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Swapping tubes in a hybrid amp

#61
sratas said:
ok...got some serious mod fever after all this talk about hybrids and tubes.

re-listened to my samples with various tubes and combinations of them.
had a hard time picking a clear favorite or drastic changes in tone. Think about subleties, even with reference headphones and good ears :) Much more easy to detect a change in the overall loudness of the sample, maybe indicating different gain in my particular circuit. The most particular ones, compared to the stock russian 2 tubes, proved to be two Tung-sol (very clear, transparent, extended, defined, with as much gain), and the Sovteks LPS (almost as much clarity as the Tung-sol, with a little bit more gain and colouration).
Pumped by all this tube talk, kept the two Tungsol 12ax7, and after a good two hours playing, I can say they can be a very good compliment. They sound a lot clearer and now I can keep the sublows flat without too much boominess, where the stock tubes forced me to cut drastically the 30 Hz frequency point on the parametric.
Let's see with the band, but I'm quite pleased.
Unfortunately, I have not any 12AT7 on hand at the moment, but I am intentioned to try one (maybe the Siemens ECC801S or a Gold Lion) when I happen to buy it.
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The brand isn't important, it's the combination of specific parameters common to all 12AT's that is responsible for the difference. Brand is way more to the right of the decimal point compared with the class of tube (12AT versus 12AX)
 
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#62
agedhorse said:
On the Walkabout, it's the tube closest to the input jack and heatsink.

I'll let you decide what the end result is and what your impressions is.
Click to expand...

Just tried this as well. I started with my usual Tung-Sol 12AX7 (Gold). This is my fav 12AX7 of all (Gold Lion, JJ, "regular" TungSol, TAD) as it seems to just sound fatter.
Swapped it for a TungSol 12AT7 (nonGold). The amp was obviously a bit quieter. Apart from that my ears don't seem to pick up the AT7 magic :(
 
#64
agedhorse said:
Replacing a 12AX with an AT or AU will not generally damage anything unless the product is so marginally designed as to be useless. A 12AU can provide greater current but only if the circuit is designed to allow it.
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In an old tube amp of mine, I was running some 12AU7s for the PI and driver stages (originally designed for a 12AT7) and was getting some redplating on the power tubes. When I replaced them with 12AT7, the redplating went away. Was this caused by using the wrong type of tube in a non-gain stage?
 
#65
gillento said:
Just tried this as well. I started with my usual Tung-Sol 12AX7 (Gold). This is my fav 12AX7 of all (Gold Lion, JJ, "regular" TungSol, TAD) as it seems to just sound fatter.
Swapped it for a TungSol 12AT7 (nonGold). The amp was obviously a bit quieter. Apart from that my ears don't seem to pick up the AT7 magic :(
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Interesting, did you play at higher volunes? Also see if the tone controls work a little more smoothly. I would have expected a little more expressive dynamics, especially at higher levels. The greater the level the greater the dynamics.
 
#66
cfsporn said:
In an old tube amp of mine, I was running some 12AU7s for the PI and driver stages (originally designed for a 12AT7) and was getting some redplating on the power tubes. When I replaced them with 12AT7, the redplating went away. Was this caused by using the wrong type of tube in a non-gain stage?
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It depends on how the bias voltage for the output stage is managed. I don't like circuits that are dependent on specific tube parameters for something as critical as bias. A PI tube failure taking out a set of power tubes is not a stable design IMO. This kind of gets back to my point of designing circuits that are forgiving of tube parameter variations, you don't want the bias of output tubes to shift with PI tube aging either. Fortunately most PI circuits address this ok.
 
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#67
agedhorse said:
It depends on howv

the bias voltage for the output stage is managed. I don't like circuits that are dependent on specific tube parameters for something as critical as bias. A PI tube failure taking out a set of power tubes is not a stable design IMO. This kind of gets back to my point of designing circuits that are forgiving of tube parameter variations, you don't want the bias of output tubes to shift with PI tube aging either. Fortunately most PI circuits address this ok.
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The bias voltage appears to be taken off the PI. Was this a more common design choice in the 70s? How hard would this system be to replace with a quartet of bias pots?
 
#68
rnolan said:
@ paulraphel, (there's no solid state anything in the signal path), you probably have to go back to the early 70s' or before for this to be the case (e.g. 1972 Marshall (no PCB just a breadboard)). Most tube circuits include ss IC opp amps etc
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Well, 1982. That's when the VTBP-201 was designed. Pretty sure there's no solid state nuthin' in the signal path. The tone controls are all passive, old Fender-ish. I believe the rectifier is ss.

FWIW, the best sound I got out of this thing, to my ears, was from some 1960s era RCA black plate 12ax7s. They're warm and slightly dark, without hurting the preamp's clarity (I liked to tone down the Dem's glassy treble a bit). These were a bargain 10 years ago when I got them, but as with a lot of things they're stupid expensive now.

To the point of how long preamp tubes last, when I called Jim Demeter about a problem, he immediately dismissed the idea I'd worn out the tubes. He basically said that in his circuits the tubes would outlast the capacitors. And indeed my unit's problem was unrelated to tubes.

@agedhorse, in preamp designs where tube differences have minimal to zero audible effect, is there a reason to use tubes at all, besides marketing?
 
#69
Tubes generate harmonics. When they are pushed, they can compress and exhibit non-linear behavior. Both these attributes can be highly desirable and a good reason to use tubes. People work very hard to try to capture these tube related qualities in a solid state amp. There is a tonal difference between tube and solid state amps. Otherwise, there wouldn't be any new tube amps. It isn't just marketing.

When you are chasing tone, this is important to some.
 
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#73
paulraphael said:
@agedhorse, in preamp designs where tube differences have minimal to zero audible effect, is there a reason to use tubes at all, besides marketing?
Click to expand...

As beans suggested, there are still reasons to use tubes. Because there is little difference between tubes doesn't mean that there is no dependence in the way a tube works or functions but that the wildly differing parameters have resolved back to some basic baseline that is desirable.

There are so many circuit variables to exploit in a tube preamp that it's really hard to generalize, but for some tube circuits it is very possible to emulate them in the solid state domain, and for other approaches it is very difficult to do so. Then there's all that space in between ;)
 
#75
one of the biggest differences is in the preamp: a 12AX7 gives a very natural compression, especially the harder its played. Solid state pres do not do this....its not nearly as obvious on single-coils (a J) as it is with a humbucker (a P)
 
#76
Rib 13 said:
one of the biggest differences is in the preamp: a 12AX7 gives a very natural compression, especially the harder its played....its not nearly as obvious on single-coils (a J) as it is with a humbucker (a P)
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Depends very much on the circuit, a 12AX7 can be used in a way that does exactly the opposite as well (if that were the goal)...
 
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#79
Years ago, they used tubes in instrumentation amps. These were amps operated in a linear region and were used in measurement and test equipment. Examples include chart recorders for plotting data, types of operational amplifiers, even computers.

As designed, they make lousy audio amplifiers. The circuit design dictates the application and the same tube can be used in a variety of applications.
 
#80
agedhorse said:
There are so many circuit variables to exploit in a tube preamp that it's really hard to generalize, but for some tube circuits it is very possible to emulate them in the solid state domain, and for other approaches it is very difficult to do so. Then there's all that space in between ;)
Click to expand...

Maybe a topic for another thread, but which qualities of tubes do you find most difficult to achieve with a solid state circuit (not counting digital modeling, etc.)?

I'm asking as the veteran (in both bass and hi fi) of tubey-sounding solid state gear and solid state-sounding tube gear.
 

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