Transmitting Triode Amps
I am looking to build a transmitting-triode based amp. Single Ended operation, >20 W power, and (relative!) afforadability is what I am looking for. I just love those large, brightly glowing devices!
I in the process of downselecting a schematic - I am currently considering a 300b transformer coupled to a 845 output valve type circuit. Only problem is that I have never actually heard such an amp. I would appreciate any feedback from anyone who has built such an amp, and any suggestions.
The two usual choices are 845 or 211.
As a first project I would be very wary because they use very high voltages >800v.
Having said that the 211 & 845 amps I have heard have been very good, like a 300b on steroids, with all of the great mids plus bass control.
Until i have a lot more experience i have set a 500V limit and most of the amps in my queue are under 300V... my path to more acoustic power is more amps (my full-range bi-polar 2-ways can accomodate 3 amps [plus the SS sub amp of course]) and more efficient speakers ...
The voltages are the reason why my first project is going to be a stereo 300b. Something like a JELabs I imagine
For a 1st try i would go with a 211 as it has a µ of 10 and is not so hard to drive as an 845. Both 211 and 845 have wonderful sonics and i could not easily decide for one of them. To my ears, the 211 seems a tiny bit more neutral and the 845 has a tinybit more magic although from plate characteristics would expect the inverse; the 845 is one of the most linear triodes ever made. But for1000V at the plate it wants to see 300 V p2p of swing and this swing has to mean business :(
(For the record, the 211 and 845 i heard were running in different amps.)
Anyway, transformer coupling is a very good idea for both tubes.
High voltages: a problem to find PS (and coupling) caps. Else, not a big problem if you are permanently aware that 1000 Volts can jump quite a distance in free air already, so don't carelessly approach the plate voltage with an uninsulated piece of metal. The surge catches you earlier than you think.
I hope Vinylsavor is lurking and throwing in his 2 cents, he built a 211-driven 211 (gorgeous!!) and can add practical advice.
I could not yet be convinced to enter such a project; the output transformer's primary has to be sooo big and eats up all the bandwidth ... probably a mental barrier only. Pondering on a different path:
A SET amp using the ED8000, a small noval miniature tube having R_p of 250Ohms and being happy with a 600 Ohms primary.
as Bernhard mentioned, I have some experience with the
211/845. Currently I use a 211 as output tube, driven by
another 211. My amp is configurable for 845 or 211 in
the output socket.
I learned, that these big triodes are very dependant on the
quality of the driver stage. Your decision to go for a DHT there
is good. I personally would not use the 300B. If you use
a thoriated tungsten output, use a similar driver tube.
If you don't want to go to the extreme of using a 211, go
for the 801A. There is something special about that thoriated
The 801A has more gain than the 300B. If you have a preamp
which can drive a transformer input, you can stick with just two
stages and a 1:4 step up transformer at the amps input.
With a DHT as driver you need to use DC heating, otherwise
hum can become a big problem. These thoriated tungsten
triodes change their sound significantly with different kinds
of filament supplies. Don't use voltage regs. Either use a
passively choke filtered supply, or a constant current source.
Have fun with your project!
High Voltahge Transformer Issues
So, it my choice is now betwen a 211 or an 845 based amp. The plate voltage on some designs I ran across is up to 1 KV - aside from the obvious personal safety issues, the problem of transformer breakdown comes up. Does anyone have any experience with this, and any guidelines/suggestions for suitable output transformers?
How about a pair of 810s? Have a pair, never used in my junk box.
the 810 is a transmitting triode meant to be used with positive grid bias, means heaps of grid current. It is meant to be used for class B, prefereably for class C operation. not a beginners project to build a well-sounding audio amplifier from it.
If you want, i can mail you the datasheet, maybe it's easier then to sell them.
The sheet doe not even show curves below bias -20V. There, the curves are cuddling together, promising a very nonlinear operation. The tube should generate considerable k3
Lots of AM stations used those as modulators, as well as transmitters.
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