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eftychia at 07:59pm on 2004-01-23Why can't I get any cool radio stations on the second floor?
WRNR comes in great in the car and pretty well in the kitchen (the receiver drifts, so it has to be tuned in again every so often when I start hearing oldies-masquerading-as-classic-rock or classic-rock-masquerading-as-oldies), but I cannot tune it in at all in the office or the bathroom.
A couple of months ago I discovered the Towson University radio station, not quite as cool as WRNR, but cool in a similar direction -- different enough to be good for variety. It comes in great in the car. It comes in well some days and so-so other days in the kitchen. And the best I can do in the office is a blurry, fuzzy signal that sounds like it's piped through the electronic equivalent of lint, to a torn speaker. (It's not the speaker -- adjacent stations are clear.) IIRC, I can't even get a really clean signal from WTOP-AM upstairs.
Isn't getting higher up supposed to help?
I can get classic-rock that repeats the same songs too often, country, conservative talk, probably 98-Rock if I look for it, some top-40 stuff, and "cool jazz" (which mostly means jazz dumbed down to be more like elevator music -- if I want to hear jazz, I want to hear jazz fercryinoutloud, and if you can mix in big-band era stuff along with the modern so much the better). And I can sortakinda tune in one or two NPR stations, but not quite cleanly. But not radio that I can turn on and listen to as background for hours and hours without getting bored or annoyed.
Maybe I should haul the Fisher upstairs, which has a pretty darned good tuner in it, to find out whether it's the location or the equipment. Only problems with that idea are that a) I'd have to rearrange the office to make room for it, b) I'd need somebody healthy to help me and I'd want my elbow to be in good shape first, and c) it's only good for about twenty minutes of clear sound before the marginal tube (I think it's one of the 12AX7 preamps but I could be wrong) gets hot enough for the sound to go all crackly. (I've wanted to completely re-tube it for years but have never had enough money at once. I should give up and just replace the two 12AX7s or the two main ... 6V6? 6L6? ... power amplifier tubes to see whether the crackle is coming from where I think it's coming from, and if that works, hope that everything else can hold out for another 42 years.
The damned thing sure is pretty though. And sounded really good before that tube went bad. I'd need to find something else to put in the front hall to sort mail and keep keys on, of course.
Okay, one more whine
I *think* that FM is ground transmitted, but perhaps I am mistaken.
Re: Okay, one more whine
Got any halloumi?
"[...] ground transmitted [...]"
I'm not sure what you mean by that. (What it sounds like is some Tesla thingie that I'm pretty sure requires much longer wavelengths. A quick Google of the phrase turned up a bunch of references to mechanical vibrations, and one reference to navigational beacons on the ground transmitting to aircraft.)
My understanding is that both AM and FM are more or less line-of-sight, with AM diffracting around obstacles and bouncing off the ionosphere more than FM (because of the frequency ranges used, I think, rather than the inherent differences between the two modulation methods), but it's been a long time since I last checked.
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It's a brick-and-wood rowhouse built in 1867.
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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
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#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.
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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.
Umm....what about the web?
Tangent
Re: Umm....what about the web?
Believe me, I've tried using Internet radio. It can be done briefly for special occasions, but as an everyday solution it doesn't fly on the hardware and connection I can afford. Spending twenty minutes to download seven minutes worth of MP3, yes; listening to a streaming webcast in real time, no.