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Do Power Tubes in Cathode Biased Amps Need to be Matched

2/21/2006 9:43 PM
Amp Guy Do Power Tubes in Cathode Biased Amps Need to be Matched
I know it's nice to have a matched pair/quad in a fixed biased amp.  
Does it matter in a cathode biased amp?  
If I had a pair/quad of tubes that was not matched, would the cathode biased amp even know the difference?
2/22/2006 11:00 AM
Re: Do Power Tubes in Cathode Biased Amps Need to be Mat
It is the same for both fixed-biased and cathode-biased amps if the biasing voltage only comes from one shared source. If you have more than one source of biasing voltage (separate adjustable bias pots for each tube in a fixed-bias scheme, or separate cathode resistors for each tube in a cathode-bias scheme), then the matched pair/quad logic does not apply as well.  
On the other hand, do not necessarily think that "it's nice to have a matched pair/quad" in any amp. Matching output tubes IF DONE CORRECTLY can minimize stress on the output transformer and help with the hum-canceling attributes of a push-pull output section. If done correctly, matched tubes given you a cleaner and stronger fundamental tone. However, mismatched output tubes will ead to more even-order harmonics in the output section, especially if there is no negative feedback loop. Even-order harmonics definitely add a desirable sweetness to notes. But mismatching also lessens the push-pull effect on hum and can stress the output transformer if you mismatch too far. Even more misleading, some tubes sold and heavily advertised as "matched" are not even matched in all the categories where matching might make a difference. In sum, do not accept it as set in stone that matched output tubes are better than non-matched tubes because the real story is not so one-sided.
2/22/2006 10:21 PM
Steve A.

    So what if the pair/quad is mismatched to the point of being from different factories (and millenia!) You were referring to tubes from the same batch, but not actually matched- right?  
    Bruce was just telling us about a control to deliberately mismatch the two phases of a harmonica amp... I guess with a harmonica amp the idea is to get as much volume as possible without getting feedback? Of course, you'd want a good sound, too, but microphone feedback seems to be the big spoiler...  
Steve Ahola
2/23/2006 8:30 AM
Al Lang

It has been know that slightly mismatched power tubes can produce nicer harmonics then dead-on matched tubes. I like to run my tubes about 5 to 8 mA's off from each other. A balancing pod or two biasing pods can bias where ever you want.  
2/23/2006 11:37 AM
Jammin' John
No Match, Thanks !
I also like my tubes to be a little mismatched !  
I use "pulls" that my hi fi friend finds for me.  
As long as they don't hummmmm,off key, and sound good I like 'em !  
2/23/2006 5:44 PM
There is a difference between an un...
There is a difference between an unbalanced signal applied to a push/pull power section and unmatched power tubes (different current draw at same DC voltage) in a push/pull circuit. Tubes that are mismatched, as to current draw, can cause excessive DC current in the OPT, unless they have separate bias adjustments. Going back to standard tolerances of the day, I consider tubes that are within 10% current draw to be balanced. Thatís just my opinion.
2/23/2006 8:25 PM
Re: Do Power Tubes in Cathode Biased Amps Need to be Mat
Steve, I have heard of people using completely different brands (Mullard, Tesla, Siemens, etc.)of the same type of output tubes (EL34) in old Marshall amps without worrying too much about matching. So it probably can be done if you are careful. However, I never tried mixing brands in my Marshall. or my other amps. Like I mentioned above and somebody mentioned below, mismatching can cause trouble for the output transformer as you start to go more extreme with the mismatch. Output trannies are too expensive for me to experiment with those extremes... at least right now. :)

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