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.
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.