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hot tubes, I mean really hot

8/10/2005 1:57 PM
hot tubes, I mean really hot
Hi all,  
So I just finished putting together an 800 clone as found in TUT 3, but I get the following problem. My power el34s get way too hot. Red hot. I checked all of my connections, and everything seems right. Using the dissipation bias method, I turn my pots as to create the most negative voltae possible on the control grid. All volumes and eq turned all the way down and no input.  
So I plug in an output tube, and turn on the amp. At first it warms up ok, heaters orange, everything good. After a few seconds, the plate starts to glow orange and the glow starts to spread across the plate. The smoke comming from the plastic base of the tube tells me no good. All of this happens within about 30 seconds, then I turn the power off. Also, as the tube starts to overheat, there is a progressivley louder, yet dull, hum from my speaker.  
When I have no power tubes, everything checks out ok. Preamp tubes light up fine, no sounds from the speaker. The voltage on the control grid reads about -5.6 to -5.4 Volts.  
I have tried using a different output tube, but still the same problem. I have tried all four tubes in at the same time, but still same problem. This circuit is supposed to allow any number of tubes to work correctly, so having one tube in at a time isnt the problem. If anyone is familiar with this sort of problem I would greatly appreciate any help. Thanks!  
Rock n Roll.  
8/10/2005 2:00 PM

Oh I forgot to mention, I cannot even try to bias my tubes, because of this. And I have not tried to use an input to see if the amp actually works. The heating up process is very rapid and does not leave me with much time to poke around in the circuit.  
Thanks again.  
8/10/2005 3:33 PM
Mark Black

Yikes! You should be seeing something more like -38 to -42V on the control grid (pin-5).  
Maybe double check the installation of your bias circuit filter caps to make sure you don't have them in backward polarity-wise (easy and common mistake).  
Good luck - Mark
8/10/2005 7:57 PM

Only 5 volts of bias? My God, those tubes ought to be launching towards orbit. Find out why you don't have -40 volts or so of bias and it will fall into place.
8/11/2005 12:22 AM

Thanks for your reply. I made a startling discovery and revealed some stupidity on my part. Big time. I have a grounding switch, and itís a dpdt. Well To make a long story short, it wasnít wired right, so my circuit never reached chassis ground. I fixed this, but still got the same problem. Small voltage at the Control Grid. I checked all capacitors, and they are in correct polarity. I noticed a discrepancy between the parts layout and schematic that I am using. One lists 100nF caps for the bias circuit, and the other lists 22nF. I will pick up the 100nF caps tomorrow and see if this changes anything, but I thought I would ask your opinion on the matter. Also, is it possible that I have ruined my capacitors, bias and otherwise, with the previous grounding problem?  
Thanks for your help.  
8/11/2005 9:53 AM
I believe it is uF (microFarads)...  
Is the bias supply voltage ok if you disconect it from the circuit?  
Just trying to help.  
8/11/2005 10:36 AM

Thanks JC,  
Yeah I would think nano Farad is too small, but thats what it says on the schem/layout. If its not too much, I will try to get both values at the store.  
Im still kind of new at this, so to get things straight, is the bias supply the 300-0-300 volts that the PT puts out? Or is that a voltage supplied from the board?  
Thanks for your help,  
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