211 / 845  características

Triodos para 1200V em placa

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845 Push-Pull

Schematic by Susumu Sakuma.

Amplifier & Power Supply

211 links

845 links

Source: Duncan Amplification

According to a copy of RCA, Toshiba tube manual and articles appeared in audio

hobbist magazine in '70s, their general rating data of 211 and 845 are;

                              211        845

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Filament voltage(V)            10        10

Filament current(A)          3.25      3.25

Maximum DC Plate voltage(V)  1250      1250

Maximum Plate Dissipation(W)   75        75

*Amplification Factor          12       5.3

* Amplification factor of 211 is about double of 845. This implies

  there would be some differences in operation data.

Here are typical operation data.

211 AF power amplifier & Modulator Class A1

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DC plate voltage(V)       750    1000   1250

DC Grid voltage(V)        -46     -61    -80

DC Plate current(mA)       34      53     60

Load resistance(K ohm)    8.8     7.6    9.2

Power output(W)           5.6      12   19.7

845 Class A1 power amplifier

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DC plate voltage(V)       750    1000   1250

DC Grid voltage(V)        -98    -155   -209

DC Plate current(mA)       95      65     52

Load resistance(K ohm)    3.4     9.0     16

Power output(W)            15      21     24

>From these data, you may know 9K ohm load could be applied to both 211 and 845,

but you have to adjust plate voltage from 1250 to 1000 as well as DC grid

voltage from -80 to -155 when you replace the tube from 211 to 845. In addition

to bias adjust, You should remember 845 requires double grid swing than 211

does.

By the way, there's strange figures (5.6W @750V, 12W@1000V) in 211 power output

data. Do you think 5.6W output is good operating point? Is there something

wrong? The answer would be that 211 allows class A2 operation. There was an

article that described 211 class A2 amp could produce 35W output at 900V plate

and -44V grid voltage with 10K load. In this case, the grid swing was 200V peak

to peak; +50 to -150V. Though there's no class A2 operation data in tube manual,

the 211 data sheet shows linear plate characteristics in positive grid area.

This means class A2 operation would be probably allowed.

Of course, A2 class operation requires low impedance driver stage to absorb grid

leak current. So the circuit must be different from that of A1 class amp.

Now, here's conclusion. Both 211 and 845 may work at about 8-10K ohm load 1000V

plate voltage with adjusting bias voltage to be appropriated if the driver stage

has enough swing and power.

This report is just my private desk work for 211/845 single tube amp

investigation, I have no experienced with 211 and 845 amp circuit design. I hope

someone in netter who has experienced 221 or 845 tube, offers his valuable bench

work report.

From Duncan amplification.