Dear all,
A friend of mine is listening music with SPARK 550 inte-amp (originally desinged for 300b) using 2A3. As I know, those are not compatible. Plate volatge 400V, heater 5.0V for 300b and 250V, 2.5V for 2A3. There was no problem to enjoy music for a month except brighter light and some blue glow at the top of bulbs. What will happen if it plays for a long time?
A friend of mine is listening music with SPARK 550 inte-amp (originally desinged for 300b) using 2A3. As I know, those are not compatible. Plate volatge 400V, heater 5.0V for 300b and 250V, 2.5V for 2A3. There was no problem to enjoy music for a month except brighter light and some blue glow at the top of bulbs. What will happen if it plays for a long time?
Two possibilities.
1. The Cathode will be cooked inn time. aka. Low electron emission. It should fade away like a red dwarf.
2. It goes into thermal run away and red plate. It could burn up like a super nova.
My guess is #1 likely will happen to this amp. If #2 happen, make sure someone is home.
1. The Cathode will be cooked inn time. aka. Low electron emission. It should fade away like a red dwarf.
2. It goes into thermal run away and red plate. It could burn up like a super nova.
My guess is #1 likely will happen to this amp. If #2 happen, make sure someone is home.
The power transformer that was designed to power 2 x 1.25A 300B filaments will have trouble powering 2 x 2.5A 2A3 filaments.
The transformer might catch fire.
Depending on the rest of the amp circuitry, the 2A3 tubes might short, causing problems with the rectifier(s) and power transformer B+ secondary. Not good.
I have designed amplifiers that could use a 2A3 or a 300B with AC filament power, without doing any circuit changes, only a re-adjust of the hum potentiometer. But that is very involved combination of parts and circuitry, and does not utilize the 300B to its full capability.
It is far too complex for a simple discussion on this thread (careful design, complex or simple, would not be safe without a 100% proper design).
After I designed and built it, it was possible to do tube rolling / tasting by merely powering down the amp each time before swapping tubes . . . 300B . . . 2A3 . . . 300B . . . 2A3 . . .
The transformer might catch fire.
Depending on the rest of the amp circuitry, the 2A3 tubes might short, causing problems with the rectifier(s) and power transformer B+ secondary. Not good.
I have designed amplifiers that could use a 2A3 or a 300B with AC filament power, without doing any circuit changes, only a re-adjust of the hum potentiometer. But that is very involved combination of parts and circuitry, and does not utilize the 300B to its full capability.
It is far too complex for a simple discussion on this thread (careful design, complex or simple, would not be safe without a 100% proper design).
After I designed and built it, it was possible to do tube rolling / tasting by merely powering down the amp each time before swapping tubes . . . 300B . . . 2A3 . . . 300B . . . 2A3 . . .
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filament circuit seems biggest risk here.
but if you solve that, µ-wise there is not huge difference. just a readjust the bias. and you are ok.
but if you solve that, µ-wise there is not huge difference. just a readjust the bias. and you are ok.
The risk depends on the amplifier.
300B Maximum ratings: 400V plate(to filament), 100mA, 36 Watts plate dissipation,
Grid resistor 50k Ohms fixed bias, 250k Ohms self bias.
2A3 Maximum ratings: 300V plate(to filament), 15 Watts plate dissipation,
Grid resistor 50k Ohms fixed bias, 500k Ohms self bias.
So, for a 2A3 with 300V plate to filament, the bias is about 50V, current about 50mA, and plate dissipation at 15 Watts. That limits the B+ to about 350V.
Some 300B amplifiers use fixed bias and B+ 400V, or self bias and B+ much higher than 450V.
300B Maximum ratings: 400V plate(to filament), 100mA, 36 Watts plate dissipation,
Grid resistor 50k Ohms fixed bias, 250k Ohms self bias.
2A3 Maximum ratings: 300V plate(to filament), 15 Watts plate dissipation,
Grid resistor 50k Ohms fixed bias, 500k Ohms self bias.
So, for a 2A3 with 300V plate to filament, the bias is about 50V, current about 50mA, and plate dissipation at 15 Watts. That limits the B+ to about 350V.
Some 300B amplifiers use fixed bias and B+ 400V, or self bias and B+ much higher than 450V.
Something will give up. If the amp is DC heated, the 2A3s may cause a short (eventually) in the DC supply that destroys the power transformer.
For AC heating, the 2A3 filaments are likely to burn up. This might take a bit of time, but it will happen.
You need to convince your friend to get a solid state amp, tube amps are likely a very poor choice for this individual. (I'm sure he just loves the way he has things setup though)
For AC heating, the 2A3 filaments are likely to burn up. This might take a bit of time, but it will happen.
You need to convince your friend to get a solid state amp, tube amps are likely a very poor choice for this individual. (I'm sure he just loves the way he has things setup though)
I was not clear enough. mtr22n's friend's amp should Not be used for tube swapping of 2A3 and 300B.
With special design, it is possible to design an amplifier that can use either 2A3 or 300B tubes. It may or may not need:
1. One or more extra switches,
2. Multiple sockets, two for the 2A3 and two for the 300B (a method of switching to different circuits / and or parts),
3. More parts, special power transformers (more secondary windings), different output transformers (multiple primary impedance taps),
4. An even more creative design that does not need switches, different sockets, different power transformers, or different output transformers.
I designed and built #4. But #4, like most of the designs #1 - #3 trades off the maximum performance of either the 2A3 or the 300B.
My design #4 was just for fun. It did Not have optimum performance for either the 2A3 or the 300B. What it did was allow listening to 2A3 or 300B without damaging the 2A3, 300B, or the amplifier.
The best alternative is to use 2 amplifiers. They may have similar circuits, but should have different parts to bring out the best qualities of the 2A3, and of the 300B. If two amplifiers seems too expensive, then consider how much it will cost for two 2A3 mono blocks, plus two 300B mono blocks.
With special design, it is possible to design an amplifier that can use either 2A3 or 300B tubes. It may or may not need:
1. One or more extra switches,
2. Multiple sockets, two for the 2A3 and two for the 300B (a method of switching to different circuits / and or parts),
3. More parts, special power transformers (more secondary windings), different output transformers (multiple primary impedance taps),
4. An even more creative design that does not need switches, different sockets, different power transformers, or different output transformers.
I designed and built #4. But #4, like most of the designs #1 - #3 trades off the maximum performance of either the 2A3 or the 300B.
My design #4 was just for fun. It did Not have optimum performance for either the 2A3 or the 300B. What it did was allow listening to 2A3 or 300B without damaging the 2A3, 300B, or the amplifier.
The best alternative is to use 2 amplifiers. They may have similar circuits, but should have different parts to bring out the best qualities of the 2A3, and of the 300B. If two amplifiers seems too expensive, then consider how much it will cost for two 2A3 mono blocks, plus two 300B mono blocks.
6A3sUMMER,
it would be interesting to hear about the tube rolling.
I am looking for 2W in the range of 400-17kHz (horn) and I am considering a SE 300B (I have two genalex gold lion PX300b's) or SE 2a3. This would be fed directly from a DC coupled DAC with a variable outputs (0 - 5K output impedance) 2V output.
I am only beginning with this project, right now starting to look for drivers for the output tubes. I would appreciate if you could give me some suggestions (topology, drivers).
it would be interesting to hear about the tube rolling.
I am looking for 2W in the range of 400-17kHz (horn) and I am considering a SE 300B (I have two genalex gold lion PX300b's) or SE 2a3. This would be fed directly from a DC coupled DAC with a variable outputs (0 - 5K output impedance) 2V output.
I am only beginning with this project, right now starting to look for drivers for the output tubes. I would appreciate if you could give me some suggestions (topology, drivers).
I am currently doing mostly push pull amps now, and a few SE designs too.
I am currently using Triode Wired Beam Power Tubes for both push pull and for SE.
They are all low power, but I am now working on a moderate power push pull.
There are many different 300B and 2A3 SE topologies, I have built many of them, and enjoyed listening to them; most can be very good if implemented properly.
Your DAC is 2V.
1. Is that 2V rms (+2.828V to -2.828V)?
2. Is the DAC output 0VDC with no signal? You do not want any DC offset if you couple direct from the DAC to the volume control to the input tube grid.
3. What is the sensitivity of your 400-17kHz horn (dB/Watt at 1 meter)?
What input tube you use will require knowing those, so you do not have too little, or too much gain.
4. What is your room size, horn sensitivity, and how loud you like to listen? That will dictate the power required.
Gain is one thing, Power is another.
5. Will you be willing to use DC to power your 2A3 or 300B filaments?
AC filaments will cause intermodulation at 2X your 50/60Hz line frequency (100Hz/120Hz). Those intermods will appear on each and every 400-17,000 Hz music note/ test tone (no matter how small the intermod is).
This is one reason I use Triode Wired Beam Power tubes now, the filament requirements are simpler (AC).
Even though the lowest signal frequency is 400Hz (plus some attenuated lower frequencies
from the crossover), I suggest using a Full Range SE Output transformer (you are going to have 50 or 60mA DC in the OPT primary).
I built some SE 2A3 and 300B and 45 amps with AC filaments, and some with DC filaments (I prefer DC filaments).
I like to use a current source in the input tube plate load.
I like to use RC coupling between the input and output stage.
I use self bias for the input tube.
I have used battery grid bias in the past for the output tube, but not any more, instead I prefer self bias.
I also built some push pull 2A3 and 300B amps, but only use Triode Wired Beam Power Tubes now (KT66, KT88, KT77, 7591, 6L6GC), and 6CK4 indirect heated triodes.
I have a SE amplifier with one triode section of an ECC82 or paralleled triode sections of a 12AY7 input tube (for lower or higher gain), current source in the input tube plate load, self bias of both input and output tube, and triode wired KT66.
My push pull amps are going through a topology change. It remains to be seen how they will end up.
Give me some answers to 1 - 5 above, and let's see what I come up with.
I am currently using Triode Wired Beam Power Tubes for both push pull and for SE.
They are all low power, but I am now working on a moderate power push pull.
There are many different 300B and 2A3 SE topologies, I have built many of them, and enjoyed listening to them; most can be very good if implemented properly.
Your DAC is 2V.
1. Is that 2V rms (+2.828V to -2.828V)?
2. Is the DAC output 0VDC with no signal? You do not want any DC offset if you couple direct from the DAC to the volume control to the input tube grid.
3. What is the sensitivity of your 400-17kHz horn (dB/Watt at 1 meter)?
What input tube you use will require knowing those, so you do not have too little, or too much gain.
4. What is your room size, horn sensitivity, and how loud you like to listen? That will dictate the power required.
Gain is one thing, Power is another.
5. Will you be willing to use DC to power your 2A3 or 300B filaments?
AC filaments will cause intermodulation at 2X your 50/60Hz line frequency (100Hz/120Hz). Those intermods will appear on each and every 400-17,000 Hz music note/ test tone (no matter how small the intermod is).
This is one reason I use Triode Wired Beam Power tubes now, the filament requirements are simpler (AC).
Even though the lowest signal frequency is 400Hz (plus some attenuated lower frequencies
from the crossover), I suggest using a Full Range SE Output transformer (you are going to have 50 or 60mA DC in the OPT primary).
I built some SE 2A3 and 300B and 45 amps with AC filaments, and some with DC filaments (I prefer DC filaments).
I like to use a current source in the input tube plate load.
I like to use RC coupling between the input and output stage.
I use self bias for the input tube.
I have used battery grid bias in the past for the output tube, but not any more, instead I prefer self bias.
I also built some push pull 2A3 and 300B amps, but only use Triode Wired Beam Power Tubes now (KT66, KT88, KT77, 7591, 6L6GC), and 6CK4 indirect heated triodes.
I have a SE amplifier with one triode section of an ECC82 or paralleled triode sections of a 12AY7 input tube (for lower or higher gain), current source in the input tube plate load, self bias of both input and output tube, and triode wired KT66.
My push pull amps are going through a topology change. It remains to be seen how they will end up.
Give me some answers to 1 - 5 above, and let's see what I come up with.
The easy swap 2A3 / 300B amp used AC filaments, a 7.5V filament winding, and a 2 Ohm 50 Watt (voltage dropping) power resistor in series with the filament. That automatically gives the right filament voltage to the 2A3 and 300B that is inserted in the socket.
The output stage plate voltage was 350V, the self bias resistor was 1k Ohm, and the OPT was 3k Ohm primary, there was a little more current drawn by the 300B. This less than optimum compromise did sound good with both the 2A3 and the 300B. Because it was a mono block, you had to listen to one tube, turn the amp off, wait for the tube to cool before pulling it, and then insert the other tube, power up, and listen. Not ideal, but lots of fun.
The output stage plate voltage was 350V, the self bias resistor was 1k Ohm, and the OPT was 3k Ohm primary, there was a little more current drawn by the 300B. This less than optimum compromise did sound good with both the 2A3 and the 300B. Because it was a mono block, you had to listen to one tube, turn the amp off, wait for the tube to cool before pulling it, and then insert the other tube, power up, and listen. Not ideal, but lots of fun.
6A3sUMMER,
thanks for your reply!
1. Yes, -2V8, to +2V8
2. Yes, here is a little explanation from the designer of the DAC (I would prefer, if no sonic penalty, to use the variable output):
"There are fixed outputs (750 Ohms) and a variable outputs (0 - 5K).
Ternary logic switches switch between -2V8, 0V and +2V8. This logic drives a passive resistor matrix that has 750 Ohms output impedance. The output is DC-coupled since it swings symmetrically around GND, so no coupling caps in the signal path either.
The +2V8 is provided by a discrete quatrode voltage regulator, the tracking -2V8 is derived from a second discrete quatrode regulator and a servo circuit that compares both plus and minus voltages and corrects the negative voltage for tracking. This way there is no DC on the outputs.
The built-in relay volume control consists of relays and resistors (also passive circuit) and controls volume in 32 calibrated steps"
105 dB/watt @1m (Ja-6681)
Living room is around 30m2, but it opens completely to a kitchen/dining room of another 40m2. The max distance to the speakers form the listening position is around 3.5m. I was aiming at around 106-108 dB pk at listening position with a crest factor of 6-8 dB, but this is probably more than I would use.
I had thought of R. Coleman regs.
I will most probably use the FC-20s form ISO, but I had spotted a pair of Tango H5S that I thought would do given the intended range. Point taken.
I've been looking mostly at some DC coupled designs, regs, gyrators, OPT options for now. I got already a pair of 300b's but I am open to 2a3, 45 or something else.
I thank you for your interest. Perhaps I should open a new thread for this or ask the mods to move this posts to a new one.
thanks for your reply!
1. Yes, -2V8, to +2V8
2. Yes, here is a little explanation from the designer of the DAC (I would prefer, if no sonic penalty, to use the variable output):
"There are fixed outputs (750 Ohms) and a variable outputs (0 - 5K).
Ternary logic switches switch between -2V8, 0V and +2V8. This logic drives a passive resistor matrix that has 750 Ohms output impedance. The output is DC-coupled since it swings symmetrically around GND, so no coupling caps in the signal path either.
The +2V8 is provided by a discrete quatrode voltage regulator, the tracking -2V8 is derived from a second discrete quatrode regulator and a servo circuit that compares both plus and minus voltages and corrects the negative voltage for tracking. This way there is no DC on the outputs.
The built-in relay volume control consists of relays and resistors (also passive circuit) and controls volume in 32 calibrated steps"
105 dB/watt @1m (Ja-6681)
Living room is around 30m2, but it opens completely to a kitchen/dining room of another 40m2. The max distance to the speakers form the listening position is around 3.5m. I was aiming at around 106-108 dB pk at listening position with a crest factor of 6-8 dB, but this is probably more than I would use.
I had thought of R. Coleman regs.
I will most probably use the FC-20s form ISO, but I had spotted a pair of Tango H5S that I thought would do given the intended range. Point taken.
I've been looking mostly at some DC coupled designs, regs, gyrators, OPT options for now. I got already a pair of 300b's but I am open to 2a3, 45 or something else.
I thank you for your interest. Perhaps I should open a new thread for this or ask the mods to move this posts to a new one.
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swak,
Just a few estimates:
1. At 3.5 meters, that is about 1/12 of the acoustic power at 1 meter (-11 dB).
If you want 106 to 108 dB with a headroom of 6 to 8 dB, that is 112 to 116 dB.
That means you need up to 116dB + 11dB = 127dB @ 1M from the horn, extremely loud, ear-splitting levels.
127dB - 105 dB/W = 22dB. That requires 158 Watts!
But not all of that power needs to be in the horn, that starts at 400Hz.
Most power is generally in the Bass (a Woofer/Midrange speaker).
I bet an SE 300B amp will probably drive the 400Hz to 17kHz, @ 105dB/W horn to satisfying levels on most music.
2. DC coupling has its advantages, and its disadvantages.
One good DC coupled amp had over 200mA in the 2A3 at warm up (!), so some very sophisticated circuitry was required to solve that.
But it still required about 2 minutes warm up, and also needed adjustment for the initial setup, as well as any time when input or output tubes were changed.
Just a few estimates:
1. At 3.5 meters, that is about 1/12 of the acoustic power at 1 meter (-11 dB).
If you want 106 to 108 dB with a headroom of 6 to 8 dB, that is 112 to 116 dB.
That means you need up to 116dB + 11dB = 127dB @ 1M from the horn, extremely loud, ear-splitting levels.
127dB - 105 dB/W = 22dB. That requires 158 Watts!
But not all of that power needs to be in the horn, that starts at 400Hz.
Most power is generally in the Bass (a Woofer/Midrange speaker).
I bet an SE 300B amp will probably drive the 400Hz to 17kHz, @ 105dB/W horn to satisfying levels on most music.
2. DC coupling has its advantages, and its disadvantages.
One good DC coupled amp had over 200mA in the 2A3 at warm up (!), so some very sophisticated circuitry was required to solve that.
But it still required about 2 minutes warm up, and also needed adjustment for the initial setup, as well as any time when input or output tubes were changed.
6A3summer,
Thanks for your reply!
I am certainly thinking of much less power for the horn🙂 I meant 106-8 peak over the whole freq. range form both speakers and not only direct signal, so like 98-100 dB-A when placing a sound meter at average at the listening position, and even that, as I mentioned, would be more than I would normally use.
I actually think even two watts from a 45 could be enough.
One thing I know for sure is that I am looking for a tested design that I can replicate as I am in no way capable of undertaking such a design myself.
Thanks for your reply!
I am certainly thinking of much less power for the horn🙂 I meant 106-8 peak over the whole freq. range form both speakers and not only direct signal, so like 98-100 dB-A when placing a sound meter at average at the listening position, and even that, as I mentioned, would be more than I would normally use.
I actually think even two watts from a 45 could be enough.
One thing I know for sure is that I am looking for a tested design that I can replicate as I am in no way capable of undertaking such a design myself.
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