• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

300B PP ph.split

I've used a Lundall phase splitter at the front end of a PP 300B amp (LL1676??) and I didn't like it except with a low impedeance TVC. I think the output impedance of the preamp becomes a significant issue (see Lynn Olsen's notes re: Karna amp and others). As another track, I've also used the Magnequest SETH amp autotransformer splitter and liked it. Sound was great, sensitive to placement around other mag fields, but otherwise good. My best PP 300B has been done using the kind of circuit Allen Wright uses in his PP1 amp. See Vacuum State Electronics schematic section for more on this. Oh, and I've also tried a Williamson style front end (sans feedback), it was okay. I auditioned that amp at the last VSAC (2003?) and it was well received in the craftsman room. So, overall, PP1 is tops, followed by the MQ autoformer (pps style), then the Williamson, last was the Lundall input transformer. Of course, YMMV!
 
"Best is no transformer at all :cool: "

Absatively! This is one of the few 300B designs I've seen where there is adequate grid drive. Inadequate grid drive is, I believe, a big part of the problem of disappointing performance from this particular VT. This aspect is well done, although I do question the benefit of having a BW that could make this serve as a longwave RF amp.

Why they went to all that bother and used the sh!tt!st possible phase splitter is beyond my comprehension. The paraphase just plain SUX, and would be suitable for a quick 'n' dirty design where you weren't particularly concerned with fidelity, as in the audio out of a SW xcvr, ferinstance. The article complains of excessive gain. So the phase splitter should have been an LTP with active tail load. That would be a vast improvement.
 

Shoog

Member
2002-08-15 10:16 pm
Eire
I second the advise to consider a center tapped choke. This produces a very clean direct sound.

I have to disagree with Miles on this. All the theory says that a transformer shouldn't work but when you actually plug one into the right circuit they sound like nothing else. Also why go to the trouble of using some of the most linear valves available and then kill their signature by wrapping large amounts of negative feedback round them. Build with KT88's if that your philosphy. 300B's offer you the most direct path to signal purity and they should be used in the simplest circuits possible. A tiny bit of Negative feedback might be appropriate to tighten up the bass response - but I would judge this very carefully in light of listening impressions.


Shoog
 

Shoog

Member
2002-08-15 10:16 pm
Eire
If the tubes are linear, then shouldn't they have almost no "signature"?
True, but nothing is without some distinct flavor beyond its measured characturistics. The figures only tell you part of the story as far as I am concerned.

Would you not admit that an over zealous use of feedback kills the very quality which good triodes bring to the mix, ie air, space, warmth, ambience and microdetail.
Infact as Miles said himself,
"Too much tends to make for a "solid statey", "Big Box store" sound. I went with hollow state to get away from that. "

Shoog
 
Shoog said:
Also why go to the trouble of using some of the most linear valves available and then kill their signature by wrapping large amounts of negative feedback round them.

Who said anything about using "large amounts" of gNFB? You never saw anything like that from me, I guarantee you. My designs use very little, as compared to SS designs. (But that's a whole 'nother story, and BJT amps need lots of gNFB.) The latest project has variable gNFB that maxes at 12db(v). For the most part, I don't use more than 6.0db of gNFB. The other design I did doesn't use much more.

As for using linear valves, optimizing the open loop performance before adding the gNFB is the major part of my design philosophy. gNFB is an enhancement, not something you use to sweep your design errors under the carpet. As for "signature", I have no idea as to what this is supposed to mean. Everything I ever heard that relates to this concept has been in a negative light, such as the "MOSFET sound" from some really ill-concieved designs. Or as related to gee-tah amps (as in the "Marshall sound", or the "Fender sound") where fidelity isn't a consideration, or necessarily desireable. I leave such arguements to the axemen, since I don't play.
 
Would you not admit that an over zealous use of feedback kills the very quality which good triodes bring to the mix

That depends on your definition of "over-zealous use." If the amp is designed to be unconditionally stable with feedback and care is taken to avoid blocking, then the use of feedback is, in my experience, a positive. This assumes speakers that want a low driving imnpedance (i.e., 99% of them)

But I will point out that I'm not aiming for effects boxes, I want my amps and preamps to not audibly alter the signal, just make it bigger. Others may have different goals and my comments are not relevant to that approach.
 

Shoog

Member
2002-08-15 10:16 pm
Eire
I would have to say that my experience of using gNFB hasn't been great and I have achieved better results with different approaches. That doesn't make me an out and out enemy of it though and I'm certain I would use it in the right place.

We will have to agree to differ on this one.

Shoog