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 Post subject: Driver transformer woes
PostPosted: Feb Wed 11, 2004 9:27 am 
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Hi all,<P>I'm working on the radio found here (the one on page 4-24)<BR> <A HREF="http://www.nostalgiaair.org/PagesByModel/729/M0016729.pdf" TARGET=_blank>http://www.nostalgiaair.org/PagesByModel/729/M0016729.pdf</A> <P>Audio wasn't as good as expected after recap. So I discovered that only one of the 2A3s was doing anything, and found that half of the output transformer primary was open. A replacement was duly ordered from PTOP and installed, and although voltages on the plates were now fine, it still didn't sound right. So I checked the grid voltages on the 2A3s. One had -45, the other had +4. Yup, half the secondary of the driver transformer is open.<P>This driver transformer is center-tapped on both the primary and secondary. There are two center taps on the secondary; each goes to one side of a pot that controls hum. I have no problem with bypassing this and using only a single center tap. But I still need help - what kind of a transformer can I get to replace it?<P>Just out of curiosity, is this going to be an all the time thing where I'm going to have to replace transformers? If so this radio restoration hobby is going to get to be a problem. This is the third transformer I've had to replace in two sets, and they're expensive. I'm not pleased.<P>Sorry to sound whiny, I'm just pretty disgusted right now...... oh well<P>regards,<P>------------------<BR>Paul


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 Post subject: Driver transformer woes
PostPosted: Feb Wed 11, 2004 1:47 pm 
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Hello<BR>I would look at the multi-tapped replacement from AES.It has a centre tapped primary (15-135K Ohm) and secondarys giving ratios of 1: 1:1.5, 1:3, 1:6. The secondary is not centre tapped but you should have no problem getting around that, as you say. It is P-T124E and is $25.<BR>I would actually measure the ratio first as you still have one good half. Also take the thing apart after... you may find an easy break near a terminal...<BR>Some one may come up with cheaper alternative or somewhere that may have an original. <BR>No! don't get despondent... in a lot of radios over the years I have only had a few bad transformers. Some manufacturers made them better than others and it also depends where the radio was kept... a warm desert climate or a southern swamp...<BR>Gary<P>------------------<BR>


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 Post subject: Driver transformer woes
PostPosted: Feb Wed 11, 2004 8:11 pm 
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<tinfoil-hat> Personally, I blame extraterrestrial aliens. They must have something to do with it.</tinfoil-hat><P>Actually, the replacement output transformer seems to be working fine; both tubes are now getting roughly equal plate voltage whereas previously only one got any at all (except I have no idea if I have the secondary hooked up right - it's multi-tapped for different voice-coil impedances and I don't know which is which, so I just picked one black lead and one colored one. Anyone who would care to impart me with clue on this point would have my undying appreciation.)<P>The problem now is the driver xformer between the type 37 driver tube and the 2A3 grids. The grids should be getting -44V bias and only one is, the other reads +0.4 (left out a decimal point in my previous post). Interestingly, it is the same socket that had both of these problems. Both 2A3 tubes are fine; I swapped them in and out of the good socket and each one worked well. <P>I'm also concerned about hum - the new output transformer does not seem to have a humbucking coil as the OEM one did, and if I have to bypass the hum adjustment pot, am I going to have problems? If so could anyone suggest improvements I could make? I don't much care about originality here - the thing needed a total restoration as it was; I just want the audio section to work well so I can add an outboard phono jack to this set to hook up my MP3 player.<P>Thanks much<P>------------------<BR>Paul


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 Post subject: Driver transformer woes
PostPosted: Feb Wed 11, 2004 9:10 pm 
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Hi Paul,<P>First of all, don't run the radio for more than a minute or two with positive voltage on the 2A3 grid... you'll destroy the tube. Also, your plate voltage readings are meaningless with no bias on one of the tubes.<P>A hum-bucking coil is normally part of the field coil assembly on an electro-dynamic speaker. It's purpose is to cancel the hum induced at the speaker by virtue of having AC on the speaker magnet. It's wired in series between the output transformer secondary and the speaker voice coil, but it's physically located on the speaker.<P>The bias arrangement for this radio is interesting. The driver transformer is not center-tapped. It has two separate secondary windings. One goes to a fixed tap on the hum control, the other returns to the hum control wiper.<P>The Hammond 124E is the only one which has the proper secondary configuration. If you're using that device, then use an ohmmeter to check back from the tube grid pins to the hum adjustment pot... you probably have an open or a bad solder joint somewhere, or the hum adjust pot itself is bad.<P>If you're trying to use a center-tapped transformer, it won't work as designed. The hum adjustment control would do nothing with this type of transformer.<P>------------------<BR>73 de Leigh W3NLB | | Leigh@AtwaterKent.Info<BR> <A HREF="http://www.AtwaterKent.info" TARGET=_blank>http://www.AtwaterKent.info</A>


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 Post subject: Driver transformer woes
PostPosted: Feb Wed 11, 2004 9:53 pm 
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Paul Dietenberger wrote:
<font>quote:</font><HR>Originally posted by Leigh:<BR><B>Hi Paul,<P>First of all, don't run the radio for more than a minute or two with positive voltage on the 2A3 grid... you'll destroy the tube. Also, your plate voltage readings are meaningless with no bias on one of the tubes.<BR></B><HR>
<P>Hi Leigh, <BR>Yeah, as soon as I saw + voltage on that grid pin the radio went down and there hasn't been a tube in the socket since. 2A3s are too pricey to waste. I had been wondering why the plate voltages of the tubes weren't exactly equal - the good socket had ~270 IIRC and the bad one read about 227. Oh well, better than the 0 I had before.....<P><B>A hum-bucking coil is normally part of the field coil assembly on an electro-dynamic speaker. It's purpose is to cancel the hum induced at the speaker by virtue of having AC on the speaker magnet. It's wired in series between the output transformer secondary and the speaker voice coil, but it's physically located on the speaker.<BR></B><BR>The OT of this radio is mounted on the speaker frame. When I unwrapped the coil the first thing I found was two layers of about 15 turns each of relatively heavy-gauge wire (about the same gauge as solid hookup wire), of which one end was attached to the voice-coil lead. I had assumed this was the hum-bucking coil. If it is, and my replacement hasn't one, then I haven't one at all anymore.<P><B>The bias arrangement for this radio is interesting. The driver transformer is not center-tapped. It has two separate secondary windings. One goes to a fixed tap on the hum control, the other returns to the hum control wiper.</B><P>You are absolutely correct. I had thought I'd be able to simply join those two together, effectively connecting the secondary windings together in series and completely bypassing the hum control, but I get the impression that that isn't a good idea.<P><B>The Hammond 124E is the only one which has the proper secondary configuration. If you're using that device, </B><P>I'm not; I don't have a replacement at all. I guess I'll have to order one.<P><B>then use an ohmmeter to check back from the tube grid pins to the hum adjustment pot... you probably have an open or a bad solder joint somewhere, or the hum adjust pot itself is bad.</B><P>I had already desoldered the leads to the hum adjustment pot and measured resistances between the leads and the grid pins of the 2A3s. The good secondary measured (going from memory here) 2500 ohms, the other was open. <P><BR><B>If you're trying to use a center-tapped transformer, it won't work as designed. The hum adjustment control would do nothing with this type of transformer.<BR></B><P>True, that's why I thought I might be cutting it out of the circuit. But I think I'll just look for the 124E, as everyone is telling me to do, and hope for the best. Thanks much to all. If anyone has any further comments please speak up. <IMG SRC="http://antiqueradios.com/forums/smile.gif"><P>Regards,<P>------------------<BR>Paul


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 Post subject: Driver transformer woes
PostPosted: Feb Wed 11, 2004 10:09 pm 
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Leigh wrote:
<font>quote:</font><HR>Originally posted by Paul Dietenberger:<BR><B>...I had been wondering why the plate voltages of the tubes weren't exactly equal - the good socket had ~270 IIRC and the bad one read about 227...</B><HR>
<P>Hi Paul,<P>That's because the bad tube was drawing much higher plate current than the good one. The IR drop through the output transformer on the bad side was higher = lower plate voltage.<P>------------------<BR>73 de Leigh W3NLB | | Leigh@AtwaterKent.Info<BR> <A HREF="http://www.AtwaterKent.info" TARGET=_blank>http://www.AtwaterKent.info</A>


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 Post subject: Driver transformer woes
PostPosted: Feb Tue 17, 2004 10:54 pm 
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See if this works. I just copied a site from a previous thread. Should be able to click on it below. If not copy and past into the header of this page:<BR> <A HREF="http://www.nostalgiaair.org/PagesByModel/729/M0016729.pdf" TARGET=_blank>http://www.nostalgiaair.org/PagesByModel/729/M0016729.pdf</A> <P>The second diagram at that site is the simple guitar amp (from Sears) similar to a Fender champ. Instead there are two stages. I would add another pot between the 6SF5 and the 6V6 for a master control.


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 Post subject: Driver transformer woes
PostPosted: Feb Tue 17, 2004 10:56 pm 
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Ooops, rite idea wrong thread. Lets try this again


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 Post subject: Driver transformer woes
PostPosted: Feb Fri 20, 2004 3:27 am 
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Location: ayer ,ma usa
Are you sure that is a hum adjustment? many 2A3 amps needed separate bias adjustments due to High Gm of 2A3; (see RCA tube manual on 2A3)that means either two 2.5V filament windings, or adjustable grid bias. Sounds like one tube is fixed grid bias, the other is variable to match to it in anode current.that was acommon approach. You may have toasted a 2A3 already with no bias. The reason the output trans opened was probably excessive current in one half of it due to the open driver transformer, and resulting lack of bias.. It may be cheaper and better to replace the driver transformer with a 6SN7 driver tube, like a williamson amp ckt. ,or use one half as audio amp, one half as phase inverter... if there is away to fit the extra tube /change another tube; I have done that with Philcos that have the PP 42 's in class B which sound awful and rapidly cook 42's(change driver 42 to 6SN7 and outputs to 6L6 triodes..about same total filament draw, and no driver trans.) anyway, 2A3's sound great...worth fighting the problem!!<P>------------------<BR>


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 Post subject: Driver transformer woes
PostPosted: Feb Fri 20, 2004 4:17 am 
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I have the original instruction sheet for this radio, which references that pot in these terms: "There is a hum adjustment, to be turned with an insulated handle screw driver, at the rear of the chassis, under the type 2A3-H tubes. With the volume control all the way to the left, turn the hum adjustment to the point of minimum hum."<P>Why or how this works, I have no idea. But both 2A3s are still testing fine on my tester, so I guess I'm safe. Thanks for the idea and words of encouragement. I haven't ordered any parts to fix that problem yet, but I look forward to hearing this thing play once it's all set to rights. <P>Regards,<P>------------------<BR>Paul


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 Post subject: Driver transformer woes
PostPosted: Feb Fri 20, 2004 7:50 am 
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Hi Paul<P> Noticed you mentioned 2A3H tubes. These are Raytheon 2A3's with a cathode. Having a cathode and only 4 pins may require a more exact hum balance adjustment?<P>------------------<BR>Norm


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 Post subject: Driver transformer woes
PostPosted: Feb Fri 20, 2004 5:34 pm 
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I wouldn't be surprised, Norm. Who knows. Maybe in 1934, the building of hum-free AC radios was still not perfected and 2A3 tubes were bleeding-edge technology. This is the oldest radio I own of any type, so I just don't know.<P>Yes, this is one of those odd 1934 Colonial-built Silvertone radios that not only used the heater-cathode 2A3H, but also the 83V rectifier that used a separate heater and cathode. I'm sure that when this thing went in for service in 1944, those tubes were unobtainium even then (there was and is a M-R 5Z3 in the 83V socket, drawing an extra amp of filament current.) But when new, all 11 tubes in this radio were of the heater-cathode type. The old output transformer had a humbucking coil, and then there's this hum adjustment. They must have felt pretty serious about eliminating hum from this set. Still, at 14uF each, the original filter caps were kind of small. They've been replaced with 22s.<P>I thought about looking for the correct tubes for this set, but with 2A3H showing up on eBay about once a month at $125 each, and a pair of tested-good 2A3 already in the set, I think I'll pass.<P>------------------<BR>Paul


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