[marv]

I have been working on the 37-660 I picked up recently and have a couple of questions. I have capped it and replaced resistors where necessary. I fired it up and measured volages and all seem to be reasonable except I measure 256V on the plate of the 6K5 where it is supposed to be 150. Also I checked the resistance of the OT and am getting some real high readings. The schematic says on the primary CT to either end should be 330 ohms and 370. I am getting 1540 and 1980. I suspect that there may be those "green spots of corrosion" creating the high readings. I had this happen last year in a 41-629. Can anmyone confirm the approximate resistance readings of the primary should be?

Any clue as to why I would have such a high plate voltage on the 6K5? Thanks.
http://www.nostalgiaair.org/PagesByModel...013220.pdf

[tab10672]

Are you measuring to chassis or B? Also is the OT disconected when reading?

[marv]

I have the speaker removed that has the OT mounted on it. The only connection on the OT is the secondary to the speaker coil. So I am measuring only the OT primary winding. The voltage measurements are to ground per the schematic.

[tab10672]

If the field coil or one of the filters were bad or oversized the 6j5 should also be high it looks like there are only a few resistors that would cause only the 6k5 plate to be high I would start from B and work my way there subtracting resistance voltage along the way.

[codefox1]

If the field coil or one of the filters were bad or oversized the 6j5 should also be high it looks like there are only a few resistors that would cause only the 6k5 plate to be high I would start from B and work my way there subtracting resistance voltage along the way.

Check the voltages on the plates of the 6F6. If B+ isn't making it that far, the power supply is practically unloaded and of course will be way up.

[marv]

My field coil on my DVM reads 2110 vs 2200 on schematic. All of my voltage readings on the tubes (except for the 6K5) appear to be normal (a bit higher than schematic which I would expect). I'll go back and check resistor #49 (6K5 plate resistor, 99k) that I changed out with a 100k to see if it reads right.

I am still hoping someone has an opinion on the resistance readings I'm getting on the primary of the OT (see opening post). According to the schematic they should be a lot less. Thanks guys.

[tab10672]

yea it sounds like its bad but as someone else said if it wasnt loading that end you should have higher plate voltages than just the one tube rite?.

[Ron Ramirez]

The original output transformer has a primary resistance of 700 ohms, plate to plate.

What is more important, however, is that this transformer needs to have a primary impedance of 12,400 ohms to match a 1.25 ohm impedance voice coil.

You can't measure impedance with your ohmmeter...but you'll need this info if/when you replace that audio output transformer. A Hammond 125D will work as a replacement; connect the voice coil to output terminals 4 and 5. It won't match exactly but will be very close.

[marv]

Thanks Ron. I realize that you can't measure impedance but a resistive measurement can indicate problems (corrosion/breaks) in the winding. I suspect that has happened with this one. I have rewound power transformers and some coils before so I am going to unwind this thing and see what I find. (More of a challenge).

[Ron Ramirez]

Right you are. Well, good luck to you; keep us posted on that transformer.

[codefox1]

Right you are. Well, good luck to you; keep us posted on that transformer.

Did you get around to this yet?

Ceck the voltages on the plates of the 6F6. If B+ isn't making it that far, the power supply is practically unloaded and of course will be way up

[morzh]

What is more important, however, is that this transformer needs to have a primary impedance of 12,400 ohms to match a 1.25 ohm impedance voice coil.

You can't measure impedance with your ohmmeter...but you'll need this info if/when you replace that audio output transformer. A Hammond 125D will work as a replacement; connect the voice coil to output terminals 4 and 5. It won't match exactly but will be very close.

Marv,

This (what Ron just said) means that your output to input windings turn ratio should be 100 (the input impedance is the output impedance time square of the turn ratio).

So when you have your Hammond replacement XFMR, look at the table and choose the taps that give you primary to secondary turn number ration equal 100.

Although as I remember, the table actually lists the input impedance vs. Voice coil resistance.

[marv]

I had to look at the windings (thats me!) In the primary side found a total of 76 breaks in the 26 layers (about 160 turns/layer). Won't be fixing this one.

[marv]

OK, I am getting ready to attempt a rewind of this thing and have one more question. I counted the turns of the bad OT and measured 3984 turns on the primary and 39 on the secondary. I think the turns ratio agrees with what Ron and Morzh stated above. By my measurent the primary used #37 wire and the secondary #20. I have some new 37 on the way but do not have any 20.

What will happen if I use #22 for the secondary maintaining the correct number of turns? Will the impeadance still be correct but result in a lower watt rating?

[exray]

OK, I am getting ready to attempt a rewind of this thing and have one more question. I counted the turns of the bad OT and measured 3984 turns on the primary and 39 on the secondary. I think the turns ratio agrees with what Ron and Morzh stated above. By my measurent the primary used #37 wire and the secondary #20. I have some new 37 on the way but do not have any 20.

What will happen if I use #22 for the secondary maintaining the correct number of turns? Will the impeadance still be correct but result in a lower watt rating?

Hi,
I'm a bit late to this thread. You might be able to sneak by with #22 but it would probably be worth the effort to get some 20 so as to not have to worry about it later. I guess re-doing 39 turns wouldn't be a terrible job if you had to rewind it.

On that high 6K5 voltage. If its still high after you get the xfmr redone then you should probably question the type of meter you're using. That's a very high impedance point and the meter they used originally to create the voltage chart probably loaded down the circuit somewhat. A DMM will read high at points like this.