Dynaco-Doctor Forum

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RE: New Tubes (6CA7)--now bias too high

Posted by xaudiomanx on February 15, 2016 at 17:49:02:
I know I am not in this discussion but as I wrote before. I've had NOS tubes that didn't bias correctly in some amps. The fact that your older set does bias properly just tells me that these particular set of tubes might need something as simple as a smaller bias resistor on the power supply. The resistor after the diode that has a cap on either side. Usually they are about 10 or 18K depending on the amp.

Also, if you can, check the bias voltage at pin 5 on each set of tubes. If they are all in the same range then it could just be the particular tubes they sent.

As far as what Peter said. I never heard of a particular set of tubes for musicians and a different set for regular music listening. I have ordered many sets of tubes for either type of use and I just use whatever was sent as long as they were matched. My particular source for all my tube needs is Jimmy McShane, especially when I need a closely matched set of tubes, like for the Dynaco. He would also be able to tell you if your tubes might be somewhat defective.

Also, individual bias pots is a great addition to any amp that is now using paired tube biasing.

A good friend/tech also told me that as long as the plates don't glow cherry red you aren't damaging the tubes and Dynaco is know for running tubes at the point of destruction with no conservation at all in an effort to get as much power output as it can. I don't know if that's true or not but I have to go with what others tell me when I am not sure. The only thing I can think of that might be compromised is the power transformer going way past its limitations as it is the weak link of any Dynaco amp.

Also, if you really want to hear the amp sing get yourself another one and use one channel from each(not bridged). Just use either the right channel or the left from each amp(doesn't make a difference). In essence you decreased the demand on the amp and doubleing the power supply without doing a thing, and that makes the power transformer work less. You can pull the power tubes out of the non-used side but if you do that make sure it is the side that doesn't power up the 7199 tube. That will help the power tranny to run a bit cooler as well. But again, not bridged.

And also remember, these amps were designed when the power source was 115AC. We now have generally 125AC. So other power supply issues arise because of the higher source voltage.

Sorry for butting in but I wanted to try and see what others say on the subject. Another good place to get others to give advice that I rely on heavily is DIY Tubes. Jimmy McShane, as well as very smart others are more than glad to help. I use them all the time for advice and they are usually correct when I present myself correctly.

Just my "0" cents!