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121.13.152.69
Folks,
Quite a few people always are looking for "high power" SE Amplifiers. Reason being, they have fairly inefficient Speakers.
I recently helped getting a custom build pair of Amp's of the table that include some circuit tricks that deliver this, while retaining all the good stuff.
Only recently we discussed Cathode Followers on thishere board:
http://www.audioasylum.com/forums/tubediy/messages/17/178125.html
I quipped in pointing to my design implemented in a Kit called Lux:
This custom Amp uses a fixed bias variant of the same circuit. That is the fixed bias (around -140V) is applied via a series resistor to the coke and 5687 Grid Resistor, instead of returning them to ground. Varying the series resistor varies the Bias, or alternatively one can use some form of automatically adjusted bias system (this build did the second).
The 300B is run a little hard (400V/80mA = 32W Dissipation) but not unduly so. Measured output is 16W RMS into 8 Ohm at around 5% THD using cheap Valve Art output Tubes.
This Amp heavily relies on the various modules from diyhifisupply (Film Cap Powersupply, Filament Supply, Active Bias System) so a full schematic is of little use unless you buy the exact modules and modify them as was done for this Amp, but the principle should still be of use in other contexts.
Due to the choke loaded cathode follower the overload behaviour is quite gentle, it takes around 6dB short term overload without extreme clipping or "choking off" of of the signal. This good overload behaviour means it is possible to "overdrive" the Amplifier quite a bit with short peaks and get a subjective level of output much greater than suggested by the actual power without obvious distortion or audible effects from overload recovery.
The custom Amp has the exact front-end from my "Yet Another 91 Type SE Amp" also found in the "Lady Day 91" Kit from diyhifisupply, basically a classic WE 91 with a WE 310A Pentode but with the output stage in fixed bias and the "turbo" circuit added.
The most surprising thing was to compare these new Amp's to some standard Lady Day 91's. Conventional wisdom would hold that the "boosted" and "more complex" Amplifiers should sacrifice something to attain their more than doubled Power (the 91 starts clipping at around 7 Watt). Interestingly enough the reverse was true.
The new Amplifer sounds significantly more vivid and has that same deliciousness and delicacy the stock 91 Lady Day has. It definitely is the better sounding Amplifier of the two, with virtually no playtime and even at low volume levels , where non of the "power boost" features of the new Amp are any help. I suspect that removing the cathode (bias) RC section from the Lux is the largest contributing factor.
Best thing though is, this Amp gets (for music, if not for test signals) output levels that subjectively place it alongside 211 and many 845 Amplifiers, while still using the much easier and cheaper to implement 300B. Kinda have your cake and eat it.
So, if someone here is in need of a fairly powerful 300B Amp try slipping a choke loaded cathode follower into a fixed bias arrangement and make sure your driver stage can swing enough voltage. The results are more than even I expected.
Now, what if we build such an amplifier with a "Super 300B"? That should again double the actual available power and if we retain the 6dB "overload margin" even most more conventional audiophile speakers should be drivable...
Ciao T
Follow Ups:
I can't agree more! For many years now I've been using a White CF to (easily) drive a pair of parallel 845s in similar fashion to get around 75W or near 40W with a single tube. The WCF is DC coupled to the previous (somewhat unconventional) stage which provides gain and sets the overall bias for the 845s. Current through each 845 is individually fine tuned by adjusting a small cathode R. 845s may be hard to drive but done this way they have absolute authority and lose nothing in finesse. Anyone who thinks 845s have no top end should try driving them with a very low impedance!
I agree that you can have your cake and eat it. Similar power and efficiency to PP but all the benefits of SET ... biased in A1 where most of the music lives and only transitioning into A2 when called upon.
I've been trying to get around to modifying my conventional 300B / 2A3 SETs along similar lines but perhaps I'll try your simpler approach as they are somewhat easier to drive.
Naz
what is the op point of your 845s (B+ and current)?
TIA,
I still have a switched network of resistors in 3 different places that I originally built in to compare the sound of different operating points of the OP tubes, and gain / bias stage (the one that feeds the White CF driver). I found pretty much the same as in most SETs to my ears and that is that increasing the current through the OP tube(s) generally sounds a tad better. My bench measurements on all of the SETs I’ve built also supports the theory of lower distortion at higher currents.
However it depends on other factors, not the least of which is how long you want your tubes to last. I’m running the 845Cs (new Ms) which will handle over 100W. I generally run them conservatively at 75 – 80 mA with cathode to plate voltage of a little over 1KV but wind the wick up to 85 – 90 mA for serious listening sessions with my audio mates.
The driver is a different story (in my case the pre driver since the WCF is pretty neutral). Here I find that lower current sounds and measures better. I always assumed that it was due to better conditions for distortion cancellation. Only recently I got around to Simming it and whilst the difference is subtle, it confirms what I’d always suspected in this particular setup.
Bottom line, better operating points will not come close to compensating for poor design. For 845s it’s all about driving them properly and of course good OP traffos and PSUs.
Naz
Obviously it couldn't be done with an EF86 as a driver, but what about an interstage transformer with the 300b fixed bias on the secondary? Wouldn't that achieve similar results yet eliminate a coupling cap and tube (yes, add expensive IT, I know). Just wondering.
Hi,
Well, the CF drives the 300B from a very low impedance, meaning it can handle some class A2 without the drive voltage collapsing, which is a big part of the extra power gained...
The paralleled 5687 run at very low current and high anode voltage manage around 300 Ohm drive impedance to the 300B. Not a lot lse can do that.
And I find IT's that run with a magnetised core to be too hard to get right, to be honest I see the coupling cap not as much of an issue (other than a religious one and one that is only in the head).
If you use an IT and Fixed Bias you have two coupling capacitors of usually modest quality (the cap on IT secondary to ground from the Bias is a coupling cap, so is the power supply decoupling capaictor for the stage driving the IT), with my little "trick" circuit you have one very low value capacitor (this particular build uses Teflon) which due to the low value required can be almost as good as it can be.
So from where I stand I can only see (and hear) drawbacks from the IT Circuit. Of course, if your religion demands IT's you can use them, but the results will be worse.
Ciao T
T.,
I'm CHEAP , by necessity. I too like the mild A2 regime, but good loading chokes are dear. "Sand" in the form of an IRFBC20 MOSFET gets my nod for the DC coupled voltage follower driver. Paralleled 6SN7 sections are downright wimpy compared to that "puppy". TANSTAAFL applies here, like always, and some PSU grief comes with the territory. Tubelab (http://www.tubelab.com/powerdrive.htm) and Keen (http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm) show the way.
I've provided a link to the IRFBC20 data sheet. Please notice the all important low and stable reverse transfer capacitance. Allen Wright curses MOSFETs out, but I find the IRFBC20 and ZVN0545A to be quite useful. Exhibiting reasonable capacitive behavior is why.
Eli D.
Hi,
Really good loading chokes are dear indeed. But with 300 Ohm Drive Impedance from the CF or inductance requirements go right down.
I do like to go way over board (100H/10mA) but even 30H will get below 2Hz -3dB Point. So the cathode loading choke has fairly relaxed requirements, many larger inductance PSU Chokes can probably work there.
The Mosfet Follower is okay driven from low output impedance stages (like the 5842/417A in the Tubelab Amp) but becomes progressively less good sonically as drive impedance goes up.
There indeed ain't no such thing as a free lunch (except in the "Free Lunch/DRD/Monkey" Amplifier circuit and even there it ain't free, it takes loads of extra +B).
But my little "trick" which for many inscrutable reasons I call "Bushido" (This explicitly is a high impedance Pentode combined with a choke loaded CF coupled directly to the output tube Grid) delivers a ***** Lunch for a beer budget, which is probably TNBTTAFL (The Next Best Thing To A Free Lunch).
Ciao T
No religious preference here. I do have a pair of ITs in a 300b amp right now (AN ones from their IT mono block kit) and I'm pretty happy with the performance (driven by paralled 6SN7). On the other hand, I've built many a traditional cap coupled designed including the "another WE91" you had posted on the DIYHIFI page, though I can't find it now. I think I used a 77 or something like that for the pentode. Those that heard it enjoyed it a lot. Good points about the coupling caps in the IT design.
Hi,
The old 91 Article is here:
http://www.audioasylum.com/cgi/t.mpl?f=tubediy&m=71898
BTW, one or two people who have heard pretty much anything I have build over time feel the Lux was the best yet.I think that 16 Watter is even better, not just at loud passages, but also way down in the quiet stuff.
Ciao T
The choke loaded CF driver topology is a real winner for SETs. I also like the EF86 a great deal, and would have used them with my own 211s if not for wanting octals. This should be a very articulate amplifier.
Is this the actual schematic for the Lux kit amp advertised by DIY Hi-Fi (which is rated at 12W Peak)?
Or is this schematic of a modification?
And/or is this a schematic to the 16W 300B amp?
"What did the Romans ever do for us?"
Hi,
This is the schematic (without values) for the Lux (which you may happen to know is based on a design by me).
The amendments for what was described above would look like this (principle only):
If the above is realised with suitable values and parts it produces 16W if the 300B is operated at 400V/80mA fixed bias.
The Lux produces around 12W with the 300B operated at 350V/80mA self bias.
The "classic" 91 Amplifier produces around 7 Watt with the 300B operated at 400V/80mA self bias.
The exact schematic would be of limited value, as it relies on several modules sold by diyhfs which ADDITIONALLY need to be modified (non-trivial) to work correctly.
But to many of the DIY'ers hanging out here the principle above should be enough to start cooking their onwn versions.
Ciao T
The cathode choke by its internal resistance applies proper cathode bias to the 5687. Then, a negative bias is applied to the cathode choke 'ground end' and combination bias of the cathode resistor plus positive fixed cathode bias applied to the 300B?
Hi,
> The cathode choke by its internal resistance
> applies proper cathode bias to the 5687.
Yup. If the choke DCR is too low just add a suitable resistor to make up the difference.
> Then, a negative bias is applied to the cathode choke 'ground end'
> and combination bias of the cathode resistor plus positive fixed
> cathode bias applied to the 300B?
Yes. The Bias for the 5687 is around 14V, so the voltage applied to the choke/grid leak resistor needs to be around 14V more negative than the Bias required by the 300B.
Ciao T
Hi Thorsten,
So the diagram plus your description indicates fixed bias for the 5687. What's going on with the 300B? Is it getting it's own fixed bias supply as well?
Cheers
Hi,
The 5687 is self biased as choke loaded cathode follower.
As it "rides" on a negative voltage of around -95V it's cathode is at around -80V which is the bias for a 300B at 400V/80mA.
Ciao T
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