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Cathode Bias on Matchless Amps


 
8/20/1999 6:28 PM
Charlie
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Cathode Bias on Matchless Amps
The schematics of the Matchless Clubman and Chieftan amps show P-P EL34 output tubes with the cathodes individually biased by a 270 ohm resistor/250 mfd. cap combination.What's the logic behind this instead of both tubes cathodes sharing a single resistor/cap ?
 
8/20/1999 8:22 PM
Benjamin Fargen
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Charlie,  
 
Multiple reasons......flexibility in matching a non-matched pair of NOS tubes(I.E. individual bias settings),Reliabilty of the cathode resistor and required wattage of the cathode resistor.  
 
Both of these amps are running at a high bias current in class A. The lower voltage drop across the individual cathode resistors allows for the use of a lower wattage cathode resistor. I believe Matchless used 2 X 12 watt resistors in these amps.....a 25+ watt resistor would be needed to run a single sharing CR. Also the size of a 25 watt resistor would not very flexible for layout reasons.  
 
 
 
Benjamin
 
 
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8/23/1999 1:13 PM
Charlie
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Benjamin,  
 
Thanks for the reply.You've confirmed what I had suspected-to keep things running cooler and some added flexibility to "Tune" the output tubes ,if needed.  
 
Charlie
 
8/21/1999 7:54 AM
pgambon
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I think there is something sid for having tubes work independently. Fender amps seem to enifit from having shared cathode bias resistors replaced wit individual ones. so note that the matchless runs the first 12ax7 with both sections wired in paralell (sp). I've often wondered if there as cross talk between the halfs of a 12ax7. I've constructed a champ type amp using two 12ax7's wired this way. It sounds real strong. I haven't done a real scientific study on this but that's my impression. Any thghts on this?
 
8/21/1999 4:47 PM
Bruce
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When paralleling preamp tubes like this you get about a +3dB boost in signal with NO additional noise figure.  
 
If the bias resistor is set to a low enough value to allow plently of current to flow in both parallel tube halves, the sound is a little richer too.  
 
In a conversation with Doc, he pointed out to me that the Matchless parallel preamp tube circuits normally do not take advantage of that and in fact bias the parallel sections up for even less current.  
 
Bruce
 
8/23/1999 2:21 PM
Charlie
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It's interesting that Matchless parallel preamp ckts. don't appear to take advantage of the +3db boost.As a matter of fact,the Chieftan Reverb has two paralled 12ax7's ( V1 & V2)in the preamp signal path. I don't recall seeing this in other amps.It certainly isn't an economy move-adds a whole extra tube! Could it be for a richer tone ( ala Matchless !)or is there something to be said for reducing "Cross talk" between 12ax7 halves as mentioned in the previous post by pgambon ?  
 
Charlie
 
8/23/1999 2:58 PM
Doc
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I doubt that there is any cross-talk in those multi-section tubes. Control grids are surrounded by their own plate structure, and the pin connector leads are relatively short. Remember, signal levels and maximum frequency levels are low in these dual-triode or triode/pentode circuits.  
 
 
 
Paralleling a pair of similar tube sections tends to "thicken" the sound a little. It may actually be smearing, due to inexact match of each section's operating characteristics.  
 
 
 
If you take a typical 12AX7 preamp (as seen in Fender circuits), which has a 100k plate resistor and a 1.5k cathode bias, and parallel two triodes but expect same voltage gain & input signal headroom, you would halve the value of each resistor. (So that would be 750-820 cathode resistor and a 47k plate resistor, each with double the wattage rating since the current through each is now doubled.)  
 
 
 
When you look at the Matchless circuit, you'll notice that the common cathode resistor is 1.5k, and the plate resistor is sometimes 220k. If a single triode were used here, and the same gain characteristics wanted, the cathode bias resistor would be 3k and the plate resistor would be 440k! That's a high gain, low operating current circuit. On the Spitfire amp, I measured a bias voltage of 1.26v across the 1.5k resistor. That translates to a combined cathode current of .84ma, or less than a half ma per tube section. The plate voltage measured 150vdc.  
 
 
 
When paralleling two (of the same type) triodes, the current gains (transconductance) add. You get twice the current gain for the same amount of noise as that created by a single triode. So you do get a higher signal-to-noise ratio, but if the two tube sections are not carefully constructed and matched there will not be identical signals adding through the plate resistor. This is ok (and obviously desireable) for guitar amps, but may not be acceptable in circuits where pinpoint accuracy is important, such as hifi phono preamps.
 
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