Cool old 1940s guitar amp with field coil speaker (field coil speaker question)

[Toxophilite]

another amp from the same friend...the guy has all the luck! and he worked in music store for 30 years
This one is an amp made by Turner of Toronto Ontario Canada
Says"only use AC current' on the front of the chassis!


5Y3
6Y6g
6J5
6C5
filed coil speaker, near mint condition, looks like a new amp!
NOT working
Heaters come on
but no noise or sound
Looks like it's full of firecrackers and crayons and a couple of tall narrow can caps(with wires coming out!)containing 2  8mfd caps each
I think a full recap is in order and I'm betting that will bring it back to life providing the original tubes are good
a three prong plug might be an idea too
Is there a way to test if a field coil speaker is still good??

[sluckey]

Is there a way to test if a field coil speaker is still good??

You can measure resistance of the voice coil. It should be some low resistance, ie, 4, 8, 12, 16Ω, etc. Look at the cone. Is it torn? Brittle? Flexible? Will it move when you gently push on it?

Now measure the resistance of the field coil. It may be anywhere from a couple hundred ohms to 5K, or even 6K. Depends on how it's connected in the power supply.

Measure voltages in the amp, each filter cap and tube pins. Try to draw a schematic, at least for the power supply and power amp.

[Toxophilite]

Here's my attempt at a schematic so far
No voltages yet and the field coil speaker baffles me.
Thus the speaker floating space and the extra coil down at the bottom right

It kind of looks like the speaker is a replacement as there are 3 cream coloured cloth wires going to it from the amp and then they are taped up to 5 (yes 5) different coloured cloth wires going to the field coil speaker
3 leads to the transformer (beige brown green0 2 to the electromagnet(??)(black and white)
I can't really get a probe on any of these wires as they go right into their respective transformers/coils
I think there's a ground and a hot going to the transformer and the coil and an audio signal going to the transformer
If I measure what seems to be the voice coil (tiny leads going to the actual cone) I get .4 ohms


The Rola speaker says it's a  10k2  1000 ohms   manufacturers code 285 722

[sluckey]

Plug amp into a lamp limiter. If the lamp does not glow brightly then plug amp straight into the wall.

Get some voltage measurements. You know there should be big AC volts on pins 4 and 6 of the 5Y3 and there should be B+ on pins 2 and 8 of the 5Y3. That RED/YEL wire from the PT is the center tap. All voltages should be measured with reference to that center tap. (NOTE! The center tap may not be connected to chassis. Look and see. If it is just measure voltages with reference to chassis.)

6Y6 is wired just like a 6V6. There should be B+ voltage on pin 3 and pin 4. If cathode biased, there should be 15V to 20V on pin 8.

There's nothing baffling about a field coil speaker. It's just like a modern speaker except it doesn't have a real magnet. The field coil is an electro magnet and does the exact same thing a permanent magnet does. In your case the field coil is probably wired just like a choke in the power supply. It's not likely that the field coil is open, but if it is, you will likely have B+ on the 6Y6 plate but you won't have any downstream B+ voltages. Hopefully the attached drawing will help you troubleshoot the power supply. If you have downstream B+ voltages the field coil is probably OK. (BTW, I suspect the speaker is original and the OT has been replaced. That would explain the taped splices.)

You can measure the resistance of the field coil by measuring across the correct two of those 3 cream colored wires. If you are not sure which two that is, just remove the tape and expose the ends of the black and white wires that you know go to the field coil.

This drawing may not be exactly what you have but it's probably close...

[Toxophilite]

Ooopsy
NO voltages on pin 2 and 8 of the rectifier
I do have 373 AC on pins 4 and 6


I think the rectifier might be pooped out..I do have another but I'm very leary of giving the unit full power without some nice new power supply caps in there
I don't really want to cook any transformers


Doesn't that seem like a prudent course of action?

[sluckey]

NO voltages on pin 2 and 8 of the rectifier

This could also be a short on the B+ line, like a filter cap. Measure resistance from pin 2 and 8 to ground. Remember that your red/yel center tap is ground and it may not be connected to chassis. Have you looked at that yet?

[Toxophilite]

Hi i noticed the center tap tap to ground pretty early on. There are at least 4 mechanical ground connections, one at each tube basically. The center tap is on the 6C5 tube ground which I thought was unusual but it is grounded. I checked the continuity from the ground point to the chassis just to make double sure


There is continuity to ground from pins 2 and 8  so maybe a short?

[sluckey]

There is continuity to ground from pins 2 and 8  so maybe a short?

How many ohms? Zero ohms is a short.

[Toxophilite]

0 ohms,  a short, I was being unnecessarily tentative

[sluckey]

Start disconnecting stuff until the short disappears. Filter caps are prime suspect.

[Toxophilite]

Will do


As usual, you're right about the OT, etc it looks like it's a pushpull OT that they're using half of!


the 3 cream wires:


1- From the pin 4 and  the first filter cap to the field coil


2- return from the field coil? The red (B+) wire from the OT is combined with the black field coil wire where they are both joined to this cream coloured wire


3- Signal from plate of 6Y6 to transformer to the green wire of the OT


The extra brown wire from the OT is not attached to anything

[Toxophilite]

disconnecting the 1st filter cap did it  Now I get 16M to ground which starts dropping by about 50 ohm increments


here's a revised schematic depicting what seems to be happening with the field coil etc
The schematic you posted was very helpful for this
Though I think I still have it wrong

[vibrolax]

I don't like the power switch on one leg of the mains line and the fuse on the other.  Are you going to re-do the mains wiring the normal way, with neutral to the PT and switch and fuse on the hot?

The other thing I saw that I wondered about is the screen connection, with the screen filter cap on the far side of the 15K resistor.  But now I recall that the 6Y6 needs a much lower maximum screen voltage than a 6V6, like 135V.

It the CT of the OT primary really ground???? That would make a very low resistance from B+ to ground, i.e. essentially a short circuit.  I'd expect the CT wire to be tied off and insulated.  Maybe the CT of the OT is tied to B+, one end to the plate, and the other end unconnected.

[Toxophilite]

welll
No plans to rewire anything as yet, though new caps all around will probably be installed as well as maybe replacing a few resistors. and likely a three pronged plug


Admittedly the whole area around the speaker and the OT is a little dodgy on my schematic, not necessarily reflecting the real deal..I'm still figuring it out really

[sluckey]

The OT is probably a universal replacement single ended transformer that has a tapped primary for different tube types. The brown wire is one of those taps and is not connected. The "green" wire to the plate is probably blue. Look closely.

Your pics only show two wires for the field coil. That's all that's most field coils have. If you have more than that, post some additional pics.

You've already found a major problem with that shorted filter cap. All the filter caps look original and need to be replaced. All the coupling caps look original too. Those types are prone to leakage and need to be replaced.

I'll bet that a cap job will make this thing sing again. But while you're in there check all those resistors too. Carbon comps tend to increase in resistance with age.

I've doctored your drawing a bit more...

[Toxophilite]

I concur
here's a couple pictures of the speaker, OT etc
This is the area that confuses me
As you can see there looks to be two wires entering the field coil(?) and two leaving then there's 3 connection son the speaker proper on comes from the OT, the middle one seems to join the field coil and the OT and the other outside one comes from the field coil...strangely there seesm to be continuity between the three attachment points on the speaker or only .4 - .6 ohms

[eleventeen]

Remember that in many cases, when you are measuring ohms and getting a sub-1-ohm reading you are reading the resistance of your test leads....in series with a short circuit.


I can only guess / NOT rule out the following:


I would imagine the two terminals right next to each other on the speaker are common. Remember, if there is a requirement to ground the speaker frame or one of the leads going to the speaker, that speaker is mounted on a piece of plywood and not inherently connected to the chassis.


The connection between the one lead of the field coil and the supply side of the OT *may* occur via THREE wires. There could be a wire (out of the rectifier) running to the FC, then BACK into the chassis, then up to the speaker mounted OT. These can be a tad frustrating to line out. Since this project is not going to be dumped into the trash no matter what happens, you can feel confident going ahead and investing in parts you KNOW you will need. I agree with Sluckey, replace all the caps and (for now) forget about the strange speaker/OT wiring.


That would be my approach.

[Toxophilite]

Yeah ,thanks
It was something I was checking out more for the schematic
As I'd never drawn one from scratch so to speak. I was enjoying the process and investigating how it all hooked up and as much as I could understand ,why.


My first thought, just looking at the amp was "replace all the caps!" A couple of the carbon resistors look a little done too
The whole amp is pretty low on  components anyway
Off to the electronics store on Monday!

[Toxophilite]

Completely recapped the amp and replaced about 4 resistors. fired it up with just the old rectifier...no dice. swapped in another 5Y3Gt I had that I knew was good. Huzzah she lives
Popped in the other tubes , brought it up on the variac


It sounds quite nice, not noisy at all.
seems to get most of it's volume between 5-10 on the pot (or as the control says medium volume, full volume))
I'm going to clean the pot
with my low output old gretschs it's clean clean clean..and clean and gets up to a decent practice volume
Nice sound too considering there's not really an eq


Thanks all
Now I don't want to give it back!

[sluckey]

Now I don't want to give it back!

I can see why! That amp looks like it's never seen any daylight. Pristine inside and out.

If you think you may ever work on that amp again you may want to measure some voltages and annotate your schematic. Could save a lot of backtracking and head scratching down the road.

Y'all take it easy on that old speaker.
 

[Toxophilite]

Hahah Not THAT loud
Good sound though, kinda feel like I could redo the Sun Sessions with it
Or maybe lay down some Barney Kessel ala the sessions he did with Julie London
That straight in, very simple amp sound
Nice!

[Platefire]

Really Nice! Just hard to believe an old amp like that could remain that clean through the years. Out of my amp building and restoration, that is one thing I haven't done that I've had a hankering to do. Get a 40's type amp and bring it up to speed. Thanks, Platefire

[TIMBO]

Good work guys, then there is hope for this one http://el34world.com/Forum/index.php?topic=19150.0
As this is an amplifier by itself I guess I'm going to have to track down a speaker with a field coil.