833A power amplifier

From 1920 to 1960 was the golden age of vacuum tube era. Lots of excellent triodes created from USA and Europe. Some of them originally designed for radio transmitters were widely used in world war II whilst the others worked for audio amplifiers. I collect direct heating triodes for fun and however they go together eventually to made excellent single end amplifiers.

The direct heating triode sounds style different but all excellent :

EIMAC 150/250
RCA 833A
WESTERN ELECTRIC 242A
WESTERN ELECTRIC 300B
RCA 8005
RCA 812A
RCA 250
RCA 245
RCA 171A
2A3

I selected RCA 833A for this project, it was originally designed for 30MHz radio transmitters and also used in X-ray machines. From the very begining rare people think about 833A for audio amps.

RCA 833A filament supply 10V 10A, Vp 3000V, Pd 1500W The best 833A ever made with special alloy plate structure The plate is rigid with less elasticity, does not like other metal plates it will only be broken rather than deformed. It can work normally under red hot plate condition. Very good sound quality

From non-official speaking, the grid works only on negative bias side is called right tube while the positive side is left tube, this is because zero bias is at the center.

I decided this machine works on both side for more power out put, the grid positive peak is about +25V at Vp 300V so that the max grid current is about 15ma :

The operating (quiescent) point is at Vp= 1200V   Vg= -12.5V   Ip= 90ma
The top point is at Vp= 300V   Vg= +25V   Ip= 180ma
Top point Ig(max)= 15ma   at Vg= +25V   and Vp= 300V
The grid driving voltage = (25+12.5)*2 = 75V pp.
The out put voltage swing is 1800V pp
The output power to 10K£[ load is:

P= (90*90/1.4142/1.4142)*10,000/1,000,000 = 40.5 Watt   or

P= V*V/R/1.4142/1.4142 = 900*900/10,000/1.4142/1.4142 = 40.5 Watt

The interelectrode capacitance grid to plate is 6.3 µµf and grid to filament is 12.6 µµf , this will load the driving tube a lot at high frequency so that a 2A3 cathode follower direct drive has to be arranged due to its very low output impedance.

The circuit design is straight forward, all direct couple to output, if the pre amp output signal does not include a DC component, the .33 £gf input capacitor can also be taken out.

The cathode resistor of 833A=50/4+10=22.5
The voltage drop at Vg max is .09*22.5= 2V
The total driving voltage is 75+2*2= 79 Vpp

                                                                    - µ Rp
The voltage gain of  V1 & V2  = ----------------------------------
                                                               Rp+ ra + (µ+1)Rk




                                                                -  33*51
The voltage gain of  V1  = ---------------------------------------   =  - 24
                                                          51+ 2.7 + 34*0.51




                                                                -  33*39
The voltage gain of  V2  = ---------------------------------------   =  - 3.7
                                                          39+ 2.7 + 34*9.1




                                                                  µ Rk
The voltage gain of  V3  = ----------------------------------
                                                            ra + (µ+1)Rk

                                                                 4.2*20
                                               = ------------------------------  =  0.8  
                                                            0.8 + 5.2*20

                                   


The voltage swing at V2 plate is      79 / 0.8 = 98.75 Vpp     

                                                   

The voltage swing at V1 plate is      98.75 / 3.7 = 26.7 Vpp     

                                                    

The rate voltage input  is      26.7 / 24 = 1.11 Vpp    

                                          

If we want to improve the operation we can introduce a Negative feedback network :

Let's try    -6 db   negative feedback

Open loop gain from input  to 833A grid = (-24)(-3.7)(0.8) = 71.04  
 

   6 = 20 Log (1+ AB)         Where A is gain, B is feed back coefficient


   6
 ------ =  Log (1+AB)
  20


   0.3    =  Log (1+ 71.04B)


        
      100.3     =  1+ 71.04B


       2 = 1 + 71.04B
  
                  1
       B = ----------- =  0.01407
                71.04


                                                             ra + Rp
Input resistance of V1 cathode  = ----------------- 
                                                               µ+1  
   

                                          2.7 +51
                               =  ----------------------
                                             34


                                =   1.58 K


        1.58 K// 510 = 385



                                      385                    µ+1 
         0.01407 = -----------------------( --------------- )
                                    385 +  Rf                 µ 
   


                                        385                   34
        (385 +  Rf) = --------------------( -------------- )
                                    0.01407                33 
 
                  
                     =  28,192


               Rf  =  28,192 -  385

                  =  27,807 

                  =  27.8 K
                  

High frequency consideration :

                                                                -  31*10
The voltage gain of 833A  = --------------------------------------   =  - 23.3
                                                          10+ 2.6 + 32*0.0225


The equivalent 833A input capacitance = Cgk + (gain+1)*Cpg + stray capacitance

                                        =  12.3 + 24.3*6.3 + 20
                     
                                        =   185.4  µµf

The output resistance of 2A3 = ra/(µ+1) +12.5 = 166.5 

               166.5 //20 K = 165 

 
                                                                             
At  - 3 db point    165 = ( 1/ 2*3.14*f*185.4)1012

                    f = 5.2 MHz   OK


The equivalent 2A3 input capacitance = Cgk + Cpg + stray capacitance

                                        =  16.5 + 7.5 + 10
                     
                                        =   34  µµf

The output resistance of V2 = ra + (µ+1)Rk = 303 K

               303 K//39 K = 34 K

   
At  - 3 db point   f = 138 KHz   OK


The equivalent V2 input capacitance = Cgk + 4.7*Cpg + stray capacitance

                                        =  3.3 + 4.7*1.4 + 10
                     
                                        =   19.88 µµf   


The output resistance of V1 = ra + (µ+1)Rk = 20 K

               20 K//51 K = 14.4 K


At  - 3 db point   f = 558 KHz   OK

DC filament supply with huge filter capacitors for 2A3 and 833A is a must otherwise it will hum. I also change the driver tube with mono plate 2A3 and WE300B to see what happen , all ok and work normally with their own sound style.

Due to all direct coupling the frequency response is totally depending on the output transformer , this machine 25Hz to 55KHz no problem with excllent sound quality.

The overall power supply arrangement is a bit more than that of general tube machines as they are independent supplies and the weight and volume are huge. This is the reason why only very rare commercial products use 833A as power tube and ask for extreme high price about US$100,000 for a mono block pair, is it reasonable or not, you judge.

 
Thank you for looking !

                                                                        Written by    Terry  Tam   I.Eng  MIEEIE  MIET  (U.K.)