KT88 custom chassis amp" that I recently bought from Roy Mottram, is set up for two tube rectifiers (5AR4 / GZ34) and two separate C354 chokes, so that each tube and choke feeds one channel and carries approximately 140-150 mA (IIRC, the driver board is around 11-13 mA). The main DC HV, from the cathodes of teh 5AR4, feeds its first cap, then immediately branches into two separate chokes and their following caps, to deliver two (mostly) separate B+ lines.
To implement this in an original style ST70 chassis and using an SDS cap board would be difficult, if not outright impossible.
This uses the Trafomatic "ST-70" Toroid Transformer set, which are very conservatively rated and seriously overbuilt. I could easily pull 50WPC out of this combination. The driver board is based on the classic grounded cathode VA and Schmidt LTPI design, and uses three 12AT7 / 6201s. THe chassis is already punched and drilled for this board, so all the hard the "dog work" is already done for me
THis chassis plate is a LOT larger than the standard ST70 chassis, being 17 X 10 inches, and it sits atop an inverted 17 X 10 X 3 inch Hammond chassis (and I'll add a wooden base for aesthetic reasons). Stuffing all of this into and onto a standard sized ST70 chassis would be a daunting, if not altogether impossible, task.
HTH
/ed B in NH
Last edited by EWBrown on Mon Dec 24, 2007 11:35 am, edited 2 times in total.
Real Radios Glow in the Dark
-
EWBrown - Insulator & Iron Magnate
- Posts: 6389
- Joined: Wed Mar 19, 2003 6:03 am
- Location: Now located in Clay County, NC !
by SteveH » Thu Dec 20, 2007 1:58 pm
Ed,
I have to go back and look at the schematic; but I think you might be onto something about just using 2 C354 chokes....I imagine that I would just put both leads off of the D+ tap on the board...
I have to go back and look at the schematic; but I think you might be onto something about just using 2 C354 chokes....I imagine that I would just put both leads off of the D+ tap on the board...
- SteveH
- KT88
- Posts: 422
- Joined: Thu Dec 14, 2006 5:38 pm
- Location: Delaware
by mesherm » Thu Dec 20, 2007 4:36 pm
If you parallel two C-354s you will get twice the current but half the inductance. So you will have 0.75 Henries @400 ma. The extra capacitance on the SDS board should make up for the inductance drop though. Keep the C-354s well apart else any mutual inductance will force you to determine which leads to connect to keep them in phase.
I have used dual C-354s after the first cap to make split power supplies to feed the right and left PP stage but that will be harder with an SDS board.
I have used dual C-354s after the first cap to make split power supplies to feed the right and left PP stage but that will be harder with an SDS board.
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
-
mesherm - KT88
- Posts: 1232
- Joined: Fri Aug 27, 2004 10:33 pm
- Location: Alvin Texas
by EWBrown » Fri Dec 21, 2007 7:24 am
With a single 5VAC filament winding, the two 5AR4s are essentially in parallel, or the alternative approach is to tie both plates together in each 5AR4 then connect one to each of the "hot" HV secondary leads (similar to using two damper tubes).
The tube-rectified DC then connects to one end of both chokes, and the "input" capacitor. and the other end of the chokes connect to their own caps and then feed B+ to the two OPT cetner taps, separately.
If one were to add an extra 5VAC filament trannie, then only the 5AR4 plates need to be in parallel, then the cathode/filaments can be separately fed into their own CLC filtering, then on to each channel independently.
Since my unit has three locations already punched for JJ dual section or similar caps, I can have up to six cap sections. I'll probably run with 32 + 32 uF connected together for 64 uF, right off the cathode(s) then to each choke, which will then have 100 uF on the other end, which then goes to each channel separately. The third dual section cap can get droppong resistors
to then feed the VTA driver board (which already has its own CRC filtering / decoupling) but an extra stage of filtering won't hurt... .
I know, a picture is worth a thousand words, but I'd have to jump through electronic flaming hoops and hurdles to attempt to post it here, especially with our "new and improved" network connection, which is more accurately described as a "hobbled, antiquated and slow" connection... The "DSL turtles" on the TV ads have the speed advantage here Yellow_Light_Colorz_PDT_09
/ed B in NH
The tube-rectified DC then connects to one end of both chokes, and the "input" capacitor. and the other end of the chokes connect to their own caps and then feed B+ to the two OPT cetner taps, separately.
If one were to add an extra 5VAC filament trannie, then only the 5AR4 plates need to be in parallel, then the cathode/filaments can be separately fed into their own CLC filtering, then on to each channel independently.
Since my unit has three locations already punched for JJ dual section or similar caps, I can have up to six cap sections. I'll probably run with 32 + 32 uF connected together for 64 uF, right off the cathode(s) then to each choke, which will then have 100 uF on the other end, which then goes to each channel separately. The third dual section cap can get droppong resistors
to then feed the VTA driver board (which already has its own CRC filtering / decoupling) but an extra stage of filtering won't hurt... .
I know, a picture is worth a thousand words, but I'd have to jump through electronic flaming hoops and hurdles to attempt to post it here, especially with our "new and improved" network connection, which is more accurately described as a "hobbled, antiquated and slow" connection... The "DSL turtles" on the TV ads have the speed advantage here Yellow_Light_Colorz_PDT_09
/ed B in NH
Real Radios Glow in the Dark
-
EWBrown - Insulator & Iron Magnate
- Posts: 6389
- Joined: Wed Mar 19, 2003 6:03 am
- Location: Now located in Clay County, NC !
by SteveH » Fri Dec 21, 2007 4:52 pm
Hrmm...I think I may be at a loss here. At first I thought that it was just one cap; but now that I look at the schematics it is two sections. Unless I can find a choke that will handle the current and fit under the chassis (doubtful), I guess I will be stuck @ 50mA/tube Yellow_Light_Colorz_PDT_21
- SteveH
- KT88
- Posts: 422
- Joined: Thu Dec 14, 2006 5:38 pm
- Location: Delaware
by Quad » Fri Jan 16, 2009 11:45 am
I am considering using KT88s on my VTA board amp.
It is SS rectified, so I am using the 325V taps for the KT77s I now have.
I plan to switch to the 360V taps for the KT88.
The Trafomatic Toroid HV is rated 300ma, and I reckon it can do better,
so it should easily drive 75ma per KT88 tube.
The CLC choke is a C354 equivalent (Hammond 156R, 1.5H 200ma).
Looking for ideas to handle the increased B+ draw.
The choices are -
a) Replace with 159V ( 1.5 H, 500mA, 27 ohms)
b) Replace with 159T ( 2.5 H, 300mA, 43 ohms)
c) Use the extra 156R I have to create independent B+ for each channel
Are there any changes required on the VTA board to handle the higher B+ of around 500V?
TIA
Prasad
It is SS rectified, so I am using the 325V taps for the KT77s I now have.
I plan to switch to the 360V taps for the KT88.
The Trafomatic Toroid HV is rated 300ma, and I reckon it can do better,
so it should easily drive 75ma per KT88 tube.
The CLC choke is a C354 equivalent (Hammond 156R, 1.5H 200ma).
Looking for ideas to handle the increased B+ draw.
The choices are -
a) Replace with 159V ( 1.5 H, 500mA, 27 ohms)
b) Replace with 159T ( 2.5 H, 300mA, 43 ohms)
c) Use the extra 156R I have to create independent B+ for each channel
Are there any changes required on the VTA board to handle the higher B+ of around 500V?
TIA
Prasad
- Quad
- KT88
- Posts: 254
- Joined: Mon Jan 14, 2008 6:54 am
- Location: India
by EWBrown » Fri Jan 16, 2009 1:27 pm
The original C354 is rated at 200 mA, so that is about a 11.2VDC drop across it at full rated current, and that equates to 2.24 watts dissipated in the choke. .
Raise the curent to 300 mA, the voltage drop rises to 16.8VDC, and the power dissipated through the choke increases to over 5 Watts, so it will cook in short order...
The 500 mA choke, at 300 mA and 27 ohms DCR will drop about 8.1VDC, and at 300 mA< that is 2.43 watts dissipation.
The limiting factor then becomes the capability of the rectifier tube, if you use one, as a 5AR4/GZ34 is pretty much pushed to the limit at 200 mA, especially the J/J and Sovtek versions.
The vintage (expensive) Mullards, and Amperex GZ34s are more capabbe, else use the solid state rectifiers on the cap board, with their increased B+ voltage.Don't even think of using a Chinese made 5AR4 here, it will go pyrotechnic in a few seconds...
The VTA-70 board (I don't have the schematic here with me), there is a 7.5K (?) dropping resistor that connects to the main ST70 power supply B+ voltage to drop the board's B+ for the LTPIs and VAs.
It is sized for approx 430-440VDC, standard ST-70 B+, so in order to accomodate 500VDC B+ you would have to increase its value, or else add in another resistor off the board, to drop the extra 50 to 60VDC,
IIRC the VTA70 board consumes around 11-12 mA total, (5.5 to 6 mA per channel) so to drop 50V the added resistor would have to be around 4.3K, for, and for 60V, around 5.1K to 5.6K, use a 2W resistor to play it safe, Or increase the existing 7.5K to 12K or 13K, 2W, to gt the same plate voltages on the VTA's 12AT7s.
I am running solely by memory here, so the current and resistor values may have to be "massaged" in order to drop the VTA board to 430-440VDC or less. As they say, measure twice, cut once...
As far as the choke, the higher current versions will work well, but just make sure that it will fit under the ST-70 chassis, as it is pretty shallow at around 1.75 inches inside depth.
The C354 and Hammond 156R just barely fit, by a hair... The "split PSU" with two separate chokes is a good method, but it does take more parts, and more precious "real estate" on and under the already overcrowded chassis.
/ed B
Raise the curent to 300 mA, the voltage drop rises to 16.8VDC, and the power dissipated through the choke increases to over 5 Watts, so it will cook in short order...
The 500 mA choke, at 300 mA and 27 ohms DCR will drop about 8.1VDC, and at 300 mA< that is 2.43 watts dissipation.
The limiting factor then becomes the capability of the rectifier tube, if you use one, as a 5AR4/GZ34 is pretty much pushed to the limit at 200 mA, especially the J/J and Sovtek versions.
The vintage (expensive) Mullards, and Amperex GZ34s are more capabbe, else use the solid state rectifiers on the cap board, with their increased B+ voltage.Don't even think of using a Chinese made 5AR4 here, it will go pyrotechnic in a few seconds...
The VTA-70 board (I don't have the schematic here with me), there is a 7.5K (?) dropping resistor that connects to the main ST70 power supply B+ voltage to drop the board's B+ for the LTPIs and VAs.
It is sized for approx 430-440VDC, standard ST-70 B+, so in order to accomodate 500VDC B+ you would have to increase its value, or else add in another resistor off the board, to drop the extra 50 to 60VDC,
IIRC the VTA70 board consumes around 11-12 mA total, (5.5 to 6 mA per channel) so to drop 50V the added resistor would have to be around 4.3K, for, and for 60V, around 5.1K to 5.6K, use a 2W resistor to play it safe, Or increase the existing 7.5K to 12K or 13K, 2W, to gt the same plate voltages on the VTA's 12AT7s.
I am running solely by memory here, so the current and resistor values may have to be "massaged" in order to drop the VTA board to 430-440VDC or less. As they say, measure twice, cut once...
As far as the choke, the higher current versions will work well, but just make sure that it will fit under the ST-70 chassis, as it is pretty shallow at around 1.75 inches inside depth.
The C354 and Hammond 156R just barely fit, by a hair... The "split PSU" with two separate chokes is a good method, but it does take more parts, and more precious "real estate" on and under the already overcrowded chassis.
/ed B
Real Radios Glow in the Dark
-
EWBrown - Insulator & Iron Magnate
- Posts: 6389
- Joined: Wed Mar 19, 2003 6:03 am
- Location: Now located in Clay County, NC !
by mesherm » Fri Jan 16, 2009 2:49 pm
Just install a 62 volt 5w Zener diode between the B+ and the VTA B+ input. That will drop the voltage down to 438 vdc.
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
-
mesherm - KT88
- Posts: 1232
- Joined: Fri Aug 27, 2004 10:33 pm
- Location: Alvin Texas
by EWBrown » Fri Jan 16, 2009 4:25 pm
I guess that I'm too much ingraned into tubeology to even think about a zener diode. Nice simple solution Yellow_Light_Colorz_PDT_11
The "prime directive "is to make the 12AT7s' various plate voltages to be identical to those on the original ST70 B+ line.
Speaking of Zener diodes, I need ot find a 25V, 10 Watt D04 format stud mount zener to fix a 30 year old test receiver at work.
/ed B
The "prime directive "is to make the 12AT7s' various plate voltages to be identical to those on the original ST70 B+ line.
Speaking of Zener diodes, I need ot find a 25V, 10 Watt D04 format stud mount zener to fix a 30 year old test receiver at work.
/ed B
Real Radios Glow in the Dark
-
EWBrown - Insulator & Iron Magnate
- Posts: 6389
- Joined: Wed Mar 19, 2003 6:03 am
- Location: Now located in Clay County, NC !
by mesherm » Fri Jan 16, 2009 5:51 pm
25 volt 10 watt D04 Zener = Mouser PN 526-NTE5199A
http://www.mouser.com/Search/ProductDetail.aspx?qs=JV7lzlMm3yKa8HTv4MsLtQ%3d%3d
http://www.mouser.com/Search/ProductDetail.aspx?qs=JV7lzlMm3yKa8HTv4MsLtQ%3d%3d
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
-
mesherm - KT88
- Posts: 1232
- Joined: Fri Aug 27, 2004 10:33 pm
- Location: Alvin Texas
by EWBrown » Fri Jan 16, 2009 6:01 pm
THanks!
THe original one was a 1N3982 (or at least in the 1N39XX series. I did find that particular one at an other vendor, but at $20 plus s/h, I figured that there was something better... 10 bucks is much beter...
This Zener is used in the power supply, part of the internal battery charger, of an Electrometrics NTR-51C test receiver. A reall odd duck, to say the least... :o Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_04 It has an amazing collection of 1970s vintage weird transistors and ICs. No CPUs, no surface mount, no tubes.
I'll probably put one on my next Mouser order and then get the boss to re-imburse me for at least the cost of the ZD, and maybe the shipping, too...
Yellow_Light_Colorz_PDT_06
/ed
THe original one was a 1N3982 (or at least in the 1N39XX series. I did find that particular one at an other vendor, but at $20 plus s/h, I figured that there was something better... 10 bucks is much beter...
This Zener is used in the power supply, part of the internal battery charger, of an Electrometrics NTR-51C test receiver. A reall odd duck, to say the least... :o Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_04 It has an amazing collection of 1970s vintage weird transistors and ICs. No CPUs, no surface mount, no tubes.
I'll probably put one on my next Mouser order and then get the boss to re-imburse me for at least the cost of the ZD, and maybe the shipping, too...
Yellow_Light_Colorz_PDT_06
/ed
Real Radios Glow in the Dark
-
EWBrown - Insulator & Iron Magnate
- Posts: 6389
- Joined: Wed Mar 19, 2003 6:03 am
- Location: Now located in Clay County, NC !
13 posts
• Page 1 of 1
Who is online
Users browsing this forum: No registered users and 45 guests