Model 680 interstage transformer. - Printable Version
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Model 680 interstage transformer. - vaguy22 - 08-16-2016
Hello Does anyone know what the D.C. resistance readings should be on the input transformer between the 42 and the 6A3s? I am not getting signal past the plate of the 42 driver tube. plate voltage on the 6A3 s is 290 grid voltage is -63 volts on both tubes. Thanks in advance. Norman
RE: Model 680 interstage transformer. - Radioroslyn - 08-16-2016
Hi Va.
Looks like pri is 475 ohms sec is 1k ohms g to g. Interesting I checked to see if the bias was too high at -63v but that about right Just for grins you can put a cap for the plate side to one of the grids. Tipically the secondary will go open and you'll loose bias one one or both output tubes.
RE: Model 680 interstage transformer. - vaguy22 - 08-16-2016
Terry: Thanks for the fast reply my pri reads 451 Ohms secondry one side reqads 431 Ohms the other side reads 520 ohms taken with digital ohmmeter. Are there any replacements availible? thanks again Norman
RE: Model 680 interstage transformer. - Radioroslyn - 08-16-2016
Resistance seems ok. What are you using to detect the audio signal? Just for laughs pull the 6A3's out and see if you have a signal on pin 3 on either tube.
RE: Model 680 interstage transformer. - vaguy22 - 08-17-2016
Radioroslyn: I am using the aux input of a stereo receiver. Getting clean signal from the plate of the 42 driver tube through a .01 mfd cap. tried the grid lead of the 6A3. no signal at all not even a weak one. Thanks for your help. Norman
RE: Model 680 interstage transformer. - Radioroslyn - 08-19-2016
That's odd that you've got the proper resistance for the winding and it's not shorted to gnd as you wouldn't have the bias voltage on the 6A3 grids but it's not passing any audio. Mechanically it's like the core is missing!
https://www.tubesandmore.com/products/transformer-hammond-audio-interstage-7k-pri-158k-sec
Looks like the ticket.
GL
RE: Model 680 interstage transformer. - vaguy22 - 08-19-2016
Got it going was'nt the interstage transformer. now just weak audio but it is crystal clear no hum or distortion so will have to find out what stage is weak Thanks for your help Norman