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Epi Valve Special voltage too high?

12/15/2005 9:16 PM
Bob G. Epi Valve Special voltage too high?
As per my earlier post, I've mapped out a majority of the schematic for this amp and much to my surprise the plate voltage on the single EL84 tube is almost 350V. The current across the 220 Ohm cathode resistor is about 47mA - which according to my calculations is driving the tube at 15W! While the plates are not glowing I am a little concerned about this high of a voltage and dissipation.  
The screen voltage at pin 9 is just under 330V. I think I read that having a significantly lower screen voltage will protect a tube with a higher than recommended plate voltage. From one of the Aiken articles I read I was also under the impression that for a single EL84 to run "properly" in Class A the plate voltage needed to be about 250V. Is it possible that this amp is not running in Class A with such a high plate voltage?  
I really would like to get the power tubes to distort nicely but it appears to difficult to do this without getting to deafening levels. This 5W amp can be really loud. I was thinking that if I could lower the plate voltage somehow it would be better for the tube as well as allow earlier power tube distortion. Is there an easy/effective way to do this short of replacing the output transformer?  
Bob G.
12/15/2005 10:05 PM
Bob G.
Correction - I meant "Power" Transformer that would need to changed to reduce the voltage from 350V to 250V (not "output" xformer).  
BTW - the output xformer is a little dinky thing. Is there any reason why this would be insufficient with a lower plate voltage? I just read a post where someone changed the output transformer and the tone improved greatly on his amp.  
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12/16/2005 6:10 PM
Steve A.

    350vdc is a little high for EL84's; I think that 320-330vdc works great. I believe that the Vox AC-30 runs at 320vdc, but there are some doubts as to whether the Vox AC-30 is actually Class A.  
    Epiphone must have gotten a great deal on 220 ohm resistors because that it was they used in the Valve Junior; the Gibson LP Jr used 270 so that is what I put in. To lower your power level you could put in a larger cathode resistor. But I'm not sure how that would affect power tube distortion- for that you might look into some sort of attenuator, like the one on my site designed by Bruce Collins. One of the tricks is to look at the four speakers in a 4x12 wired up as two parallel strings of two speakers each in series. You replace 3 of those speakers with power resistors of the appropriate value and wattage, and wire up the remaining "virtual" speaker to the output jack. Add in a small choke to let the resistors respond more like a speaker and you are ready to rock! ;)  
Steve Ahola
12/16/2005 9:01 PM
Bob G.
Couldn't I add a resistor (what value/rating) between the diode bridge and the transformer to reduce the plate voltage? I've read about people adding zeners in series to drop the voltage but I'm not sure if that's better/easier.  
Also, I noticed on your schematic for the Les Paul Jr. that the plate voltage is about 312V (is that what it is on your Valve Junior?). I guess if you're keeping current low enough via the cathode resistor you're OK but what about the 250V max required for single-ended class A that Aiken writes about. Is that not necessary?  
I've seen/heard the AX84 High Octane which is at 250V and that sounds incredible. Would that be largely attributed to the supply voltage?  
12/16/2005 9:38 PM
Steve A.

    Adding a series resistor before the plates adds sag, because the voltage drop depends on the current being drawn (the more current the more the voltage drops). A little bit of that is okay and adds a tube rectifier-like effect to your power supply. Too much and it will sag like an old mattress. :(  
    On the other hand a zener will introduce a fairly exact voltage drop regardless of the current being drawn, although you have to be careful not to exceed the current ratings.  
    As for your question about Randall Aiken, I noticed that he is hanging out with the "big boys" like Doug Roccaforte on the A.G.A. newsgroup, so you might check over there... :D  
Steve Ahola
12/18/2005 4:02 AM
This being a SE Class A amp, though, there's little change in current draw, so there'd be no9t much change in voltage drop, and therefore little sag. So a series resistor might work fine. Can't hurt to try. :)
12/19/2005 10:07 PM
Bob G.
Thanks, I was thinking that a resistor might work fine - but determining the right value might take some time. A quick estimate leaves me thinking something <= 1k might work. But that may require a 5W resistor.  
I was also thinking that because the current draw is presently a steady 47mA I could also use 4 x 12V, 1W zeners in series to drop the voltage down from 350V to about 300V at the plate (this would in turn lower the current and should result in the cathode resistor staying at 220 Ohm and lower the Power dissipated by the single EL84 from the present 15W to just under 12W.  
Either way it looks like I need to do some testing here and to find a good location for adding these components.  
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