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Converting sig gen to solid state
I have an old Heath SG-6 signal generator. It uses two triodes.
I'd like to convert it to solid state. The switched coils, varible cap and dial mechanism are very nice. Any suggestions on what kind of oscillator circuit I should use? I imagine the difficult part will be coming up with a feedback circuit that will cover the 160 kHz to 50 Mhz. Any ideas? Has anyone seen any articles on this kind of conversion? By going solid state and adding a few buffer stages I'm hoping I could make this thing a lot more stable. 73 Bill M0HBR CU2JL N2CQR http://planeta.clix.pt/n2cqr |
Bill Meara wrote: I have an old Heath SG-6 signal generator. It uses two triodes. I'd like to convert it to solid state. The switched coils, varible cap and dial mechanism are very nice. Any suggestions on what kind of oscillator circuit I should use? I imagine the difficult part will be coming up with a feedback circuit that will cover the 160 kHz to 50 Mhz. Any ideas? Has anyone seen any articles on this kind of conversion? By going solid state and adding a few buffer stages I'm hoping I could make this thing a lot more stable. 73 Bill M0HBR CU2JL N2CQR http://planeta.clix.pt/n2cqr You should be able to replace the triodes with FETs, this was done occasionally when FETs first became available. You'll probably need to fiddle with some of the component values. Don't forget to reduce the HT voltage. 8-) Leon -- Leon Heller, G1HSM Email: My low-cost Philips LPC210x ARM development system: http://www.geocities.com/leon_heller/lpc2104.html |
Bill Meara wrote: I have an old Heath SG-6 signal generator. It uses two triodes. I'd like to convert it to solid state. The switched coils, varible cap and dial mechanism are very nice. Any suggestions on what kind of oscillator circuit I should use? I imagine the difficult part will be coming up with a feedback circuit that will cover the 160 kHz to 50 Mhz. Any ideas? Has anyone seen any articles on this kind of conversion? By going solid state and adding a few buffer stages I'm hoping I could make this thing a lot more stable. 73 Bill M0HBR CU2JL N2CQR http://planeta.clix.pt/n2cqr You should be able to replace the triodes with FETs, this was done occasionally when FETs first became available. You'll probably need to fiddle with some of the component values. Don't forget to reduce the HT voltage. 8-) Leon -- Leon Heller, G1HSM Email: My low-cost Philips LPC210x ARM development system: http://www.geocities.com/leon_heller/lpc2104.html |
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Bill Meara ) writes:
I have an old Heath SG-6 signal generator. It uses two triodes. I'd like to convert it to solid state. The switched coils, varible cap and dial mechanism are very nice. Any suggestions on what kind of oscillator circuit I should use? I imagine the difficult part will be coming up with a feedback circuit that will cover the 160 kHz to 50 Mhz. Any ideas? Has anyone seen any articles on this kind of conversion? By going solid state and adding a few buffer stages I'm hoping I could make this thing a lot more stable. 73 Bill M0HBR CU2JL N2CQR http://planeta.clix.pt/n2cqr As Leon said, you should be able to change the triode to an FET, with minimal change. In other words, leave the oscillator as it is, and merely change biasing. I once took a Collins PTO, bought at a hamfest for $2.50, took out the tube and wired in an FET across the tube socket. Obviously, I reduced the "B+" to around 12V or so. I can't remember if I even had to change the plate resistor (there might not have been one, but instead an RFC). Michael VE2BVW |
Bill Meara ) writes:
I have an old Heath SG-6 signal generator. It uses two triodes. I'd like to convert it to solid state. The switched coils, varible cap and dial mechanism are very nice. Any suggestions on what kind of oscillator circuit I should use? I imagine the difficult part will be coming up with a feedback circuit that will cover the 160 kHz to 50 Mhz. Any ideas? Has anyone seen any articles on this kind of conversion? By going solid state and adding a few buffer stages I'm hoping I could make this thing a lot more stable. 73 Bill M0HBR CU2JL N2CQR http://planeta.clix.pt/n2cqr As Leon said, you should be able to change the triode to an FET, with minimal change. In other words, leave the oscillator as it is, and merely change biasing. I once took a Collins PTO, bought at a hamfest for $2.50, took out the tube and wired in an FET across the tube socket. Obviously, I reduced the "B+" to around 12V or so. I can't remember if I even had to change the plate resistor (there might not have been one, but instead an RFC). Michael VE2BVW |
Your solid state devices will either be really light-weight or will have
significantly higher capacitances than the tubes, so it would be very difficult to get your oscillator to track. I've got an RF-1 which I've contemplated upgrading. Before I went to solid state I'd give the filaments a regulated supply (like budgie's post) _and_ put in a regulated plate supply. A solid-state wideband buffer would probably be a quite good idea; a 0-50MHz broad-band amp into 50 ohms should be fairly easy. "Bill Meara" wrote in message om... I have an old Heath SG-6 signal generator. It uses two triodes. I'd like to convert it to solid state. The switched coils, varible cap and dial mechanism are very nice. Any suggestions on what kind of oscillator circuit I should use? I imagine the difficult part will be coming up with a feedback circuit that will cover the 160 kHz to 50 Mhz. Any ideas? Has anyone seen any articles on this kind of conversion? By going solid state and adding a few buffer stages I'm hoping I could make this thing a lot more stable. 73 Bill M0HBR CU2JL N2CQR http://planeta.clix.pt/n2cqr |
Your solid state devices will either be really light-weight or will have
significantly higher capacitances than the tubes, so it would be very difficult to get your oscillator to track. I've got an RF-1 which I've contemplated upgrading. Before I went to solid state I'd give the filaments a regulated supply (like budgie's post) _and_ put in a regulated plate supply. A solid-state wideband buffer would probably be a quite good idea; a 0-50MHz broad-band amp into 50 ohms should be fairly easy. "Bill Meara" wrote in message om... I have an old Heath SG-6 signal generator. It uses two triodes. I'd like to convert it to solid state. The switched coils, varible cap and dial mechanism are very nice. Any suggestions on what kind of oscillator circuit I should use? I imagine the difficult part will be coming up with a feedback circuit that will cover the 160 kHz to 50 Mhz. Any ideas? Has anyone seen any articles on this kind of conversion? By going solid state and adding a few buffer stages I'm hoping I could make this thing a lot more stable. 73 Bill M0HBR CU2JL N2CQR http://planeta.clix.pt/n2cqr |
In article , "Tim Wescott"
writes: Your solid state devices will either be really light-weight or will have significantly higher capacitances than the tubes, so it would be very difficult to get your oscillator to track. I've got an RF-1 which I've contemplated upgrading. Before I went to solid state I'd give the filaments a regulated supply (like budgie's post) _and_ put in a regulated plate supply. A solid-state wideband buffer would probably be a quite good idea; a 0-50MHz broad-band amp into 50 ohms should be fairly easy. Going to all-regulated supplies for a vacuum tube thing turns out to be rather easy. I've just finished breadboarding that for a 1960s era HF SWBC receiver to improve its frequency stability. Tube oscillators need a stable "B+" or plate supply that, in older days, was enabled by gas shunt regulators (OA2 or OB2, for example). Heater voltage variation is less of an effect on frequency stability but such does occur when the AC primary voltage varies to the extremes of its +/- 10% possible range. On checking the HV supply of this little receiver, the OB2 strike voltage had increased beyond manufacturer's spec so it did not always strike and start regulating with power-on. The lone OB2 in spares, also 40 years old, was just the opposite, striking below spec. That prompted looking into a series regulator for the whole B+ using an ST (Fairchild also makes them) TIP47 series pass NPN with a 250 V breakdown and a 2N3440 250 V NPN as the error amplifier. Not a problem to have it regulate everything at +120 VDC plate supply, using only half of the original filter capacitor quad. Regulating the filaments just barely works with a conventional 7812 positive IC regulator. My version is a combo series-parallel string of 6.3 and 12.6 V nominal filaments, the whole taking 0.6 A. With less than 10% ripple on full-wave rectification and 12.6 VAC RMS nominal, the 2.0 V minimum of series drop necessary is maintained even at 105 VAC primary transformer input. The filament voltage is maintained constant at 12.1 VDC within 1 percent at all conditions of AC primary voltage. To get a regulated minus, a 7912 (if it can be found) will do with rectifier and capacitor polarity reversed, handy for biasing other circuits with a negative voltage. With a 6.3 V filament arrangement, that would be on the hairy edge of regulating at -10% AC line voltage. Filter capacitor needs to be high enough for only 5% ripple and rectifier diodes can't be much more than 0.7 V forward drop to fit the minimum 7806 regulator 2 V head- room. Note: The original AC filament voltage was made within certain (seldom specified) limits; if that is on the low side also, the 7806 may poop out during droops in ripple voltage. 7906s were originally made in the series for negative voltage regulation, also good for providing a minus bias voltage. Tube filaments for indirectly-heated cathodes don't care about DC polarity. Slightly-lower than nominal voltage will improve heater life. Higher voltage NPNs for series-pass applications are getting scarce. So are higher voltage zeners. TIP48 has slightly higher breakdown than TIP47, can work for higher B+ values although there's a need to check voltages for +10% AC line conditions rather than -10%. Exceeding breakdown voltage can be catastrophic to the transistor junction in just a single AC cycle. Len Anderson retired (from regular hours) electronic engineer person |
In article , "Tim Wescott"
writes: Your solid state devices will either be really light-weight or will have significantly higher capacitances than the tubes, so it would be very difficult to get your oscillator to track. I've got an RF-1 which I've contemplated upgrading. Before I went to solid state I'd give the filaments a regulated supply (like budgie's post) _and_ put in a regulated plate supply. A solid-state wideband buffer would probably be a quite good idea; a 0-50MHz broad-band amp into 50 ohms should be fairly easy. Going to all-regulated supplies for a vacuum tube thing turns out to be rather easy. I've just finished breadboarding that for a 1960s era HF SWBC receiver to improve its frequency stability. Tube oscillators need a stable "B+" or plate supply that, in older days, was enabled by gas shunt regulators (OA2 or OB2, for example). Heater voltage variation is less of an effect on frequency stability but such does occur when the AC primary voltage varies to the extremes of its +/- 10% possible range. On checking the HV supply of this little receiver, the OB2 strike voltage had increased beyond manufacturer's spec so it did not always strike and start regulating with power-on. The lone OB2 in spares, also 40 years old, was just the opposite, striking below spec. That prompted looking into a series regulator for the whole B+ using an ST (Fairchild also makes them) TIP47 series pass NPN with a 250 V breakdown and a 2N3440 250 V NPN as the error amplifier. Not a problem to have it regulate everything at +120 VDC plate supply, using only half of the original filter capacitor quad. Regulating the filaments just barely works with a conventional 7812 positive IC regulator. My version is a combo series-parallel string of 6.3 and 12.6 V nominal filaments, the whole taking 0.6 A. With less than 10% ripple on full-wave rectification and 12.6 VAC RMS nominal, the 2.0 V minimum of series drop necessary is maintained even at 105 VAC primary transformer input. The filament voltage is maintained constant at 12.1 VDC within 1 percent at all conditions of AC primary voltage. To get a regulated minus, a 7912 (if it can be found) will do with rectifier and capacitor polarity reversed, handy for biasing other circuits with a negative voltage. With a 6.3 V filament arrangement, that would be on the hairy edge of regulating at -10% AC line voltage. Filter capacitor needs to be high enough for only 5% ripple and rectifier diodes can't be much more than 0.7 V forward drop to fit the minimum 7806 regulator 2 V head- room. Note: The original AC filament voltage was made within certain (seldom specified) limits; if that is on the low side also, the 7806 may poop out during droops in ripple voltage. 7906s were originally made in the series for negative voltage regulation, also good for providing a minus bias voltage. Tube filaments for indirectly-heated cathodes don't care about DC polarity. Slightly-lower than nominal voltage will improve heater life. Higher voltage NPNs for series-pass applications are getting scarce. So are higher voltage zeners. TIP48 has slightly higher breakdown than TIP47, can work for higher B+ values although there's a need to check voltages for +10% AC line conditions rather than -10%. Exceeding breakdown voltage can be catastrophic to the transistor junction in just a single AC cycle. Len Anderson retired (from regular hours) electronic engineer person |
"Bill Meara" wrote in message
om... I have an old Heath SG-6 signal generator. It uses two triodes. I'd like to convert it to solid state. The switched coils, varible cap and dial mechanism are very nice. Any suggestions on what kind of oscillator circuit I should use? I imagine the difficult part will be coming up with a feedback circuit that will cover the 160 kHz to 50 Mhz. Any ideas? Has anyone seen any articles on this kind of conversion? By going solid state and adding a few buffer stages I'm hoping I could make this thing a lot more stable. 73 Bill M0HBR CU2JL N2CQR http://planeta.clix.pt/n2cqr Hi Bill, I'm in the process of doing something similar. I have (had) an old "University" RF oscillator, 6 ranges, covers 100kHz-130MHz. Based on a dual triode (oscillator and buffer) and a pentode for audio oscillator for modulation. This thing looked cheap(ish) and didn't work very well, would start and stop at various frequency settings. So I stripped it and decided to keep the tuning cap, range switch, and tuning coils. Rebuilt it with a JFET (J310) but had to change the circuit. Coils are centre tapped and fed the supply voltage, so the Anode current flowed through the coil. When the FET was placed into the circuit this overdrove the gate and oscillation was erratic and unstable. Supplying through a resistor improved things quite a bit. Problem I have now is with the low frequency ranges. The coils for this are large (they are air cored with a tuning slug?) so are high resistance as they have lots of wire. I may try to increase gain to compensate or else replace with a toroid. As someone else mentioned, I haven't checked tracking so I don't know how well the tuning dial will match the frequency. James. |
"Bill Meara" wrote in message
om... I have an old Heath SG-6 signal generator. It uses two triodes. I'd like to convert it to solid state. The switched coils, varible cap and dial mechanism are very nice. Any suggestions on what kind of oscillator circuit I should use? I imagine the difficult part will be coming up with a feedback circuit that will cover the 160 kHz to 50 Mhz. Any ideas? Has anyone seen any articles on this kind of conversion? By going solid state and adding a few buffer stages I'm hoping I could make this thing a lot more stable. 73 Bill M0HBR CU2JL N2CQR http://planeta.clix.pt/n2cqr Hi Bill, I'm in the process of doing something similar. I have (had) an old "University" RF oscillator, 6 ranges, covers 100kHz-130MHz. Based on a dual triode (oscillator and buffer) and a pentode for audio oscillator for modulation. This thing looked cheap(ish) and didn't work very well, would start and stop at various frequency settings. So I stripped it and decided to keep the tuning cap, range switch, and tuning coils. Rebuilt it with a JFET (J310) but had to change the circuit. Coils are centre tapped and fed the supply voltage, so the Anode current flowed through the coil. When the FET was placed into the circuit this overdrove the gate and oscillation was erratic and unstable. Supplying through a resistor improved things quite a bit. Problem I have now is with the low frequency ranges. The coils for this are large (they are air cored with a tuning slug?) so are high resistance as they have lots of wire. I may try to increase gain to compensate or else replace with a toroid. As someone else mentioned, I haven't checked tracking so I don't know how well the tuning dial will match the frequency. James. |
"Avery Fineman" wrote in message ... In article , "Tim Wescott" writes: Your solid state devices will either be really light-weight or will have significantly higher capacitances than the tubes, so it would be very difficult to get your oscillator to track. -- snip -- On checking the HV supply of this little receiver, the OB2 strike voltage had increased beyond manufacturer's spec so it did not always strike and start regulating with power-on. The lone OB2 in spares, also 40 years old, was just the opposite, striking below spec. Antique Electronics Supply has good replacement tubes -- but solid-state regulators for old tube equipment is more fun, kinda like putting EFI on your flathead Ford engine (no kidding, it's done). -- snip more -- With a 6.3 V filament arrangement, that would be on the hairy edge of regulating at -10% AC line voltage. Filter capacitor needs to be high enough for only 5% ripple and rectifier diodes can't be much more than 0.7 V forward drop to fit the minimum 7806 regulator 2 V head- room. I would use a new LDO regulator, if I used one at all. On thinking about it after my post, it occurred to me that for the filaments I would be happy with a well-filtered DC supply to keep the filament hum from modulating my RF. If all you have is a 6.3V supply you need really low-drop diodes, however, and your filament transformer's RMS current will go up no matter what. Note: The original AC filament voltage was made within certain (seldom specified) limits; if that is on the low side also, the 7806 may poop out during droops in ripple voltage. 7906s were originally made in the series for negative voltage regulation, also good for providing a minus bias voltage. Tube filaments for indirectly-heated cathodes don't care about DC polarity. Slightly-lower than nominal voltage will improve heater life. But can decrease cathode life if the cathode current is high -- even oxide cathodes don't like being temperature starved. Check some of the warnings and recommended start up procedures for 10kW transmitter tubes for some _really_ tight cathode care-and-feeding instructions. -- snip even more -- |
"Avery Fineman" wrote in message ... In article , "Tim Wescott" writes: Your solid state devices will either be really light-weight or will have significantly higher capacitances than the tubes, so it would be very difficult to get your oscillator to track. -- snip -- On checking the HV supply of this little receiver, the OB2 strike voltage had increased beyond manufacturer's spec so it did not always strike and start regulating with power-on. The lone OB2 in spares, also 40 years old, was just the opposite, striking below spec. Antique Electronics Supply has good replacement tubes -- but solid-state regulators for old tube equipment is more fun, kinda like putting EFI on your flathead Ford engine (no kidding, it's done). -- snip more -- With a 6.3 V filament arrangement, that would be on the hairy edge of regulating at -10% AC line voltage. Filter capacitor needs to be high enough for only 5% ripple and rectifier diodes can't be much more than 0.7 V forward drop to fit the minimum 7806 regulator 2 V head- room. I would use a new LDO regulator, if I used one at all. On thinking about it after my post, it occurred to me that for the filaments I would be happy with a well-filtered DC supply to keep the filament hum from modulating my RF. If all you have is a 6.3V supply you need really low-drop diodes, however, and your filament transformer's RMS current will go up no matter what. Note: The original AC filament voltage was made within certain (seldom specified) limits; if that is on the low side also, the 7806 may poop out during droops in ripple voltage. 7906s were originally made in the series for negative voltage regulation, also good for providing a minus bias voltage. Tube filaments for indirectly-heated cathodes don't care about DC polarity. Slightly-lower than nominal voltage will improve heater life. But can decrease cathode life if the cathode current is high -- even oxide cathodes don't like being temperature starved. Check some of the warnings and recommended start up procedures for 10kW transmitter tubes for some _really_ tight cathode care-and-feeding instructions. -- snip even more -- |
In article , "Tim Wescott"
writes: "Avery Fineman" wrote in message ... In article , "Tim Wescott" writes: Your solid state devices will either be really light-weight or will have significantly higher capacitances than the tubes, so it would be very difficult to get your oscillator to track. -- snip -- On checking the HV supply of this little receiver, the OB2 strike voltage had increased beyond manufacturer's spec so it did not always strike and start regulating with power-on. The lone OB2 in spares, also 40 years old, was just the opposite, striking below spec. Antique Electronics Supply has good replacement tubes -- but solid-state regulators for old tube equipment is more fun, kinda like putting EFI on your flathead Ford engine (no kidding, it's done). There still exist vacuum tubes off someone's shelf even though the prices for small receiving-type tubes are $5 to $12 each (with any sort of guarantee of performance).. It was a bit of a shock to find those old, seemingly trustworthy OB2s to be quite off specification, especially with the strike voltage off on the high side. The receiver I was restoring was my own design and it depended on the shunt regulator to help the stability of two local oscillators that were L-C tuned, not quartz. In the early 1960s and the conception of this receiver, regulation of B+ (as we quaintly referred to HV then, heh heh) was only for expensive things such as high-quality test instruments. A shunt regulator is fine for critical things like oscillator stability but it does waste power which is converted to heat, variable heat depending on AC power line stability. Changes in local heating (within the cabilnet, chassis) means another random variable to throw off oscillator stability. Series regulators waste less heat (general statement, not an absolute) and can hold the HV more constant over a wider current demand range. I had accomplished a fairly sensitive and tuning-stable SW receiver for my father using only 6 tubes for active devices, solid-state rectifiers, and a single gas shunt regulator. Filament power was a mere 7.6 W total; a common 5Y3 rectifier needs 10 W for its filament by itself (2 A at 5 V). Internal heating was minimal and there was no such nonsense as "waiting for it to warm up," to reach some thermal equilibrium above room temperature. That "warm up wait" doesn't assure anything about internal radio stabiity since room temperature may be variable and the internal power dissipation varies with everything from AC line voltage to plate current variation from AGC stages changing with signal strength. Tube series regulators waste power (and thus dissipate heat) because the cathode-plate voltage is relatively high to stay within a linear operating range. NPN transistors are more forgiving for series regulation work since their emitter-collector voltage can be lower for the same series current. Two NPNs in my prototype series regulator (plus one zener) waste only 5 W for at least 2% voltage stability over AC line (I can tweak that to at least 1$), An OB2 shunt regulator wastes 1.9 W alone and the total dissipation from all in the old supply is about 8 W. I'm already ahead of the game in reducing wasted heat by going to transistors and zeners. -- snip more -- With a 6.3 V filament arrangement, that would be on the hairy edge of regulating at -10% AC line voltage. Filter capacitor needs to be high enough for only 5% ripple and rectifier diodes can't be much more than 0.7 V forward drop to fit the minimum 7806 regulator 2 V head- room. I would use a new LDO regulator, if I used one at all. On thinking about it after my post, it occurred to me that for the filaments I would be happy with a well-filtered DC supply to keep the filament hum from modulating my RF. If all you have is a 6.3V supply you need really low-drop diodes, however, and your filament transformer's RMS current will go up no matter what. I had NO "hum in the RF" to begin with...the filament regulation is an afterthought since varying filament voltage in small receiving tubes used as oscillators will result in varying fixed-tuned frequency. How much variation depends on a whole bunch of other factors of the oscillator circuit design. Using a variable autotransformer for AC mains adjustment may lead to (sometimes) shocking discovery that 6.3 V filaments do NOT stay at 6.3 when the AC mains varies +/- 10%. Frequency counter measurement of the LOs in my case showed some drift with AC mains voltage change that could only be accounted for by the filament voltage changing. The 7800s (positive) and 7900s (negative) all-in-one series regulators are very common. Their minimum voltage drop for regulation specs is only 2 VDC for most in each series. Minimum extra space, minimum components added. Using a negative output and regulator yields a stable bias supply which can be applied to other circuits or additions. The 7800s and 7900s are legacy devices and still in production. Note: The original AC filament voltage was made within certain (seldom specified) limits; if that is on the low side also, the 7806 may poop out during droops in ripple voltage. 7906s were originally made in the series for negative voltage regulation, also good for providing a minus bias voltage. Tube filaments for indirectly-heated cathodes don't care about DC polarity. Slightly-lower than nominal voltage will improve heater life. But can decrease cathode life if the cathode current is high -- even oxide cathodes don't like being temperature starved. Check some of the warnings and recommended start up procedures for 10kW transmitter tubes for some _really_ tight cathode care-and-feeding instructions. MOST homebrew projects involve lower power. Running 6.3 V NOMINAL filaments at 6.0 V isn't going to degrade anything. In my case it has 12.6 V NOMINAL filament voltage reduced to 12.0 V exact. Filament voltages are always given as "nominal" since the indirectly-heated tubes almost always get their filament voltage from transformers. Since ordinary transformers don't regulate, the percentage change of secondary winding voltage is the same as primary winding voltage change....for most of North America that is 115 VAC RMS +/-10 V or about +/-10%. A 6.3 V filament-heater can thus range from 5.7 V to 6.9 V just from the line. Reduction in the high-temperature thermionic devices' voltage will result in longer operating life. That was true over a half century ago. A friend of mine into model railroading has been running lots and lots of #327 28 V rated aircraft indicator lamps in his track layout, all running at about 18 VAC. NO outages yet due to filament failure in what must be about 3 dozen or so used for 35+ years. Still quite bright enough in ordinary room lighting. I prefer solid-state all around, including LEDs. I'm keeping most of the vacuum guts of this little receiver still in vacuum tube style just for sentiment's sake. Everyone's mileage varies. Len Anderson retired (from regular hours) electronic engineer person |
In article , "Tim Wescott"
writes: "Avery Fineman" wrote in message ... In article , "Tim Wescott" writes: Your solid state devices will either be really light-weight or will have significantly higher capacitances than the tubes, so it would be very difficult to get your oscillator to track. -- snip -- On checking the HV supply of this little receiver, the OB2 strike voltage had increased beyond manufacturer's spec so it did not always strike and start regulating with power-on. The lone OB2 in spares, also 40 years old, was just the opposite, striking below spec. Antique Electronics Supply has good replacement tubes -- but solid-state regulators for old tube equipment is more fun, kinda like putting EFI on your flathead Ford engine (no kidding, it's done). There still exist vacuum tubes off someone's shelf even though the prices for small receiving-type tubes are $5 to $12 each (with any sort of guarantee of performance).. It was a bit of a shock to find those old, seemingly trustworthy OB2s to be quite off specification, especially with the strike voltage off on the high side. The receiver I was restoring was my own design and it depended on the shunt regulator to help the stability of two local oscillators that were L-C tuned, not quartz. In the early 1960s and the conception of this receiver, regulation of B+ (as we quaintly referred to HV then, heh heh) was only for expensive things such as high-quality test instruments. A shunt regulator is fine for critical things like oscillator stability but it does waste power which is converted to heat, variable heat depending on AC power line stability. Changes in local heating (within the cabilnet, chassis) means another random variable to throw off oscillator stability. Series regulators waste less heat (general statement, not an absolute) and can hold the HV more constant over a wider current demand range. I had accomplished a fairly sensitive and tuning-stable SW receiver for my father using only 6 tubes for active devices, solid-state rectifiers, and a single gas shunt regulator. Filament power was a mere 7.6 W total; a common 5Y3 rectifier needs 10 W for its filament by itself (2 A at 5 V). Internal heating was minimal and there was no such nonsense as "waiting for it to warm up," to reach some thermal equilibrium above room temperature. That "warm up wait" doesn't assure anything about internal radio stabiity since room temperature may be variable and the internal power dissipation varies with everything from AC line voltage to plate current variation from AGC stages changing with signal strength. Tube series regulators waste power (and thus dissipate heat) because the cathode-plate voltage is relatively high to stay within a linear operating range. NPN transistors are more forgiving for series regulation work since their emitter-collector voltage can be lower for the same series current. Two NPNs in my prototype series regulator (plus one zener) waste only 5 W for at least 2% voltage stability over AC line (I can tweak that to at least 1$), An OB2 shunt regulator wastes 1.9 W alone and the total dissipation from all in the old supply is about 8 W. I'm already ahead of the game in reducing wasted heat by going to transistors and zeners. -- snip more -- With a 6.3 V filament arrangement, that would be on the hairy edge of regulating at -10% AC line voltage. Filter capacitor needs to be high enough for only 5% ripple and rectifier diodes can't be much more than 0.7 V forward drop to fit the minimum 7806 regulator 2 V head- room. I would use a new LDO regulator, if I used one at all. On thinking about it after my post, it occurred to me that for the filaments I would be happy with a well-filtered DC supply to keep the filament hum from modulating my RF. If all you have is a 6.3V supply you need really low-drop diodes, however, and your filament transformer's RMS current will go up no matter what. I had NO "hum in the RF" to begin with...the filament regulation is an afterthought since varying filament voltage in small receiving tubes used as oscillators will result in varying fixed-tuned frequency. How much variation depends on a whole bunch of other factors of the oscillator circuit design. Using a variable autotransformer for AC mains adjustment may lead to (sometimes) shocking discovery that 6.3 V filaments do NOT stay at 6.3 when the AC mains varies +/- 10%. Frequency counter measurement of the LOs in my case showed some drift with AC mains voltage change that could only be accounted for by the filament voltage changing. The 7800s (positive) and 7900s (negative) all-in-one series regulators are very common. Their minimum voltage drop for regulation specs is only 2 VDC for most in each series. Minimum extra space, minimum components added. Using a negative output and regulator yields a stable bias supply which can be applied to other circuits or additions. The 7800s and 7900s are legacy devices and still in production. Note: The original AC filament voltage was made within certain (seldom specified) limits; if that is on the low side also, the 7806 may poop out during droops in ripple voltage. 7906s were originally made in the series for negative voltage regulation, also good for providing a minus bias voltage. Tube filaments for indirectly-heated cathodes don't care about DC polarity. Slightly-lower than nominal voltage will improve heater life. But can decrease cathode life if the cathode current is high -- even oxide cathodes don't like being temperature starved. Check some of the warnings and recommended start up procedures for 10kW transmitter tubes for some _really_ tight cathode care-and-feeding instructions. MOST homebrew projects involve lower power. Running 6.3 V NOMINAL filaments at 6.0 V isn't going to degrade anything. In my case it has 12.6 V NOMINAL filament voltage reduced to 12.0 V exact. Filament voltages are always given as "nominal" since the indirectly-heated tubes almost always get their filament voltage from transformers. Since ordinary transformers don't regulate, the percentage change of secondary winding voltage is the same as primary winding voltage change....for most of North America that is 115 VAC RMS +/-10 V or about +/-10%. A 6.3 V filament-heater can thus range from 5.7 V to 6.9 V just from the line. Reduction in the high-temperature thermionic devices' voltage will result in longer operating life. That was true over a half century ago. A friend of mine into model railroading has been running lots and lots of #327 28 V rated aircraft indicator lamps in his track layout, all running at about 18 VAC. NO outages yet due to filament failure in what must be about 3 dozen or so used for 35+ years. Still quite bright enough in ordinary room lighting. I prefer solid-state all around, including LEDs. I'm keeping most of the vacuum guts of this little receiver still in vacuum tube style just for sentiment's sake. Everyone's mileage varies. Len Anderson retired (from regular hours) electronic engineer person |
I have an old Heath SG-6 signal generator. It uses two triodes.
I'd like to convert it to solid state. Bill- Others have supplied more good suggestions than I thought were possible, but I couldn't resist adding to it. With lower voltage solid state circuits, you probably will need additional amplification to provide the same output you now have. Keeping the output level constant over the entire range could be a challenge. I seem to recall that there are high voltage FETs available that might be candidates for direct substitution for the tubes. Keeping in mind that a reactive load could stress the FET at double the supply voltage, you might have to reduce B+ slightly. The suggestion to regulate B+ seems a good idea, but perhaps you only need to do it for the oscillator triode(s). I once replaced a 105 volt VR tube in a Heath HW-100, with a ten watt zener. It was a reverse-polarity diode, so the stud mounted directly to the chassis. That approach might be easier than regulating higher current for the entire circuit. If you try the approach of keeping the tubes with a regulated DC filament, I wonder if a vacuum tube would be susceptible to the same "filament notching" that panel lamps experience? According to a report I read many years ago, it seems that lamps operating from DC fail sooner on average than lamps operating from AC. Failure analysis showed a notch in the filament develops only on the DC lamps, with failure occuring at that point. 73, Fred, K4DII |
I have an old Heath SG-6 signal generator. It uses two triodes.
I'd like to convert it to solid state. Bill- Others have supplied more good suggestions than I thought were possible, but I couldn't resist adding to it. With lower voltage solid state circuits, you probably will need additional amplification to provide the same output you now have. Keeping the output level constant over the entire range could be a challenge. I seem to recall that there are high voltage FETs available that might be candidates for direct substitution for the tubes. Keeping in mind that a reactive load could stress the FET at double the supply voltage, you might have to reduce B+ slightly. The suggestion to regulate B+ seems a good idea, but perhaps you only need to do it for the oscillator triode(s). I once replaced a 105 volt VR tube in a Heath HW-100, with a ten watt zener. It was a reverse-polarity diode, so the stud mounted directly to the chassis. That approach might be easier than regulating higher current for the entire circuit. If you try the approach of keeping the tubes with a regulated DC filament, I wonder if a vacuum tube would be susceptible to the same "filament notching" that panel lamps experience? According to a report I read many years ago, it seems that lamps operating from DC fail sooner on average than lamps operating from AC. Failure analysis showed a notch in the filament develops only on the DC lamps, with failure occuring at that point. 73, Fred, K4DII |
Knight Kit made a solid state sig. generator model KG-686. Maybe you could get some ideas from that. Jim Bill Meara wrote in message om... I have an old Heath SG-6 signal generator. It uses two triodes. I'd like to convert it to solid state. The switched coils, varible cap and dial mechanism are very nice. Any suggestions on what kind of oscillator circuit I should use? I imagine the difficult part will be coming up with a feedback circuit that will cover the 160 kHz to 50 Mhz. Any ideas? Has anyone seen any articles on this kind of conversion? By going solid state and adding a few buffer stages I'm hoping I could make this thing a lot more stable. 73 Bill M0HBR CU2JL N2CQR http://planeta.clix.pt/n2cqr |
Knight Kit made a solid state sig. generator model KG-686. Maybe you could get some ideas from that. Jim Bill Meara wrote in message om... I have an old Heath SG-6 signal generator. It uses two triodes. I'd like to convert it to solid state. The switched coils, varible cap and dial mechanism are very nice. Any suggestions on what kind of oscillator circuit I should use? I imagine the difficult part will be coming up with a feedback circuit that will cover the 160 kHz to 50 Mhz. Any ideas? Has anyone seen any articles on this kind of conversion? By going solid state and adding a few buffer stages I'm hoping I could make this thing a lot more stable. 73 Bill M0HBR CU2JL N2CQR http://planeta.clix.pt/n2cqr |
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