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BR-9 amplifier questions


murray
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Hi:

 

I was given a BR-9 amp in the mid-80's in need of a new voice coil. I assume an entire re-cone would go along with that.

 

I didn't know where to get that done so had someone temporarily put a PM speaker and filter choke in it, and set the field coil Jensen speaker aside in a box...moved several times...and didn't know where it was.

 

Found it this weekend, and there is also a place that repairs all manner of speakers, including field coil types less than an hour from me. I stopped there once to inquire about estimated cost and absolutely no estimates or even guessing without seeing it.

 

So, I'll try to follow up on that. I'm here to ask:

 

1) Does anyone have a wild guess what would be a REASONABLE price (US$) once they DO quote me?

 

2) Anyone know anything about the wire color code for the field coil? A bit faded in color, 4 wires, best I can describe them is Red/Black, Red/Yellow, Black cotton over white, and Yellow (or Dirty White) cotton over Black. If this makes no sense I'll take another look...it's what I scribbled on a piece of paper while doing acrobatics in the basement trying to reach the speaker.

 

3) Anyone ever measure any characteristics of the output transformer mounted on the speaker frame (or is that called the 'basket'?).

 

The metal 'frame' or 'basket' has numbers F8 U C5689 220240

the tape around the field coil also says C5689

The cone itself says 4921W3

 

The speaker label says Jensen Chicago and some other logo stuff I don't remember.

 

I'll guess it's roughly 8" diameter, maybe smaller if the cone 'spider' is the functional measurement.

 

I think 1947 sticks in my head for some reason, but that's a guess at this very moment, and not terribly important to the questions...

 

I am happy to see my animated avatar works here (easily amused)...most sites petrify the poor little guy...good thing it doesn't have sound too.

 

Thanks

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...more info

 

The Red/Black - Yellow/Red (I may have had the color 'bias' reversed) pair measure about 780 ohms with a DMM. The 'larger-insulation-diameter' pair with solid color cotton insulation measure less than 1 ohm, and so do my ohm meter leads...so that concerns me a little...just seems too small to serve a purpose if that's a legitimate measurement.

 

Murray

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  • 3 weeks later...

Well, I got some answers for myself, and want to share them in case anyone else can use the info.

 

The field coil structure has two separate windings as 4 wires.

 

It appears to be one of several variations in field coil magnet speaker construction, one winding serving as the electromagnet field coil (instead of a permanent magnet), and the second is likely a humbucking winding in series with the voice coil.

 

The field coil winding is also used as the filter choke/inductor in the high voltage power supply.

 

The field coil winding is described as 1000 ohms on the commonly found BR-9 schematic on the WWW, and no description of the 2nd winding. Another site describes the humbucking winding as only a few turns and 'low resistance'.

 

I took the speaker first to a coilwinding lab contact who says my favors will disappear if I say who they are, so that is what it is...

 

The DC resistance of the field coil was 747.4 ohms, and the inductance at 120 Hz with 10Vrms across it was 9.09 9 H. At 5 Vrms, it was 8.55 H.

 

The impedance at 120 Hz and 10 V rms for the field coil which consists of the inductance plus the coil wire resistance was 8.705k,@ phase angle 51.9 degrees, or R=5370 ohms, L = 9.09H.

At 5 Vrms, Z = 8.193 k, @ phase angle 51.9 degrees, or R = 5051 ohms, L = 8.551 H

 

I take the dependency on applied voltage to mean the core isn't quite fully magnetized at 5-10 Vrms, and at a higher applied ripple voltage (someone suggested 30% of the plate voltage value), I suspect the inductance would be higher than the 8.55-9 H.

 

Who cares, etc?

 

There are many articles on the web on how to replace a field coil speaker with a PM (permanent magnet) one. They usually suggest using a resistor equal to the field coil resistance, and ideally inserting a filter choke with an inductance having the same impedance as the field coil, nominally the 1000 ohms, measured as 747 in my case.

 

I have not seen any published values of inductance previously. What these measurements tell me is that the impedance of the field coil as a power supply filter choke is higher than the 1000 ohms resistance...probably no surprise to anyone who is following along thus far. I chose 120 Hz is it's the ripple frequency in the US for the full wave center-tap rectifier configuration...100 Hz in countries with 50 Hz mains.

 

It's possible the inductance could be lower with DC flux in the core, but as stated above, the inductance is probably low due to a limit of 10 Vrms available from the signal generator used.

 

If I were to replace a field coil speaker with a PM one, I would aim for a filter choke with an inductance of at least 10 H at roughly 80 mA DC (guesstimate for all tubes powered in the BR-9) choke plus additional resistance totaling a minimum of that 1000 (or 750) ohms, of appropriate power rating to avoid overheating.

 

Oh, the humbucker winding is probably truly only a few turns...it measures 114 milli-ohms, and 39 uH at 1000 Hz (chosen as a mid-band audio frequency). Initially I feared it was a shorted winding, but a shorted winding on a shared ferrous core would cause the other winding to be reduced to near zero (leakage inductance only), and it is not, evidenced by the 9-ish H inductance. The shop said they are indeed very low resistance.

 

I'm doing the opposite, removing a resistor and filter choke and PM speaker someone put in for me around 1990 before I understood what I would eventually need to do...25-ish years later.

 

I found a local speaker repair facility that has done many many such field coil speaker voice coil rewinds and recones, and surprisingly, the quote ($45) was less than some mail-order ones ($65 and up) I got...and no shipping...

 

I assume they will replace the cone if they are rewinding the voice coil, but I trust whatever they decide is appropriate. I would assume old paper cones need replacement, but some articles say that is at least more more aspect removed from 'original'...

 

When I get it back, I won't be able to try it out in the amp right away...needs new filter capacitors and power cord, but once I know exactly what I paid for and how much, I'll follow up...

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$US 46.20 with tax. www.circuitshop.com, Caledonia MI.

 

Wound new voice coil per old one (I get about 3.8 ohms DC measurement on new one), replace cone (paper with felt dust cap), spider, etc., & Litz wire leads to voice coil terminals pressed into the metal frame. Shop said the 240 part of mfr/date code would have to be 40th week 1942 since permanent magnet speakers had replaced field coil types by 1952.

 

I didn't think the BR-9 was made as early as 1942, so maybe the speaker isn't original (older). I'll have to look for other component date codes but I can't quite reach the amp at the moment. I took lousy blurry pics with my phone.

post-6377-061846300 1400346728_thumb.jpg

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  • 2 weeks later...

The amp face looks like early 50's style, and the description in some literature called it a 'throwback' design with field coil speaker...so I think this means 1952 instead of 1942 is possible and the statement I was given about field coils being gone by 1952 might be generally correct but wrong for the BR-9...no way for me to know, however.

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  • 2 months later...

It's apparently one of the 'transitional' ones that has the GA-9 circuit: parallel-single-ended 6V6GT outputs, one 6SJ7 input, no interstage transformer.

 

I e-mailed the manufacturer of the power transformer to inquire if they might have made the output one.

 

They knew what it was as they had designed one on request from Gibson, but did not end up being the supplier.

 

GA9-O was supposed to be 2.5k:3 ohms with a single-ended DC bias current rating of 70 mADC. The 3 ohm secondary was to go with the common 3.2 ohm voice coil of the era. Turns ratio was 27.5:1 (which suggests 2.5k:3.3 ohms)

 

It was suggested that Rola made alot of the transformers as they were owned by the same company that owned Jensen for a long time.

 

 

I would like to find a GA9-O transformer, good or bad, or dimensions and wire color, or a suggestion for a replacement.

 

I'm looking at 5k:8 ohm with 70 mA DC current rating possibilities, thinking of it as a 2.5k:4 scenario...possibly Magnetic Components Inc/Classic Tone 40-18031, or one of the Edcor XSE or GXSE series, depending what their current ratings are.

 

Thanks

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In case there is anyone other than me interested in this topic, another update...eusa_hand.gif

 

MCI ClassicTone 40-18031 5k primary & 8 ohm secondary with a 4 ohm speaker will approximate the 2.5k:4 goal, but will run a little warmer with the higher current.

 

Edcor recommended their 15W 2.5k:4 GXSE15 series part, and it happens to have a 120 mADC rating and 40-18000 Hz frequency response vs. the typical 70-80 to 15000 Hz response typical of smaller ones. I wonder if this is more of a 'mid-fi' one...that may be harder to overload than a 'puny' one...or just a step further away from trying to approximate the original characteristics.

 

I may get an Edcor one for future experiments, but I think the ClassicTone 40-18031 may be my best bet for trying to match the original.

 

I wish it wasn't $13-16 to ship a $30.66 part, but it's still a pretty small investment overall to get the amp back up & running (capacitors don't count as I already had them).

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  • 4 months later...

I've read your posts with great interest due to my recent purchase of a 1951 or 1952 BR-6f. I haven't found much info on this series/era of amplifiers, so I'm interested in learning all that I can on these amps.

 

My amp is almost totally original. The only items not original include the handle, 4 capacitors, and I "think" 1 resistor (it's the only one with a tolerance band on it).

 

Thank you for posting.

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I've read your posts with great interest due to my recent purchase of a 1951 or 1952 BR-6f. I haven't found much info on this series/era of amplifiers, so I'm interested in learning all that I can on these amps.

 

My amp is almost totally original. The only items not original include the handle, 4 capacitors, and I "think" 1 resistor (it's the only one with a tolerance band on it).

 

Thank you for posting.

 

 

Hi:

 

I saw a Gibsonette in a store this week and peeked in the back at the tubes and the transformer, then looked it up...very frustrating how often designs were changed under the same product model name...I guess if one considers how many Les Paul variations there are, it's no surprise...

 

There were two different circits for BR-9 (BR-9 original and later BR-9's had GA-9) circuitry.

 

The Gibsonette (GA-8) apparently had three or four variations! At least one schematic out there actually was marked with a GA-9-O out put transformer, so I may have actually seen one without recognizing it...I'll go back with schematics & ask & the store will let me look a little closer.

 

A good site to explore for your project is http://music-electronics-forum.com

 

It seems to have absorbed (has archives for) ampage.org, which is where I found schematics yesterday (there are always multiple sources, and the more you find, the better equipped you are to recognize anomalies in an amp).

 

I assume you found this:

 

http://schematicheav...sonamps/br6.pdf

 

and might as well list this too...

 

http://schematicheaven.net/gibson.html

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  • 4 years later...
On 5/17/2014 at 12:09 PM, murray said:

$US 46.20 with tax. www.circuitshop.com, Caledonia MI.

 

Wound new voice coil per old one (I get about 3.8 ohms DC measurement on new one), replace cone (paper with felt dust cap), spider, etc., & Litz wire leads to voice coil terminals pressed into the metal frame. Shop said the 240 part of mfr/date code would have to be 40th week 1942 since permanent magnet speakers had replaced field coil types by 1952.

 

I didn't think the BR-9 was made as early as 1942, so maybe the speaker isn't original (older). I'll have to look for other component date codes but I can't quite reach the amp at the moment. I took lousy blurry pics with my phone.

post-6377-061846300 1400346728_thumb.jpg

 

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  • 3 months later...

I  realize this thread is very old but I ran across it just this morning...I have replaced OT's in Gibson amps that used 2 6v6's in parallel output/single-ended configuration with an OT from Allen Amps.  From the Allen Amps website:

"TO11S Heyboer output transformer rated 4,000 ohms primary to 4 or 8 ohms secondary. It is the same as the above TO11C but mounted on its side the short way to prevent transformer coupled hum. It will now be the standard OT for the 10W Chihuahua. Excellent choice for a 6L6 or EL34 single-ended amp using either a 4 or an 8 ohm speaker load. 3-1/8" mounting centers. It is the same physical size as my TO22 transformer"

 

 

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