28-11-2021, 10:15 AM
I hope I'm posting in the right forum here...
I've been restoring an old 9R-59DS. It suffered from intolerable 50Hz hum. The output xformer had already been moved under the chassis away from the power xformer. I noticed that the hum was present at the same volume regardless of the AF gain setting. On removing V7, which is an ECC85 that serves as audio preamp and BFO, the noise disappeared. The conclusion I came to was that there was a heater/cathode short, since the heater was the only source of 50Hz, rather than 100Hz. I replaced it with one scavenged from an old set and the hum vanished. Unfortunately, the replacement is too worn out for the BFO to work, so I have a NOS one on order.
I remember reading elsewhere of someone with a 9R-59 with a similar humming BFO. My question is, therefore, how common are heater/cathode shorts on ECC85s/6AQ8s? Are there any brands that are less likely to succumb to this?
Thanks in advance for any accumulated wisdom...
I've been restoring an old 9R-59DS. It suffered from intolerable 50Hz hum. The output xformer had already been moved under the chassis away from the power xformer. I noticed that the hum was present at the same volume regardless of the AF gain setting. On removing V7, which is an ECC85 that serves as audio preamp and BFO, the noise disappeared. The conclusion I came to was that there was a heater/cathode short, since the heater was the only source of 50Hz, rather than 100Hz. I replaced it with one scavenged from an old set and the hum vanished. Unfortunately, the replacement is too worn out for the BFO to work, so I have a NOS one on order.
I remember reading elsewhere of someone with a 9R-59 with a similar humming BFO. My question is, therefore, how common are heater/cathode shorts on ECC85s/6AQ8s? Are there any brands that are less likely to succumb to this?
Thanks in advance for any accumulated wisdom...
My name is Steve, but I answer to most things like "Oi, you!".
