FWIW, I noticed much the same thing with an old pair of LESA speakers I owned which had open backs. Is there any possibility of doing even a "bad" bass reflex with this design? I assume you probably are not equipped to measure this driver's TS parameters, but perhaps you can find one of the old ubiquitous BR articles and see if you can come up with something approximately ok for this driver. You might really want to post this in the Loudspeaker forum.. Someone like GM or panomaniac over there should be able to help you.
Without knowing the TS parameters of the driver everything we do is a guessing game. Perhaps the Qts of the speaker is lower than we think it is, in which case, you can close the back to increase the Q (and the Fcb), or you can turn it into a reflex system. If you want a classic jukebox sound I would expect you would want a Q of about 1.5. But be aware that by whatever amount you raise the Q you also raise the resonant frequency. If Q = 0.7 and you raise it to 1.4 then the resonant frequency will go from 40 or 45 to 80 or 90.
If you have never experimented with WinISD I would recommend that completely. It is a good design tool and you can learn a lot by just playing with theoretical speakers to see what happens when you change cabinet volume or port size.
Wade
If you have never experimented with WinISD I would recommend that completely. It is a good design tool and you can learn a lot by just playing with theoretical speakers to see what happens when you change cabinet volume or port size.
Wade
Thanks for the sugguestions guys. I think I will split this thread's speaker section off to the loudspeaker forum and concentrate on the amp here. I have played a lot with WINisd, but only with known TS 'parems. I don't know if I would be able to do a bass reflex design or not. While I *migh* have room for the power amp next to the radio on the upper shelf, my cabinet walls certainly aren't very thick or sealed. I could do some bracing however, as well as sealing up the small holes. Doesn't have to be perfect. Maybe I should look into another type of driver for this thing? Maybe a cheap old coaxial 15"?
one can usually have EV's, UTAH's, stephens trusonic's, universities and the like from ebay for decent prices (>$80.)
Hoping my radio arrives soon. It may need some work. Someone cut the power cord off ( which God knows why, but people do all the time with these old consoles,) Also looks like someone swaped a 6ba6 and 6al5 with each other... and wrote in marker OVER the chassis markings the corresponding tube numbers. Weird. Pictures will follow as soon as I get it!
Right now I'm trying to figure out how to do the arch. I already have a source for ready to finish oak laminate, but I can't figure out what to use as a base first. I am planning on about 4 arches cut out of oak for the braces, and then wrapping it with some decent backer to bring the thickness up to around 1/8" -1/4". I'm experimenting with perf board right now...
one can usually have EV's, UTAH's, stephens trusonic's, universities and the like from ebay for decent prices (>$80.)
Hoping my radio arrives soon. It may need some work. Someone cut the power cord off ( which God knows why, but people do all the time with these old consoles,) Also looks like someone swaped a 6ba6 and 6al5 with each other... and wrote in marker OVER the chassis markings the corresponding tube numbers. Weird. Pictures will follow as soon as I get it!
Right now I'm trying to figure out how to do the arch. I already have a source for ready to finish oak laminate, but I can't figure out what to use as a base first. I am planning on about 4 arches cut out of oak for the braces, and then wrapping it with some decent backer to bring the thickness up to around 1/8" -1/4". I'm experimenting with perf board right now...
I had an astrosonic once, those 15" alnicos can do some serious wall shaking in the original cabinet. I believe it was about 6' - 7' long, about 1.5' deep, and about 2.5' tall. There was a piece of craptastic 'holey board' on the back, and if I remember correctly, there wasn't too much space behind the woofer, no more than 2'. It fired sideways out of the end of the console. There was an internal crossover to 2x Jensen RP103 knock-offs (i think motorola brand, dunno). Those cabinets were very thick solid wood though, not sure how that would affect the base extension. With bass turned up it shook the walls in not just my apartment but attached apartments as well (shoddy 1920s construction, though). Have you mucked around with polarity? If you're biamping and the signals are not in phase you will lose bass response, among other sonic qualities.
Have not tried reversing the polarity, but I don't think that has any affect on a mono system does it?? I'm only using one woofer.
No doubt that with both of the 15's that it shook the house, the poeple ( 16 year old kids..) at goodwill said that it "freakin rocked dude."
Anyways, thats interesting you say only 2' behind them. Maybe that affects the Q of the speaker? Maybe my chamber is actually too large
No doubt that with both of the 15's that it shook the house, the poeple ( 16 year old kids..) at goodwill said that it "freakin rocked dude."
Anyways, thats interesting you say only 2' behind them. Maybe that affects the Q of the speaker? Maybe my chamber is actually too large
I know that when I had to move and leave the astrosonic behind, I thought, I'll just open 'er up and take the speakers out. Magnavox used these terrible screws which can only be removed with a nut driver that I didn't have (I ended up using needlenoses pliers and getting badly scraped knuckles). The area inside was one of those cramped areas–it was probably less than 2' of open space. However all of the areas inside the cabinet were somewhat connected, but the very large tuner/record storage space etc divided the cabinet in the middle (I think there was a dividing wall in there somewhere, but mostly just clutter). If it maybe needs less rear resonance, you could try just jamming the thing full of pillows. Sometimes open backed cabs lose base in some environments, especially in rooms that allow the front wave and back wave to interact. The polarity comment was because I thought there were 2 amps in this cabinet, not just one. You're sure this amp puts out bass at all? Some amps which weren't intended for hifi just do away with bass because they didn't want to have to design the power supply to handle it or use big output transformers. You could increase some coupling caps and see what it'll handle without motorboating or getting flobby.
I haven't built the amp for the jukebox yet, this is a jukebox being made from scratch. I am using a 1970's receiver as I have said in earlier posts. It has tons of bass and power on tap, enough to bottom the woofer out if turned up enough. I've got probably 15-20 watts RMS on tap from this amp and it drives my old sansui's to sub bass regions indoors, so yeah its got bass responce.
I will try the pillow thing that you sugguested, may help damp the cabinet's resonations too. The woofer is powerful enough that my baffle is flexing with just the 4 screws in it, so I'm thinking I might have to upgrade to a 3/4" baffle with more screws holding the thing in place.
BTW, I use those horns on top of my hi-fi speakers. They sound pretty good for how cheap they probably are
I will try the pillow thing that you sugguested, may help damp the cabinet's resonations too. The woofer is powerful enough that my baffle is flexing with just the 4 screws in it, so I'm thinking I might have to upgrade to a 3/4" baffle with more screws holding the thing in place.
BTW, I use those horns on top of my hi-fi speakers. They sound pretty good for how cheap they probably are
Well a quick update, need to take more pics, but I got the radio. It was in beautiful shape, needed filters as I expected. Replaced them and my B+ went from 200v to 278v
The bass opened up on the maggie with some tube power!! This makes me happy. Probably isn't damping it as much as my SS amp was.
Anyways, I am astonished at the power that the radio's built in 6v6SE stage is giving. It can kick some serious butt, it sounds like its running around 7 to 10 watts out to the speaker. This scares me, because a 6v6 shouldn't put out more than 4 watts undistorted in SE. The stage is run at about 270v on the plate with 380r/25uf cathode bias. It has plate to plate NFB from the 6v6's plate to the plate of the 6SJ7 with a 2.2m resistor. The JAN/tungsol 6v6 tested 85%, but I was worried about it because it sounded so loud, so I put in a JJ instead. Surprise... with the JJ my output power seemed to increase more.
Am I missing something here? My little 6bq5 SE magnavox doesn't put out anywhere near this kind of power running at the same voltages, but with some NFB. This does not make sence, since the 6bq5 is supposed to have slightly more power with less distortion- on paper at least. My experience with 6v6 amps is that their all very much louder than the 6bq5 counterparts. Why is this?
The bass opened up on the maggie with some tube power!!
This makes me happy. Probably isn't damping it as much as my SS amp was. Anyways, I am astonished at the power that the radio's built in 6v6SE stage is giving. It can kick some serious butt, it sounds like its running around 7 to 10 watts out to the speaker. This scares me, because a 6v6 shouldn't put out more than 4 watts undistorted in SE. The stage is run at about 270v on the plate with 380r/25uf cathode bias. It has plate to plate NFB from the 6v6's plate to the plate of the 6SJ7 with a 2.2m resistor. The JAN/tungsol 6v6 tested 85%, but I was worried about it because it sounded so loud, so I put in a JJ instead. Surprise... with the JJ my output power seemed to increase more.
Am I missing something here? My little 6bq5 SE magnavox doesn't put out anywhere near this kind of power running at the same voltages, but with some NFB. This does not make sence, since the 6bq5 is supposed to have slightly more power with less distortion- on paper at least. My experience with 6v6 amps is that their all very much louder than the 6bq5 counterparts. Why is this?
You need to measure the actual output power, until you do the actual conjectures you made are meaningless.
It is likely that the speaker in your jukebox is just a whole lot more efficient than your regular speakers.
Are both amplifiers operating in pentode mode or have you triode strapped the Magnavox? Relative size of output transformers also plays a role, the radio might just have another whole octave or so of usable bass response.
The 6BQ5 apparently has a slight advantage in linearity, but the two tubes are similar performers otherwise, and in their respective heydays were meant to address approximately the same market space. The 6V6 has become well regarded in recent years amongst hobbyists presumably due to perceived or real performance differences between it and the 6BQ5.
Measure, measure, measure!
It is likely that the speaker in your jukebox is just a whole lot more efficient than your regular speakers.
Are both amplifiers operating in pentode mode or have you triode strapped the Magnavox? Relative size of output transformers also plays a role, the radio might just have another whole octave or so of usable bass response.
The 6BQ5 apparently has a slight advantage in linearity, but the two tubes are similar performers otherwise, and in their respective heydays were meant to address approximately the same market space. The 6V6 has become well regarded in recent years amongst hobbyists presumably due to perceived or real performance differences between it and the 6BQ5.
Measure, measure, measure!
Retrovox: I have not triode strapped the maggie, and the test was done on a pair of nova 6's , so same speakers with different amps. The radio's transformer was tiny and I didn't bother hooking it up as I had a hammond 125cse laying around. Its a little larger than the maggie x-frmrs, still haven't tried it on the amp but I have with other 6bq amps and the power was still less.
That being said, how do I measure poutput power? Can I just measure maximum RMS ac voltage before audible clipping, and then convert that to watts? As a little note, I measured my magnavox at about 3v AC per channel before audible clipping at 50hz, and 4.5v at about 100hz. I don't know if its RMS ac voltage or not, my DMM is a cheap radio shack meter.
That being said, how do I measure poutput power? Can I just measure maximum RMS ac voltage before audible clipping, and then convert that to watts? As a little note, I measured my magnavox at about 3v AC per channel before audible clipping at 50hz, and 4.5v at about 100hz. I don't know if its RMS ac voltage or not, my DMM is a cheap radio shack meter.
I tried rolling some 6v6's today. Here are the results:
Jan/tungsol 6v6GT: cathode voltage ~18v
Westinghouse 6v6GT: cathode voltage ~17v
JJ 6v6: wait a minuite...cathode voltage ~25v
The JJ pumps out considerable more power than the other two tubes. The JJ has a plate structure more like a 7591 instead of a 6v6, in some cases their larger. The one I have came from one of those silly matched pairs with a "sugguested bias point of 30ma"
Whats going on here?
Jan/tungsol 6v6GT: cathode voltage ~18v
Westinghouse 6v6GT: cathode voltage ~17v
JJ 6v6: wait a minuite...cathode voltage ~25v
The JJ pumps out considerable more power than the other two tubes. The JJ has a plate structure more like a 7591 instead of a 6v6, in some cases their larger. The one I have came from one of those silly matched pairs with a "sugguested bias point of 30ma"
Whats going on here?
I=ER where I is the current (in this case your cathode current) E is the voltage you measure across a resistor R. (In this case cathode resistor.)
To measure output power get a meter that can measure rms - yours is probably ok for sine wave signals even though at the low end of the price range it probably does not have a true rms precision rectifier. Usually you measure at 1kHz for a good sense of midband power, but it is probably worthwhile to measure at 100Hz or even 50Hz if the amplifier is up to it. I doubt the meter will work well at more than a couple of kHz although I would normally recommend testing at 5 or 10kHz as well. Use an appropriate load resistor value and measure across it.
The key is to drive the output to just the verge of clipping, usually a thd analyzer is used along with a scope. What you can do without an analyzer is drive it just to clipping using a scope to observe and back off the input signal until the clipping has just gone away. (fiddly) Measure the rms voltage with your meter, square it, and divide by the load impedance. E^2/R If you don't have a scope there is no way to determine the clipping point...
In a pinch you can use the scope for amplitude measurements, depending on your skill level you may get better results particularly above 1kHz. To get rms volts from your scope take the peak to peak measurement and multiply that by 0.3535 to get rms or half that and multiply by .707, square that number and divide by the load impedance. (Epp x .3535)^2 divide this by your load resistance.
FWIW I would be surprised if you got over 5W at 1kHz, and expect substantially less at 50Hz due to opt limitations.
To measure output power get a meter that can measure rms - yours is probably ok for sine wave signals even though at the low end of the price range it probably does not have a true rms precision rectifier. Usually you measure at 1kHz for a good sense of midband power, but it is probably worthwhile to measure at 100Hz or even 50Hz if the amplifier is up to it. I doubt the meter will work well at more than a couple of kHz although I would normally recommend testing at 5 or 10kHz as well. Use an appropriate load resistor value and measure across it.
The key is to drive the output to just the verge of clipping, usually a thd analyzer is used along with a scope. What you can do without an analyzer is drive it just to clipping using a scope to observe and back off the input signal until the clipping has just gone away. (fiddly) Measure the rms voltage with your meter, square it, and divide by the load impedance. E^2/R If you don't have a scope there is no way to determine the clipping point...
In a pinch you can use the scope for amplitude measurements, depending on your skill level you may get better results particularly above 1kHz. To get rms volts from your scope take the peak to peak measurement and multiply that by 0.3535 to get rms or half that and multiply by .707, square that number and divide by the load impedance. (Epp x .3535)^2 divide this by your load resistance.
FWIW I would be surprised if you got over 5W at 1kHz, and expect substantially less at 50Hz due to opt limitations.
I have found evidence on some guitar forums of the JJ 6v6 sounding louder and drawing more bias current. The fact that its closer to a 6L6 than a 6V6 makes me wonder, as you can push it way past its ratings with no red plating. Reports of people putting them in as drop in replacements for PP 6L6's and only experiencing a slight power decrease.
Anyways, I thought it was about time for an update.
So far, I have cleaned up the radio and mounted the face in the cabinet. I also added an RCA in on the back after removing the phono RIAA eq. Right now I have to get over the grillework before proceeding, any ideas? Here is what I have mocked up so far:
http://getchellaudio.googlepages.com/102_0230.JPG
In case I didn't mention it already, I sanded off the honeycombimg onthe front of the knobs. They look a lot better now. I also repainted the gold inserts.
Anyways, I thought it was about time for an update.
So far, I have cleaned up the radio and mounted the face in the cabinet. I also added an RCA in on the back after removing the phono RIAA eq. Right now I have to get over the grillework before proceeding, any ideas? Here is what I have mocked up so far:
http://getchellaudio.googlepages.com/102_0230.JPG
In case I didn't mention it already, I sanded off the honeycombimg onthe front of the knobs. They look a lot better now. I also repainted the gold inserts.
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