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Gold_xyz 7th February 2008 05:24 PM

300B PP ph.split
Hi to all,

I have tried to decipher the value of the components, but (for me)
it is almost impossible. :(

some idea ?

JLH 7th February 2008 07:52 PM

I cannot make out the values either.

Now, just my own personal opinion, but I would use a transformer phase spliter if it was me. I have found the more simple the signal chain, the better the sonic results. However, if you really like that schematic, go for it.

Rgs, JLH

Yvesm 7th February 2008 08:24 PM


Use a center tapped tarnsformer at the input of the amp, then drive a differential gain stage.

The input transformer has almost no power to transfer nor DC current to flow thru ;)

Best is no transformer at all :cool:


smbrown 7th February 2008 08:45 PM

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!

Miles Prower 7th February 2008 08:53 PM

"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.

jon_010101 7th February 2008 11:23 PM

I agree that the paraphase gives way too much gain for this application. Great square waves though! Still, I'd personally wrap a big negative feedback loop around that amp to increase the 1.7 damping factor and reduce the distortion a bit. :smash:

Miles Prower 8th February 2008 05:17 AM


I definitely believe that gNFB is a good thing. I don't know how big the feedback loop would have to be. Too much tends to make for a "solid statey", "Big Box store" sound. I went with hollow state to get away from that.

Shoog 8th February 2008 04:11 PM

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.


SY 8th February 2008 05:55 PM

If the tubes are linear, then shouldn't they have almost no "signature"?

Shoog 8th February 2008 10:33 PM


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. "


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