That will teach you to bring a hand truck next time you go to a flea market. Leave it back in the car, just in case you need it. The extra $15 would have been worth another mile walk.
At least you didn’t fall that many feet this time and were able to get up and walk away. Rebuild that table with PT. Just what you need - something else you’ve got to DO instead of actually working on your project.
At least you didn’t fall that many feet this time and were able to get up and walk away. Rebuild that table with PT. Just what you need - something else you’ve got to DO instead of actually working on your project.
The flea market trip that found the guitar amp was not planned. My wife and mother in law were going to the flea market and I was at work. I got a call from the flea market with a simple question that resulted in me skipping out of work for a few hours. I was driving a 2 door Dodge Neon at the time and the amp would not fit in the car and stuck out of the bungie corded trunk, so I had to take it home before returning to work.
The picnic table's fate is indetermined at this time. Part of me wants to drag it around to the burn pit and watch it DIE! The other part of me says if the rain stops I'll need something to place the cabinet on so I can finish the paint job. I know better than to sit on the other bench though.
The picnic table's fate is indetermined at this time. Part of me wants to drag it around to the burn pit and watch it DIE! The other part of me says if the rain stops I'll need something to place the cabinet on so I can finish the paint job. I know better than to sit on the other bench though.
Throw this baby in between the two channels - but made with tubes, of course. Should fatten things up - maybe sounds like one channels speaker is a couple meters over there, but its from the same box. Wouldnt even need the dual pot, just switch in/out with a thin/fat labeled control. That's my story - I'd bet that the different voicing between channels was enough to cause some phase delay, leading to the unique sound.
We got the "42nd electronics show and swap meet" this Saturday; I'm going to try to go. Doubtful I'd find anything, like I might have 20 years ago, but its fun to look. Went to a going out of business give away, at a local electronics repair place, only brought home a couple of cardboard boxes and some acoustic foam I'll use for packing. Guy said business was good actually, he just lost his techs with - apparently - no one around here to replace them. An analog understanding of what's wrong with something is a very rare commodity. As are analog engineers...
We got the "42nd electronics show and swap meet" this Saturday; I'm going to try to go. Doubtful I'd find anything, like I might have 20 years ago, but its fun to look. Went to a going out of business give away, at a local electronics repair place, only brought home a couple of cardboard boxes and some acoustic foam I'll use for packing. Guy said business was good actually, he just lost his techs with - apparently - no one around here to replace them. An analog understanding of what's wrong with something is a very rare commodity. As are analog engineers...
Things that do that deserve to die… I just had the satisfaction of smashing an electric chain saw to death on the concrete driveway. Chain wouldn’t stay properly tensioned anymore for more than 30 seconds at a time (and come off track of course). Leaked oil as fast as I could put it in anyway. There’s a good gas one at the other site, but I had the opportunity to clear out some of this recent growth before it gets out of hand.
You can find something else to put the cab on to paint it. Milk crates are good. Id be building a new table with all pressure treated and throwing the cheap crap on the fire. Youve thrown horribly failed speaker cabs on the fire before, right?
Now calmed down, gotta sit in on a meeting at work, which is finally starting.
Among others. I had thrifted an OEM packaged belt drive TT that...just wouldnt run at speed, no matter how many belts I tried. 60 Hz strobed the motor with a neon lamp; that little set screw came around to a rock solid fixed position every rotation.
We have a high deck with a concrete driveway below - great for cracking the seams on speaker cabinets to break them down. Mr TT was rendered broom worthy after just one toss...for harming my wallet 3 times, to no avail.
I did manage to sell the dust cover, which I removed before sending it on its way out of my further consideration, beyond clean up.
I built lots of speaker cabinets many years ago, but haven't built any in the past 10 years until I started tinkering with these guitar cabinets. It's too soon to tell if any of the recent construction projects deserve to meet the burn pit.
I have come to the frustration point with some of my electronic projects occasionally. These usually get sentenced to "death by power supply." I cover the stuff that could explode with a piece of Lexan, connect the circuit up to my 650 volt 1.7 amp power supply and hit the switch.
Back during the Hundred Buck Amp Challenge I made a little test amp that used some old radio tubes. It worked, but didn't make as much power as I thought it should, and just didn't sound that great. Now, the 50C5 output tubes have a maximum plate voltage rating of 135 volts and I had already pushed them to 200 volts and didn't get any satisfaction. At this time I was using my old Fluke 407D power supply which goes to 555 volts, so I clipped the 250 volt power supply bypass cap out of the circuit, turned the power supply to max and flipped the HV-ON switch expecting a nuclear fireball. Again, no satisfaction, I couldn't even blow this thing up! Both output tubes were in the bright red glow mode, so I was going to let them die a slow melting death. I heard a faint hum in the speaker, which is odd since there is no AC voltage anywhere in this amp......could it be from the still plugged in guitar preamp?
Yes, not only was it still working, it flippin ROCKED. OK, stay of execution GRANTED. I dialed back the power supply until the ability to rock waned, which was somewhere above 300 volts, then started turning it back up. 335 volts is easy to make from an isolation transformer, so that's where I stopped. Who knew that 50C5 tubes could eat 340 volts and live....just keep the screens around 120 volts. That board got tossed in the box of dead circuits when I left Florida. I pulled it out of that box once I had set up a new lab here and fired it up again. It still worked, but I quickly lost interest.
I'm always looking for a way to make something good for low $$$. The 50C5 tube fits lots of old radios that are still being treasured by some collectors, so the 50C5 tubes have risen in price as the supply dwindles. The current price is about $10 each. Every time I am in the Orlando Florida area I visit Stan at ESRC. I stopped in at his place in late December of 2019 during a trip that involved taking a Girl Scout Troop to Disney World oner the week between Christmas and New Years. A discussion led me to a list of tube that Stan had LOTS of. He had over 10,000 50B5 tubes. The 50B5 is the same tube as the 50C5 except for the pinout. It seems that UL's new creepage rules were violated by the 50B5, so it went extinct overnight and the 50C5 took its place. Stan also had lots of 26HU5's. Since my van was already stuffed with all the Girl Scout stuff, I grabbed a box full of 26HU5's and 10 50B5's. I also stashed some more 26HU5's and 50B5's in his warehouse for later purchase pending some testing.
Once I got back home, I lit up some of the 26HU5's and confirmed that they will indeed make big power. They were easy to test since they drop into a 26LW6 socket with no changes. I decided to dig out the old 50C5 board and rewire the sockets for 50B5's and replace a few crispy looking resistors. I tested it, then ripped the output stages up and converted them to UNSET wiring. The 12AU6 driver - mosfet PI - push pull 50B5 in UNSET configuration test board cranked out 20 watts on 340 volts for 12 hours straight without even a hint of redness.
Unfortunately, Stan has unexpectedly passed away before I ever talked to him again. The 50B5 UNSET test board went back into the box of dead circuits........until about 2 weeks ago. It is now working again and is being connected to my 2 X 12AX7 guitar preamp for some testing.
I have come to the frustration point with some of my electronic projects occasionally. These usually get sentenced to "death by power supply." I cover the stuff that could explode with a piece of Lexan, connect the circuit up to my 650 volt 1.7 amp power supply and hit the switch.
Back during the Hundred Buck Amp Challenge I made a little test amp that used some old radio tubes. It worked, but didn't make as much power as I thought it should, and just didn't sound that great. Now, the 50C5 output tubes have a maximum plate voltage rating of 135 volts and I had already pushed them to 200 volts and didn't get any satisfaction. At this time I was using my old Fluke 407D power supply which goes to 555 volts, so I clipped the 250 volt power supply bypass cap out of the circuit, turned the power supply to max and flipped the HV-ON switch expecting a nuclear fireball. Again, no satisfaction, I couldn't even blow this thing up! Both output tubes were in the bright red glow mode, so I was going to let them die a slow melting death. I heard a faint hum in the speaker, which is odd since there is no AC voltage anywhere in this amp......could it be from the still plugged in guitar preamp?
Yes, not only was it still working, it flippin ROCKED. OK, stay of execution GRANTED. I dialed back the power supply until the ability to rock waned, which was somewhere above 300 volts, then started turning it back up. 335 volts is easy to make from an isolation transformer, so that's where I stopped. Who knew that 50C5 tubes could eat 340 volts and live....just keep the screens around 120 volts. That board got tossed in the box of dead circuits when I left Florida. I pulled it out of that box once I had set up a new lab here and fired it up again. It still worked, but I quickly lost interest.
I'm always looking for a way to make something good for low $$$. The 50C5 tube fits lots of old radios that are still being treasured by some collectors, so the 50C5 tubes have risen in price as the supply dwindles. The current price is about $10 each. Every time I am in the Orlando Florida area I visit Stan at ESRC. I stopped in at his place in late December of 2019 during a trip that involved taking a Girl Scout Troop to Disney World oner the week between Christmas and New Years. A discussion led me to a list of tube that Stan had LOTS of. He had over 10,000 50B5 tubes. The 50B5 is the same tube as the 50C5 except for the pinout. It seems that UL's new creepage rules were violated by the 50B5, so it went extinct overnight and the 50C5 took its place. Stan also had lots of 26HU5's. Since my van was already stuffed with all the Girl Scout stuff, I grabbed a box full of 26HU5's and 10 50B5's. I also stashed some more 26HU5's and 50B5's in his warehouse for later purchase pending some testing.
Once I got back home, I lit up some of the 26HU5's and confirmed that they will indeed make big power. They were easy to test since they drop into a 26LW6 socket with no changes. I decided to dig out the old 50C5 board and rewire the sockets for 50B5's and replace a few crispy looking resistors. I tested it, then ripped the output stages up and converted them to UNSET wiring. The 12AU6 driver - mosfet PI - push pull 50B5 in UNSET configuration test board cranked out 20 watts on 340 volts for 12 hours straight without even a hint of redness.
Unfortunately, Stan has unexpectedly passed away before I ever talked to him again. The 50B5 UNSET test board went back into the box of dead circuits........until about 2 weeks ago. It is now working again and is being connected to my 2 X 12AX7 guitar preamp for some testing.
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To hell with a power supply - frustrating electronics get plugged straight into 120. Including a video card I had nothing but trouble with, and speakers that are NFG. Don’t bother covering up potentially exploding parts - just stay at the other end of a 100 foot extension cord.
If a speaker cab is headed for the bonfire I’ll usually know in about 10 minutes.
If a 50C5 is really a 6W6 derivative stuffed into a little bottle it stands to reason it can take some voltage. Till the 7 pin socket arcs over. Let it collect enough dust and add some moisture I bet it would. Clean its probably fine, but I’ve seen the insides of those radios get pretty grungy and they still work at 110 on the plates. Keeping the screen at 120 is just treating it like a sweep tube. As long as you can get the required plate currents….
If a speaker cab is headed for the bonfire I’ll usually know in about 10 minutes.
If a 50C5 is really a 6W6 derivative stuffed into a little bottle it stands to reason it can take some voltage. Till the 7 pin socket arcs over. Let it collect enough dust and add some moisture I bet it would. Clean its probably fine, but I’ve seen the insides of those radios get pretty grungy and they still work at 110 on the plates. Keeping the screen at 120 is just treating it like a sweep tube. As long as you can get the required plate currents….
Sorry to read about your friend. Sounds like your head is doing OK via that last post! It's quite a challenge, like trying to beat the Chinese at it, with their "Monoprice" tube combos. Another challenge is to build something no one has thought of, but that's hard too as everyone has thought of just about everything there is, that one can do.
Watched a YT video where this fellow went hunting in Goodwill for things that can distort a guitar signal. He came back with a tape deck and an old RS mixer, showed how they sounded cranked to 10 with his guitar. Apparently he didnt know about the old RS telephone handset amplifier with the suction cup pickup. I guess finding one of those in a GW is rare these days.
I've decided to hold back on applying my 4 channel tube amp for guitar. I'd rather build a FRFR cabinet using cheap class D amps with DSP - and put the computer / studio rack in a box - or a 12AX7 preamp - ahead of something like that, architecture wise. Now, if you could make the single person luggable FRFR sound like it's 16 ft wide and 8" tall, that would be something I havent seen done - yet!
The power supply that I have is an old style 60 Hz switcher. The feedback loop is very slow, especially since it contains 1000 uF of capacitance internally across the output terminals. The current limiter will kick in AFTER the short that triggered it no longer exists. Shortly after that the control loop will overcompensate with a huge voltage overshoot.
Let's just say one of my first, and probably the most popular of my "inventions" in high school electronics class was the kamikaze cord AKA the parts tester. Wall plug on one end alligator clips on the other end. All the parts it tested were bad. I bought a box full of these 470 uF 50 V caps at a hamfest in the late 70's for a project that I eventually made a dozen or more of. Unfortunately the caps turned out to be crap and I had to recall the units and replace them all. The rest became remote controlled firecrackers. The pin spacing just happens to match a US wall outlet or extension cord. I found a few in a box about a month ago.
Another one of my inventions was the damper tube.....not the kind you find in a TV set, but the kind that erupts like a volcano spewing hot water and steam! WTF?
Take any old tube of the 7 pin, 9 pin, or Compactron variety, or anything with the vacuum seal at the top. Hold it under water in the bathroom sink and snip the seal off with a pair of wire cutters. The vacuum in the tube will suck water inside filling up the tube. Bring it back into the classroom, plug it into the tube tester and turn the heater setting to about 2 X rated, and all the other knobs up to max. Mount Tubulus will erupt shortly. Sneak one into somebody's project and they are in for a surprise. Stick a couple into the tube testers at closing time in the Olson Electronics store, after the power has been shut off, and the boss will get a surprise when he makes the morning rounds.
Look at the typical operation and the plate curves and judge for yourself. A 6AQ5 is a 7 pin 6V6 with reduced maximum specs. Note the 1100 volt maximum peak plate voltage rating on the 7 pin 6AQ5 for TV vertical sweep use. The 50B5 has the same pinout as the 6AQ5, but somehow the sockets arced over in a 115 volt radio from plate to heater but lived OK in a TV set. UL demanded a change, so the plate was moved to pin 7 and the 50B5 became the 50C5. The older octal radios used a 50L6 tube which IS a 6W6 with a 50 volt heater.
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Speaking of 50V heaters, wonder what you could do with a pair of 7695's? Not a buck a tube but not too bad.
tubesandmore.com/products/vacuum-tube-7695-beam-power-amplifier
https://tubedata.wernull.com/sheets/168/7/7695.pdf
tubesandmore.com/products/vacuum-tube-7695-beam-power-amplifier
https://tubedata.wernull.com/sheets/168/7/7695.pdf
I have been waffling back and forth on the series string heaters. Wiring all the heaters of an amp in series and running them from the secondary of an isolation transformer, while developing +170 and +340 volts of screen and B+ voltage from that same secondary definitely cuts the cost of the power transformer in half, or more. It also makes the choice of tubes a lot more restrictive. Most of the 50 volt tubes need 150 mA of heater current, while most if not all of the high Gm TV IF amp pentodes need 300 mA or more, so these can not simply be wired in series.
I have exactly one 7695 tube, and I have never applied power to it. The plate inside it looks like a 6W6 with fins, so it can dissipate more heat than the 6W6. There is also a 50FE5 tube that looks like the same tube in an octal base. I don't have any at the moment.
For now, I am experimenting with the old board containing 50B5's because it is mostly built and working. I may build a one off guitar amp from whet it becomes, or it may go nowhere. I plan on building myself "the last guitar amp" which is sill in the thinking and dreaming stage. A one off for myself can use whatever tube I like, as long as I can collect enough of them to outlast me. Anything that may be duplicated needs to use tubes that are available. Most of the current tube vendors will not tell me how many of any given tube they have in stock. Sometimes I can guess by asking "how many do I have to buy to get them for X$" ( a number that's considerably less than their published price). I had conputer access to Stan's inventory list and as of Jan 2020 he only had 3 of the 7695 tubes. At the same time he had over 15000 50B5's and sold 16 of them in 2019. I was about to buy a couple hundred for 50 cents each.
I have exactly one 7695 tube, and I have never applied power to it. The plate inside it looks like a 6W6 with fins, so it can dissipate more heat than the 6W6. There is also a 50FE5 tube that looks like the same tube in an octal base. I don't have any at the moment.
For now, I am experimenting with the old board containing 50B5's because it is mostly built and working. I may build a one off guitar amp from whet it becomes, or it may go nowhere. I plan on building myself "the last guitar amp" which is sill in the thinking and dreaming stage. A one off for myself can use whatever tube I like, as long as I can collect enough of them to outlast me. Anything that may be duplicated needs to use tubes that are available. Most of the current tube vendors will not tell me how many of any given tube they have in stock. Sometimes I can guess by asking "how many do I have to buy to get them for X$" ( a number that's considerably less than their published price). I had conputer access to Stan's inventory list and as of Jan 2020 he only had 3 of the 7695 tubes. At the same time he had over 15000 50B5's and sold 16 of them in 2019. I was about to buy a couple hundred for 50 cents each.
How about the old tool motor speed control / dimmer scheme? Essentially pulse width modulate the AC cycle. I have a Dremel outboard "speed control" that seems to work just fine backing off the rated wattage of a heater plate under my wife's plant starts - like the tube heater, a resistive load obviously. I assume it could easily - as a scheme - run a 50V tube heater off 120.
I looked inside and there's hardly any components to it; the DIAC, TRIAC; a cap and a couple resistors. A second arrangement could run 3-4 12V heaters in series off the same isolation secondary. A parallel connection with no differing heater current worries.
Unsure of the cost of such parts, but with the plethora of common wall mount dimmers around, one would think easily salvaged. Just replace the control with a fixed resistor after dialing it in.
I looked inside and there's hardly any components to it; the DIAC, TRIAC; a cap and a couple resistors. A second arrangement could run 3-4 12V heaters in series off the same isolation secondary. A parallel connection with no differing heater current worries.
Unsure of the cost of such parts, but with the plethora of common wall mount dimmers around, one would think easily salvaged. Just replace the control with a fixed resistor after dialing it in.
Yes, it's possible to use a triac based device to regulate the heat energy in a tube heater, and the parts are cheap. Unfortunately the abruptly turned on pulses of energy created by the triac will radiate harmonics of the line frequency all throughout your circuit. been there tried that.
It is also possible to use just a capacitor in series with the tube heaters. The capacitive reactance works somewhat like a resistor without most of the heat dissipation to reduce the energy applied to the tube heater. The drawback here is that you are relying on a clean sine wave on your AC line. Usually it's close enough to work, but a distorted line voltage will put more heat in the heater.
Either of these methods can cause unhappiness in the isolation transformer. Most iso transformers are minimalist designs intended to work at their rated VA into a purely resistive load with the operating temperature near their max spec. Construction is such that they will buzz and get warm if misloaded. The venerable Triad N-68X will support a 50 VA resistive load without much bother. Use it to power my 4 tube 4 watt amp with about 12 watts of resistive heaters and about 15 watts of DC load from a full wave diode bridge, and it runs at about the same temp with a slight buzz on some transformers. It is powering the 50B5 amp seen in post #127 which runs 18 watts of heaters and a full wave voltage doubler to make 340 volts on my 126 volt line power. It does NOT like doing this and I used a 100 VA transformer for the overnight test. Any kind of asymmetrical (half wave) load can cause saturation effects which can fry the transformer even when lightly loaded.
One can use math to determine the amount of reactance / resistance to drop some heater voltage, but that only works on a clean sine wave. I use an IR thermomenter to adjust the circuit for the same heat in the tubes as pure DC.
The big old 1KW 600 volt 1.7 amp power supply seen on my workbench uses a triac "dimmer" circuit on the primary side of a HUGE power transformer to regulate the output voltage. This works, but there is a LOT of filtering on both the input and output of that supply, including a CLC filter on its output with 1000 uF of capacitance. Run that supply into a heavy load at about 300 volt of DC output and it buzzes like a chain saw and shakes the whole workbench!
It is also possible to use just a capacitor in series with the tube heaters. The capacitive reactance works somewhat like a resistor without most of the heat dissipation to reduce the energy applied to the tube heater. The drawback here is that you are relying on a clean sine wave on your AC line. Usually it's close enough to work, but a distorted line voltage will put more heat in the heater.
Either of these methods can cause unhappiness in the isolation transformer. Most iso transformers are minimalist designs intended to work at their rated VA into a purely resistive load with the operating temperature near their max spec. Construction is such that they will buzz and get warm if misloaded. The venerable Triad N-68X will support a 50 VA resistive load without much bother. Use it to power my 4 tube 4 watt amp with about 12 watts of resistive heaters and about 15 watts of DC load from a full wave diode bridge, and it runs at about the same temp with a slight buzz on some transformers. It is powering the 50B5 amp seen in post #127 which runs 18 watts of heaters and a full wave voltage doubler to make 340 volts on my 126 volt line power. It does NOT like doing this and I used a 100 VA transformer for the overnight test. Any kind of asymmetrical (half wave) load can cause saturation effects which can fry the transformer even when lightly loaded.
One can use math to determine the amount of reactance / resistance to drop some heater voltage, but that only works on a clean sine wave. I use an IR thermomenter to adjust the circuit for the same heat in the tubes as pure DC.
The big old 1KW 600 volt 1.7 amp power supply seen on my workbench uses a triac "dimmer" circuit on the primary side of a HUGE power transformer to regulate the output voltage. This works, but there is a LOT of filtering on both the input and output of that supply, including a CLC filter on its output with 1000 uF of capacitance. Run that supply into a heavy load at about 300 volt of DC output and it buzzes like a chain saw and shakes the whole workbench!
I thought you already settled on the metal 6L6 and 6V6 for The Last Guitar Amp. People who will only buy new production or Audio Only NOS through regular tube vendors or music stores can still build one - with the glass versions.
Then again, if you build with 50C5-ish tubes, some form of them is generally available. When 50C5s get rare and go through the roof a 12W6 could be dropped in the circuit - with a different heater supply.
Its not only triac-controlled EI cores that will do that. Anything where there is a lot of internal stress can make a bunch of racket. Try listening to a big multi kilowatt iron pig amplifier playing music at war volume into a dummy load. The toroid in my CA 18’s barely makes a buzz sitting there idle. At war volume, you’d think it was going to shake its way out of the rack. Generally you won’t hear this, but in testing where the power isn’t going into a speaker you do. Smaller toroids do this to some extent, but it seems the bigger they go the louder they will get at full current through the windings.
Then again, if you build with 50C5-ish tubes, some form of them is generally available. When 50C5s get rare and go through the roof a 12W6 could be dropped in the circuit - with a different heater supply.
Its not only triac-controlled EI cores that will do that. Anything where there is a lot of internal stress can make a bunch of racket. Try listening to a big multi kilowatt iron pig amplifier playing music at war volume into a dummy load. The toroid in my CA 18’s barely makes a buzz sitting there idle. At war volume, you’d think it was going to shake its way out of the rack. Generally you won’t hear this, but in testing where the power isn’t going into a speaker you do. Smaller toroids do this to some extent, but it seems the bigger they go the louder they will get at full current through the windings.
I have decided that I will build an all metal tube amp with technology that existed in the early 50's. it will not be the Last Guitar Amp, since I'm still trying to figure out what that is.
The tubes used in old AA5 radios have not been made in a long time. With a few exceptions, they are not available in large quantities and the price will keep climbing as the supplies shrink. The 50B5 is an exception since they were regulated out of existence almost overnight. The last tube radio lineup went to a 100 mA heater string to reduce heat (the old plastics got brittle and crumbled). As the 100 mA lineup started ramping up, RCA invented the transistor that ran from line voltage without a transformer, and poof those tubes had no home. They have been, and still are on the dollar menu since the dollar menu cane into existence. By a van load and they are 35 cents each. My problem with them is that the wimpy heater won't let me squeeze more than 10 watts from a pair of the 32ET5's without melting them.
Many years ago, I stumbled across a "honey, I shrunk the tubes" TV sweep tube that's smaller than a 6V6GT but will give me 80 watts per pair without turning red. I asked my usual question to Stan and walked away with 300 of them for $125. There were still 640 of them in his inventory when he passed.
TV sweep tubes tend to have a different, harsher tone when they slam into clipping than say, a 6V6 or even a 6L6 in a guitar amp application. I will be experimenting with a setup that can run multiple different pairs of output tubes for different sonic signatures. That seems to be the rage now in $4000 boutique guitar amps, and even some that are becoming more mainstream.
That may all become moot if variable ratio UNSET works out in real world cranked guitar applications.
The tubes used in old AA5 radios have not been made in a long time. With a few exceptions, they are not available in large quantities and the price will keep climbing as the supplies shrink. The 50B5 is an exception since they were regulated out of existence almost overnight. The last tube radio lineup went to a 100 mA heater string to reduce heat (the old plastics got brittle and crumbled). As the 100 mA lineup started ramping up, RCA invented the transistor that ran from line voltage without a transformer, and poof those tubes had no home. They have been, and still are on the dollar menu since the dollar menu cane into existence. By a van load and they are 35 cents each. My problem with them is that the wimpy heater won't let me squeeze more than 10 watts from a pair of the 32ET5's without melting them.
Many years ago, I stumbled across a "honey, I shrunk the tubes" TV sweep tube that's smaller than a 6V6GT but will give me 80 watts per pair without turning red. I asked my usual question to Stan and walked away with 300 of them for $125. There were still 640 of them in his inventory when he passed.
TV sweep tubes tend to have a different, harsher tone when they slam into clipping than say, a 6V6 or even a 6L6 in a guitar amp application. I will be experimenting with a setup that can run multiple different pairs of output tubes for different sonic signatures. That seems to be the rage now in $4000 boutique guitar amps, and even some that are becoming more mainstream.
That may all become moot if variable ratio UNSET works out in real world cranked guitar applications.
Only thing I can think of is a PFC circuit, which would only work for the B+ part, I assume something like that is far beyond the scope of a realistic investment for the project. Even with the tranny being happy to have sine wave current load, you're still stuck with the tube selection and matching up the heater string voltage and current.
I was hoping a variable heater power could lead to a cathode emission control, where a user could play around with it, perhaps changing the sound of the amp in a musically useful way. It was a simple discovery on the tube tester that I could make a bad tube test good with a somewhat hotter cathode.
Some here say "you cant do that"; ok, permanently, overnight, or for the duration of a particular song? I still recall the 12AX7 in the series strung, off line stereo amp my father bought from Lafayette, those little filaments going up into the cathode would burn white every time you turned it on; 100s of times I assume in the years it played, without that tube ever failing.
I truly have no idea what such a control would do sound wise, as a result of varying the transconductance of the tubes. Perhaps just a dirty way to shift around the operating point of all the preamp tubes at once? A Brown-Orange-Yellow control?
Yeah, the PFC circuit would cost more than using a slightly bigger transformer.
While most kids my age were pumping gas or flipping burgers for employment while still in high school, I worked in a TV repair shop fixing TV sets. In a time long before flat screens and any LED's except red, All TV sets had a "picture tube" AKA the Cathode Ray Tube. Like any tube it will wear out, but unlike most tubes these things cost about half as much as a new TV, and that's for a rebuilt one. Yes, shops specialized in rebuilding color CRTs. When the emission started getting low, you attached a "booster" between the socket and the CRT to squeeze a bit more life out of it. The booster did exactly that. It had a small transformer inside that stepped the 6.3 volt heater voltage up to about 8 volts. Often these things could get another 6 months to a year out of the CRT.
I know that accidentally killing heater power in an output tube that's being run hard can initiate a tube arc as the cathode cools.
Somewhere on one of my old hard drives I have / had a whitepaper from Bendix detailing the life expectancy of some of the premium "Red Bank" military tubes. It seems that running the heater outside the +/- 5% range they specified shortened the life of the tube. One side was worse than the other, and caused failure by a different mechanism, but I don't remember the details. Bits and pieces of this paper are in some of their tube specs. The data sheet for these Hi-Rel tubes have exactly the same warning about heater voltage, but none go into any details. Those are the ony Bendix tubes that I have played with that I could remember the type numbers. Yes, the 6384 makes for a blow proof 6L6GC with some socket rewiring. The 7403 has some really nice triode curves that go to 850 volts, and half the 6094's I had were bad so I never tried them in anything.
Some European 12AX7's flash brightly at first turn on, and as you say, they don't die.
For reasons that I don't fully understand the pair of 12AX7's in my ADA MP1 guitar preamp are run off of 8 volts DC. See schematic. Next time I have my guitar preamp board hooked up I'll plug the heater power cable into a variable DC supply to see how much difference it makes.
While most kids my age were pumping gas or flipping burgers for employment while still in high school, I worked in a TV repair shop fixing TV sets. In a time long before flat screens and any LED's except red, All TV sets had a "picture tube" AKA the Cathode Ray Tube. Like any tube it will wear out, but unlike most tubes these things cost about half as much as a new TV, and that's for a rebuilt one. Yes, shops specialized in rebuilding color CRTs. When the emission started getting low, you attached a "booster" between the socket and the CRT to squeeze a bit more life out of it. The booster did exactly that. It had a small transformer inside that stepped the 6.3 volt heater voltage up to about 8 volts. Often these things could get another 6 months to a year out of the CRT.
I know that accidentally killing heater power in an output tube that's being run hard can initiate a tube arc as the cathode cools.
Somewhere on one of my old hard drives I have / had a whitepaper from Bendix detailing the life expectancy of some of the premium "Red Bank" military tubes. It seems that running the heater outside the +/- 5% range they specified shortened the life of the tube. One side was worse than the other, and caused failure by a different mechanism, but I don't remember the details. Bits and pieces of this paper are in some of their tube specs. The data sheet for these Hi-Rel tubes have exactly the same warning about heater voltage, but none go into any details. Those are the ony Bendix tubes that I have played with that I could remember the type numbers. Yes, the 6384 makes for a blow proof 6L6GC with some socket rewiring. The 7403 has some really nice triode curves that go to 850 volts, and half the 6094's I had were bad so I never tried them in anything.
Some European 12AX7's flash brightly at first turn on, and as you say, they don't die.
For reasons that I don't fully understand the pair of 12AX7's in my ADA MP1 guitar preamp are run off of 8 volts DC. See schematic. Next time I have my guitar preamp board hooked up I'll plug the heater power cable into a variable DC supply to see how much difference it makes.
Attachments
While wasting time randomly following links around the internet I found a site full of good vacuum tube guitar amp information:
The output tube in (mild) red glow mode got my attention.
https://robrobinette.com/Amp_Stuff.htm
The output tube in (mild) red glow mode got my attention.
https://robrobinette.com/Amp_Stuff.htm