845/211 driver circuits
I am partial to the 211 and 845 output tubes. I am curious as to all the different drive circuits that have been used in these designs.
I have been experimenting with a few different driver circuits using common tubes that do not draw major current such as the 6BL7/ 6BX7 tubes.
Circuit 1 - 6SL7 (Both sections paralleled - grounded cathode) direct coupled to a 6SN7 (Both sections paralleled - cathode follower) driving a 211;
1050 plate voltage
Custom 16k OPT
14 watts RMS @ 5% THD
Circuit 2 - 6SN7 (SRPP) direct coupled to a 6SN7 (Both sections paralleled - grounded cathode) driving an 845;
810 plate voltage
19.5 watts RMS @ 5% THD
Scary B+ always kept me away from these...
I have pretty good results using 417A/5842 into a 6AH4 triode for driving an 845. A single 12HL7 will just about get you there in terms of gain. I'd like to try one of those with something like a 12B4 cathode follower.
Here's a preliminary design I'm working on that makes use of the 845.
This one uses the 6DR7, a dissimilar dual triode for use as a vertical deflection oscillator/amp. The small signal half is a high-u triode that gives a decent gain margin for this. The large signal section can stand up to the high voltages that the large input swing (310Vp-p) that the 845 requires. Active plate loading here maximizes the output swing and linearity.
These DHTs have a nasty habit of pulling grid current even before Vgk actually swings positive. To handle that, and also source the current needed to charge up Cgk + Cmiller + Cstray, as well as handle the occasional transient clip, use a source follower as a grid driver.
Also, incorporate variable gNFB.
Stay away from a driver that is RC coupled to the 845 / 211. An IT or cathode follower driver will provide sufficient drive for the output tube. Sounds much better.
If you aren't adverse to solid state devices, check out tubelab's web pages for good stout drivers for big triodes. I think he calls it "power drive".
3.2K is low for an 845. It likes to see an OT of around 7K ohms. 16K ohms is good for the 211. Higher OPT impedence reduces output power only slightly and it reduces distortion quite a bit. It also increases damping factor. Both of these things are good.
If you are doing this single ended you will have problems with filament hum on the big triodes. Getting a low noise filament supply will take some doing.
Thanks for all the reply's.
I am not necessarily against mixing different technologies, but would like to stay all tube based for this experiment. No problem with hum from my 211 / 845 DHT amps, just have to have a good (beefy) filament x-former, good caps and a decent DC circuit.
I am working my way up to using other tubes for the final driver, but wanted to start off with basic tubes and see (and hear) the results . Circuit 2 is driving the 845 as is referenced in the RCA 845 data sheet, except the x-former is supposed to be 3.4k. Extrapolating the data for the plate voltage and bias points I used, 19.5 watts RMS is right on target using the low impedance OPT.
Yes, more primary impedance is better, but I wanted to verify the Chinese 845 operating points by using this configuration. Seems like the Chinese 845 are close to the originals as we all have suspected. Not a bad tube for the money.
At 1 watt, the 845 output THD (with driver circuit 2) is only 0.2%. Not bad for a couple of common octal tubes.
One of the best driver you can use is the Alan Kimmel choke coupled MuStage. Look up the last two issues of VTV magazine on it. The choke coupled MuStage does it all (1)Wide bandwidth (2)Low distortion & very linear (3)Very good PSRR(4)Low output impedance with High drive currant
(5) No driver system is more consistent and versatile than the choke couple MuStage.
. You can use a lot of input tubes that would not work as well,like a EF86 Mesh plate or a Russian 310A.For a wild combo try the pentode of a 6bm8 input 7591 driver,you could get well over 800vPP with that combination. If you what to use your 6sl7 try a EL86 for a driver.
One last thought about choke couple MuStage,it work really well with a interstage transformer because
it has the low output impedance with good currant. Give it a try you will not reget it.
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