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Problems biasing 5881s for desired tone...


 
12/10/1997 9:09 AM
Ken O.
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Problems biasing 5881s for desired tone...
This is the scenario (BTW I am new to this forum and I have to tell you all that this place is as close to nirvana as I will probably ever get!); I am in the process of designing a ~50W guitar amp. I am starting with the power section first and have not even considered the pre-amp yet other than the basic architecture. My problem is that I definitely want to use 5881s fixed bias into AB1 (I'm trying to keep the count at two because I want to be able to drive them pretty hard) but I'm not sure about how to choose the bias point for the two tubes in order to get the desired power output. I will most likely use a 50W Hammond output transformer (1650?). If I use the primary impedance (Raa/4) of the XFMR to determine the bias point I get a current that is too high for one tube. This brings up another question; Is the value of quiescent current derived from the XFMR power rating and impedance the combined current of the two tubes or the value for each side? Does the voltage follow this rule also? After I get these issues straightened out it should just be a matter of choosing the grid voltage. This would seem to be dictated by the voltage drive and quiescent current values already derived. Is it just a matter of drawing a load line on the plate characteristics using Ra (not Raa) and V+ and picking off the grid voltage where the load line and current value meet? I have Kevin O'Connor's "The Ultimate Tone" but it doesn't really give any design equations for output power ect, ect, or design procedures (actually hints are what I'm looking for. I'd rather understand that just follow blindly). The book is great by the way but I like equations. Call me a masochist! Thanks in advance for any help.  
 
 
Ken O.
 
12/10/1997 9:38 AM
J Fletcher
Well I'll just give the simple answer,because I haven't looked at load lines since college,15 years ago.To get 50 watts from a pair of 5881's,you'll need about 475volts on the plates and screens,into a primary impedance of about 4000 ohms.The standing current per tube can be anywhere from 20 ma to 35 ma.I personally wouldn't use the Hammond transformer,I would buy one from Mojo or New Sensor designed for a guitar amp.Hope that's of some use to you.
 
12/10/1997 11:33 AM
Ken O.
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Here is a point that is confusing me somewhat; I have two different data sheets for a 5881 and they both give different values for max plate voltage and current. One says 360V and 150mA and the other says 400V and 200mA. Which is right? Do you know of a source (preferable on the internet...ie. readily available) of accurate tube data sheets? Also I'm assuming that 475V is the voltage on the center tap of the OPT and not the plate voltage which would be lower due to the primary Z of the OPT. Is this correct? Thanks again.  
 
Ken O.
 
12/10/1997 12:00 PM
J Fletcher
I've run Sovtek 5881's for about a year with 500 volts on the plates and screens,no problems yet.The dc voltage drop across the transformer primary between CT and plate is usually only 5 volts or so.The dc resistance of the primary is in the range of 50 to 80 ohms per side,the impedance is much higher naturally.To me,"right" is what happens in the real world of rock and roll,is reliable,is repeatable and sounds good.As far as data sheets go I personally have learned more from studying schematics of guitar amps,and guitar amps themselves,than from any other source.Fenders from the late 50's for instance had 5881's and had 460 volts on their plates.
 
12/10/1997 12:45 PM
Steve M.
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Hi,  
 
Something needs to be addressed: Which 5881 are we talking about here? The Sovtek 5881 is not a real 5881 like the Tung-sol or other American NOS, and therefore, does not have the same ratings. From what I understand, the plate resistance is lower for the Sovtek 5881, and gives less power output than a real 6L6/5881 in the same circuits.  
 
Regards,  
 
Steve
 
12/10/1997 1:08 PM
Ken O.
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Hmmmm...good question. The only thing I know about that is that the original 5881s were an industrial version of the 6L6GBs. The 6L6GC was a later version of the 6L6 with higher plate voltage (500V as opposed to 360V) and power dissapation (30W as opposed to 23W). If this is so, why do some data sheets include the 5881 specs and curves in with the 6L6GC data? Is everything else the same with the exception of the two parameters I've mentioned above? I'm assuming that the tubes I'm talking about are all American versions. What kind of ratings does the Sovtek have?  
 
Ken O.
 
12/10/1997 1:46 PM
Steve M.
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Ken,  
 
I've never seen data on the Sovtek 5881, but I've been told that it is used in the Russian MiG fighter jet. It is not a "real" audio tube at all. It just happens to be close enough to 6L6 ratings to be a viable substitute. The "5881" name was marketing hype introduced by Mike Mathews at Sovtek/New Sensor. It is unfortunate that he chose to call the tube 5881 because it causes confusion among people.  
 
FWIW, I've had experience with both types of tubes, and I'll say that the Tung-sol 5881 was a nice pleasant midrange and smooth top. The Sovtek 5881 is good for clean sounds but as soon as you start to push it, the top end gets a little ragged. That's not to say that it is a bad audio tube, it does sound pretty good in amps that are designed around it. The Sovtek 5881 also is very sturdy and reliable, it can easily take voltages of 500 volts and last in the circuit. Some hifi guys say that it is an *excellent* hifi tube. It also has the advantage of being fairly inexpensive, less than half what a decent NOS 6L6GC would cost.  
 
YMMV  
 
Steve
 
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