| Author | Message | ||
     Brian Vargas (brianvargas) Username: brianvargas Registered: 07-2013 |
Hello all, A friend of mine dropped off his amp for some basic inspection. When last used, this amp melted a power tube and blew a fuse according to my friend. One of the 3 remaining tubes in the amp appears heat warped as well. Note, these were EH tubes. I have purchased JJ's for my work on this amp. I have opened this unit up and found it to be quite clean considering the age of the amp. After spending a bit of time with the schematic and a meter to familiarize myself with the guts, I can say the preamp section looks good, but I need to confirm the last stage of channel 2. Connections in bias circuit are good. I am hoping you folks can help me with some questions and provide any insights as I move forward in this process. I respect the safety issues of working on tube amps, can discharge caps appropriately etc (not to be dismissive). FWIW, I am an electrical engineer by education and career, have 3 tube amps of my own that I have biased (the easy cathode current X plate voltage way) and become familiar with, but my overall tube amp experience is basic. I am looking to expand my knowledge and experience. Via email, Steve K. has already shed a great deal of light about this particular amp! Thank you! I feel very lucky to have found this forum! Ok, the questions. Tell me if I understand a portion of the circuit correctly... 1. The cathodes are all tied to a single point (labeled y) where they are connected to ground through 2 parallel 10R 1W resistors making the resistance from y to ground 5R. 2. The schematic suggests the voltage at y should be 1/2VAC making the cathode current 25mA. 3. Steve has suggested using PLATE current for bias current measurements. The plate and cathode currents will be different because of the grounded suppressor, correct? I guess I want to know, if I can't get my hands on a probe that will supply me with the plate current could I ballpark with cathode current in some way... 4. ...a caveat...I will also be using the "signal->amp->dummy load with scope on the output" method of looking at the point of crossover distortion elimination. I have several 8R 200W power resistors I will fashion into a 16,8, and 4R setup, a tektronix 465, and a B&K generator FWIW. 5. I own a Compu Bias probe that measures cathode current and plate voltage to calculate dissipation, but as Steve suggests plate current is better here...and the Compu Bias can't be used anyway because its abs. max for plate voltage is 600V. SMOKE! Care to point me in a direction for an off the shelf, reliabe device that can handle measuring plate current for this instance that will not be invasive to the circuit? 6. Any further observations from the photos? 7. Anything you would like to add? Thanks for any and all help. I am here to learn! Here's the photo link. Be sure to use the entire link. It seems the auto activate URL's is confused by the @. http://www.flickr.com/photos/98693286@N03/with/9256051076/ If I need better photos just let me know. Thanks a ton! | ||
| Brian Vargas (brianvargas) Username: brianvargas Registered: 07-2013 |
read this from one of Mike's previous posts... "Well, the easiest method is to get some kind of bias adapter. It goes in the socket of your power tube and the socket plugs into it. The two leads coming out go to your digital meter and read either mA's or mV's, depending on which one you have. Then you can get a true reading of cathode current(which also has a little plate current built in too-I use that for a safety margin). The other easy way is to install 1 ohm resistors on each socket from pin 8 to ground-or in this case, to the input of that 10 ohm resistor. This allows you to look at the cathode current/voltage PER TUBE. This is a little more tube safe than just getting a total current per pair. Mike." so, cathode current is fine to use. Great! | ||
| Brian Vargas (brianvargas) Username: brianvargas Registered: 07-2013 |
and in the same thread I see where Mike mentions that the 10R resistors only give you an indication of the current for a PAIR of tubes when what you really want is individual current value. crystal clear. See what a little time a research provides! |