Sincerely,
Martin Dannys
:I have recently aquired a matching pair of Northern Electric Company R4039-C power amplifiers from a local church rummage sale. They each have a pair of westinghouse 6L6G tubes and a 5X4G rectifier. There is a 5pin input and two 4 pin oututs on each amp.
:Through my research i believe they originated from the tone cabinet of a 1930's - 1940's vintage electric organ. I am looking convert these two behemoths into a pair of hi-fi mono block amplifiers.
:I have been unable to find the schematic listed on the website. Any idea where I could find the schematics for these units?
:I was wondering if I might also solicit any advice on their conversion to hi-fi, as I am new to tube audio and there is much to learn.
:Any information provided would be much appreciated.
:
:Sincerely,
:
:Martin Dannys
Well, there is an above link that will likely get you a schematic. I just typed like three paragraphs, but when I copied them to go get the link, I erased them by copying the link. That pisses me off!
Anyway, since they are amplifiers, they will likely be of very simple design, and you can draw a schematic yourself if you like. The only impedances you will be able to get on transformers are DC impedances. If the amplifiers seem to work fine right now, or will work fine when all of the condensers are replaced, you can assume that the transformers are in good order and can take fairly accurate DC measurements to be used as future reference, should any trouble come up.
It is likely, since these were church amplifiers, that the remote speakers used had their own output transformers, and each amplifier does not have its own output transformer. If this is so, you may purchase output transformers of high quality, designed for use with the 6L6 push-pull set-up, from Antique Electronic Supply, also known as www.tubesandmore.com. Order their catologue. It is a lot more fun to look through than their web site, in my opinion. Their web site is updated far more often than the yearly catologue, though, so you may find things in the web site that are not listed in the catologue.
The only way you will be able to get accurate control grid voltages is with a vacuum tube volt meter. If you are lucky enough to own one, then you are very privileged. They are extremely sensitive meters that surpass your average multi-meter. You will not be able to get accurate control grid voltages unless all bypass condensers are replaced with new ones that don't leak. If the original ones don't leak, then you are equally lucky. Leakage is considered bad when it is in the millions of ohms, as this is enough to throw off control grid voltages. There must be ABSOLUTELY NO LEAKAGE. Leakage can be determined with an ordinary multi-meter purchased at Radio Shack, or perhaps a finer Simpson meter, though these are very expensive.
All voltage readings taken must be taken only after it is determined that no condensers are leaky, and all resistors are right on value. Replace the electrolytics if they have dried up or are leaky, though with electrolytics, 10 MEG leakage is acceptable. Once you have recorded all the voltages with a properly working amplifier, and you even have two to compare with, you will have these voltages to compare with when the amplifiers go bad.
The one real drawback of operating each amplifier or taking voltage readings is if it requires an electrodynamic speaker. You will need the original schematic to determine the appropriate field coil resistance. If an electrodynamic speaker is required, you may make up a resistance to replace the field coil, once the resistance has been determined. It must be capable of about 25 watts. You may find Jensen and other electrodynamic makes on eBay. People typically list the speakers as "field coil" speakers, for some strange reason, even though this is not their proper name. You will need to know the proper field coil impedance requirement before you purchase two speakers. They must be exactly of the same make and impedance if you wish to use these amplifiers for stereo reproduction. Use the Jensen or other make 10, 12, or 15 inch speaker as your woofer, and with a cross-over network, add your squaker and tweeter.
In place of the original tone control, you may add bass, mid-range, and treble controls, using various methods. If I had a copy of your original diagram, I could draw up the circuits required. The treble control will only be a variation of the tone control, but will use a different value condenser. The mid-range control will use negative feedback. The bass control can either use negative feedback or simply a resistor that changes circuit "tension" on the input section.
Thomas