RL-70H-6 Replacement - Videokarma.org TV - Video

Jonathan · #1 10-03-2007, 02:40 PM

Hello,

In my 1939 RCA K60 there is an RL-70H-6 loudspeaker. It sounds like total crap.

Reason being is that it's frequency limited. The cone and speaker surround are one in the same, with ridges in the cone to make it more flexible and act as the surround. I'm almost certain this is the reason for the limited bandwidth. I play my mp3s/cds through either the victrola or television input of the radio which sound very bandwidth limited. I hate to destroy the integrity of this beautiful radio, but I feel I'm not getting the full frequency range the amplifier can put out. In my research, I think I have two options:

1. Modify the original loudspeaker by cutting out the surround (which is part of the cone) and gluing in a modern rubber replacement. But where am I going to find one for a 12" speaker? The cone is in decent shape.

2. Buy a new permanent magnet speaker, audio transformer, and field coil. Since the old speaker used the field coil as it's magnet, I need to find a field coil that has the same specs of the old one. I believe a Hammond 155J should do the trick perfectly. It's 15H, so it can store a lot of energy, 300mA max current flow, as the B+ draw is 67mA according to RCA schematics it should be fine, and it's resistance is 1.026k which is within tolerance of the original 1.06K field coil resistance. Hammond makes a "universal" audio transformer which through different pin arrangements you can connect any speaker. RCA schematics say the power output of the amp is 2.5W typical, 4W max.

Any tips or suggestions would be greatly appreciated.

Thanks.

Jonathan

Tom Bavis · #2 10-03-2007, 03:31 PM

Why not connect an external speaker? You'll want to do that anyway to prove that the internal one is the problem...

I don't think changing the surround will help, and I think you should leave the speaker original...

The numbers don't add up for the choke - cant be rated for 300 mA - 67 mA in 1026 Ohms is 4.5 W - that's too much - better to use a lower resistance choke and a series resistor.

Jonathan · #3 10-03-2007, 07:05 PM

Tom,

I think you're right. Here is a link to Hammond's site with low to medium current power chokes. You are completely right, as I must have read wrong. 155J is rated for 30mA, not 300mA. The resistance of the original field coil is 1.06k ohm. The RCA schematic says total "B" current (probably B+) is 67mA. Do you suggest getting something that can do at least 100mA and use a high power resistor to make up the slack? Is it more important to match the inductance of the choke or the resistance?

Also, I was thinking about connecting an external speaker to the audio transformer and disconnecting the internal speaker to see if there is a difference. In fact, I'll do that tonight. But I wonder what the frequency response is of the original audio transformer. Back then, a wide frequency response wasn't required, even for prewar television or victrolas. I think the amplifier should have a fairly wide frequency response. I forget how to calculate the amplifier's bandwidth. Hammond offers audio transformers for general replacement of tube audio transformers and hi-fi replacements. General replacements go from 100Hz to 15kHz. Hi-fi replacements have a bandwidth of 20kHz to 20kHz, but weigh 11 pounds. :P Plus, I'll have to use a smaller speaker in any case, and possibly make some sort of bracket so it fits.

Thanks.

Jonathan

Jonathan · #4 10-06-2007, 06:12 PM

I connected a small 8 ohm external speaker in place of the internal speaker. It sounded much better, mainly better frequency response. I was thinking of using a 4 ohm 6" speaker with a frequency response of around 100Hz to 15kHz. That should give me decent sound without much expense.

Since I don't have an inductance meter, does anyone know the inductance of the field coil on the RL-70H-6 RCA loudspeaker? I'll get a new Hammond choke with inductance and current capability the same or close to the old one and use a resistor to match up the resistance of the old one to 1060 ohms. Does that sound right?

Thanks.

Jonathan

Tom Bavis · #5 10-06-2007, 07:14 PM

Inductance doesn't matter much - at worst you can increase the capacitor AFTER the choke to improve filtering. If a 10H 100 ma choke is too big, get a 5 or 8H one - and at least 80 mA rating - and add resistance as needed - make it a 10W resistor.

Jonathan · #6 10-06-2007, 11:50 PM

Perfect, I'll try it out and let you know.

Thanks.

Jonathan

Tube Radio · #7 10-07-2007, 02:14 PM

To make it more Hi-FI remove R11, C18, and C29. You may have to add a tweeter to get any decent HF response as those type speakers do not have very good HF response. Also increase the size of C26 and C28 to .1uf. The origional speaker should have excellent low frequency response. I would advise keeping the origional speaker for when (not if) you revert the console back to origional. Been down that road with my K-80 console and after around $50.00 later I have the console pretty much back origional. I didn't keep the origional working speaker when I tried to make mine HI-FI which IMHO was a waste of time to try and do that, but if you do it and like how it sounds when you are finished then that's all that matters. I don't know if this is true with your console since yours has a single output tube, but in mine which has push-pull audio I got feedback when I removed the caps I mentioned and had the tone control/input selector set for max HF response when I turned the volume control up past halfway.

Chad Hauris · #8 10-07-2007, 09:12 PM

The sound systems of these old radios are designed to provide frequency response best suited to AM radio and 78 RPM records since that was all there was!
We have had good results improving treble response on field coil speaker type jukeboxes by just adding a tweeter to the voice coil circuit and using the orginal field coil speaker as the woofer. We used a non-polar electrolytic in series with the tweeter to block low frequencies to it (about 10 mfd) and just connected it into the voice coil circuit of the original speaker.
You may even be able to make a "coaxial" two-way speaker by carefully mounting the tweeter to the front of the field coil speaker using thin metal or wood strips.

Jonathan · #9 10-12-2007, 02:54 PM

After some thought, I think I'll keep the original speaker. It does have an excellent bass response. I'll get a tweeter (possibly 8 or 16 ohm impedance, as the impedance of the speaker in there is 2 ohms) and mount it inside somehow. I'll also try out those changes TubeRadio suggested. Hopefully I won't have any feedback issues either. It should broaden the frequency response. The challenge will be making the tweeter fit.

Where does the original speaker cut off at? Around 5kHz?

Thanks.

Jonathan

Tube Radio · #10 10-17-2007, 09:31 AM

Not too sure where the amp cuts off the frequency response. I would advise you connect a small value cap (selected so that frequencies above 16 Khz are attenuated) from the plate of the first audio tube and ground. That should solve any potential feedback issues. Another method would be to insert a variable resistor in series with the cap connected to the plate of the first audio tube and ground. Mount the resistor on the back of the chassis. Now you will have a treble control which will be useful to balance the sound between the 12 inch speaker and the tweeter. As to mounting the tweeter what you could do is find a metal strip that will go between two of the speaker mounting bolts on opposite sides of the speaker and mount the tweeter to that. Tweeter will now be in front of the main speaker cone so the speaker shouldn't be too large from front to back. What would probbably work best is a Magnavox tweeter (6 inches @ 3.2 ohms IIRC) which may be found in almost any late 50's Magnavox console (may also be labeled Columbia as Magnavox made a few models for them). You will have to experiment on the value of cap you put in series with the tweeter determined by what sounds best to you.