How would the midax and trebax sound on a tube amp using a pair of 6BM8 in push pull or other power tubes in push pull?
The high sensitivity of the Midax and Trebax made them compatible with the low power valve amplifiers of the day.
Interestingly, I've discovered that the Midax was included in the "small box" Vox AC 50 valve amp used by the Beatles.
You can see the Midax magnets peeping out through circular holes in the backs of John and George's cabs.
https://www.voxac50.org.uk/midax.htm
Interestingly, I've discovered that the Midax was included in the "small box" Vox AC 50 valve amp used by the Beatles.
You can see the Midax magnets peeping out through circular holes in the backs of John and George's cabs.
https://www.voxac50.org.uk/midax.htm
At some point I'll build a push pull tube amp using the 6BM8 as that will only require the two tubes and two transformers.
The RCA woofers aren't that efficient, but I currently power them with this kit that puts out 15 watts into 4 ohms, although it's driving a 16 ohm load so power output may be under 10 watts.
https://store.qkits.com/electronic-kits/audio-kits/15-watt-mono-audio-power-amplifier-kit-fk607.html
So a push pull tube amp will be good enough.
Instead of building an amp I may see if there's a push pull mono tube amp I can get for a reasonable price then all I have to do is rebuild it versus designing an amp and finding a chassis for it.
Oh also I have two or three XO/5000 as well.
I suppose whoever sold the midax, trebax and both types of crossover may have started with a two way and later added the midax to upgrade to a three way.
One of the midax I have is the earlier smooth finish version.
The RCA woofers aren't that efficient, but I currently power them with this kit that puts out 15 watts into 4 ohms, although it's driving a 16 ohm load so power output may be under 10 watts.
https://store.qkits.com/electronic-kits/audio-kits/15-watt-mono-audio-power-amplifier-kit-fk607.html
So a push pull tube amp will be good enough.
Instead of building an amp I may see if there's a push pull mono tube amp I can get for a reasonable price then all I have to do is rebuild it versus designing an amp and finding a chassis for it.
Oh also I have two or three XO/5000 as well.
I suppose whoever sold the midax, trebax and both types of crossover may have started with a two way and later added the midax to upgrade to a three way.
One of the midax I have is the earlier smooth finish version.
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Looks like I may be getting a Heathkit A9 amplifier for this speaker.
I also have a small 6 ohm dome tweeter.
I may put a 10 ohm resistor in series with it and a small cap so that it reproduces above 13kHz just to see how it sounds with the drivers.
I also have a small 6 ohm dome tweeter.
I may put a 10 ohm resistor in series with it and a small cap so that it reproduces above 13kHz just to see how it sounds with the drivers.
Maybe, however theres no real wall where the speaker is.
Only a "wall" made up of a wood frame and copper screen material.
Back a little over 10 years ago parts express had a deal on a box of 200 onkyo dome tweeters (made for certain models of GM vehicles) for $10 so I bought a box and it's very easy for me to add a dome to most any speaker.
I usually put a 13Khz low pass in series with the stock tweeter and a 13kHz high pass in series with the dome as the usual issue is that the stock tweeter doesn't reproduce the upper treble very good. That also preserves as much of the stock speaker sound as possible.
Of course the trebax may be just fine in the upper treble, but the environment I'm in is a little noisy due to test station fans nearby so the dome tweeter will definitely help.
Only a "wall" made up of a wood frame and copper screen material.
Back a little over 10 years ago parts express had a deal on a box of 200 onkyo dome tweeters (made for certain models of GM vehicles) for $10 so I bought a box and it's very easy for me to add a dome to most any speaker.
I usually put a 13Khz low pass in series with the stock tweeter and a 13kHz high pass in series with the dome as the usual issue is that the stock tweeter doesn't reproduce the upper treble very good. That also preserves as much of the stock speaker sound as possible.
Of course the trebax may be just fine in the upper treble, but the environment I'm in is a little noisy due to test station fans nearby so the dome tweeter will definitely help.
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For the dome tweeter it is 6 ohms so to keep the impedance close to 15 ohms I will use a 10 ohm series resistor.
For a 6 db/octave high pass of 14.6k I will need a .82uF cap.
Now would it be ok to use 6 dB/octave or should I use 12 dB/octave?
All I'm after is a slightly better reproduction of the upper treble close to 16kHz and the dome will also extend the speaker's response to beyond 16kHz.
I have a hole in the box that I had tried to cut for the trebax. Maybe this dome tweeter will fit there. There's a screw hole in the rear where a cable strain relief is screwed to. If need be I can bend a thin piece of metal to give me a way to mount it to the box properly if the hole is too big for the tweeter to be wedged in.
Now I may cut a piece of wood or cardboard to span the top rear of the box so that it is more sturdy.
The grill for the woofers will be some of the copper screen material I salvaged when another screen room was being dismantled for the primary purpose of patching small holes in the screen of mine that someone had merely put silver tape over. I'll use a good double sided tape to secure it to the box. I would have secured it using the screws that I used to mount the woofer, but I used some Nylon insert locknuts as that will keep the nuts from backing off without me having to tighten the screws so much that it flattens the cardboard or pulls the screw through the cardboard.
I may try to get a picture or two of the box Tuesday.
For a 6 db/octave high pass of 14.6k I will need a .82uF cap.
Now would it be ok to use 6 dB/octave or should I use 12 dB/octave?
All I'm after is a slightly better reproduction of the upper treble close to 16kHz and the dome will also extend the speaker's response to beyond 16kHz.
I have a hole in the box that I had tried to cut for the trebax. Maybe this dome tweeter will fit there. There's a screw hole in the rear where a cable strain relief is screwed to. If need be I can bend a thin piece of metal to give me a way to mount it to the box properly if the hole is too big for the tweeter to be wedged in.
Now I may cut a piece of wood or cardboard to span the top rear of the box so that it is more sturdy.
The grill for the woofers will be some of the copper screen material I salvaged when another screen room was being dismantled for the primary purpose of patching small holes in the screen of mine that someone had merely put silver tape over. I'll use a good double sided tape to secure it to the box. I would have secured it using the screws that I used to mount the woofer, but I used some Nylon insert locknuts as that will keep the nuts from backing off without me having to tighten the screws so much that it flattens the cardboard or pulls the screw through the cardboard.
I may try to get a picture or two of the box Tuesday.
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It is common to blend in super tweeters gradually by way of a 6 dB/octave filter. A 1.0 uF capacitor may be just the job!
That would give me a high pass of 10kHz. A bit lower than I prefer as that cuts too much into the stock tweeter's range.
The treble control for the amp is centered at 15kHz so perhaps I may not need the dome tweeter as I could just boost the treble some. I'll know for sure when I get the amp and get it restored.
Here's the recapped crossover. Those red colored caps for some reason just look right in the crossover. Maybe it's because some caps made across the pond were red in color.
Here's the speaker. I suppose if I built the same size box out of wood, the woofers would sound even better. The dome tweeter is just sitting in there at the moment for the picture. Will connect it up in a bit and see how it sounds. If too loud I'll just add series resistance and lower the cap value until it's right.
I won't make the dome permanent as I want to try the speaker with the Heathkit A9C amp once I get it and get it restored. If I still need the dome I'll make it permanent at that time. I ran out of double sided tape Thursday so I couldn't finish attaching the grill
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The dome tweeter with the series 10 ohm resistor and .47uF cap adds just a little top end sparkle without affecting the basic sound of the existing drivers.
Here's the speaker. I suppose if I built the same size box out of wood, the woofers would sound even better. The dome tweeter is just sitting in there at the moment for the picture. Will connect it up in a bit and see how it sounds. If too loud I'll just add series resistance and lower the cap value until it's right.
I won't make the dome permanent as I want to try the speaker with the Heathkit A9C amp once I get it and get it restored. If I still need the dome I'll make it permanent at that time. I ran out of double sided tape Thursday so I couldn't finish attaching the grill
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The dome tweeter with the series 10 ohm resistor and .47uF cap adds just a little top end sparkle without affecting the basic sound of the existing drivers.
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The physical fit of the caps looks just right too - neat job.
To be honest, the crossover looks a lot neater than the cardboard enclosure, which is definitely a grill on proposition!
Is that a few repaired tears I see on the RCA woofer cones?
The cardboard box indeed looks bad, but it looks much better than any attempted building of a wood box I could do. Plus I really wouldn't want to bring in a largish wood box to work.
Yes a few repaired tears.
I didn't always store those woofers properly.
I used some humiseal as I didn't have any Elmer's school glue at work or home as it was the only thing other than super glue or epoxy I had at work.
The speaker isn't turned up loud enough to make the cones move much though.
The grill will only cover the woofers as I don't know how the copper screen material will affect the sound from the midax and dome tweeter. Plus the screen is merely to protect the woofers from accidentally being punctured.
The woofers do seem to have plenty of bass down to about 40Hz when they start to roll off. That could be a combination of the drivers and the cardboard box, however the thicker box is better than the thin cardboard boxes I've used in the past at home.
Now if I ever run across another pair of the same RCA woofers, I'll have two proper cabinets made to the same dimensions as the cardboard box only taller to accommodate the trebax and dome tweeter.
I'd then find a second Heathkit A9C amp and fit the amps in the boxes so that they are hidden. I'd then get something like a Schitt Saga S preamp and use that as it has 5 inputs and is remote controlled.
I may see what cap and coil value is needed to make it a 12 dB/octave crossover as the dome may integrate better that way. Sounds fine as is, but can't hurt to try the 12 dB/octave crossover as I can easily make the inductor value and I probably have the cap value or something close. That said I'd need to know if the existing crossover is Linkwitz-Riley, Butterworth or Bessel before I try the 12 dB/octave. For Linkwitz-Riley I'd need a .381uF cap and a .39uH inductor for a crossover of 13,050Hz. I can use a .33uF cap in parallel with a .05uF cap for the .38uF cap and I should be able to get the inductor, but if not I can get the next higher value and unwind as necessary to equal .39mH.
I should put a low pass in series with the trebax, but I'd then have to move the l-pad resistors to the trebax terminals. That requires a .4uF cap and a .365uH inductor for a 13,050Hz low pass. I'll have to see if .2uF caps are available to make a .4uF cap.
Doing that will keep the trebax from reproducing some of the same range as the dome.
Now I cannot remember, but is the crossover wired to take into account the fact that the 12 dB/octave slope causes a 180 degree phase shift or do I need to swap the input wires around or is it just fine as is?
Yes a few repaired tears.
I didn't always store those woofers properly.
I used some humiseal as I didn't have any Elmer's school glue at work or home as it was the only thing other than super glue or epoxy I had at work.
The speaker isn't turned up loud enough to make the cones move much though.
The grill will only cover the woofers as I don't know how the copper screen material will affect the sound from the midax and dome tweeter. Plus the screen is merely to protect the woofers from accidentally being punctured.
The woofers do seem to have plenty of bass down to about 40Hz when they start to roll off. That could be a combination of the drivers and the cardboard box, however the thicker box is better than the thin cardboard boxes I've used in the past at home.
Now if I ever run across another pair of the same RCA woofers, I'll have two proper cabinets made to the same dimensions as the cardboard box only taller to accommodate the trebax and dome tweeter.
I'd then find a second Heathkit A9C amp and fit the amps in the boxes so that they are hidden. I'd then get something like a Schitt Saga S preamp and use that as it has 5 inputs and is remote controlled.
I may see what cap and coil value is needed to make it a 12 dB/octave crossover as the dome may integrate better that way. Sounds fine as is, but can't hurt to try the 12 dB/octave crossover as I can easily make the inductor value and I probably have the cap value or something close. That said I'd need to know if the existing crossover is Linkwitz-Riley, Butterworth or Bessel before I try the 12 dB/octave. For Linkwitz-Riley I'd need a .381uF cap and a .39uH inductor for a crossover of 13,050Hz. I can use a .33uF cap in parallel with a .05uF cap for the .38uF cap and I should be able to get the inductor, but if not I can get the next higher value and unwind as necessary to equal .39mH.
I should put a low pass in series with the trebax, but I'd then have to move the l-pad resistors to the trebax terminals. That requires a .4uF cap and a .365uH inductor for a 13,050Hz low pass. I'll have to see if .2uF caps are available to make a .4uF cap.
Doing that will keep the trebax from reproducing some of the same range as the dome.
Now I cannot remember, but is the crossover wired to take into account the fact that the 12 dB/octave slope causes a 180 degree phase shift or do I need to swap the input wires around or is it just fine as is?
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Another reason I stayed away from the exotic expensive film caps is because those are usually much larger in size than regular film capacitors and they wouldn't have fit at all.
Given I may be moving the l-pad for the trebax external to the crossover I may do the same with the midax as well. That way I'll never again need to open up the crossover box for anything.
One midax has two ring terminals so I may remove those and use them on this midax as I currently have the wires stuffed inside the holes where plugs would have gone.
Given I may be moving the l-pad for the trebax external to the crossover I may do the same with the midax as well. That way I'll never again need to open up the crossover box for anything.
One midax has two ring terminals so I may remove those and use them on this midax as I currently have the wires stuffed inside the holes where plugs would have gone.
The data sheet for the crossover indicates the driver polarity at its outputs and I would presume phase shift is compensated for in the internal wiring.
Oh ok.
I did look at the second crossover and all negative terminals are connected together.
It could be that phase didn't really matter as much back then far as all drivers being in phase or out of phase so long as all drivers were connected the same way. Then again depending when these drivers and crossovers came out, mono was still the thing and phase wouldn't have mattered anyways other than all drivers being connected with the same phase.
I may reverse the input connections tomorrow just to see if there's a difference in sound.
So for Butterworth I need a .54uF cap and a .277mH inductor for the high pass and a .58uF cap and .26mH inductor for a crossover of 13kHz.
Now if I go with a 14,000Hz crossover I'll need a .5uF cap and .257mH inductor for the high pass and a .535uF cap and .24mH inductor for the low pass.
I can get a .5uF cap easy and I can possibly use a .54uF cap for the .535uF.
The inductors I can make easy by taking inductors higher than the needed value and using the DATS V3 unwinding them until I get the proper inductance.
I did look at the second crossover and all negative terminals are connected together.
It could be that phase didn't really matter as much back then far as all drivers being in phase or out of phase so long as all drivers were connected the same way. Then again depending when these drivers and crossovers came out, mono was still the thing and phase wouldn't have mattered anyways other than all drivers being connected with the same phase.
I may reverse the input connections tomorrow just to see if there's a difference in sound.
So for Butterworth I need a .54uF cap and a .277mH inductor for the high pass and a .58uF cap and .26mH inductor for a crossover of 13kHz.
Now if I go with a 14,000Hz crossover I'll need a .5uF cap and .257mH inductor for the high pass and a .535uF cap and .24mH inductor for the low pass.
I can get a .5uF cap easy and I can possibly use a .54uF cap for the .535uF.
The inductors I can make easy by taking inductors higher than the needed value and using the DATS V3 unwinding them until I get the proper inductance.
Now since the crossover frequency is so high at 14kHz, I may not need a low pass on the trebax as I think from something I saw its upper frequency limit is 16kHz, however I see some official adverts from back then stating 20kHz as the upper limit. Which is correct?
To me it doesn't sound like much above 14kHz is being reproduced, but it could just be how the trebax sounds.
Now in order to get an inductance I didn't have to alter I chose a high pass frequency of 14,350Hz which gives me an inductor of .25mH and a cap of .489uF. I can use a .47uF and two .01uF caps in parallel, however given the 5% tolerance of the Audyn caps, two 1uF in series will work fine or I can use one .47uF. Plus if I go with the .47uF and don't like how it sounds at 12 dB/octave I can simply disconnect the inductor.
In this instance the higher crossover frequency is desired as I only need a little help on the very top end of the treble.
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Given I'm going with the .47uF cap I may see if I have an inductor I can unwind to .25mH then if I like the sound I only have to buy a .47uF film cap unless one of the Wima .47uF film caps will work just fine which I have a few of.
To me it doesn't sound like much above 14kHz is being reproduced, but it could just be how the trebax sounds.
Now in order to get an inductance I didn't have to alter I chose a high pass frequency of 14,350Hz which gives me an inductor of .25mH and a cap of .489uF. I can use a .47uF and two .01uF caps in parallel, however given the 5% tolerance of the Audyn caps, two 1uF in series will work fine or I can use one .47uF. Plus if I go with the .47uF and don't like how it sounds at 12 dB/octave I can simply disconnect the inductor.
In this instance the higher crossover frequency is desired as I only need a little help on the very top end of the treble.
EDIT:
Given I'm going with the .47uF cap I may see if I have an inductor I can unwind to .25mH then if I like the sound I only have to buy a .47uF film cap unless one of the Wima .47uF film caps will work just fine which I have a few of.
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