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On Mixing output tubes -- Is this a good idea?

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jedinvest

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I have just replaced a bias resistor and EL34 tube on my Anthem AMP-1. Since I am getting fed up with having blown bias resistors, but getting good at replacing them, I just find a spare tube and plug in in with a new bias resistor and then bias each tube to about 35 ma.
I noticed that the amplifier is sounding awfully good. I also am wondering if this is the result of mixing output tubes. I mean a GrooveTubes for good bass and a EH for a clear midrange, all on one output side. The other side has a EH for that good mid-range again and a Svetlana for silky highs.
Anything wrong with this? Is it maybe advantageous?
Also, I don't bias often, actually only when I install the new tubes. Is this a big mistake? I think biasing is a big headache with about fifteen little screws to undo, and then watchout for that high-voltage!

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Re: On Mixing output tubes -- Is this a good idea? - Lew 07:39:34 01/22/04 (3)
In theory for optimal performance of a circuit with several output devices in parallel, you want those devices to be as closely matched as possible. By mixing tubes of different origin, you reduce the chance that the disparate tubes are well matched. The notion that mixing tube brands will obtain the perceived special sonic virtues of each brand is, to put it gently, wrong-headed. This is not to say that by dumb luck your amp could not sound good with tubes of mixed origin.
- Re: On Mixing output tubes -- Is this a good idea? - jedinvest 11:11:45 01/22/04 (2)
  That is the conventional wisdom. As far as running tubes in parallel, that is probably the case as you describe. However, running a different tube, or set of tubes, on each leg of the Push-Pull output, should not have any problem if the tubes are biased individually. If you consider that each tube has its own distortions and the cross-over region distortion is probably one of the biggest, then there is nothing wrong at all in this arrangement.
  What is in question is whether a tube with good bass when mated with a tube with a good top-end gives you both good bass and good top-end or neither. I don't have the patience to find this out immediately, but in the future I will consider the opportunity when it arises to try out these combinations.
  - Re: On Mixing output tubes -- Is this a good idea? - Lew 07:07:36 01/23/04 (1)
    I will hazard a guess that the results of mixing tubes with perceived different sonic character (and I use the word "perceived" deliberately) will be serendipitous; could be good, could be bad. For sure there is no harm in experimenting, especially if you can bias each tube individually. As you probably know, it is well and good to set the DC bias to be equal for all tubes, but still each tube may behave differently in terms of current demand when driven with an audio signal. Folks talk about mixing capacitor types in order to form a copacetic coupling capacitor; that approach does not work well in my experience.
    - Re: On Mixing output tubes -- Is this a good idea? - Failsafe54 19:14:46 02/03/04 (0)
      I have a set of CJ Premier 12's and recently got the bug to change tubes, at the time I was using the stock tubes that CJ supplies.
      I purchased a complete set of cryo-treated tubes that included 4 (6550) Valve Arts, 2 (6FQ7) RCA Cleartops and 1 (5751) JAN for each amp. After listening for a few days I realized that the new tubes had better bass and air but I had lost some detail and focus. So I mixed the tubes, using 2 CJ and 2 Valve Art 6550's and one CJ JAN and one RCA Cleartop 6FQ7. The result was just wonderful, got the detail and focus plus the bass and air. Mixing the driver tubes (6FQ7) made the biggest difference, followed by the power tubes. I don't know if it's a good idea from an electrical standpoint but I have about 200 hours on the new setup and my bias has remained dead-on.
Re: if you are replacing bias resistors often... - Russ57 15:30:42 01/21/04 (2)
then heck yes you should check your bias more often. Once a new tube has run in for a 100 or 200 hours then you can relax and check it less often. But for the first week I'd check it daily or at least every couple of days. Then maybe once a week and then after a month or two only every now and then. Better safe than sorry for me but my amp is very easy to check and takes maybe all of 5 seconds.

Russ
- And which amp do you use that is so easy to bias? (nt) - jedinvest 16:51:56 01/21/04 (1)
  - Re: It started life as a Dynaco - Russ57 04:03:57 01/22/04 (0)
    but the only thing that's stock Dynaco are the transformers. I don't know what amp you have but checking the bias implies that it is a fixed bias amp and usually one measures the voltage drop across a resistor and adjusts a pot to get a certain voltage reading. If that is the case with your amp you could mount some small test jacks and run wires to them and make it quick and easy. Of course you would have to place the adjustment pot where it could be accesible.
    Russ

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