|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
206.75.129.36
In Reply to: RE: Well...... posted by vinnie2 on April 29, 2016 at 14:21:40
I am confused by a few things here1. There is no cathode in any of these tubes. I assume plate to cathode means plate to one of the filament pins, or to the centre tap of the hum balance pot.
2. If you are getting only 33V plate to filament, the issue is with bias, the tube is conducting too much because there isn't enough bias voltage to throttle it down. Remember a triode tube is more or less a variable reaistor which is controlled by grid voltage... More grid voltage gives a higher resistance and less gives a lower one... This varying resistance is used to control power, hence amplification.
Play with the value of the self bias resistors (centre tap of him pot to ground) to set the plate voltage, not with the plate load reaistor. For example, to get a higher plate to filament voltage you would increase the value of the self bias resistor. . The plate to filament voltage should be about half the B+ voltage for the first two stages, and for the output stage should be almost full B+
Make sure to measure all the resistors a second time, maybe one or two are off in value and throwing the design out of whack.
Edits: 04/30/16Follow Ups:
"More grid voltage gives a higher resistance and less gives a lower one"I think you meant,
The more negative the voltage at the grid is, WRT the cathode, the higher the resistance of the tube (less current will flow through the tube)
And the less negative the voltage at the grid is, WRT the cathode, the lower the resistance of the tube (and more current will flow through the tube).
So;
More negative grid voltage gives a higher resistance and less negative grid voltage gives a lower one.
Pardon my intrusion, I just want to make sure no one gets confused here.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/30/16
Yes, I meant if the absolute value of the grid to filament or cathode voltage increases, (with the grid always being more
Negative than the cathode or filament) the resistance of the tube from filament to plate correspondingly increases, hopefully in a linear fashion.I think that 20k plate resistor looked pretty good on the load line someone else plotted, it's the bias which needs to be adjusted to set the operating point.
One concern I think will be a problem with this amplifier once all the bias issues are sorted out is AC hum... Especially from the first stage.
I would strongly recommend the original poster to read the first few chapters of any RCA tube manual. It gives an excellent overview of this subject in much better language than I could come up with :)
Additional references which I would strongly suggest to saturate yourself with before designing amplifiers are the radiotron designers handbook 4th edition, and the audio cyclopaedia. And also anything by Norman Crowhurst. But even the lowly RCA tube manual has page after page of valuable info in the sections before the tube data begins.
Edits: 04/30/16
You will have to get in line behind Tre for suggesting I study all the different texts there are on tube theory and design. The only problem is this is a hobby for me and I much prefer learning things by doing when it comes to electronics. I tend to nod off when trying to wade my way through a lot of the theory.
I am not really trying to design an amp as such, I am just making some modifications to a design that Drummer Will came up with and posted on this forum a while back. I built his 26 preamp and was very impressed with it, so I wanted to try a version of his 26/26/45 amp too. I already had one channel working very nicely on my breadboard a while back, but then I took a break to do other things for a while before starting the final version. Maybe too long. I also never actually breadboarded both channels and I think that is part of the problem too. If I had I might have noticed these problems earlier.
I am going back to the breadboard again, so I will be starting with the load line Palustris drerw up that you mention. I had good luck with that before.
Edits: 04/30/16
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: