First off, most car radios are way better made than most table top radios in both sensitivity and selectivity.
Second, your building may have some kind of metal in it that shields the signal.
External antennas usually help.
There is a vanishingly small chance that I have a few 12AX7's in my junk box but I can't guarrantee they're not microphonic. If you have 6V6s you can swap in 6L6s without any problem, but not the other way around. The 6L6s are are built to take higher voltages... although I did run an old tube amp with 6V6s instead of the required 6L6s - I put a 4" fan next to the tubes and hoped they didn't fry.
"[...] most car radios are way better made than most table top radios in both sensitivity and selectivity."
#blink# Huh. I had no idea. I guess it makes some sense in that car radios are going to move in and out of good reception areas, and pass through places were signals overlap, so they've got a tougher problem to solve, but I suppose I just expected that anything they could design into a car radio in that regard might as well get tossed into a tabletop radio where you don't have to squeeze everything into a box that fits in that little space in the dashboard. So I guess the design features that make car radios more sensitive & selective must add significantly to the cost then?
Argh. I have enough trouble understanding electronics without tossing economics into the mix!
"[...] building may have some kind of metal in it [...]"
Unlikely, but I can't claim to know for sure. 1867 row house, brick exterior walls and everything else wood as far as I can tell.
"The 6L6s are are built to take higher voltages"
Ah, that explains why they have such similar part numbers. I'd wondered just how different they were, since I always saw them used the same way as each other in guitar amps.
But I just went downstairs and grabbed the rear panel of the stereo (which I never screwed back on just so I could grab it easily the next time I wanted to look) and it turns out I misremembered the power amplifier tubes. It looks like the amplifier is two 12AX7 and two 6HU8 (one each per side). There's a box with a 6AQ8, a 6BA6, and a 6BE6 in it along with a picture of what I think is supposed to be the tuner mechanism (I don't have a schematic, just a Tube Layout Diagram). Elsewhere there's a 6AU6 and a 6HS6 near components labelled "Ratio Detector" and "FM Limiter", and another 12AX7 and 6BA6 off by themselves (how many parts are there in a tube phonograph-preamp circuit circa 1962?).
Hmm. Some of those do fall into the 6[A-Z]*6 pattern. I should borrow a tube manual from madbodger and try to learn what the letters mean.
I've got no idea which is more likely -- a bad 12AX7 or a bad 6HU8 -- but the reason I think it's in the main amplifier section is that crackling in a speaker is the only symptom and it happens whether there's any real signal coming in or not (e.g. when I turn the selector to "Tape" or "Phono" and provide no input there)
I know 12AX7s are still pretty easy to get ahold of, even if I don't have the budget to order them -- or at least they were the last time I got a Musician's Friend catalog, which, come to think of it, has been a while. I don't know whether any of these others are still being made, or only available when someone uncovers a pallette of dusty new-old-stock. (Hmm ... lemme see if Sovtek has a web site ... the last time I glanced at the tube market was around the time Sovtek had a factory fire or something, and people were still saying to stay away from Chinese tubes back then. Have the Chinese gotten better, or do I still want to stick to Russian -- or ancient American -- tubes?)
What causes a tube to become microphonic? I'm familiar with the failure mode, but don't know the mechanism of it.
Car radios are way more expensive because of the added selectivity, sensitivity and small size.
Tube numbers can be damn near identical and be very different inside. The 6*6 is an execption to the rule.
I'm guessing that the most likely tube failing is the 12AX7. The 6HU8 is probably a driver for the power tubes while the 12AX7 (sometimes 12AT7s) are preamplifiers. The crackle is often some kind of minor voltage fluctuation that would be picked up more by the preamp tubes.
If the crackle is on one channel you might want to just swap the 12AX7s around and see if the crackle changes channels. That may give you an idea of who's the culpret.
As why tubes turn microphonic - I really don't remember.
The next time I have a chance to plug the beast in, I'll try swapping tubes from channel to channel. I can't remember whether I did that the last time I was poking at it or not.
Yeah, car radios tend to be quite good, and therefore disproportionately expensive (when new, since they "come with" a car, usually). Since new cars usually contain radios, used car radios are available cheaply. I have a box of several -- you're welcome to some if you'd like. Just feed hook up an antenna and speakers and feed it 12 volts, and you've got a quite nice little stereo. I've seem car radios built into speakers (along with 12 volt power supplies, sometimes) as portable systems. Works a treat.
6L6 tubes first appeared in 1936, and are arguably the tube in longest continuous production (still made today, in at least four countries). Numerous variants appeared, many using the same pinout (7AC). Sometime in history, beam tubes (like the 6L6) and real pentodes (like the EL34) got lumped together, and more recent tube manuals give all of 'em the 7AC pinout (the original pentode version was 6S). I have a little amplifier (currently out on loan) that will accept any of the 7AC/6S type tubes (tho the high/low voltage switch can only be set to "low" for some of them). It's great to sound out the different tubes (and there are a lot of them).
As for your tube lineup, your amp is a bit of an oddball. The 6HU8s are dual pentodes, and are the output tubes. These are a bit of an oddball tube, and there aren't many substitutes. They were probably used for cost reasons, as two 6HU8s are going to be less expensive than a quartet of 6BQ5/EL84 output tubes. Unfortunately, I don't have any in stock, and they're $40 apiece tubes today. Most of the other tubes I do have, or have suitable substitutes.
Yes, tube numbering does have patterns, but it's not completely regular (of course). The numbering system that applies to these tubes (variously known as RMA or RETMA) goes essentially like this: the first number is the approximate filament voltage (usually, there are an assortment of interesting exceptions). The 6 tubes expect 6.3 volts. The last number is (approximately) the number of elements in the tube. The letters in the middle are assigned serially. So a 12AX7 wants 12.6 volts (though the tapped heater can also be run on half that, by wiring the sections in parallel), has 7 elements (heater, two cathodes, two grids, two plates), and is around the 40th such. The 12AY7 was assigned right after it, presumably.
The failure modes for the different kinds of tubes vary (tho any tube can fail any way it wants). The low power tubes like 12AX7 have small, closely spaced elements, and tend to be plagued by microphonics (loose bits) and shorts. Power tubes such as the 6HU8 are more robustly constructed, and thereby are subject to different ills, such as cathodes wearing out and gassiness. The crackling sound is likely either a bad resistor, leaky capacitor (or dirty path on a tube socket), or a noisy tube. This is handy, as these are all cheap and easy fixes using parts I have lying around. Those power tubes would be another matter.
As for tube sources, NOS tubes are generally better made than modern ones. However, tubes are much more variable than most people realize, and just getting a few samples and being willing to tweak your circuit will let you get pretty much any sound you want. Russian tubes are being made at several factories, and various events have made it tricky to find out which brands are made at what factories. Sovtek tubes tend to come from the Reflector factory, and are fairly reliable, but have a lot of consistency problems. Svetlana tubes have generally been quite well made, but recent examples are showing more variability. Chinese tubes were originally pretty poor, but the factories over there are quick to upgrade their facilities, and modern efforts are quite respectable.
As for your antenna, try unrolling a bunch of aluminum foil along two adjacent walls, forming a big "L" and hook that to your radio. If you can ground your radio somehow, this will likely help a lot. Another trick is to capacitively couple the antenna lead into your phone line, effectively using the phone wires as your antenna. However, this can bring in more noise than signal, and often does.
"I have a box of several -- you're welcome to some if you'd like. Just feed hook up an antenna and speakers and feed it 12 volts, and you've got a quite nice little stereo."
They don't want more current than I can suck out of most suplus wall-warts, do they? One with line-outs to go to an equalizer would be a bonus because then I could send it through my mixer with the rest of my audio, but if that feature doesn't show up on hand-me-downs, I've got a pair of 8 Ω speaker cabinets I could rearrange the office to make room for (and if I dig enough, maybe I can find another pair if the radio wants 4 Ω).
"6L6 tubes first appeared in 1936, and are arguably the tube in longest continuous production (still made today, in at least four countries)."
Yow.
"As for your antenna, try unrolling a bunch of aluminum foil along two adjacent walls, forming a big "L" and hook that to your radio. "
I'll try that. Heh. Come summertime, I'll have foil on the windows to keep out the evil heat-the-house-up rays, and foil on the walls to attract the happy bring-me-music rays.
(no subject)
Second, your building may have some kind of metal in it that shields the signal.
External antennas usually help.
There is a vanishingly small chance that I have a few 12AX7's in my junk box but I can't guarrantee they're not microphonic. If you have 6V6s you can swap in 6L6s without any problem, but not the other way around. The 6L6s are are built to take higher voltages... although I did run an old tube amp with 6V6s instead of the required 6L6s - I put a 4" fan next to the tubes and hoped they didn't fry.
-m
(no subject)
#blink# Huh. I had no idea. I guess it makes some sense in that car radios are going to move in and out of good reception areas, and pass through places were signals overlap, so they've got a tougher problem to solve, but I suppose I just expected that anything they could design into a car radio in that regard might as well get tossed into a tabletop radio where you don't have to squeeze everything into a box that fits in that little space in the dashboard. So I guess the design features that make car radios more sensitive & selective must add significantly to the cost then?
Argh. I have enough trouble understanding electronics without tossing economics into the mix!
"[...] building may have some kind of metal in it [...]"
Unlikely, but I can't claim to know for sure. 1867 row house, brick exterior walls and everything else wood as far as I can tell.
"The 6L6s are are built to take higher voltages"
Ah, that explains why they have such similar part numbers. I'd wondered just how different they were, since I always saw them used the same way as each other in guitar amps.
But I just went downstairs and grabbed the rear panel of the stereo (which I never screwed back on just so I could grab it easily the next time I wanted to look) and it turns out I misremembered the power amplifier tubes. It looks like the amplifier is two 12AX7 and two 6HU8 (one each per side). There's a box with a 6AQ8, a 6BA6, and a 6BE6 in it along with a picture of what I think is supposed to be the tuner mechanism (I don't have a schematic, just a Tube Layout Diagram). Elsewhere there's a 6AU6 and a 6HS6 near components labelled "Ratio Detector" and "FM Limiter", and another 12AX7 and 6BA6 off by themselves (how many parts are there in a tube phonograph-preamp circuit circa 1962?).
Hmm. Some of those do fall into the 6[A-Z]*6 pattern. I should borrow a tube manual from
I've got no idea which is more likely -- a bad 12AX7 or a bad 6HU8 -- but the reason I think it's in the main amplifier section is that crackling in a speaker is the only symptom and it happens whether there's any real signal coming in or not (e.g. when I turn the selector to "Tape" or "Phono" and provide no input there)
I know 12AX7s are still pretty easy to get ahold of, even if I don't have the budget to order them -- or at least they were the last time I got a Musician's Friend catalog, which, come to think of it, has been a while. I don't know whether any of these others are still being made, or only available when someone uncovers a pallette of dusty new-old-stock. (Hmm ... lemme see if Sovtek has a web site ... the last time I glanced at the tube market was around the time Sovtek had a factory fire or something, and people were still saying to stay away from Chinese tubes back then. Have the Chinese gotten better, or do I still want to stick to Russian -- or ancient American -- tubes?)
What causes a tube to become microphonic? I'm familiar with the failure mode, but don't know the mechanism of it.
(no subject)
Car radios are way more expensive because of the added selectivity, sensitivity and small size.
Tube numbers can be damn near identical and be very different inside. The 6*6 is an execption to the rule.
I'm guessing that the most likely tube failing is the 12AX7. The 6HU8 is probably a driver for the power tubes while the 12AX7 (sometimes 12AT7s) are preamplifiers. The crackle is often some kind of minor voltage fluctuation that would be picked up more by the preamp tubes.
If the crackle is on one channel you might want to just swap the 12AX7s around and see if the crackle changes channels. That may give you an idea of who's the culpret.
As why tubes turn microphonic - I really don't remember.
-m
Re:
Some more grist for the mill
new, since they "come with" a car, usually). Since new cars usually contain radios, used
car radios are available cheaply. I have a box of several -- you're welcome to some if
you'd like. Just feed hook up an antenna and speakers and feed it 12 volts, and you've
got a quite nice little stereo. I've seem car radios built into speakers (along with 12 volt
power supplies, sometimes) as portable systems. Works a treat.
6L6 tubes first appeared in 1936, and are arguably the tube in longest continuous
production (still made today, in at least four countries). Numerous variants appeared,
many using the same pinout (7AC). Sometime in history, beam tubes (like the 6L6)
and real pentodes (like the EL34) got lumped together, and more recent tube manuals
give all of 'em the 7AC pinout (the original pentode version was 6S). I have a little
amplifier (currently out on loan) that will accept any of the 7AC/6S type tubes (tho
the high/low voltage switch can only be set to "low" for some of them). It's great to
sound out the different tubes (and there are a lot of them).
As for your tube lineup, your amp is a bit of an oddball. The 6HU8s are dual pentodes,
and are the output tubes. These are a bit of an oddball tube, and there aren't many
substitutes. They were probably used for cost reasons, as two 6HU8s are going to be
less expensive than a quartet of 6BQ5/EL84 output tubes. Unfortunately, I don't have
any in stock, and they're $40 apiece tubes today. Most of the other tubes I do have, or
have suitable substitutes.
Yes, tube numbering does have patterns, but it's not completely regular (of course).
The numbering system that applies to these tubes (variously known as RMA or RETMA)
goes essentially like this: the first number is the approximate filament voltage (usually,
there are an assortment of interesting exceptions). The 6 tubes expect 6.3 volts. The
last number is (approximately) the number of elements in the tube. The letters in the
middle are assigned serially. So a 12AX7 wants 12.6 volts (though the tapped heater
can also be run on half that, by wiring the sections in parallel), has 7 elements (heater,
two cathodes, two grids, two plates), and is around the 40th such. The 12AY7 was
assigned right after it, presumably.
The failure modes for the different kinds of tubes vary (tho any tube can fail any way it
wants). The low power tubes like 12AX7 have small, closely spaced elements, and tend
to be plagued by microphonics (loose bits) and shorts. Power tubes such as the 6HU8
are more robustly constructed, and thereby are subject to different ills, such as cathodes
wearing out and gassiness. The crackling sound is likely either a bad resistor, leaky
capacitor (or dirty path on a tube socket), or a noisy tube. This is handy, as these are
all cheap and easy fixes using parts I have lying around. Those power tubes would be
another matter.
As for tube sources, NOS tubes are generally better made than modern ones. However,
tubes are much more variable than most people realize, and just getting a few samples
and being willing to tweak your circuit will let you get pretty much any sound you want.
Russian tubes are being made at several factories, and various events have made it
tricky to find out which brands are made at what factories. Sovtek tubes tend to come
from the Reflector factory, and are fairly reliable, but have a lot of consistency problems.
Svetlana tubes have generally been quite well made, but recent examples are showing
more variability. Chinese tubes were originally pretty poor, but the factories over there
are quick to upgrade their facilities, and modern efforts are quite respectable.
As for your antenna, try unrolling a bunch of aluminum foil along two adjacent walls,
forming a big "L" and hook that to your radio. If you can ground your radio somehow,
this will likely help a lot. Another trick is to capacitively couple the antenna lead into
your phone line, effectively using the phone wires as your antenna. However, this can
bring in more noise than signal, and often does.
Re: Some more grist for the mill
"I have a box of several -- you're welcome to some if you'd like. Just feed hook up an antenna and speakers and feed it 12 volts, and you've got a quite nice little stereo."
They don't want more current than I can suck out of most suplus wall-warts, do they? One with line-outs to go to an equalizer would be a bonus because then I could send it through my mixer with the rest of my audio, but if that feature doesn't show up on hand-me-downs, I've got a pair of 8 Ω speaker cabinets I could rearrange the office to make room for (and if I dig enough, maybe I can find another pair if the radio wants 4 Ω).
"6L6 tubes first appeared in 1936, and are arguably the tube in longest continuous production (still made today, in at least four countries)."
Yow.
"As for your antenna, try unrolling a bunch of aluminum foil along two adjacent walls, forming a big "L" and hook that to your radio. "
I'll try that. Heh. Come summertime, I'll have foil on the windows to keep out the evil heat-the-house-up rays, and foil on the walls to attract the happy bring-me-music rays.