The unit has no model number, but the circuit corresponds exactly with the schematic for the Gulbransen Models 200, 291, 292, 295, 9950 (New type) listed in Riders page 1-6 Gulbransen. I believe it is a 9950. There is an 80, 4 stages of RF using 226's, then a 224 detector, a 226 1st AF, and two 245's in push pull. The first audio transformer has a primary winding measuring 1600 ohms, the secondary connects to the grids of the 245's at each end, with a center tap returned to ground. half of the secondary winding is open, the remaining half measures 3000 ohms. The second audio transformer has a center tapped primary that is connected to what looks like B+, each end is connected to a plate of the 245's. The primary winding reads 600 ohms, 300 for each half. The secondary winding reads 1 ohm and connects to the voice coil of a large dynamic speaker. I assume that this winding is shorted.
Both transformers look like they need to be replaced. Any suggestions on where I might find suitable replacements, and particularly what it is that I should be looking for?
Thanks as always.
Lode Swinnen
I have good news for you. The second transformer should be ok. It's an output transformer and the secondary will be very low resistance.
The first is a driver or audio interstage transformer. Replacements are available from AES, tubesandmore.com and Play Things of Past, oldradioparts.com . Be sure to select one with a center tapped secondary. DC resistance isn't very important. A53-C or A63-C will both work. The first one being less expensive and second with better frequency response.
Norm
: Both audio transformers in a Gulbransen console unit need replacement, but I don't know how to pick suitable replacements from what is available through AES or other sources. Would very much appreciate suggestions.
: The unit has no model number, but the circuit corresponds exactly with the schematic for the Gulbransen Models 200, 291, 292, 295, 9950 (New type) listed in Riders page 1-6 Gulbransen. I believe it is a 9950. There is an 80, 4 stages of RF using 226's, then a 224 detector, a 226 1st AF, and two 245's in push pull. The first audio transformer has a primary winding measuring 1600 ohms, the secondary connects to the grids of the 245's at each end, with a center tap returned to ground. half of the secondary winding is open, the remaining half measures 3000 ohms. The second audio transformer has a center tapped primary that is connected to what looks like B+, each end is connected to a plate of the 245's. The primary winding reads 600 ohms, 300 for each half. The secondary winding reads 1 ohm and connects to the voice coil of a large dynamic speaker. I assume that this winding is shorted.
: Both transformers look like they need to be replaced. Any suggestions on where I might find suitable replacements, and particularly what it is that I should be looking for?
: Thanks as always.
: Lode Swinnen
Lode Swinnen
: Hi Lode
: I have good news for you. The second transformer should be ok. It's an output transformer and the secondary will be very low resistance.
: The first is a driver or audio interstage transformer. Replacements are available from AES, tubesandmore.com and Play Things of Past, oldradioparts.com . Be sure to select one with a center tapped secondary. DC resistance isn't very important. A53-C or A63-C will both work. The first one being less expensive and second with better frequency response.
: Norm
:
: : Both audio transformers in a Gulbransen console unit need replacement, but I don't know how to pick suitable replacements from what is available through AES or other sources. Would very much appreciate suggestions.
: : The unit has no model number, but the circuit corresponds exactly with the schematic for the Gulbransen Models 200, 291, 292, 295, 9950 (New type) listed in Riders page 1-6 Gulbransen. I believe it is a 9950. There is an 80, 4 stages of RF using 226's, then a 224 detector, a 226 1st AF, and two 245's in push pull. The first audio transformer has a primary winding measuring 1600 ohms, the secondary connects to the grids of the 245's at each end, with a center tap returned to ground. half of the secondary winding is open, the remaining half measures 3000 ohms. The second audio transformer has a center tapped primary that is connected to what looks like B+, each end is connected to a plate of the 245's. The primary winding reads 600 ohms, 300 for each half. The secondary winding reads 1 ohm and connects to the voice coil of a large dynamic speaker. I assume that this winding is shorted.
: : Both transformers look like they need to be replaced. Any suggestions on where I might find suitable replacements, and particularly what it is that I should be looking for?
: : Thanks as always.
: : Lode Swinnen
Posted by Lode Swinnen on December 11, 2001 at 23:17:59:
Hello. I followed Norm's suggestion and obtained a modern A63C equivalent transformer and connected it in place of the original, which had half of the secondary open. The transformer linked a 26 1st AF to two 45's in push pull The radio worked, and very well!
However, within a couple of minutes, the sound distorted and faded out. Switching the radio off even for a moment restored function, but only for a few minutes each time. I had already replaced all the capacitors except for the small capacitors in the RF stages. Although the resistors measured OK, I replaced them all too. Of note, a large power resistor was open and looked burned when I first started. It connects the center tap of the 45 filament transformer winding to ground. I had replaced it with a 780 ohm resistor of similar size.
Half of the new transformer's secondary winding is now open. The grid of one 45 becomes increasingly positive wrt ground as the fading sets in, reaching about 30 volts. Probably the result of the open winding rather than the cause. The plate voltages on the 45's are indicated as 230 on the schematic, but measure almost 300 V as soon as the set is switched on. The 26 plate voltages are also high - 150 instead of the expected 116V.
Sorry about this long description, but I am now truly stuck. My guess is that unless I fix what is causing the transformers to burn out, putting in another transformer will lead to the same result. The 45's tested good though weak. I cannot test for gas. would anyone have any idea what might overload the grid circuit and lead to transformer burnout?
Thanks
Lode Swinnen
: Both audio transformers in a Gulbransen console unit need replacement, but I don't know how to pick suitable replacements from what is available through AES or other sources. Would very much appreciate suggestions.
: The unit has no model number, but the circuit corresponds exactly with the schematic for the Gulbransen Models 200, 291, 292, 295, 9950 (New type) listed in Riders page 1-6 Gulbransen. I believe it is a 9950. There is an 80, 4 stages of RF using 226's, then a 224 detector, a 226 1st AF, and two 245's in push pull. The first audio transformer has a primary winding measuring 1600 ohms, the secondary connects to the grids of the 245's at each end, with a center tap returned to ground. half of the secondary winding is open, the remaining half measures 3000 ohms. The second audio transformer has a center tapped primary that is connected to what looks like B+, each end is connected to a plate of the 245's. The primary winding reads 600 ohms, 300 for each half. The secondary winding reads 1 ohm and connects to the voice coil of a large dynamic speaker. I assume that this winding is shorted.
: Both transformers look like they need to be replaced. Any suggestions on where I might find suitable replacements, and particularly what it is that I should be looking for?
: Thanks as always.
: Lode Swinnen