Magnavox Georgian 151b Mod

[Paul1453]

A new victim has arrived in my shop.  I picked up a Magnavox Georgian 151b for $20 yesterday.  Stripped her down and cleaned her up, now what to do with it?  I know I don't have a good handle on this output transformer thing yet, but this one appears to have come with the OT mounted on the 12" speaker.  But, even though this is the original design part I'm guessing you all will tell me I need a different one for a Guitar amp.    Well what do you think?  Can you point me to a 6V6 schematic that you have used and really liked the results?  From what I've found so far I only need one of the 6V6s for a 5W amp.  Using both in Push/Pull can make 12W+???  Looking to keep most of my projects down in the 5W range as home practice amps.

[shooter]

here's a quicky search link
http://www.nostalgiaair.org/PagesByModel/331/M0010331.pdf

the link came from antique radio, the write up stated PP 6v6 with a field coil speaker, just from the 2 minute search and info, sounds like a 5E3 build, or if you want less ump, go SE 5F2 ish, probably one of my best lo-watt amps.  go fishing for field coil speaker articles and you will get the just between that method and the more common OT used today

[Paul1453]

Thank you!  The first Wiki article I found said the field coil speaker was used in some high end Hi-Fi gear.  I would like to try to use this speaker on this if possible.  Not worth the trouble, or the sound will not be good for a guitar amp?  Seems like a decent speaker, would be a shame not to be able to use it.

[shooter]

http://www.radioremembered.org/edspeaker.htm

if you want to try and keep it, I'd start here.  BE WARNED, some of the tests/checks this guy is doing WILL kill you if you're not CONFIDENT working on HV

I'd start with the dcr - ohms check -  and go from there, knowledge is key here

[Paul1453]

Seems like the field coil speaker may be making a comeback in Guitar amps.  How to get that sweet over-driven output tube sound without the ear-splitting volume or Hot Plate dummy load device?  Use a Fluxtone field coil speaker.  http://fluxtonespeakers.com/how-it-works.html
http://fluxtonespeakers.com/

[Willabe]

The Fluxtone is not really a field coil speaker.

[Paul1453]

The Fluxtone is not a field coil speaker.

Are you sure?  They don't look like Alnico magnet speakers to me.
http://fluxtonespeakers.com/product-list.html
OK, so isn't varying the strength of the magnetic field the basic idea of a field coil speaker?

Don't know for sure about the fluxtones, but I do know my Magnavox speaker doesn't have a magnet in it.
Fluxtones are insanely expensive, IMO.

[Willabe]

I think they work by varying the magnetic field's strength that the voice coil sees.

I thought they was still a magnet in them, but maybe your right. 


 

[Willabe]

Hmmmm, spec sheet list's magnet type and magnet weight. But it also give a dc resistance and coil inductance #'s. There sure looks like there's a big wire coil wrapped around where the voice coil is.

Man, do they cost the $$$$$$$$$$$$   

I think I was thinking about another type of variable power speaker, I forget who makes it, was it Eminence? 
 
 

[HotBluePlates]

...  I picked up a Magnavox Georgian 151b for $20 yesterday.  Stripped her down and cleaned her up, now what to do with it?  ... this one appears to have come with the OT mounted on the 12" speaker.  ...

... the write up stated PP 6v6 with a field coil speaker, just from the 2 minute search and info, sounds like a 5E3 build ...

...  The first Wiki article I found said the field coil speaker was used in some high end Hi-Fi gear.  I would like to try to use this speaker on this if possible.  ...

A bit of advice beyond this one amp: If you're buying old gear to refurbish/repurpose, assume you will be keeping the power supply, power output stage and phase inverter (if present) largely intact. While it's pretty easy to gut/replace a preamp, the power transformer, output transformer, output tube type (and in this case, speaker + field coil) are all designed to work together. When you pull those pieces apart, you'll bang your head against a wall until you arrive at the same place as where the power supply/output stage started. Not that I learned all this the hard way by making old gear inoperable thinking I could just take a power or output transformer for something else ... 

The field coil is the electromagnet for the speaker, but also a choke in the power supply. So PT voltage & current is already sized to allow for drop across the field coil (often ~1kΩ DCR) while still giving appropriate supply voltages for the output tube and OT primary impedance. Of course, the OT is mounted on the speaker. So my recommendation is keep these sections (and phase inverter) intact. Find a preamp you like and add it in front of the phase inverter.

[HotBluePlates]

Seems like the field coil speaker may be making a comeback in Guitar amps. ...  http://fluxtonespeakers.com/how-it-works.html
http://fluxtonespeakers.com/

It's been a while since we broke down how the Fluxtone does what it does. http://el34world.com/Forum/index.php?topic=7384.msg65487#msg65487

Fluxtone is a side-show for this discussion, because that's all about creating a different "master-volume" solution other than those already in use (master volume, variable supply voltages, variable power output schemes, etc).

I doubt we'll see vintage-style field coil speakers again, if only because high voltage has to leave the chassis to energize the field coil, and I doubt UL, CSA, ETL, etc will start thinking that's a good idea in the modern world...

[sluckey]

OK, so isn't varying the strength of the magnetic field the basic idea of a field coil speaker?

The basic idea of a field coil is to make an electro magnet that acts exactly the same as a permanent magnet in a modern speaker. Way back then there were no cheap ceramic magnets. So, electro magnets were the cheap alternative.

[Willabe]

Just read through the link HBP posted, I stand corrected, it is a field coil speaker.

[Paul1453]

A bit of advice beyond this one amp: If you're buying old gear to refurbish/repurpose, assume you will be keeping the power supply, power output stage and phase inverter (if present) largely intact. While it's pretty easy to gut/replace a preamp, the power transformer, output transformer, output tube type (and in this case, speaker + field coil) are all designed to work together. When you pull those pieces apart, you'll bang your head against a wall until you arrive at the same place as where the power supply/output stage started. Not that I learned all this the hard way by making old gear inoperable thinking I could just take a power or output transformer for something else ... 

The field coil is the electromagnet for the speaker, but also a choke in the power supply. So PT voltage & current is already sized to allow for drop across the field coil (often ~1kΩ DCR) while still giving appropriate supply voltages for the output tube and OT primary impedance. Of course, the OT is mounted on the speaker. So my recommendation is keep these sections (and phase inverter) intact. Find a preamp you like and add it in front of the phase inverter.

I appreciate this advice, and all the other helpful info received here.  At least this Magnavox came in a relatively nice looking cabinet for it's age.  The parts alone should sell on Ebay for more than the $20 I've got into it, if this modding dream proves to much for me to handle.  I've learned a lot already and am not ready to throw in the towel yet.  Hopefully I get up to speed rather quickly with all your help. 

EDIT: Added missing quote tags -- HBP

[Paul1453]

The field coil is the electromagnet for the speaker, but also a choke in the power supply. So PT voltage & current is already sized to allow for drop across the field coil (often ~1kΩ DCR) while still giving appropriate supply voltages for the output tube and OT primary impedance. Of course, the OT is mounted on the speaker. So my recommendation is keep these sections (and phase inverter) intact. Find a preamp you like and add it in front of the phase inverter.

So you are suggesting I do something like this.  Please excuse the crude cut and paste job.  I know the values and connections will need modification, but this is the idea I should follow to use the PP 6V6 output section with the field coil speaker?

[shooter]

Pretty sure that's what HBP means, but before you jump head first, might wanna know IF the speaker/field coil, PS etc are good.  Haven't priced them, but I believe a good working field coil speaker might set you back a few bucks.   

[Paul1453]

before you jump head first, might wanna know IF the speaker/field coil, PS etc are good.  Haven't priced them, but I believe a good working field coil speaker might set you back a few bucks.

How to test then?  PT will measure very low resistance between the windings, even if it has melted some together, so unless some show open I'm not sure how to test it without hooking it up and measuring voltages.  For the speaker, I can measure both sides of the output transformer again looking for open to indicate bad.  Nothing on this looked burnt or completely melted like I would expect from a serious short.  Hoping to move forward thinking these parts are good.  If either of these major parts are bad, it will be time for a re-evaluation of a completely different tried and true design, no?

[jjasilli]

FWIW:  http://www.chambonino.com/work/gibson/gib1.html

[shooter]

Depending on your skill set, determining a good/bad PS is pretty straight fwd.  Once that is done, then moving to the speaker is next, since it's actually part of the PS, but 1st it needs good volts coming outta the 5y3 along with good filaments, AND a 3-prong conversion!

[Paul1453]

FWIW:  http://www.chambonino.com/work/gibson/gib1.html

That's the ticket!  Right down to the field coil speaker.   How else would I have know my Maggie was a classic Gibson Bronco in disguise?  You are the man jjasilli!
Thank you very much!

[PRR]

> field coil speaker was used in some high end Hi-Fi gear.

No.

1890 through 1940, the only GOOD speaker magnet was an electro-magnet (field coil).

1940-1950 speaker design switched almost entirely to permanent magnets, first Alnico then ceramic (and now poly/moly/sammy exotic ceramics for some work). J. B. Lansing was very excited by Alnico and what he could do with it.

This despite the fact that a field-coil can do double-duty as a filter choke. (The rise of lower-price electrolytic filter caps took some of the sting out of the loss.)

> isn't varying the strength of the magnetic field the basic idea of a field coil speaker?

Didn't used to be. Holding the field *constant* was a design goal. Changing the field strength screws-up the careful balance of all speaker parameters.

In the Old Days, guitarists would be the last to like that, because you can only play softer (louder leads to smoke).

Yeah, every dumb idea comes around in a new era, and I agree there may be some point to a speaker so lame you can throw horsepower in and get flea-power out. Hey, I can buy a Corvette and drive around town with the parking brake dragging.

> Stripped her down and cleaned her up, now what to do with it?

Well, if you hadn't already ripped it up, you cudda just played it. A high-class radio and a guitar amp are practically the same thing! Main difference is instead of an AM tuner you have a guitar preamp.

See the Volume pot? Disconnect the top leg, inject audio to it, you have a 12 Watt audio power amp.

This will "play" with a geetar right to the volume pot but it will be hard work. An LPB booster or two in front is a quick fix. Another fix is to re-purpose a radio-tuner tube as a guitar preamp. Often you can rip-loose the IF tube and rig it as a standard audio stage. In this case there's an even better trick: the "DET-AVC" diode is really a 6J5 triode (cheaper by the gross). Re-wire that using the part values from "1st AF" stage, it'll play nice. If no spare audio triode, triode-wire the IF pentode and that'll work.

The Magnavox speaker probably has killer tone. Magnavox was a pioneer in loudspeakers and were years ahead of other makers. Modern guitar speakers ARE the Magnavox plan, just built cheaper of tougher materials. That may be a problem: that speaker is durable playing 1W-2W of UN-distorted sound in a large parlor or tavern, but in over-driven guitar work it will probably tear itself apart (if age hasn't got it yet).

[Paul1453]

> field coil speaker was used in some high end Hi-Fi gear.

No.

1890 through 1940, the only GOOD speaker magnet was an electro-magnet (field coil).

1940-1950 speaker design switched almost entirely to permanent magnets, first Alnico then ceramic (and now poly/moly/sammy exotic ceramics for some work). J. B. Lansing was very excited by Alnico and what he could do with it.

This despite the fact that a field-coil can do double-duty as a filter choke. (The rise of lower-price electrolytic filter caps took some of the sting out of the loss.)

> isn't varying the strength of the magnetic field the basic idea of a field coil speaker?

Didn't used to be. Holding the field *constant* was a design goal. Changing the field strength screws-up the careful balance of all speaker parameters.

In the Old Days, guitarists would be the last to like that, because you can only play softer (louder leads to smoke).

Yeah, every dumb idea comes around in a new era, and I agree there may be some point to a speaker so lame you can throw horsepower in and get flea-power out. Hey, I can buy a Corvette and drive around town with the parking brake dragging.

> Stripped her down and cleaned her up, now what to do with it?

Well, if you hadn't already ripped it up, you cudda just played it. A high-class radio and a guitar amp are practically the same thing! Main difference is instead of an AM tuner you have a guitar preamp.

See the Volume pot? Disconnect the top leg, inject audio to it, you have a 12 Watt audio power amp.

This will "play" with a geetar right to the volume pot but it will be hard work. An LPB booster or two in front is a quick fix. Another fix is to re-purpose a radio-tuner tube as a guitar preamp. Often you can rip-loose the IF tube and rig it as a standard audio stage. In this case there's an even better trick: the "DET-AVC" diode is really a 6J5 triode (cheaper by the gross). Re-wire that using the part values from "1st AF" stage, it'll play nice. If no spare audio triode, triode-wire the IF pentode and that'll work.

The Magnavox speaker probably has killer tone. Magnavox was a pioneer in loudspeakers and were years ahead of other makers. Modern guitar speakers ARE the Magnavox plan, just built cheaper of tougher materials. That may be a problem: that speaker is durable playing 1W-2W of UN-distorted sound in a large parlor or tavern, but in over-driven guitar work it will probably tear itself apart (if age hasn't got it yet).

I understand your Corvette analogy but I think that your scenario more appropriately fits using a Hot Plate or other resistor attenuators.  I see the field coil speaker as fitting the Corvette with an ultra low geared transmission.  In effect, you can still hear your engine roar as you race through the gears, but the neighbors don't call the cops because you topped out at 30mph instead of 180mph.

Both of my finds had broken two prong electrical cords and were in unknown condition.  I felt it was safer for both me and the salvageable parts to just start from a clean slate.  50+ years of dust, smoke, and oily grime makes working on gear quite unpleasant.  I get quite filthy as I tear them apart and clean them up into something that is reasonable to work on.  If I ever do get cheap gear in working order, I will remember your advice and try to keep the original functions intact while adding the geetar part.  ;^)

[Paul1453]

Maggie, as I've affectionately renamed her to, has moved to the top of my repair priority list.  To change her into a Gibson BR-6F it appears to only be missing the 6SJ7 tube, which I got a pair of from Ebay for $6.99 including shipping.  It would be nice to show my wife and children that Dad is not going senile in his old age, and that what appears to be old junk to most people can really be transformed into some nice sounding guitar amps. 
Now for some questions:

Turret board or P2P wiring?  I think I could replicate this circuit P2P, as that is what I am more familiar with, but I think the turret board layout looks cleaner and may simplify troubleshooting if necessary.  Is there software that can create a schematic based on the component placement and wiring connections placed on this kind of a board?  How do some of you design and check your board layouts?

Tube placement and shielding?  I have plenty of room on this chassis to separate the tubes if that will help.  I also have some big tuning circuit coil covers which could be made to fit over the 6SJ7 and 6SN7 tubes as shielding.  Putting them all close together can use less wire to tie things together, but also makes it more difficult for my clumsy and shaky hands to get in there and solder things together.  I'm thinking of leaving the 6v6's in their original location, but moving the 6SN7 and 6SJ7 back some to implement the shielding covers and make it easier for me to solder things together underneath.

Any modifications you would suggest?  I was thinking it would be kind of cool if I could somehow add the little dial light bulbs it had it in a way that would modulate their intensity based on the guitar's output signal.  A parallel path on both sides of the 6V6's output to the transformer maybe?  Quite possibly that's just a bad idea, feedback?

[shooter]

Turret board or P2P wiring

most opt for the turret option, and for the reasons you mentioned.  Doing a *clean* PP is an art.

I use expessSCH and expressPCB for most of my work, it's free   Folks use Visio also, pricey but hands down good!

I always start with paper and pencil, scribble, scratch, modify, till I'm close, then move to the computer

since you already have the Gibson schematic, pencil whip your layouts, Doug has a turret board program that you can print out, then do the pencil, eraser thing til you're sure.
Read as much as you can find about *best practice* for the layouts, wire routing etc

Get a good working chassis, then come back with flashing lites, there are lots of "ways", ldr, opto-couplers, etc.