I=C dv/dt
Slew Rate (dv/dt) = 2 * PI * Freq(max) * Vmax
So, you need to know the miller capacitance of the tube and the maximum voltage swing and maximum frequency you wish to operate at.
You also need to calculate the current to the grid resistor based on it's resistance and grid voltage swing.
Slew Rate (dv/dt) = 2 * PI * Freq(max) * Vmax
So, you need to know the miller capacitance of the tube and the maximum voltage swing and maximum frequency you wish to operate at.
You also need to calculate the current to the grid resistor based on it's resistance and grid voltage swing.
Keep an eye at the resonant frequency of the Miller cap and trafo inductance. If second triode anode circuit is inductive at such frequency, the circuit will gently oscillate.
Keep me honest. Here are my calculations:
Assume max frequency = 20K
Voltage swing GM70 = 850V
GM70 plate-to-grid capacitance = 12pF
GM70 gain = 6.7
GM70 miller capacitance = 12pF * (6.7+1) =92.4pF
I = 2 * pi * 20,000 * 900V * 92.4 = 10.4mA
Correct?
Assume max frequency = 20K
Voltage swing GM70 = 850V
GM70 plate-to-grid capacitance = 12pF
GM70 gain = 6.7
GM70 miller capacitance = 12pF * (6.7+1) =92.4pF
I = 2 * pi * 20,000 * 900V * 92.4 = 10.4mA
Correct?
Not yet mentioned is that the slew rate limit is the very raggedy edge of not slewing. Linear operation requires many times as much current capacity, the more the merrier.
All good fortune,
Chris
All good fortune,
Chris
How much shunt capacitance does the interstage transformer add? How much circuit strays?
All good fortune,
Chris
All good fortune,
Chris
I assume you already have the interstage transformer so you cannot change its inductance. Therefore the anode current of the driver will set the low frequency limit for 115V peak driver voltage output.
What is your lowest frequency at full power? What is the inductance of your interstage? What is the max DC current you can have across the primary of the interstage? All these things will set limits at low frequency.
What is your lowest frequency at full power? What is the inductance of your interstage? What is the max DC current you can have across the primary of the interstage? All these things will set limits at low frequency.
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BTW, all these calc's assume the interstage transformer is 1:1. If so, the proposed loadline doesn't allow a voltage swing of 115 Volts peak. If it's a step-up transformer, then the current requirements scale with the step-up ratio.
All good fortune,
Chris
All good fortune,
Chris
Yesterday you said you have removed the 100K grid leak resistor on the Aa tube a few weeks ago. Where did you get this idea ?
On your car; do you also open the motor block remove some parts just for fun. Then start the engine and expect to have much more power ......
Sorry if i embarrass you, but you make such scary dangerous things on your amps, without thinking about it. If i look to your threads from the last 2 years; they are full of such experiments. First people try to help you, then they stop .... Guess why ?
Your are asking such ambitious technical questions, while you don't know basic simple things how tubes work in a circuit.
From the answers you get: you don't realize what is good and usefull for you. It's like an endless search.
Buy the Morgan Jones - Valve Amplifers book, try it, it may help you.
You are stuck to the idea that an all dht SE amp is the absolute best.
Fine. For your amp with dht tubes you need 3 tubes. There are no high gain dht tubes. The Aa is a good low current tube: max cathode = 5mA. It is no good driver tube for GM70. Second tube as driver could be a 46 in triode: 25-30mA, at 200-230V. Gain=5.6
The Aa has a plate resistance of ca 30KOhm. You need a load of 3 times = ca 100KOhm. You can't do that with an inductive load (plate choke or IT transformer) Use a gyrator pcb from ALE / Bartola. Aa plate voltage + ca 100V gyrator voltage.
Direct coupling from Aa to the 46 would be much too dangerous for you. Use a copper V-cap but with fixed bias on the 46. This way you have DC on both foils of the capacitor, best way to use coupling caps. (no cap charging)
The 46 (triode) has ca 2,3KOhm plate resistance. If your IT is still the 5K Slaggle, you can use it if it can handle 25mA DC.
Your current circuit: driving the input grid of GM70 with the Aa high plate resistance 30KOhm tube + low 3mA current. You can't use tiny low current tubes to drive big transmitting tubes. Are you loading the 30K Aa tube with the 5K Slaggle IT ?
You can insert a interstage transformer loaded 46 to the GM70: a medium plate resistance 2.3K + 25mA current, plus 5.6 times gain: you would get more power and much better frequency extension. For the Aa (300VDC) and 46 (240VDC) you could use the same psu (from rectf. 83). The first psu transformer would be too weak for the Aa + 46 tubes.
A 300B might be a little better driver for the GM70. But you can't drive it with the Aa's tiny 3mA. You don't want to use 4 dht tubes in the amp.
If you want change something in your amp; post it first here and ask people if it is a good idea.
You was very lucky in the past. You could have damaged your amp, your loudspeakers or harm yourself.
On your car; do you also open the motor block remove some parts just for fun. Then start the engine and expect to have much more power ......
Sorry if i embarrass you, but you make such scary dangerous things on your amps, without thinking about it. If i look to your threads from the last 2 years; they are full of such experiments. First people try to help you, then they stop .... Guess why ?
Your are asking such ambitious technical questions, while you don't know basic simple things how tubes work in a circuit.
From the answers you get: you don't realize what is good and usefull for you. It's like an endless search.
Buy the Morgan Jones - Valve Amplifers book, try it, it may help you.
You are stuck to the idea that an all dht SE amp is the absolute best.
Fine. For your amp with dht tubes you need 3 tubes. There are no high gain dht tubes. The Aa is a good low current tube: max cathode = 5mA. It is no good driver tube for GM70. Second tube as driver could be a 46 in triode: 25-30mA, at 200-230V. Gain=5.6
The Aa has a plate resistance of ca 30KOhm. You need a load of 3 times = ca 100KOhm. You can't do that with an inductive load (plate choke or IT transformer) Use a gyrator pcb from ALE / Bartola. Aa plate voltage + ca 100V gyrator voltage.
Direct coupling from Aa to the 46 would be much too dangerous for you. Use a copper V-cap but with fixed bias on the 46. This way you have DC on both foils of the capacitor, best way to use coupling caps. (no cap charging)
The 46 (triode) has ca 2,3KOhm plate resistance. If your IT is still the 5K Slaggle, you can use it if it can handle 25mA DC.
Your current circuit: driving the input grid of GM70 with the Aa high plate resistance 30KOhm tube + low 3mA current. You can't use tiny low current tubes to drive big transmitting tubes. Are you loading the 30K Aa tube with the 5K Slaggle IT ?
You can insert a interstage transformer loaded 46 to the GM70: a medium plate resistance 2.3K + 25mA current, plus 5.6 times gain: you would get more power and much better frequency extension. For the Aa (300VDC) and 46 (240VDC) you could use the same psu (from rectf. 83). The first psu transformer would be too weak for the Aa + 46 tubes.
A 300B might be a little better driver for the GM70. But you can't drive it with the Aa's tiny 3mA. You don't want to use 4 dht tubes in the amp.
If you want change something in your amp; post it first here and ask people if it is a good idea.
You was very lucky in the past. You could have damaged your amp, your loudspeakers or harm yourself.
If you use 1;2 step-up then the driver will see the GM70 Miller capacitance multipied by 4.. Still, you will need 125-150H to get full swing down to 30Hz. More, with a step-up you cannot have a bifilar design.
I do nòt think this tube is good for driving the GM70 with an interstage. It runs at too low current for this application, typically 3-4 mA max. So unless you have a 1:1 intestage with good frequency response and about 250-300 H primary inductance to get down to 30Hz with lowish (acceptable but not great) distortion, it is not going to work well. If you relax the lower frequency limit to 50Hz it might be a lot easier to find a suitable IT but there are far better tubes for this task.
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On the other hand, nobody wants to hear a GM70 making MAXimum level at 20kHz. (Neither do their tweeters.) Speech/music tends to roll-off by 2kHz, giving a useful 10:1 margin from 20kHz slew. It will clip bass/midrange first. (Which will also be uncomfortable...)
And it is really difficult to get into good slewing without large NFB, which is not customary in simple tube designs. Small-signal and large-signal responses tend to track, if not "enhanced" with heavy NFB. I dunno if this particular plan qualifies.
And on the other other hand, getting anywhere near slewing, meaning when output is no longer controlled by input, but instead by the charging rate of a capacitor, is still caca doodoo. This is just a silly circuit.
All good fortune,
Chris
All good fortune,
Chris
So to this point I have refrained from throwing my hat in the ring. I am not sure how many of the commenters let alone the OP have any experience with the GM70 DHT. (Their technical concerns are on point.) I have the (mis)fortune to know something about it. These amplifiers ran full range until late summer 2018 and now do bass duty - they are capable of operation over 30Hz - 20kHz +/-1dB. (See below) They have been in continuous service since 2010 except for downtime for repairs and upgrades, and I average about 1500hrs per year of use.
The first thing I would mention is the GM70 is more than a bit frightening to work with, the envelope runs at close to 200°C, filament consumption is 60W, plate dissipation should be north of 80W for good performance, and it requires much more than the 750V or so you make available after you subtract the bias voltage. It is also quite short lived, I am about to install my 13th pair in 13 years of operation sometime in the next couple of months. Running them beyond 1200 - 1400 hours is risky due to the possibility of destructive runaway if one of the filaments breaks, and they will.. (This will probably destroy your output transformer before you have had time to react.)
You can drive them with a single tube, but not the Aa, and you will not be able to enter A2 with any reasonable single tube driver. I use 5K 1:1 Monolith Magnetics bi-filar wound 30mA 60H IT in these amplifiers, driven by a triode connected D3A (another name for VHF oscillator) running at 18mA and 200V. The output stage is fixed bias and with plate current running around 100mA at 1.05kV on a good day I can get almost 20W out of one of these monoblocks. (with 7K output transformer) It is about the most inefficient way to make 20W that I can come up with - of course if I had designed a driver that could source more than a few mA of grid current I could have 35 - 40W of A2 power - never went there.
Have you considered the fact that you cannot heat these with AC - the hum would be intolerable. I use the Coleman regulators on big heat sinks and feed them with 25V pre-regulators - this results in 15W of dissipation in the Coleman regulators and another 20W or so in the pre-regulators.
I also take issue with the choice of 866A rectifiers, at this point I won't even allow them in my house, should one break you have a real health hazard on your hands. IMO these belong in the dustbin of history. I use SIC rectifier based bridges into CLCLC filters. The GM70 can comfortably tolerate >1kV on the plate without filament power applied, the filaments being tungsten are not prone to cathode stripping either.
The GM70 is a cheap tube given its performance, but it is not cheap to use, getting good results from it requires complex, dangerous and expensive electronics to go with it.
Because of the voltages involved I have never made the design of this amplifier available here, and won't. I and a number of others have built these. (I built one for a friend a couple of years ago.) Anything using a GM70 really is the province of the most experienced builders. I will never design nor build another amplifier using this tube.
Something else to consider is where these come from, and the likely future supply since they are no longer made.
You might want to look at the EML520B or related types. EML also makes the 20B with a mu of about 20 (Yeah I use these too) which with a 520B gives you a viable two DHT design, particularly if you use an input transformer with about 6dB of voltage gain. These tubes will have a long enough service life to defray their cost. Filament supplies and plate power supplies are less demanding to design and build since voltages and currents are in a range most hobbyists can handle safely. Two sets of EML520B would have cost me about the same or a bit less than the GM70 over the run of years and for roughly the same operating hours. (Power output also would be in the same range.)
You can drive them with a single tube, but not the Aa, and you will not be able to enter A2 with any reasonable single tube driver. I use 5K 1:1 Monolith Magnetics bi-filar wound 30mA 60H IT in these amplifiers, driven by a triode connected D3A (another name for VHF oscillator) running at 18mA and 200V. The output stage is fixed bias and with plate current running around 100mA at 1.05kV on a good day I can get almost 20W out of one of these monoblocks. (with 7K output transformer) It is about the most inefficient way to make 20W that I can come up with - of course if I had designed a driver that could source more than a few mA of grid current I could have 35 - 40W of A2 power - never went there.
Have you considered the fact that you cannot heat these with AC - the hum would be intolerable. I use the Coleman regulators on big heat sinks and feed them with 25V pre-regulators - this results in 15W of dissipation in the Coleman regulators and another 20W or so in the pre-regulators.
I also take issue with the choice of 866A rectifiers, at this point I won't even allow them in my house, should one break you have a real health hazard on your hands. IMO these belong in the dustbin of history. I use SIC rectifier based bridges into CLCLC filters. The GM70 can comfortably tolerate >1kV on the plate without filament power applied, the filaments being tungsten are not prone to cathode stripping either.
The GM70 is a cheap tube given its performance, but it is not cheap to use, getting good results from it requires complex, dangerous and expensive electronics to go with it.
Because of the voltages involved I have never made the design of this amplifier available here, and won't. I and a number of others have built these. (I built one for a friend a couple of years ago.) Anything using a GM70 really is the province of the most experienced builders. I will never design nor build another amplifier using this tube.
Something else to consider is where these come from, and the likely future supply since they are no longer made.
You might want to look at the EML520B or related types. EML also makes the 20B with a mu of about 20 (Yeah I use these too) which with a 520B gives you a viable two DHT design, particularly if you use an input transformer with about 6dB of voltage gain. These tubes will have a long enough service life to defray their cost. Filament supplies and plate power supplies are less demanding to design and build since voltages and currents are in a range most hobbyists can handle safely. Two sets of EML520B would have cost me about the same or a bit less than the GM70 over the run of years and for roughly the same operating hours. (Power output also would be in the same range.)
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I still don't understand the attraction of DHTs as driver stages. For output stages, type 211, 304TL, even 845, (never tried GM70), great curves, I'm sold. But for drivers? There are plenty of modern valves with at least as good curves and actual separate cathodes (!), which minimize the grief factor by a big factor. What's the attraction?
All good fortune,
Chris
All good fortune,
Chris