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I recently bought a Dynaco ST-70 clone with the VTA board. It came with two GE 5814A phase splitter tubes and a Sylvania 5963 tube in the input position on the VTA board. I'm wondering why the different tubes for the board? I know that they are all 12AU7 equivalents, but why not use a matched 12AU7 trio? I tried a matched trio of Electro-Harmonix 12AU7's in there but the 5814A pair with the 5963 sound much better. The trio of EH 12AU7's made the amp sound more like a solid state amp. I'm a bit new to tube amps and I'm hoping someone can explain to me why this is. I love the sound from this amp, by the way, it's a great match with my Klipsch KLF-20's.
Edits: 02/27/14Follow Ups:
mrhardyman covered some ground that I agree with. The VTA board is excellent, though we favor the 6SN7 version. The VTA board comes with generous filtering for each of the three tubes and a needed replacement bias supply.
The other thing you should do is beef up the main power supply if it hasn't been done. We replace the input 20uF capacitor with a 50 uF unit, and the next capacitor, also a 20uF unit, with 500uF. It gives the amp surprisingly good bass.
To be clear, we are a dealer that uses the VTA board. We are not a dealer, and we do not get a special price on the boards. We just like them and use them.
Why does the center tube have such an effect on the overall sound? I'm trying to learn how to set this amp up and what tubes do what. I'll look into the 12BH7's, are they a direct drop-in replacement for the 12AU7's with the VTA board? I think I read somewhere about using them as an alternative to the 12AU7's with good results. I've got a trio of RCA Cleartops but found them a bit bright with my system. At present I'm using a matched pair of RFT 5814A's on the sides with the Sylvania 5963 in the center with pleasing results.
Edits: 02/28/14
The center tube does voltage amplification duty for both channels. It is resistor loaded. The other tubes are in a constant current long tailed pair configuration with one of the grids grounded. It is the phase splitter/driver that connects to the finals. The center tube's topology is different from the other tubes' topology. The characteristics of the center tube's voltage gain will more audible than the splitter/drivers', since any common mode distortion will largely cancel in the output stage, and there's little or no gain in the stage. The 12BH7 requires more heater power than the 12AU7, but that probably won't be a problem.
Let us know how it sounds.
I've had great luck tube rolling with mine. The center position is the most critical and needs to have both sections match. This particular tube really 'voices' the amp. I've used Psvane, RCA cleartops, new Tungsols and others. For me the new Tungsol right now is my favorite. Very quiet, extremely musical and makes this amp sing. The Psvane is equally good. YMMV.
The side tubes don't have to be 12AU7s and a really nice substitute is using the 12BH7 tube in those positions. It doesn't change the voicing much but it seems to handle voltage swings much better. I've had really good luck using the EH 12BH7. Not NOS but just as good and really cheap in comparison.
I received a Tung-Sol 12AU7 today to try in the center. Odd that this tube is identical in every way except the labeling to an Electro-Harmonix 12AU7/ECC82 that I already have. I've read about this somewhere before, that there are several different brands of tubes coming out of Russia that are the same tubes with different labels. Now I know for sure. Sounds the same too.
Edits: 03/05/14
Same factory. New Sensor bought the names. It's a puzzlement!
On the other hand, I've bought the Psvane 12AU7s for my various and sundry preamp circuits and they are all rather marvelous!
The Tung-Sol in the center spot works very well, is balanced, open, articulate and, most of all, quiet. I can't tell the difference between it and the Psvane "T" series, even though the price is so similar. Very dissimilar build.
I don't have a background in tube glory. It is just very nice to be able to buy new tubes that are the sonic equivalent of legendary 'NOS' and that are consistent in build quality. My few forays into NOS is a mixed bag. Some are wonderful, some are just old.
Give the tube some time. Mine really didn't settle for about 200 hours. Mind now, I let most of my DACs and Preamps run 24/7. They just sound better when the internal temps are up and running. Every time I fire up a cold piece in the signal chain, it is not pretty.
Best sound so far is with the RTF 5814a's in the sides and a GE JG 5814a in the center. The GE's bass went way lower and the midrange was fuller than with the Tung-Sol. Also the soundstage was very well spread out with sparkle in the highs. I tried the RTF's with the Sylvania 5963 in the center and got a very diffuse soundstage with more top end but it sounded a bit harsh. The GE got rid of the harshness, narrowed the soundstage somewhat, and really brought out the bass and midrange.I can say that I'm fairly happy with this setup. Haven't tried the 12BH7's yet, they're next on my list.
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