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Regulating screen grid voltage?


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7/26/1999 3:06 AM
Stan
Regulating screen grid voltage?
Hello,  
 
Is there a trick to regulating the screen grid voltage on a 6L6? I had it ~ 1V under the plate V but then it drifted up about 1-2 V above the Plate.Is it crucial to have it under the plate V? Could this be causing my Dumbell clone to distort too early? All other DC votages are ok. Initially, I just put the pre-amp section in and it sounded great! But after converting to the Dumble power section, it sounds like shit. It seems as though I can hear my clean guitar in the distortion with to much high end. Tried the bassman trick with a cap across the PI plates which cut the highs a bit but distortion channel is definitely not as sweet as before. I still have the 20 watt OT but it was way louder before distorting with the previous output section than the one I have in now. I still have yet to put it on a scope to see just where the signal is distorting. If anyone knows of any remedy for my dilemma , I sho wood apeeceeate it.  
 
Stan
 
7/26/1999 7:14 AM
Stan
>Could this be causing my Dumble clone to distort too early? (clean channel)  
 
>It seems as though I can hear my clean guitar in the distortion with to much high end(distortion channel)  
 
Just want to make my previous post a little more clear.  
 
Also, Tubes are about 428V around 35mA(plates),430V (screen grids).Lead dressing and soldering are very clean. I'll keep going through the circuit to make sure it is not a wiring mistake or failed component. Incidentally, I noticed the B+ is very high thruout the amp during start-up then drifts back down to correct operating voltages, is this normal? Any tidbit of info would be great.  
 
Thanks in advance, Stan  
 
 
7/26/1999 6:42 PM
Gil Ayan

If everything was OK until you changed the power section, there are two possibilities: one is that you made a mistake and there is something wrong with the wiring. Assuming that is NOT the case, then let me ask you if, by any chance, you had more negative feedback before than you do now?  
 
 
 
When using some wimpy transformers, a good example of which could be that of a Deluxe Reverb, having no feedback makes the amp sound bad IMHO, too bright and raspy. But, that can be cured with some negative feedback... Maybe the amount of feedback in the circuit you copied is not enough to mask out the distortions introduced at the power section. Just food for thought.  
 
 
 
Gil
 
7/26/1999 11:58 AM
ken g
quote:
"Is there a trick to regulating the screen grid voltage on a 6L6? I had it ~ 1V under the plate V but then it drifted up about 1-2 V above the Plate.Is it crucial to have it under the plate V? Could this be causing my Dumbell clone to distort too early?"
 
 
 
 
Don't worry if Vs exceeds Vp by a couple of volts.  
 
 
 
I don't think this is where the distortion is being created.  
 
 
 
What kind of output stage did you have before that sounded good? Why not go back to that one?  
 
 
 
On the phase splitter, are the quiescent plate voltages approximatly 1/2 to 2/3 of the available B+ votlage?  
 
 
 
~KG~
 
7/26/1999 6:16 PM
jason

ken,  
 
How about screen voltage being lower than plate voltage. In regards to screen voltage with respect to plate voltage, how low is to low. The reason I'm asking is I have a SE amp I built (based loosely around a fender tweed) that can run a 6L6 or an EL34. The only problem is that with a 6L6 my screen voltage is about 1 volt lower than plate voltage, with an EL34 the screens are about 20 volts lower. I like the way the EL34 sounds and am thinking of trying to optimize it for that tube and saying to hell with the 6L6.  
 
Any thoughts?  
 
Thanks,  
 
jason
 
7/26/1999 6:44 PM
Ken Gilbert
quote:
"How about screen voltage being lower than plate voltage. In regards to screen voltage with respect to plate voltage, how low is to low. The reason I'm asking is I have a SE amp I built (based loosely around a fender tweed) that can run a 6L6 or an EL34. The only problem is that with a 6L6 my screen voltage is about 1 volt lower than plate voltage, with an EL34 the screens are about 20 volts lower. I like the way the EL34 sounds and am thinking of trying to optimize it for that tube and saying to hell with the 6L6."
 
 
 
 
I don't think you have any optimizing to do! If you like the amp with the EL34's in there, leave them in. EL34's historically draw more screen current than BT's, like the 6L6, probably because it is easier to keep g1 and g2 aligned (as they should be) with beam plates rather than the true pentode's supressor grid.  
 
 
 
In any event, I can't see what you're getting at in terms of optimization for the EL34's. From your post, you seem happy with the sound, so what exactly did you have in mind in terms of "optimizing?"  
 
 
 
To answer your other question, there is no screen voltage that is too low, really, as long as there's current flowing through the tube. The screen voltage will primarily determine the attraction of the electrons from the cathode to the plate--i.e. the average current flow. Of course, the control grid also has (a much greater) control over this plate current, which is how the tube works to amplify, but the screen voltage can be manipulated along with grid voltage to give the desired response.  
 
 
 
In HIFI designs, the screen current is normally kept as low as possible, since higher screen voltages lead to higher distortions. This is not relevant to musical instrument amps.  
 
 
 
More power is available with higher screen voltages since the electrons will be strongly attracted from the cathode (well, the virtual cathode really, which is made up of a space charge of electrons) towards the plate. Thus, to keep the currents down, a greater negative bias is required at the control grid. That means that there is a much higher peak plate current possible when the Vg1 is 0V (right at the onset of grid current), which means more power output.  
 
 
 
The same thing could be done by pushing the grid positive, which leads to the same greater peak plate currents, but that is a much "harder" method of achieving greater power output. It is easier to instead increase the negative bias and increase the screen voltage, which keeps the quiescent current the same while increasing Ip (pk).  
 
 
 
Hope this has helped some. Remember, there are no "correct" voltages for screens, other than avoiding situations that place them at a MUCH higher potential than the plate for extended periods (which will quickly over-dissipate them due to secondary emission effects).  
 
 
 
~KG~  
 
 
 
 
7/26/1999 8:15 PM
jason

Ken,  
 
Thanks for the reply.  
 
I guess what I was getting at with "optimizing" is that most guitar tube amp schematics show the plate voltage and screen voltage within about a volt of eachother, that's what I was refering to. True, I do think it sounds great but was just curious if this could increase that "good sound" even more. Looks as though I'm curious as to the tonal effects of screen voltage. Would it be correct in assuming that plate current increases with an increase in screen voltage?  
 
Thanks,  
 
jason
 

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