I have just completed my project on the HATT AUDIO PA-833A power amplifier. All the transformers are mounted on a large size casing with top plate of 3mm thick. The size of casing is bigger than the original for better ventilation. The 833A tube is red hot but the power transformer is not too hot after several hours of operation.

The 833A tubes are NOS made by Philips in the 80s. The driving tubes are RCA 6EM7. You can select 80W or 100W output capacity by simply flipping a toggle switch (orange 80W and red light 100W)

現在各位可在市面上看到不少用833A管子作輸出級的放大器,十年前是並不常見的。日本WAVAC出產的833A後級是當時比較出名的機種,但普及性知名度仍是不太高。

You can now see quite a number of manufacturer using the big power tube 833A on their products, but the tube was not commonly used 10 years ago. As I can remember only WAVAC in Japan use 833A on their top model mono amplifiers and it does not bring out a lot of attention at that time.

這支833A管子其實是設計用在發射機上的,以前曾廣泛用在電台廣播系統上, 隨著不停的消耗找些新品不容易而且價錢亦升至$10000壹對, 幸好現在仍可以找到中國及蘇聯生產的代用品, 價格低到令人難以置信。

The 833A tube is not difficult to find as it was designed as a transmitter tube and widely used on many broadcasting stations in the pass. While most of them were consumed during these years and you cannot find much NOS on the market now, and a new pair may fetch for a thousand bucks now, it is good to know that there are still plenty of stock on Chain made and Russia made 833A  equivalent tubes., and they cost much lower than you thought.

當我找到HATT AUDIO的時候,他們亦同時生產適用於833A管子的火牛,而我訂了壹套。線路結構和HA-4212大同小異, 只不過多了些回輸電阻, 調壓線路只用於輸出高於80瓦的時候。

When I find Hatt Audio Japan they were making transformers for 833A tube as well and I had ordered one set (total 4 pieces) for my project. The circuit configuration is very similar to the HA-4212 amplifier with some only some additional resistor for feedback, and the bias circuit is only employed for power output of more than 80 watts.

在設計833A放大器時最重要及困難之處是那個要命推動級, 雖然833A的放大率不算低,但可借它的柵極需要一個輸出阻抗超低的推動級,而一般的管子是應付不了的, 這裹需要的是數佰歐姆甚至越低越好,  選用交連牛似乎是唯一的法, 因為它可以用初次級的圈數比去進一步降低推動級的阻抗,而次級線圈的低直流電阻可以容計當833A推至正壓時柵流正常流動,因而獲得更大的輸出功率。

The most difficult and important part on the design of a 833A power amplifier is the driver stage. Though the gain of the 833A is comparatively high, it’s grid will reflect an extremely driving impedance on the driver stage that most tubes will not cope with. A driving impedance of a few hundred ohms is generally required and it works even better on lower values. The use of an inter-stage transformer seems to be a must as it can be designed to have a step down ratio to further lower the output impedance of the driver stage,  the fairly low secondary DC resistance allow bias current to flow when the 833A is driven positive to squeeze out more juice from the tube.

One design taken from Tubebbs:

I never actually draw the SE833A schematic when I builded it. Here is a description:
Input stage: 12AY7/6072 SRPP capacitor coupled to the input of the driver stage.
Driver stage: KT66 as cathode follower using the primary of Electra-Print 3033 interstage transformer. With about 300V on the plate of the KT66 the cathode will be biased about 20V. The cathode of the KT66 is     directly coupled to the Grid of the 833A. Other tube like KT77/KT88 can be substituted here but will make the 833A draw more current. It can be compensated by inserting bias resistor to the cathode of 833A.
Output stage: 833SE @ 1300V, 250ma, it will put out 135 watts when clipped . At this voltage the tube dissipate about 325 watts, the plate turn orange color. Force air cooling is a must. You can also use 800v –         1000V on the plate with bias current of 250ma to get 45 watts – 70 watts. The output transformer used is the EP CU5KB. It will take up to 300ma. I am running withe the cathode of the 833A directly to ground. If      the 833A draw more current (depending of driver tube used) cathode bias resistor can be added to decrease bias current.

The EP 3033 will not work as an interstage transformer in Shishido circuit since it is a 1:1 and will not drive the grid 833A deep enough to get significant power.

There you have the circuit description of the 833SE. Use every caution if you want to build an amp using this high voltage.

This amp sounded much better than the 845SE (Tamura 2013 output) and 845PP (Tamura 2012 output). In fact this amp sounded better than any 845 amp that I have builded so far ( I still have to build another      845 PSE using CU5KB and 3033 to compare with). It does have the “big stage sound”of the 845, and more “neutral”. The mid and high is very sweet.

You can find a lot of products from various makers and DIY as well: