• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Transmitting Triode Amps

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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.

Good luck

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lilolee said:
The two usual choices are 845 or 211

All the commercial 211 & 845 amps i have (a sample set of 3) sound quite good. That said i am a rookie at tube amp building and when a box (actually 5 boxes) of tubes came my way, i sold off the pr of 845s & 211s because the voltages needed to make these things sing are scary.

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 ...

Originally posted by lilolee
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.

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
tungsten sound.

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?

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

no you don't get corrected. The 807 can be considered as a KT-66 variant with an anode cap and a UX5 base. A beam tetrode.
It has been used with success in Williamson-style amplifiers designed for the KT-66. It has the same smooth sonics when wired in triode mode. It has a mu of about 8 then.

And it can be had dirt-cheap compared to the KT-66.

But: it is no high-voltage DH triode. IN SE-mode i would not expect more than 5 watts triode-wired.
Being a heretic

I like the 813. It's not a triode, but is similar in characteristics to the 845 when triode wired. It needs less voltage drive than an 845, but a bit more power to the grid and can use the same transformers. There are a few people on audioasylum that've built them, and more in construction or contemplating it. I have a few, but 50W (PP) is waaaay more power than I need for my horns so it might make a bass head later. That will look the biz!. The 813 lights up even brighter than the 845. Besides there are lots of US made NOS 813s around, and excellent cheap new production coming out of China and Russia (GM13)

<a href="http://members.optusnet.com.au/~xx308/RCA813-100%20curves.jpg">813 triode curves</a>. Nice huh?
<a href="http://frank.nostalgiaair.org/sheets/079/8/813.pdf">813 datasheet in pdf</a>
Re: svetlana 572 anyone?

gilid said:
What do people think of the 572 family? It appears to be an interesting alternative to the 211/845, certainly from a voltage & cost perspective.

Yes they're nice to use, sound fantastic, not quite such high voltages.
Unfortunately I believe they are now out of production.

**********vested interest************
I have 6 for sale on the Trading Post!!!!

Gilid, if you wan't some more advice / info in the '572's, I'll gladly give my experiences.
Either post here or e-mail me.

I have run these in SE class A1, getting about 19W, and then in SE class A2 getting about 24W.
The A1 circuit was nothing special, actually quite similar to Ryoichi Sekido's circuit: http://www.cityfujisawa.ne.jp/~rsekido/audio/SV572-3SE/
but I ran into 4.5K transformers which I had modified (re-stacked laminations) from a pair of 6550 PPP transformers, connecting anode to anode to get 4.5K. Fortunately these were Sowter output transformers which had no varnish.

On the A2 circuit, I direct coupled to the grid from a really strong cathode follower (6550) running at 40mA. This was to provide a really low Z source, to minimise distortion that will occur at the point that grid gurrent starts to flow.
The thing to remember is that these low mu valves need a very large voltage swing to drive them fully. This means that the driver (or pre-driver in the case of the cathode follower driver) must have a choke or transformer in it's anode. Otherwise you need a high voltage valve also for the driver.

Unless you want to experiment, I would recommend Ryoichi Sekido's circuit.
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