Jcm800 Downscale

[soundguruman]

I think the speakers from the console stereos from the 50s 60s and 70s sound wonderful for guitar. I always stop and grab the speakers out of them when I see one on trash day and have lots of them now, I need more speaker cabs to put them in! I once met an old preacher guy that had a storage unit full of these old consoles, there must have been 30 or so.. They consoles imo were nothing special but the speakers can sound great, they have super thin and lightweight paper cones and only need a watt or two to be extremely loud. All those speakers he had if installed in 4x12 cabs could have made a pretty big wall of sound... What exactly do you call the speakers with no magnets in there like the fender blue dot kind? Field coil speakers? Some of them are like and that some have magnets.

Speakers w/ no magnet are called passive radiators.
They will improve the efficiency a bit, and extend the low frequency.

[Alan0354]

I do know that JBL speaker burn out very easily.
And, they are essentially hi fi speakers that are made by Sansui, Japan.
And, I do know that when they burn out, the "warrantee" is fairly useless.
And I do know that the frequency the high frequency drivers are set to, is too low.
This low frequency causes the diaphragms to blow pretty frequently.
AND, this provides a never ending stream of replacement diaphragms, $$$.
JBL refuses to honor the warrantee, unless the speaker is returned to the original point of sale.
Which might be, 2000 miles away, or on the other side of the Earth.

The sound has a really crisp upper mid range, which may or may not be good for guitar.
It has a nice sound for PA, stereo, if you alter the crossover frequency.
Some players love these speakers...
but I will stick with EVM 12L, they are much harder to kill, and take all the abuse I can muster.

I even thought the original Fender Twin blue label speaker were very good sounding also. I had the Marshall plexi with green back, I don't particular think they are superior to the Fender Twin speaker. The Twin speaker can take the abuse. I had occasion cranked it to full for a long session and the speakers got really hot, but it kept on ticking.

Hifi components and guitar amp components don't mix. Hifi stuff is way too sensitive and blow it with guitar easily. I was very disappointed with the K120 sound, and I don't think the Altec is any better. Yes they have 3" coil so my guess they hold up a little better. But they are still hifi and you can see the cone jump like mad. I don't know why people in the 70s consider those are upgrades to the original speakers. Celection, Goodman etc. were consider the cheaper ones at the time. The kicker is those alnico speaker, those were the cheapest of the cheaps.

[Austin]

Speakers w/ no magnet are called passive radiators.
They will improve the efficiency a bit, and extend the low frequency.

Sry I meant no "permanent" magnets, like this I guess they use eltro-magnet coil that gets energized with or opposite to the voice coil. My princeton has one, but there are speakers like this in the consoles too sometimes.

[J M Fahey]

JM, do you make your own cones and voice coils?

I remember an ad from a long time ago by JBL talking about their edge wound voice coils. They make it look like they used a rectangular wire. Weber talks about edge wound voice coils and makes it look like it's just a round wire slightly squished down into an oval. What's the real story?

I tried to make my own cones and had a pertial success, but they were unusable for guitar.
They came out way too thick and "fluffy".
Passable for woofers, useless for guitars, so I buy them.
I know the process but since cheap good chinese ones are available, the investment is just not justified.

I do make my own conventional (roundwire) voice coils, if necessary, but if the same is commercially available it's usually not justified.
It's necessary to home make the vintage type ones though, (roundwire/nitro adhesive/paper former)

Pro edgewound voicecoils (JBL/EV/Pvy. Scorpion/etc.) use real rectangular section extruded aluminum wire.
Too complex to homemake, although a friend of mine has a small factory specializing in that.

I heard also about flattened round wire and actually bought at a used machine merchant a "jeweller's laminator" which among other things does that, but problem is not giving it shape, but winding it "on side" afterwards, so I disn't solve tha main problem.
Besides I was interested in enameled aluminum wire, which was (and still is) unavailable here..

[J M Fahey]

I am even curious, do you design the frame, the dimension of the magnet and all the hardware, or do you buy existing parts? I have no knowledge how you even start making your own speaker. I thought you need a lot of overhead to make speakers.

Yes, of course.
As far as the frame, I have made them sandcast in aluminum, sheet turned, stamped and even used a modified car wheel cap as the basis for a 15" frame, go figure.
No big deal, it's just a mechanical structure.
I also design and build the magnetic structures and that is a bigger problem, although I already have it solved by now.
For larger sizes I have the disks cut to rough shape with oxygen torches , then lathe shaped to exact measures.
The center polepiece is cold rolled cylinder bar steel; it comes in 20ft lengts and has to be previously cut into 3 ft pieces so the lathe man can work with them.
Parts are easy, only problem is the softest steel is needed (SAE 1008 or 1010) and it both is somewhat hard to find (because its mechanically the weakest) and being soft produces miles of curled shavings, which tend to lock the lathe, sometimes breaking the cutting tool, so I often use second best, SAE12L14 which they love.

AS you see, 90% of the work is the "mechanical" part.
Meaybe because I'm so used to it, it's not big deal, but it forces you to make certain minimum amounts to compansate.
I usually make 30 12" or 15", or 50 10" or 60/70 6"/8" or it's not worth it.
And the whole deal takes around 1 month, because I have to wait for jobs to be made, the zinc plating guys can't be trusted, the epoxy paint guys even less, and so on.

Oh well.

[soundguruman]

field coil speakers sound better than regular speakers.
too bad there are no high power ones.
In hi fi, energized fields sound way better than regular speakers like magnepans...

[Alan0354]

Yes, of course.
As far as the frame, I have made them sandcast in aluminum, sheet turned, stamped and even used a modified car wheel cap as the basis for a 15" frame, go figure.
No big deal, it's just a mechanical structure.
I also design and build the magnetic structures and that is a bigger problem, although I already have it solved by now.
For larger sizes I have the disks cut to rough shape with oxygen torches , then lathe shaped to exact measures.
The center polepiece is cold rolled cylinder bar steel; it comes in 20ft lengts and has to be previously cut into 3 ft pieces so the lathe man can work with them.
Parts are easy, only problem is the softest steel is needed (SAE 1008 or 1010) and it both is somewhat hard to find (because its mechanically the weakest) and being soft produces miles of curled shavings, which tend to lock the lathe, sometimes breaking the cutting tool, so I often use second best, SAE12L14 which they love.

AS you see, 90% of the work is the "mechanical" part.
Meaybe because I'm so used to it, it's not big deal, but it forces you to make certain minimum amounts to compansate.
I usually make 30 12" or 15", or 50 10" or 60/70 6"/8" or it's not worth it.
And the whole deal takes around 1 month, because I have to wait for jobs to be made, the zinc plating guys can't be trusted, the epoxy paint guys even less, and so on.

Oh well.

I am not very good in making mechanical stuffs. To me, this is very hard!!! Just reading all these make my head spin.........Really hats off to you, I can't do it. I just stay with my electronics and buy the speaker when needed.

[jazbo8]

I would think he went on to more productive endeavors, such as Gretchen.

So why are we bothering? We are problem solvers, if we were not we would not have entered this field. There is a lot of 'what if' sprinkled in each of us, some people do crossword puzzles, I find them boring, but figuring how to make something that works well gets my brain going. Learning from others and a sense of discovery does not hurt either.

That makes sense...

I think so far, your experiment with the 6AU6 seemed promising, let's see what we can come up with, I will have some free time after I am done with my TOS (from Hoffman Forum) build hopefully in a week or two...

Jaz

[soundguruman]

That makes sense...

I think so far, your experiment with the 6AU6 seemed promising, let's see what we can come up with, I will have some free time after I am done with my TOS (from Hoffman Forum) build hopefully in a week or two...

Jaz

The creative force is the best force, may that force be with you.

[loudthud]

This thread may be off the subject but at 29 Likes, I think it has more than any other thread

[soundguruman]

Let's go for gold.

[printer2]

I really have to question my priorities. I already have an amp I want to build, my 5E3/Blackface cross, came up with a intresting 5E3/18 Watt circuit that might be fun, work on the house, heck learn how to play my guitar. But it is that damn curiosity I have to take this a bit further.

My plan of attack. I'll be using a 30V transformer have some kicking around, it has a 6V winding but it can only supply 600mA without the voltage dropping too much. enough for two 6AU6's. The other four will be run off the 30V in a series string with 25L6's across it as the output tubes (close enough to a 6W6 just different heater voltage). A 10W 70V line transformer for output, think it can be run at 500 ohms and up in P-P. Since I do not play guitar more than the most basic level I will have to put it on something a little more travel friendly, don't want to waste a chassis for this and want to make it easy to work on and modify (and cheap) so brass finishing nails on plywood it is. Worry about shielding later. That is about it for now, just going to wing it from here.

Rough idea of what I may try. Probably use a fixed bias supply to play with first. Leaving out the first stage of gain, this should be enough for me.

[Alan0354]

What you have here is a typical old Marshall schematic designed for high B+. Read some of the posts that have schematics or links, they left out the cathode resistors and bias a little differently. You are using 70V supply, higher than what they are talking about, but it is still low, you want to follow some of the suggestions.

It is a bad idea not to have any sort of ground plan backing and just nail on wood. If you are not familiar with electronics, not only you'll have noise pickup, you likely run into ground loop problem because it is so easy to wire the ground the wrong way and create ground loop. At least put copper foil on top of the wood. Buy wide copper tape nd tape on the wood to create a ground plane to simulate the metal chassis. Also, even if you get this going, you are going to travel around with high voltage wires exposed?

[printer2]

What you have here is a typical old Marshall schematic designed for high B+. Read some of the posts that have schematics or links, they left out the cathode resistors and bias a little differently. You are using 70V supply, higher than what they are talking about, but it is still low, you want to follow some of the suggestions.

It is a bad idea not to have any sort of ground plan backing and just nail on wood. If you are not familiar with electronics, not only you'll have noise pickup, you likely run into ground loop problem because it is so easy to wire the ground the wrong way and create ground loop. At least put copper foil on top of the wood. Buy wide copper tape nd tape on the wood to create a ground plane to simulate the metal chassis. Also, even if you get this going, you are going to travel around with high voltage wires exposed?

I'm a little familiar with electronics, I will be adding a ground plane under the bottom of the plywood and I'll be making a quickie box for traveling.


Pretty much Marshall, decided to stick with it given the OP's desire. Not sure how you get 70V, I am using a 30V PT with full wave rectification should be a maximum of 42V. Can I assume you are talking of the output cathode resistors on leaving them out and biasing them differently? Or are you talking preamp stages? I was thinking of the 40V limit, thought cathode bias would be just as valid given that a fixed bias supply still eats into the voltage limit. I was thinking of working with a 24V transformer and going fixed bias but given the number of preamp tubes I would be a bit short on voltage running the preamp tube heaters in a string. Dropping the cathode follower would work but decided against it. Also part of the reason I am skipping the first stage in the JMC800. Thinking of floating the heaters on the B+, can see no reason why not given the voltage.

[Alan0354]

I'm a little familiar with electronics, I will be adding a ground plane under the bottom of the plywood and I'll be making a quickie box for traveling.


Pretty much Marshall, decided to stick with it given the OP's desire. Not sure how you get 70V, I am using a 30V PT with full wave rectification should be a maximum of 42V. Can I assume you are talking of the output cathode resistors on leaving them out and biasing them differently? Or are you talking preamp stages? I was thinking of the 40V limit, thought cathode bias would be just as valid given that a fixed bias supply still eats into the voltage limit. I was thinking of working with a 24V transformer and going fixed bias but given the number of preamp tubes I would be a bit short on voltage running the preamp tube heaters in a string. Dropping the cathode follower would work but decided against it. Also part of the reason I am skipping the first stage in the JMC800. Thinking of floating the heaters on the B+, can see no reason why not given the voltage.

I read you talk about 70V line transformer, so I pull out the 70V. For 30V, it is that much lower. I just read the posts on this thread, I have not done any of these, but you'll notice they bias very differently from your schematic. All I said is read to posts on this thread, change accordingly so you can hit it in the ball part to start.