Who says that you have to do phase splitting interstage? A nice balanced input transformer will split phase with lower distortion than any tube stage and give you galvanic isolation and common mode noise rejection as a bonus. Operating the tranny at 1-2V levels characteristic of amp inputs means that you can use a small, high quality unit like the Jensen JT11-P1.
Whole 'nother world. I sure didn't expect anything like that. 
I thought that xfmrs in the signal path was something to get away from
I thought that xfmrs in the signal path was something to get away from
Miles Prower said:Whole 'nother world. I sure didn't expect anything like that.
I thought that xfmrs in the signal path was something to get away from
I'm sure there's no shortage of transformers out there you'd want to get away from. But in my experience, the right transformer in the right impelementation gives outstanding results.
Of course they can't do it all on their own. Being passive devices they offer no power gain so for that they need to be mated with active devices of some sort. But hey, nothing's perfect. 🙂
se
Hey Miles! I have been doing a few tube amps in the last year, after doing "sand-state" for a long time. I don't use expensive transformers yet, but I prefer them over capacitors now! I now prefer to use a transformer between the single-ended driver and the PP triode output. And NO feedback.... or coupling caps! If you have the transformers, try them!
Somehow, the Dark Side has convinced everyone that transformers are evil.... but they are far more musical than caps!
Somehow, the Dark Side has convinced everyone that transformers are evil.... but they are far more musical than caps!
> a "soft start" sequencer
No. Over-volted guitar amps should have a standby switch. Between sets, you switch to standby, which removes the B+ from the tubes while leaving the heaters on (heaters never burn out). If "take five" turns into "take 25" the bar owner will scream the band back on stage: flicking the standby switch brings the amp up instantly. Also, standby kills any buzz from an unattended amp (though modern jukeboxes mask amp buzz). And finally: always start the amp in standby and give it 10-60 seconds before you go to ON so the cathodes hot-up before the HV comes on.
> makes sense not to hit the cold heaters with the full-on heater voltage. No sense blowing them unnecessarily.
You can put 60V on a 6V heater. Not for 10,000 hours, but for a very long time. It glows like a lightbulb, not even a short-life lightbulb. Cold-banging heaters is NOT a problem.
> There is no such thing as "too much iron"
To a guitarist, there is "too much iron". You may play Podunk on wednesday, Cowtown thursday, Lodi Liquors on friday night, and saturday is the Emerald Ballroom (upstairs, and the elevator hasn't worked since 1953). Lug a lot of iron around every night and you'll be in no shape to play.
There is one Fender with a grid transformer. This gives three tubes total for 10mV sensitivity and tone-control too: a 12AX7 and two 6V6/6AQ5s. Adding a tube splitter would require more power transformer for the heater, so the weight may be a wash, especially since this is a very low-performing amp and a pretty lame grid transformer.
> OPT that looks like it has quite a generous amount of core. Bigger than some power xfmrs.
HiFi output iron should be bigger than the amp's power transformer: HiFi power bandwidth should go somewhat below 50Hz, and power iron runs 50/60Hz. Also power iron can run 30% distortion (it sucks an ugly waveform from the utility company) while output iron has to run lower power density for decent THD.
However guitar output transformers are rarely as big as the power iron. By definition, they won't be fed anything below 82Hz (low note on a guitar). And (because of that upstairs gig) the speaker system is rarely good even that low. And since Fender guitar traditionally works with a Fender Bass and a bass-drum, the guitarist rarely goes for that lowest 82Hz note. So 80-120Hz power bandwidth is ample. Because the speaker drops-off, it is musically useful to have rising THD on the lowest notes: the distortion spectrum implies a fundamental that isn't actually strong.
> I thought that xfmrs in the signal path was something to get away from
In guitar amps: the sound isn't the problem, but you don't want a lot of excess iron to carry.
The real problem: if an amp is too heavy, some day you will grab a little transistor amp for a small unimportant gig. And not get booed off the stage. Next afternoon, the little sandy-amp is still in the trunk, the good tube amp is upstairs over the garage, and you go to another gig with the sandy-amp. Pretty soon you do all your gigs with sand, and Music has taken another step downhill into the future.
Of course if this is for Permanent Installation, or you have the luxury of a roadie-crew, weight may not matter. Few guitarists reach such nirvanas.
> input transformer will split phase
Guitars are lower than line-level. Typical output is 100mV, but amp full-up sensitivity has to be 10mV for light fingering or for gross overdrive. The pickup tends to be 2K-20K impedance, but as little as 100K load can affect the tone; 470K is a typical input impedance. Winding iron so it will not load a guitar is rough (Jensen has some DI transformers that do it, though at large step-down ratio). And building the complete amp, input to 6L6s/7027s, push-pull, including tone controls, would be mind-twisting and a lot of small tubes to feed. Also you WANT some even-order distortion: guitar played through low-THD systems is "thin". A skilled player works the distortion curve in ways similar to how a singer varies vocal timbre. The difference between sung notes medium or loud is not so much the strength of the fundamental (it changes little), but the relative strengths of the overtones. The string-plucker has some control of overtones with plucking style, but the soft amp adds an additional color tool. Most traditional musical instruments can be sorted into either even- or odd-order harmonic structure; the electric guitar can be played either way. (Yes, I suppose we could build a balanced amp and short-out one side of one stage to get even-order on demand...)
No. Over-volted guitar amps should have a standby switch. Between sets, you switch to standby, which removes the B+ from the tubes while leaving the heaters on (heaters never burn out). If "take five" turns into "take 25" the bar owner will scream the band back on stage: flicking the standby switch brings the amp up instantly. Also, standby kills any buzz from an unattended amp (though modern jukeboxes mask amp buzz). And finally: always start the amp in standby and give it 10-60 seconds before you go to ON so the cathodes hot-up before the HV comes on.
> makes sense not to hit the cold heaters with the full-on heater voltage. No sense blowing them unnecessarily.
You can put 60V on a 6V heater. Not for 10,000 hours, but for a very long time. It glows like a lightbulb, not even a short-life lightbulb. Cold-banging heaters is NOT a problem.
> There is no such thing as "too much iron"
To a guitarist, there is "too much iron". You may play Podunk on wednesday, Cowtown thursday, Lodi Liquors on friday night, and saturday is the Emerald Ballroom (upstairs, and the elevator hasn't worked since 1953). Lug a lot of iron around every night and you'll be in no shape to play.
There is one Fender with a grid transformer. This gives three tubes total for 10mV sensitivity and tone-control too: a 12AX7 and two 6V6/6AQ5s. Adding a tube splitter would require more power transformer for the heater, so the weight may be a wash, especially since this is a very low-performing amp and a pretty lame grid transformer.
> OPT that looks like it has quite a generous amount of core. Bigger than some power xfmrs.
HiFi output iron should be bigger than the amp's power transformer: HiFi power bandwidth should go somewhat below 50Hz, and power iron runs 50/60Hz. Also power iron can run 30% distortion (it sucks an ugly waveform from the utility company) while output iron has to run lower power density for decent THD.
However guitar output transformers are rarely as big as the power iron. By definition, they won't be fed anything below 82Hz (low note on a guitar). And (because of that upstairs gig) the speaker system is rarely good even that low. And since Fender guitar traditionally works with a Fender Bass and a bass-drum, the guitarist rarely goes for that lowest 82Hz note. So 80-120Hz power bandwidth is ample. Because the speaker drops-off, it is musically useful to have rising THD on the lowest notes: the distortion spectrum implies a fundamental that isn't actually strong.
> I thought that xfmrs in the signal path was something to get away from
In guitar amps: the sound isn't the problem, but you don't want a lot of excess iron to carry.
The real problem: if an amp is too heavy, some day you will grab a little transistor amp for a small unimportant gig. And not get booed off the stage. Next afternoon, the little sandy-amp is still in the trunk, the good tube amp is upstairs over the garage, and you go to another gig with the sandy-amp. Pretty soon you do all your gigs with sand, and Music has taken another step downhill into the future.
Of course if this is for Permanent Installation, or you have the luxury of a roadie-crew, weight may not matter. Few guitarists reach such nirvanas.
> input transformer will split phase
Guitars are lower than line-level. Typical output is 100mV, but amp full-up sensitivity has to be 10mV for light fingering or for gross overdrive. The pickup tends to be 2K-20K impedance, but as little as 100K load can affect the tone; 470K is a typical input impedance. Winding iron so it will not load a guitar is rough (Jensen has some DI transformers that do it, though at large step-down ratio). And building the complete amp, input to 6L6s/7027s, push-pull, including tone controls, would be mind-twisting and a lot of small tubes to feed. Also you WANT some even-order distortion: guitar played through low-THD systems is "thin". A skilled player works the distortion curve in ways similar to how a singer varies vocal timbre. The difference between sung notes medium or loud is not so much the strength of the fundamental (it changes little), but the relative strengths of the overtones. The string-plucker has some control of overtones with plucking style, but the soft amp adds an additional color tool. Most traditional musical instruments can be sorted into either even- or odd-order harmonic structure; the electric guitar can be played either way. (Yes, I suppose we could build a balanced amp and short-out one side of one stage to get even-order on demand...)
a "soft start" sequencer
> makes sense not to hit the cold heaters with the full-on heater voltage. No sense blowing them unnecessarily.
You can put 60V on a 6V heater. Not for 10,000 hours, but for a very long time. It glows like a lightbulb, not even a short-life lightbulb. Cold-banging heaters is NOT a problem.
If that's true, then how is it that max legal ham rigs include such a soft start sequencer? Although it shouldn't be a big problem to include a circuit that would automatically turn on the heaters, then connect the HV. Especially since I'm going solid state in the PS.
HiFi output iron should be bigger than the amp's power transformer: HiFi power bandwidth should go somewhat below 50Hz, and power iron runs 50/60Hz. Also power iron can run 30% distortion (it sucks an ugly waveform from the utility company) while output iron has to run lower power density for decent THD.
That's what I'm shooting for: HiFi, since I can'tuse a guitar amp.
> makes sense not to hit the cold heaters with the full-on heater voltage. No sense blowing them unnecessarily.
You can put 60V on a 6V heater. Not for 10,000 hours, but for a very long time. It glows like a lightbulb, not even a short-life lightbulb. Cold-banging heaters is NOT a problem.
If that's true, then how is it that max legal ham rigs include such a soft start sequencer? Although it shouldn't be a big problem to include a circuit that would automatically turn on the heaters, then connect the HV. Especially since I'm going solid state in the PS.
HiFi output iron should be bigger than the amp's power transformer: HiFi power bandwidth should go somewhat below 50Hz, and power iron runs 50/60Hz. Also power iron can run 30% distortion (it sucks an ugly waveform from the utility company) while output iron has to run lower power density for decent THD.
That's what I'm shooting for: HiFi, since I can'tuse a guitar amp.
Steve Eddy said:
This weekend, I hope. I'm just finishing drawings- I'm not as fast with schematic drawing as you.
Miles Prower said:Whole 'nother world. I sure didn't expect anything like that.
I thought that xfmrs in the signal path was something to get away from
Modern transformers from Lundahl, Cinemag, Jensen, etc., are not your grandfather's transformers. They are also a simpler and more elegant way to solving a problem...if not the cheapest way. On the other hand, if you're lucky, you can cop UTC or ADC transformers for less money off of ebay. But, if you're silly, you can be suckered into bidding wars that are pretty common for classic vintage transformers.
With Lundahl, you also have KK Audio's forum, where Kevin Carter can offer good advice concerning which transformer works best under different circumstances.
PRR said:
Now that's exactly what I wanted to ask about - how does running the heater without B+ affect tube life?
> then how is it that max legal ham rigs include such a soft start sequencer?
Are we doing receiving tubes or transmitting tubes?
Specifically: are we working 500V or 1,500V?
Above about 500V, secondary electrons have enough velocity to bombard the cathode hard enough to destroy oxide coatings. So we can't use rich-emission oxides. We have to switch to thoriated tungsten, which is not as liberal as oxide and can be "stripped" in a few seconds running at high current while below rated temperature.
The thread started with mention of 807s, which are oxide-cathode and routinely cold-started. If your thoughts have drifted to something bigger, you should find out what care and treatment is required.
Thoriated, and especially pure tungsten (used at even higher voltages) are much more fussy than oxide. With just tungsten, you have to hold it between too-low emission and melt-down, something like 1,800 degrees. 1,700 and the gain/output drops, 1,900 and the filament evaporates in a few hours. A dose of thorium really increases emission, so it can run a little cooler, but too-cool and the thorium won't seep-up from the bulk tungsten. But mess with the rare-earth oxides and you can get gobs of emission at quite low temperature, like 1,000 degrees. (My books are upstairs so these numbers may be wrong, and I don't recall what flavor degrees these numbers are...) Oxide cathodes are pretty un-critical of abuse, heater voltage or cold-start surge, so long as you don't have 1,000V electrons flying around.
Ham radio also often turns the transmitter off frequently for better reception, while requiring instant-on, so a tricky power controller is needed for basic operation.
how does running the heater without B+ affect tube life?
For receiving tubes with oxide-coated cathodes: hardly at all. Small tubes should last more than 10,000 hours; I have 12AT7s with 100,000 hours on them. You might think that it is like an incandescent lamp, but oxide-coated cathodes run so VERY cool that leaving them on is not going to shorten their lives. I've replaced many-many over-worked power tubes, but heater-burnout is a very rare thing. I've seen open heaters about three times, and in two of those cases I knew the gear had been physically abused (one thrown into a deep dumpster) which can fracture heaters.
Are we doing receiving tubes or transmitting tubes?
Specifically: are we working 500V or 1,500V?
Above about 500V, secondary electrons have enough velocity to bombard the cathode hard enough to destroy oxide coatings. So we can't use rich-emission oxides. We have to switch to thoriated tungsten, which is not as liberal as oxide and can be "stripped" in a few seconds running at high current while below rated temperature.
The thread started with mention of 807s, which are oxide-cathode and routinely cold-started. If your thoughts have drifted to something bigger, you should find out what care and treatment is required.
Thoriated, and especially pure tungsten (used at even higher voltages) are much more fussy than oxide. With just tungsten, you have to hold it between too-low emission and melt-down, something like 1,800 degrees. 1,700 and the gain/output drops, 1,900 and the filament evaporates in a few hours. A dose of thorium really increases emission, so it can run a little cooler, but too-cool and the thorium won't seep-up from the bulk tungsten. But mess with the rare-earth oxides and you can get gobs of emission at quite low temperature, like 1,000 degrees. (My books are upstairs so these numbers may be wrong, and I don't recall what flavor degrees these numbers are...) Oxide cathodes are pretty un-critical of abuse, heater voltage or cold-start surge, so long as you don't have 1,000V electrons flying around.
Ham radio also often turns the transmitter off frequently for better reception, while requiring instant-on, so a tricky power controller is needed for basic operation.
how does running the heater without B+ affect tube life?
For receiving tubes with oxide-coated cathodes: hardly at all. Small tubes should last more than 10,000 hours; I have 12AT7s with 100,000 hours on them. You might think that it is like an incandescent lamp, but oxide-coated cathodes run so VERY cool that leaving them on is not going to shorten their lives. I've replaced many-many over-worked power tubes, but heater-burnout is a very rare thing. I've seen open heaters about three times, and in two of those cases I knew the gear had been physically abused (one thrown into a deep dumpster) which can fracture heaters.
The thread started with mention of 807s, which are oxide-cathode and routinely cold-started. If your thoughts have drifted to something bigger, you should find out what care and treatment is required.
No, still going with 807s. OK, that takes care of that problem, and uncomplicates the power supply considerably. The operating voltage isn't going to be that high, and I doubt I'll be needing anything more powerful than the 807s.
No, still going with 807s. OK, that takes care of that problem, and uncomplicates the power supply considerably. The operating voltage isn't going to be that high, and I doubt I'll be needing anything more powerful than the 807s.
PRR said:
Yes, I am aware that hetaer failures in small tubes are mostly for mechanical causes. Thanks for the answer - i am designing an input stage for a hybrid and was thinking about a 'standby' heater supply. I also expect that without B+ and actual emission, the current/voltage on the heater becomes fairly uncritical - for instance, standby could run heaters at half current? I'm trying to avoid having the main power transformer on at all times, I do have a stand-by supply available but it cannot run the heaters at full voltage. For actual heating, as I'm using P (series heater supply) tubes, I have to make a current source anyway (so no problem re cold start), it just runs out of voltage headroom at standby.
I realise it is easy to get -365V from a venter tapped power stransformer, but since you already have a CCS in the first stage, and a negative power rail for it, why not use a CCS in the second stage? Unless you have a whole lot of HV caps lying around, it is likely to cost less and work better 😉
Never heard SS that outperforms Tubes
I would use a 6P5GT driving a Sylvania 6SN7WGT phase splitter driving a quad set of Push-Pall triode connected 807s operating at 400 volts DC. Output is about 25 to 30 watts of the best sounding audio you may have ever heard.
Input sensitivity is about 4 volts or great for a line amp or preamp.
I would use a 6P5GT driving a Sylvania 6SN7WGT phase splitter driving a quad set of Push-Pall triode connected 807s operating at 400 volts DC. Output is about 25 to 30 watts of the best sounding audio you may have ever heard.
Input sensitivity is about 4 volts or great for a line amp or preamp.
> as I'm using P (series heater supply) tubes, I have to make a current source anyway
You do not have to run "series" heaters on constant-current!
What the "series" rating means is: you CAN run them in a situation where voltage is not well defined, current is well defined.
Early tubes were heated with fixed-voltage sources: lead batteries or wall-power transformers. The heaters were rated for a specific voltage, but the current demand could vary all over the place. It would be similar to what the data said (so you knew how big a battery or transformer to get) but could be +/-30% depending what kind of heater-wire was handy.
Then (in the US) came "AC-DC" radios with series-string heaters. Standard was three 12V 0.15A small tubes, 35V 0.15A output, 50V 0.15A rectifier. If all five tubes had their rated voltage/current ratios, the voltages would divide properly. But say the three little tubes actually draw (at rated voltage) 0.12A, the two big tubes happened to draw 0.17A. Since the big tubes drop most of the voltage, the current is going to be high. The little tubes are over-heated.
Actually, heater current was controlled enough that these radios usually worked. Also positive temperature coefficient tends to equalize things a bit. And in fact the 35V and 50V tubes were made FOR series-string use, and the makers knew they had to pass current similar to their 12V small tubes. And all these tubes are fairly similar construction.
But now came TV sets, and the RCA low-price chassis with series-string. You have a small-cathode sync-split and a fat-cathode H-sweep tube. Say one is built with thin heater, the other with thick heater. And remember that a heater's cold resistance is a LOT lower than its hot resistance. At switch-on, large current flows, falling off as the heaters get hot. If the biggest tube has a slow-to-heat heater, current flow will stay large long after the small thin-heater tube is hot and high resistance. Its voltage and heater power can soar far above nominal until the big tube warms up.
So RCA (and everybody) developed "controlled warm-up" heaters. They all heat-up (heater resistance rise) at about the same rate, so no tube gets over-heated while the others are still warming. And of course the rated current is tightly controlled. So you can mix and match any of these tubes (of the same heater current) in a series string, and be OK.
> no problem re cold start
Same actual heater, same problem either way. It isn't a problem. It is much less of a problem than hot-starting incandescent lamps: lamps run white-hot with about a 10:1 ratio of hot/cold resistance, tubes run just-red and only (roughly) 3:1 or 4:1 ratio.
Feeding heaters with limited current "will" reduce start-up surge. Also prolong warm-up. And I don't think heater start-up is a real problem. If the heater is so fragile that it is going to snap from 3X rated current for a few seconds, then it is going to snap soon anyway.
Tubes and especially heaters are a lot more robust than many people today seem to think. In the Old Days, we never pampered them like this. And tubes were not cheap then! Excepting the exotic/faddish types, tubes are generally cheaper today, allowing for inflation, than they were in the 1950s. 12SL7 is the same dollar-price now as I paid in 1970, when gasoline was $0.35/gallon and a pack of cigarettes was $0.50-$0.75. A burnt-out tube meant many packs/gallons out of the budget, but still tube-failures were not such a problem that we thought of fetherbedding them.
> was thinking about a 'standby' heater supply.
Why?
It isn't really good to leave a cathode "half hot" for a long time. It should either be full-hot so thorium can migrate to the surface as fast as it evaporates, or stone-cold to keep the thorium locked into the cathode coating. It may not really matter on oxide cathodes.
We used to do that too. Who is old enough to recall "Instant On" TV sets? Tube TV sets took 10 to 30 seconds to warm-up and show a picture. "Instant On" killed the B+ and ran the heaters on half-wave AC. This kept them cooler but not cold when "off". When you switched-on the B+ and applied full-wave AC, you got a picture in 2 to 10 seconds. This was a Big Deal.
But, the picture wasn't all that good for the first minute. Tubes really do have to come to full temperature. Keeping them half-alive only shortens that period a few seconds. Grids and plates and their surfaces have to warm-up from full cathode heat plus hot electrons banging into them.
If you have a real need for "Instant On", consider just leaving the tubes hot all the time. Low-level tubes as used in preamps live forever, if you run them conservatively. My VTVM drifts, and I used it irregularly every day for years: its 12AU7 has been hot for 20 years. At one point I wondered about that and swapped in a low-miles 12AU7: works the same, except the "old" one drifted less so I put it back. I'm pretty demanding of that VTVM, its tube has to be "good enough for audio". It runs the 12AU7 at about 60V 3mA 180mW, well within 12AU7 ratings but not atypical for audio. (Some TV tuner tubes turn out to NOT be able to run their ratings for long periods: in TV Tuner use, almost all watchable signal are above AGC level which reduces tuner-current; the rating is for the weak-signal noise-level and nobody wants to watch a lot of buried-on-noise TV.)
You do not have to run "series" heaters on constant-current!
What the "series" rating means is: you CAN run them in a situation where voltage is not well defined, current is well defined.
Early tubes were heated with fixed-voltage sources: lead batteries or wall-power transformers. The heaters were rated for a specific voltage, but the current demand could vary all over the place. It would be similar to what the data said (so you knew how big a battery or transformer to get) but could be +/-30% depending what kind of heater-wire was handy.
Then (in the US) came "AC-DC" radios with series-string heaters. Standard was three 12V 0.15A small tubes, 35V 0.15A output, 50V 0.15A rectifier. If all five tubes had their rated voltage/current ratios, the voltages would divide properly. But say the three little tubes actually draw (at rated voltage) 0.12A, the two big tubes happened to draw 0.17A. Since the big tubes drop most of the voltage, the current is going to be high. The little tubes are over-heated.
Actually, heater current was controlled enough that these radios usually worked. Also positive temperature coefficient tends to equalize things a bit. And in fact the 35V and 50V tubes were made FOR series-string use, and the makers knew they had to pass current similar to their 12V small tubes. And all these tubes are fairly similar construction.
But now came TV sets, and the RCA low-price chassis with series-string. You have a small-cathode sync-split and a fat-cathode H-sweep tube. Say one is built with thin heater, the other with thick heater. And remember that a heater's cold resistance is a LOT lower than its hot resistance. At switch-on, large current flows, falling off as the heaters get hot. If the biggest tube has a slow-to-heat heater, current flow will stay large long after the small thin-heater tube is hot and high resistance. Its voltage and heater power can soar far above nominal until the big tube warms up.
So RCA (and everybody) developed "controlled warm-up" heaters. They all heat-up (heater resistance rise) at about the same rate, so no tube gets over-heated while the others are still warming. And of course the rated current is tightly controlled. So you can mix and match any of these tubes (of the same heater current) in a series string, and be OK.
> no problem re cold start
Same actual heater, same problem either way. It isn't a problem. It is much less of a problem than hot-starting incandescent lamps: lamps run white-hot with about a 10:1 ratio of hot/cold resistance, tubes run just-red and only (roughly) 3:1 or 4:1 ratio.
Feeding heaters with limited current "will" reduce start-up surge. Also prolong warm-up. And I don't think heater start-up is a real problem. If the heater is so fragile that it is going to snap from 3X rated current for a few seconds, then it is going to snap soon anyway.
Tubes and especially heaters are a lot more robust than many people today seem to think. In the Old Days, we never pampered them like this. And tubes were not cheap then! Excepting the exotic/faddish types, tubes are generally cheaper today, allowing for inflation, than they were in the 1950s. 12SL7 is the same dollar-price now as I paid in 1970, when gasoline was $0.35/gallon and a pack of cigarettes was $0.50-$0.75. A burnt-out tube meant many packs/gallons out of the budget, but still tube-failures were not such a problem that we thought of fetherbedding them.
> was thinking about a 'standby' heater supply.
Why?
It isn't really good to leave a cathode "half hot" for a long time. It should either be full-hot so thorium can migrate to the surface as fast as it evaporates, or stone-cold to keep the thorium locked into the cathode coating. It may not really matter on oxide cathodes.
We used to do that too. Who is old enough to recall "Instant On" TV sets? Tube TV sets took 10 to 30 seconds to warm-up and show a picture. "Instant On" killed the B+ and ran the heaters on half-wave AC. This kept them cooler but not cold when "off". When you switched-on the B+ and applied full-wave AC, you got a picture in 2 to 10 seconds. This was a Big Deal.
But, the picture wasn't all that good for the first minute. Tubes really do have to come to full temperature. Keeping them half-alive only shortens that period a few seconds. Grids and plates and their surfaces have to warm-up from full cathode heat plus hot electrons banging into them.
If you have a real need for "Instant On", consider just leaving the tubes hot all the time. Low-level tubes as used in preamps live forever, if you run them conservatively. My VTVM drifts, and I used it irregularly every day for years: its 12AU7 has been hot for 20 years. At one point I wondered about that and swapped in a low-miles 12AU7: works the same, except the "old" one drifted less so I put it back. I'm pretty demanding of that VTVM, its tube has to be "good enough for audio". It runs the 12AU7 at about 60V 3mA 180mW, well within 12AU7 ratings but not atypical for audio. (Some TV tuner tubes turn out to NOT be able to run their ratings for long periods: in TV Tuner use, almost all watchable signal are above AGC level which reduces tuner-current; the rating is for the weak-signal noise-level and nobody wants to watch a lot of buried-on-noise TV.)
- Status
- Not open for further replies.