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833a Amp 75W SE

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Hello, I would design gladly a 833a amp. Perhaps does someone have thereby experience or even a good connection diagram? I would use 845 a triode gladly as driver ( however I would do gladly without the two transducers in the preliminary stage). The whole is to become SE amp. The 833 anode voltage is to be operated and with -60V bias with a Raa of ~20k. it is around the 75 -100W to carry out in such a way with 3000V.

MfG Dominic from Germany :smash:
I will probably build this Amp. However with another driver, I think however I the Amp also times in the orginale will try out. Later then 845 drivers after Unison a S8, thus I have good experiences of sounded made ago.
It carries for instance 13W out (With small change which that heat the ECC concerns.:clown: )

Unison S8 "Driver"
833a 75W Schematic

Which hold you from orginal the circuit and/or from the idea the S8 as drivers to use in such a way.:)

Gladly I would try also times the Wavac out 833 circuit. There does someone know which more drueber?

MfG Dominic
I have some schematic's from Lancroft Audio, but they are not so perfect like i would want. I was thinking, building a very good set amp with a 300B and in place of a OPT, using an interstage trafo to drive the 833A, I was thinking of using the 300BXLS (powered at 472V and 120mA), drived by a µ-stage of D3A penthode and 27 triode (Attilla from Triode Dick) and in place of a beefy OPT, using of an interstage transformer. And i have an electron tubes book T1 from Philips, 1985 and would the TB4/1250 not been better than the 833A? The TB4/1250 looks better than the 833A, maybe little more stable?
I think that if you think, an 833A from Wavac, rated at 150w classA i think that it is most A2 driven, so sounds not so good.
I have a schematic of the HE805 from Wavac, and the schematic diagram don't look spectacular high-end, more like an overkilled dissapointment (in technical therms if you study the schematic diagram)...
Suggestions for 833-A SET

Last spring I built my 833-A SET using a 1000 Volt plate supply and the grid at ground potential or zero grid bias. I drive it with a 245 transformer coupling with a Hammond 126B with the secondaries to ground and the grid. This is more than enough drive. The Hammond 1642SE is a perfect match for the output of the 833-A. The 10 Volt 10 Amp for the cathode heater was a little trickier to get it to work without hum. I could only get it to work with aggressively filtered DC: a 20 amp rated 10 Volt transformer to a bridge rated at 50 Amps so that it does not smell from heating up, an 83,000 microfarad capacitor, a 50 Watt 0.1 Ohm power resistor, and a 4.5 Farad digital automotive capacitor that has to be pre-charged before you fire up the 833-A cathode heaters.

I have had no problems with my 833-A amplifiers since I started using them last April and they fill in all the bass you can't otherwise get from a more modest SET. I still run my tweeters and midrange speakers with 245 tubes but you don't need to tri-amp or bi-amp your speakers the way I do.

If you are anyone else builds one - and the way I built mine works tried and true, let me know how it sounds with Magnepan speakers.
I use a Hammond 126B, the primaries between the B+ and the plate of a 245 triode just as I would a speaker transformer. The secondary of the 126B is connected between the 833A cathode and the 833A grid for zero grid potential which is perfect for putting 1000 Volts on the 833A plate through a Hammond 1642SE output transformer. to the speakers. Just be sure you use aggressively filtered DC to heat the 833A filament.
The Hammond 126B is no problem for -90 Volts grid bias. it is 5K Ohms in primary and secondary and it is designed to tolerate 400 Volts and pot tested to 1000 Volts. It can carry 30 ma DC which is more than a 245 triode will draw. The Hammond 1642SE is listed to work up to 3500 Volts but it is only rated at 75 Watts before it will distort and 300 ma DC current. I do not know if this will be exceeded with the power an 833A will produce during loud passages of a full orchestra.
Dominic can you please let my now witch IT driver for 845, i means 850V/1kV +-75mA

Dr Barney, i use 126B i find its good IT if not more of 250/300V between primary and secondary @30mA good for 45 , its a bifiliar IT

for 845 driving 833A you need more volts and more current and a bifiliar IT cannot do this way i should ask three time same thing???

forget 126B please
833a with 845 driver

Hi @all,

Ohh man, This is a very old post from 2006 :-D I have planned to use the 845 with 75% max power dissipation and 1kV anode voltage. An Interstage was required but very expensive, the same applies to the output transformer. 10k and 200mA DC.

I never started the requested 833A project, but I have made experiences with the RS1026. It is a Power Triode like the 833a. I find the RS is more usable than the 833a. A1 operation for small output power is possible and I got it for free^^

I use this Operating Point I think (it was 2009):


k2A1= very very high
K3A1= low

This high power dissipation is required because this tube has no getter but zirconium parts I think. To drive I use a 211 as Cathode Follower (60W, 60mA), the RS need 50mA Grid Current for max Gain (An interstage transformer with stepdown ratio would do it better but I have no one). As a preamp, I used a D3a as a triode, later as pentod for more k3 to get an more balanced sound.

With this configuration it gives 12W in Class A1 and round about 60W in Class A2. The measuring results after the 211 were OK but after the RS very bad It could be a problem with the high DC current in conjunction with a to small air gap in the output transformer).

When listening to music then a completely different picture than when measuring (only in mono).
I hear it with LP and a DIY RIAA am with two 5654 pentods per channel. The Speaker was an B&W 803.

Motörhead - Out of the sun: Lemmys Voice Range A revelation
Dire Straits - Brothers in arms: wow

Unfortunately, with an power consumption from 1000w for a couple I did not continue at this Point.
I have learned here that poor measurement results have nothing or little to do with the sound that I like.



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I replaced the transformer between the 245 output and the 833-A grid with an LL1930 step-down transformer series primary and series secondary and it put the sensitivity just right, plenty loud without distortion and no noise got amplifier. Before I was using a potentiometer to turn down the 1:1 126B Hammond transformer and although this worked, it was akin to using latex gloves as a solution to having to write with a leaking fountain-pen. The LL1930 is run in parafeed with a 40 Henry choke and a boutique 8.2 micro-farad Mundorf silver and gold foil in oil capacitor. This gives me a very pure full-bodied sound. The 833-A grid input impedance of the grid to plate capacitance of 6.3 pf times 35, the average amplification factor in series with the grid to cathode capacitance of 12.3 pf computes to 11.65 pf for 683,066.829 Ohms at 20kHz and proportionally more at lower frequencies. This is an inconsequential load for the 600 Ohm output of the LL1930 transformer so don't be alarmed if this transformer with its sensitive mu metal core is very small. It sounds beautiful.

Some argue the damping factor is greater when the 833-A is run at 1000 Volts but this is no issue if it is used with Magnepan speakers I will eventually connect perhaps after selling my GR Research OB 7 speakers advertised on ebay and audigon.
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