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I have never seen this tube before. Each channel has an input transformer feeding an interstage transformer directly coupled to the 20B. No driver tube. Apparantly, Tron Electric uses a similar typology in its Voyager amp. I'm sure neither the Tube Lab or the Tron version is cheap. The former uses all EML tubes (both the power tube and the rectifier) + Hashimoto interstage and output transformer and Lundahl input transformers and n one of that is low cost.
Anyone ever build/hear such an amp?
"You don't need to be a Weatherman to know which way the wind blows"
Follow Ups:
line-up some time ago. Ultimate Audio has done a review on it; I think I still have the review somewhere if you would like me to have a look and email it (Edit: it is linked from the Wavelength Audio site, see Gordon's post below). I think Wavelength still has spud amps in their line-up, though I am not sure if if the 20A is one of them.I have thought about building such an amp, though despite the single tube they are probably expensive to get working well. Well designed and built circa 14K primary transformers don't come cheap (Hashimoto has one though) and I would rather go all-nickel parafeed for such a beast, alas... Three other concerns for a DIY effort: (1) I don't know if such a beast would be suitably dynamic and have enough musical meat on it's bones, (2) with series feed, would the high gain and impedance make it more or less susceptible to power supply ripple, and (3) what DC filament scheme to use?
I might again consider building one of these someday... my pending speakers should only need about 2W to meet most of my needs so could significantly reduce the step-up, especially if my source puts out a little more voltage than the standard.
Cheers.
Edits: 02/16/12
They are nice amps. Amazingly quiet, and not just for a tube amp but in general. They are quieter noise floor than most of my solid-state amps too.
nicely built, simple enough design, and well executed. I was thinking of selling mine (along with some NOS no-longer-available AVVT 20B tubes) but haven't gotten around to it yet.
They use a step-up (I forget the brand offhand), then a Magnequest EXO-45 output tranny, and Magnequest choke. DC fils of course. If I was going to hot-rod them I'd probably try a better version of EXO-45 (silver for example, or cobalt) and choke, and maybe a higher-end lundahl step-up or MQ cobalt step-up. I actually have some cobalt EXO-45 and was going to try swapping them in but never got around to it.
-Ed
Gang,
Yea I used the B, the A has too much Rp (Plate Impedance) and requires a higher load and output power would be nothing.
RCA input went into a 1:4 step up and then directly to the Grid, we used custom MagneQuest output transformers in parallel feed and it got about 3W's output power, but really required a well oiled preamplifier.
These sold big when the Horn era hit the US and all those huge amps had tons of hum.
Thanks
Gordon
J. Gordon Rankin
Apropos someone who has heard such a configured amplifier, see postings from topoxforddoc; for example, see link.
I've considered building a single stage amp with their 20B. With its 3.3k plate impedance, it needs a 10k primary impedance output transformer. 10k:8 ohm yields 35:1 step down ratio. The tube has a mu of 20. So you need some form of input gain, be it another tube or input step-up transformer to achieve unity gain, let alone positive gain.
How would one hope to drive the Miller capacitance through a step up transformer?
It's got to be high in a DHT with a mu of 20.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Unfortunately the capacitances are not specified. The description talks of the greater tube element spacings, which would on the face of it reduce capacitance, however in order to maintain the transconductance the grid and cathode area must increase (relative to miniature tubes).
A Type 10 is a similarly large tube with a relatively high mu (8) and has a grid to plate capacitance of 7pF; scaling by the mu ratio (greater plate spacing increases mu) gives 3pF. A 6SN7 is 4pF. I conclude this is about the right general area, so the Miller capacitance is going to be not far from 60-80pF, which is pretty much the same as a 300B or 2A3. Those tubes are easily driven by an interstage with a 10K winding. The only real limitation is the low primary impedance, which few tube preamps are able to drive well.
The step up ratio does not need to be that large. The 20B at factory match (medium bias) conditions is biased at -7.5v, which is 5.3vRMS. Many preamps can supply this voltage directly without stepup. Almost all preamps can supply over 2 volts RMA, so a step-up of 2:1 is adequate - that would be 2500:10K.
Just running some numbers for fun.
Paul ... I just assembled a 801A spud amp. just that tube and an input tranny 1:8 (to be installed)
sweet.
used to run something similar into his Lowther horns. He said they could not reproduce a tidy square wave but sounded sublime. I believe him.
Enjoy!
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