PowerDrive for Point-to-Point 300B Monoblocks
Before I start cutting metal for my 300B monoblocks, I would like to UPGRADE my “Simple 300B” mono amp (CCS on 5842, DC DHT filaments, but no PowerDrive MOSFET) to a true TSE-style amp by adding the MOSFET “coupling” circuit and the adjustable 5842 bias.
HOWEVER, I am using a Heathkit IP-32 power supply, which is limited to -100VDC bias voltage; the TSE circuit calls for -150 IIRC.
ALSO, I am confused about the relationship between “grid” voltage before & after the MOSFET; I read the Power Drive Cookbook a couple of times, which didn’t help.
(A) Can I merely drop the values of the “grid” resistors (R14, 20K ohms, and R15, 121K ohms) by approximately 1/3 to be able to use the -100V bias supply I have at hand?
That would mean ~ 13K ohms on the post-MOSFET grid, which I’d probably get with two 3 watt resistors in series. It would require ~80K ohms for the pre-MOSFET “grid” resistor, which should be easy enuf.
(B) I can’t see any reason to alter the values in the voltage splitter circuit, as the 100K ohm pot should have enough span; is that correct?
(C) The “Simple 300B” operating points are relatively conservative (for the TuneLab forum, anyway):
B+ is just a hair under 400V
Bias voltage under a 100K grid resistor is about -75 volts.
Total current is ~85 mA, minus ~12mA for 5842 means ~73mA thru 300B.
Which leads me to wonder: what value grid resistor is TOO LOW?
BTW, the “Simple 300B” sounds fantastic, especially now with an “Open Back Box/Baffle” using Eminence Beta 12LTA full-range, and a good DAC on the Spotify “Premium” streaming. Easily the most relaxing music I’ve ever “made”. Frankly, hard to believe the MOSFET will make much improvement, but a boy’s gotta try new things....
Before I start cutting metal for my 300B monoblocks, I would like to UPGRADE my “Simple 300B” mono amp (CCS on 5842, DC DHT filaments, but no PowerDrive MOSFET) to a true TSE-style amp by adding the MOSFET “coupling” circuit and the adjustable 5842 bias.
HOWEVER, I am using a Heathkit IP-32 power supply, which is limited to -100VDC bias voltage; the TSE circuit calls for -150 IIRC.
ALSO, I am confused about the relationship between “grid” voltage before & after the MOSFET; I read the Power Drive Cookbook a couple of times, which didn’t help.
(A) Can I merely drop the values of the “grid” resistors (R14, 20K ohms, and R15, 121K ohms) by approximately 1/3 to be able to use the -100V bias supply I have at hand?
That would mean ~ 13K ohms on the post-MOSFET grid, which I’d probably get with two 3 watt resistors in series. It would require ~80K ohms for the pre-MOSFET “grid” resistor, which should be easy enuf.
(B) I can’t see any reason to alter the values in the voltage splitter circuit, as the 100K ohm pot should have enough span; is that correct?
(C) The “Simple 300B” operating points are relatively conservative (for the TuneLab forum, anyway):
B+ is just a hair under 400V
Bias voltage under a 100K grid resistor is about -75 volts.
Total current is ~85 mA, minus ~12mA for 5842 means ~73mA thru 300B.
Which leads me to wonder: what value grid resistor is TOO LOW?
BTW, the “Simple 300B” sounds fantastic, especially now with an “Open Back Box/Baffle” using Eminence Beta 12LTA full-range, and a good DAC on the Spotify “Premium” streaming. Easily the most relaxing music I’ve ever “made”. Frankly, hard to believe the MOSFET will make much improvement, but a boy’s gotta try new things....
PowerDrive for Point-to-Point 300B Monoblocks
Before I start cutting metal for my 300B monoblocks, I would like to UPGRADE my “Simple 300B” mono amp (CCS on 5842, DC DHT filaments, but no PowerDrive MOSFET) to a true TSE-style amp by adding the MOSFET “coupling” circuit and the adjustable 5842 bias.
HOWEVER, I am using a Heathkit IP-32 power supply, which is limited to -100VDC bias voltage; the TSE circuit calls for -150 IIRC.
ALSO, I am confused about the relationship between “grid” voltage before & after the MOSFET; I read the Power Drive Cookbook a couple of times, which didn’t help.
(A) Can I merely drop the values of the “grid” resistors (R14, 20K ohms, and R15, 121K ohms) by approximately 1/3 to be able to use the -100V bias supply I have at hand?
That would mean ~ 13K ohms on the post-MOSFET grid, which I’d probably get with two 3 watt resistors in series. It would require ~80K ohms for the pre-MOSFET “grid” resistor, which should be easy enuf.
(B) I can’t see any reason to alter the values in the voltage splitter circuit, as the 100K ohm pot should have enough span; is that correct?
(C) The “Simple 300B” operating points are relatively conservative (for the TuneLab forum, anyway):
B+ is just a hair under 400V
Bias voltage under a 100K grid resistor is about -75 volts.
Total current is ~85 mA, minus ~12mA for 5842 means ~73mA thru 300B.
Which leads me to wonder: what value grid resistor is TOO LOW?
BTW, the “Simple 300B” sounds fantastic, especially now with an “Open Back Box/Baffle” using Eminence Beta 12LTA full-range, and a good DAC on the Spotify “Premium” streaming. Easily the most relaxing music I’ve ever “made”. Frankly, hard to believe the MOSFET will make much improvement, but a boy’s gotta try new things....
Before I start cutting metal for my 300B monoblocks, I would like to UPGRADE my “Simple 300B” mono amp (CCS on 5842, DC DHT filaments, but no PowerDrive MOSFET) to a true TSE-style amp by adding the MOSFET “coupling” circuit and the adjustable 5842 bias.
HOWEVER, I am using a Heathkit IP-32 power supply, which is limited to -100VDC bias voltage; the TSE circuit calls for -150 IIRC.
ALSO, I am confused about the relationship between “grid” voltage before & after the MOSFET; I read the Power Drive Cookbook a couple of times, which didn’t help.
(A) Can I merely drop the values of the “grid” resistors (R14, 20K ohms, and R15, 121K ohms) by approximately 1/3 to be able to use the -100V bias supply I have at hand?
That would mean ~ 13K ohms on the post-MOSFET grid, which I’d probably get with two 3 watt resistors in series. It would require ~80K ohms for the pre-MOSFET “grid” resistor, which should be easy enuf.
(B) I can’t see any reason to alter the values in the voltage splitter circuit, as the 100K ohm pot should have enough span; is that correct?
(C) The “Simple 300B” operating points are relatively conservative (for the TuneLab forum, anyway):
B+ is just a hair under 400V
Bias voltage under a 100K grid resistor is about -75 volts.
Total current is ~85 mA, minus ~12mA for 5842 means ~73mA thru 300B.
Which leads me to wonder: what value grid resistor is TOO LOW?
BTW, the “Simple 300B” sounds fantastic, especially now with an “Open Back Box/Baffle” using Eminence Beta 12LTA full-range, and a good DAC on the Spotify “Premium” streaming. Easily the most relaxing music I’ve ever “made”. Frankly, hard to believe the MOSFET will make much improvement, but a boy’s gotta try new things....
Sorry for the Double Post. As we used to say in the Oil Business, if you got trouble, it's because you're either Drinking Too Much, or Not Enough. In 40 years, I never met anyone who thought they were drinking too much...
Correction: Just found an ancient post (2012) which says that the grid resistor is 47K ohms in this amp. It's playing some Brazilian Sexy-Time Music from Spotify at the moment, but I'll check it tomorrow.
Correction: Just found an ancient post (2012) which says that the grid resistor is 47K ohms in this amp. It's playing some Brazilian Sexy-Time Music from Spotify at the moment, but I'll check it tomorrow.
Addendum: I found the following post from George on the TubeLab site, dated 11/19/09.
"R14 and R25 set the operating current for the mosfets. The parts list shows a value of 20K ohms. This is good for boards that operate from 275 to 325 volts of B+. <snip> Boards that operate from 360 volts to 400 volts should use 36K ohms. This will reduce the dissipation at higher voltages."
SO, if my bias supply is -100V instead of the -150V of the TSE board, I suppose I should use a grid resistor of 0.666 x 36K ohm = ~24Kohms.
Is THAT correct?
"R14 and R25 set the operating current for the mosfets. The parts list shows a value of 20K ohms. This is good for boards that operate from 275 to 325 volts of B+. <snip> Boards that operate from 360 volts to 400 volts should use 36K ohms. This will reduce the dissipation at higher voltages."
SO, if my bias supply is -100V instead of the -150V of the TSE board, I suppose I should use a grid resistor of 0.666 x 36K ohm = ~24Kohms.
Is THAT correct?
Hullo! Anybody home?
I'm slapping together a little "coupling" board with screw terminals for
(a) MOSFET
(b) bias resistors
(c) coupling cap
that I can install between the 5842 and 300B.
One question: it appears that the MOSFET middle pin (DRAIN) goes to B+.
Is that correct?
I derived that from a shot of the PCB in the assy instructions in which the middle pins are clearly tied together, and B+ is the only thing common between the two MOSFETS, but wanted to make sure...
I'm slapping together a little "coupling" board with screw terminals for
(a) MOSFET
(b) bias resistors
(c) coupling cap
that I can install between the 5842 and 300B.
One question: it appears that the MOSFET middle pin (DRAIN) goes to B+.
Is that correct?
I derived that from a shot of the PCB in the assy instructions in which the middle pins are clearly tied together, and B+ is the only thing common between the two MOSFETS, but wanted to make sure...
PowerDrive Board
A quick prototype Power Drive Board.
Once I get the resistor "ladder" adjusted (e.g. so that the 100K pot is in the middle of the range for ~70mA), I can do another smaller board...
Any responses to the questions above?
I am not above begging, if that will help...
A quick prototype Power Drive Board.
Once I get the resistor "ladder" adjusted (e.g. so that the 100K pot is in the middle of the range for ~70mA), I can do another smaller board...
Any responses to the questions above?
I am not above begging, if that will help...
Attachments
(A) confirmed that the grid resistor is 47k
(B) measured the max (ie minimum) bias voltage: it’s about -130 rather than the specified -100Vdc. The bias voltage is taken between two OA2 regulator tubes, which are 150 volt nominal regulators, so nominal value at one end of the adjustment pot should be about -150.
However, on the old Heathkit there’s no max adjustment pot on the bias supply, so I guess you get what you get through the old resistors in the bias supply circuit. Also, line voltage is probably a little higher than the 117vac assumed in the manual, so there’s that...
Should be able to safely use -125vdc (a nice round number), which (if my logic is not backwards) implies a grid resistor of 5/6 x 36K ohms = about 30K ohms. Can do.
(B) measured the max (ie minimum) bias voltage: it’s about -130 rather than the specified -100Vdc. The bias voltage is taken between two OA2 regulator tubes, which are 150 volt nominal regulators, so nominal value at one end of the adjustment pot should be about -150.
However, on the old Heathkit there’s no max adjustment pot on the bias supply, so I guess you get what you get through the old resistors in the bias supply circuit. Also, line voltage is probably a little higher than the 117vac assumed in the manual, so there’s that...
Should be able to safely use -125vdc (a nice round number), which (if my logic is not backwards) implies a grid resistor of 5/6 x 36K ohms = about 30K ohms. Can do.
Questions!
Outstanding Questions*:
(1) What values should I use for the grid and Bias “network” resistors with only -125vdc bias supply? Alternately, start with standard values and adjust after I see where it biases-up?
(2)Can somebody confirm that MOSFET Pin #2 (the middle pin, DRAIN) goes to B+? Based on photos of the board, pretty sure it does, but DON’t want to blow up a MOSFET right out of the gate...the schematic isn’t clear, ie no “source, drain, emitter” labels, nor “1,2,3” pin numbers.
*not especially GOOD questions, just unanswered.
Outstanding Questions*:
(1) What values should I use for the grid and Bias “network” resistors with only -125vdc bias supply? Alternately, start with standard values and adjust after I see where it biases-up?
(2)Can somebody confirm that MOSFET Pin #2 (the middle pin, DRAIN) goes to B+? Based on photos of the board, pretty sure it does, but DON’t want to blow up a MOSFET right out of the gate...the schematic isn’t clear, ie no “source, drain, emitter” labels, nor “1,2,3” pin numbers.
*not especially GOOD questions, just unanswered.
Point-to-Point 300B TSE Mono-block Update
I installed the “coupling” board (above) with the recommended resistor values, that is:
Grid resistor: ~32K (5/6 of the recommended 36K for 400V B+, as Heathkit maxes-out at -125vdc bias).
Voltage divider, top-to-bottom: 130K / 100K pot / 22K to ground.
Pre-MOSFET grid resistor: ~120K
MOSFET is STF3LN80K, for the record.
The poor little power supply would bias to ~70mA or so, but would only get to about 260-270vdc B+ with the bias pot cranked to 11. Clearly, sucking too much potential down the drain.
Changed the voltage divider resistors to, top-to-bottom: 47k / 100K pot / 100K to ground; essentially inverted the voltage divider. Did NOT change either “grid” resistor (pre- or post-MOSFET)
This time, biased right up at low 70s mAs; volts across the 10 ohm plate resistor agreed with the Heathkit milliammeter with B+ just shy of 400vdc.
C- is spot-on -100Vdc.
I believe that I could get the bias voltage up to (that is, DOWN to) -125vdc by further increasing the resistance below the pot, or maybe reducing the pre-MOSFET grid resistor, but it sounds so gooood right now I ain’t touching it for a day or two...
Playing Lionel Hampton’s Paris Session (1956), and the Chief Cook & Bottle-Washer yells from the kitchen “Oooo, THAT sounds good.” She NEVER comments on sound quality.
My only COMPLAINT is that the bass is so good with the MOSFET PowerDrive (akin to a 6L6 P-P amp, but transparent...) that I’m gonna have to dampen my flabby full-range cones (Eminence Beta 12LTA). Maybe that shellac coating that folks use. Might also need a phase plug if I’m gonna play that Rock & Roll music that the kids are talking about. Well, WERE talking about 30 years ago...
This amp sounds so good, now I want to hear a 46 in there, or even a 2A3, which I have NEVER gotten to sound good. The switch between power tubes is pretty easy, as I have the Pete Millett filament supply hooked to a 10T pot on the deck; easy-Peary.
Man, I can clearly hear the mallets hitting the vibe bars...wow.
Any suggestions for measurements or further tweaks? I am all ears, said the budding audiophile.
I installed the “coupling” board (above) with the recommended resistor values, that is:
Grid resistor: ~32K (5/6 of the recommended 36K for 400V B+, as Heathkit maxes-out at -125vdc bias).
Voltage divider, top-to-bottom: 130K / 100K pot / 22K to ground.
Pre-MOSFET grid resistor: ~120K
MOSFET is STF3LN80K, for the record.
The poor little power supply would bias to ~70mA or so, but would only get to about 260-270vdc B+ with the bias pot cranked to 11. Clearly, sucking too much potential down the drain.
Changed the voltage divider resistors to, top-to-bottom: 47k / 100K pot / 100K to ground; essentially inverted the voltage divider. Did NOT change either “grid” resistor (pre- or post-MOSFET)
This time, biased right up at low 70s mAs; volts across the 10 ohm plate resistor agreed with the Heathkit milliammeter with B+ just shy of 400vdc.
C- is spot-on -100Vdc.
I believe that I could get the bias voltage up to (that is, DOWN to) -125vdc by further increasing the resistance below the pot, or maybe reducing the pre-MOSFET grid resistor, but it sounds so gooood right now I ain’t touching it for a day or two...
Playing Lionel Hampton’s Paris Session (1956), and the Chief Cook & Bottle-Washer yells from the kitchen “Oooo, THAT sounds good.” She NEVER comments on sound quality.
My only COMPLAINT is that the bass is so good with the MOSFET PowerDrive (akin to a 6L6 P-P amp, but transparent...) that I’m gonna have to dampen my flabby full-range cones (Eminence Beta 12LTA). Maybe that shellac coating that folks use. Might also need a phase plug if I’m gonna play that Rock & Roll music that the kids are talking about. Well, WERE talking about 30 years ago...
This amp sounds so good, now I want to hear a 46 in there, or even a 2A3, which I have NEVER gotten to sound good. The switch between power tubes is pretty easy, as I have the Pete Millett filament supply hooked to a 10T pot on the deck; easy-Peary.
Man, I can clearly hear the mallets hitting the vibe bars...wow.
Any suggestions for measurements or further tweaks? I am all ears, said the budding audiophile.
Tinkering
That No Tinkering interlude didn’t last long.
Got under the chassis and installed the original voltage divider resistors, but inverted: 22K top / 100K / 130K to ground.
Now it biases-up at 400vdc B+, 70 mA, with the 100K pot exactly in the middle of the range. However, the C- supply is still at -100Vdc.
I suspect l’m heating-up the pre-MOSFET grid resistor unnecessarily.
Next incremental step, I’ll try dropping that resistor in stages to reduce the bias current, which should bring the C- up above -100 (than is, a higher negative value).
Any comments?
That No Tinkering interlude didn’t last long.
Got under the chassis and installed the original voltage divider resistors, but inverted: 22K top / 100K / 130K to ground.
Now it biases-up at 400vdc B+, 70 mA, with the 100K pot exactly in the middle of the range. However, the C- supply is still at -100Vdc.
I suspect l’m heating-up the pre-MOSFET grid resistor unnecessarily.
Next incremental step, I’ll try dropping that resistor in stages to reduce the bias current, which should bring the C- up above -100 (than is, a higher negative value).
Any comments?
G.A's TSE-II with 300b's have been on Altec 604e's and this sound is a revelation. The system has a pair of Brook12A (10w) monoblocks for 515b's in a separate cabinet. This all Altec system sounds the best I've had so far.
I would like to build TSE-II with 2A3's, Edcor GXSE15-5K and RCA 2A3's running ~ 350v. Could someone please tell me if this is a bad idea. The RCAs are legit and mostly 1947 production. I run the same on the Brook12A push-pull amp.
If 350v is ok, should I stick with 60ma? Can I go higher than 350v ? The idea is to biamp the 604e with Tubelab's on the HF and LF. What do you think ?
I would like to build TSE-II with 2A3's, Edcor GXSE15-5K and RCA 2A3's running ~ 350v. Could someone please tell me if this is a bad idea. The RCAs are legit and mostly 1947 production. I run the same on the Brook12A push-pull amp.
If 350v is ok, should I stick with 60ma? Can I go higher than 350v ? The idea is to biamp the 604e with Tubelab's on the HF and LF. What do you think ?
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2A3s in TSE (I or II)
Some folks love the sound of 2A3s. IF they are going to sound good, the TSE would be a great place to try them. Plus, if it doesn't work, it's pretty easy to change to another DHT.
Personally, I have never been able to make 2A3s sound as good as single-plate DHTs, including (in ascending output) 71/46/45/300B.
In the monoblock that I recently re-jiggered into a TSE-II clone, I AGAIN tried the 2A3s; same result of a slightly muddy sound, at least relative to the single plates.
I have a sneaking suspicion that 2A3 single plates (if you could find & afford them) would sound every bit as good as the other single-plates.
NOTE HOWEVER that I have never used USA 2A3s like your RCAs, only Russian and Chinese clones. That alone MAY be the difference between my results and the results of folks who rave about the 2A3.
I definitely think it's worth a try with good chances of success, particularly as HF amp in a bi-amp configuration. Plus, this set-up will be an excellent space heater in the winter.
With real USA 2A3s, suggest keeping the operating points quite conservative, particularly as you won't need much power for the tweeters. Might be tempted to eliminate any "padding" you have on your tweeters to reduce the power requirement further.
Personally I wouldn't go much above 300vdc B+ and 50mA with a real USA 2A3 (they ain't building any more), but that's me, and this IS the TubeLab forum, so....
Note that I'm not opposed to running cheap power tubes near (or over) the ragged edge (see: 6AV5) but wouldn't do that with your RCAs...
Also, I would run the 2A3s filaments slightly BELOW 2.5V for extended life.
Even a small reduction in filament voltage apparently extends filament life significantly. I have documents on that somewhere...
Some folks love the sound of 2A3s. IF they are going to sound good, the TSE would be a great place to try them. Plus, if it doesn't work, it's pretty easy to change to another DHT.
Personally, I have never been able to make 2A3s sound as good as single-plate DHTs, including (in ascending output) 71/46/45/300B.
In the monoblock that I recently re-jiggered into a TSE-II clone, I AGAIN tried the 2A3s; same result of a slightly muddy sound, at least relative to the single plates.
I have a sneaking suspicion that 2A3 single plates (if you could find & afford them) would sound every bit as good as the other single-plates.
NOTE HOWEVER that I have never used USA 2A3s like your RCAs, only Russian and Chinese clones. That alone MAY be the difference between my results and the results of folks who rave about the 2A3.
I definitely think it's worth a try with good chances of success, particularly as HF amp in a bi-amp configuration. Plus, this set-up will be an excellent space heater in the winter.
With real USA 2A3s, suggest keeping the operating points quite conservative, particularly as you won't need much power for the tweeters. Might be tempted to eliminate any "padding" you have on your tweeters to reduce the power requirement further.
Personally I wouldn't go much above 300vdc B+ and 50mA with a real USA 2A3 (they ain't building any more), but that's me, and this IS the TubeLab forum, so....
Note that I'm not opposed to running cheap power tubes near (or over) the ragged edge (see: 6AV5) but wouldn't do that with your RCAs...
Also, I would run the 2A3s filaments slightly BELOW 2.5V for extended life.
Even a small reduction in filament voltage apparently extends filament life significantly. I have documents on that somewhere...
(a) Just noticed you are in Bangalore. Pls ignore "space heater" comment.
(b) Just noticed that George made good comments about operating points for vintage 2A3s in TubelabSE - best MOSFET choice?
"I talked with a user who used Sovtek 2A3's in his Tubelab SE. He had hum problems with them that went away when he switched to 300B's. He was running the Sovteks far above the maximum ratings at 360 to 370 volts. I set my amp up to the same conditions, and put in some Shuguang 2A3's. This was the best sound that I have ever heard from 2A3's. I even tried some vintage RCA (twin plates) at this voltage and they sounded good too. I have purchased his 2A3's to see if they hum here, but have not tested them yet.
I can't recommend running 2A3's at that voltage, certainly not rare or expensive tubes, but I have no problem exploring the upper limits of cheap Chinese tubes. The Shuguang 300B's don't sound half bad either, but they really like 350 volts or so to come alive. I am using a Hammond 272JX which is 300 - 0 - 300 volts, but they are Hammond volts, so on my voltmeter I read 330 - 0 - 330."
(b) Just noticed that George made good comments about operating points for vintage 2A3s in TubelabSE - best MOSFET choice?
"I talked with a user who used Sovtek 2A3's in his Tubelab SE. He had hum problems with them that went away when he switched to 300B's. He was running the Sovteks far above the maximum ratings at 360 to 370 volts. I set my amp up to the same conditions, and put in some Shuguang 2A3's. This was the best sound that I have ever heard from 2A3's. I even tried some vintage RCA (twin plates) at this voltage and they sounded good too. I have purchased his 2A3's to see if they hum here, but have not tested them yet.
I can't recommend running 2A3's at that voltage, certainly not rare or expensive tubes, but I have no problem exploring the upper limits of cheap Chinese tubes. The Shuguang 300B's don't sound half bad either, but they really like 350 volts or so to come alive. I am using a Hammond 272JX which is 300 - 0 - 300 volts, but they are Hammond volts, so on my voltmeter I read 330 - 0 - 330."
Thanks hareynolds, some great info there. George's post hits the nail on the head with the 360v comment.
Not much experience with tubes, my reference is the 300bs running at 350v/60ma. Power is a funny concept, there's no let up on GRUNT with Altecs at any power levels. Its ridiculous.
Dumb question probably - with 350v/60ma and 5K OPT, can I get 4W with 2A3s ?
Not much experience with tubes, my reference is the 300bs running at 350v/60ma. Power is a funny concept, there's no let up on GRUNT with Altecs at any power levels. Its ridiculous.
Dumb question probably - with 350v/60ma and 5K OPT, can I get 4W with 2A3s ?
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