• 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.

Single tube, Single ended, 20W, impossible?

Just use a dozen 12HL7 in parallel (were on the $1 list once) to get 120 Watt Pdiss, 252000 gm, 150 Ohm Rp in triode. Single stage, modest B+. Super triode curves too.
 

Attachments

  • TFH_in_triode.jpg
    TFH_in_triode.jpg
    97.9 KB · Views: 188
Yup, excellent triode curves for 4P1L. Just need a bunch in parallel. pics 1 & 2

Could also go for a new series Schade N Fdbk on a big beam pentode, like a 36LW6. Pics 3 is using 21HB5A as a similar sample. Looks like a DHT 211 I think.
 

Attachments

  • 4P1L_p6Vstep_10mA.jpg
    4P1L_p6Vstep_10mA.jpg
    89.2 KB · Views: 122
  • 4P1L_triode.gif
    4P1L_triode.gif
    26.8 KB · Views: 128
  • 21HB5A_Corr_Schade.jpg
    21HB5A_Corr_Schade.jpg
    641.7 KB · Views: 123
Member
Joined 2006
Paid Member
Im sure there is some bazillion watt triode with a bazillion gain out there that you could run at 20 watts, but you would become your power companies new best friend.
If you take a closer look at it, there are not too many toobs to choose from. As to power, it depends where you liv. For instance, i need at least 15ooo kWh just to keep my house at 18°C during the winter, and amp dissipation would not change one yota on my bill. During summer i want sand, preferable between my toes, on a beach
 
Last edited:
My just for fun contribution.

https://frank.pocnet.net/sheets/111/8/845.pdf
845 schem.png
845 output waveforms.png
fft 1.25W .png


.SUBCKT 845 A G K
+PARAMS: MU=5.355 ERP=1.5
  • KK1=6323 KP=85.64 KVB=65.8 vg0=3 va0=0
  • CGA=13.5E-12 CGK=6E-12 CAK=6.5E-12 RGI=4000;
.func V_6() {KP*( (1/MU)+((V(G,K)-vg0)/sqrt(V(A,K)**2+KVB**2)) )}
E8 8 0 VALUE={(V(A,K)-va0)/KP*LN(1+EXP(V_6()))}
Eam am 0 VALUE= {2*Pow(V(8),ERP)/KK1}
GA A K VALUE={V(am)}
D3 5 k DX ; FOR GRID CURRENT
R1 g 5 {RGI} ; FOR GRID CURRENT
Rak A K 1G
Rgk G K 1G
C1 G K {CGK}
C2 G A {CGA}
C3 A K {CAK}
.MODEL DX D(IS=1N RS=1)
.ENDS
 

Attachments

  • 20W single tube.asc
    3.1 KB · Views: 42
Last edited:
Finding a 150:100k audio step up transformer capably swinging 134 volts p-p: consider switching to bourbon.
That is not actually the harder part if there is no DC current in it. It's a reverse 26:1 transformer where you can use mumetal, for example, that has got permeability 10+ times better than GO laminations. Enough turns to deal with the voltage and a good winding geometry. The harder part is driving the 845 through such SUT trnaformer...35-40 nF at least up to 20KHz..
 
I guess the guy who built that power amp with the 211 was driving it with a preamp using the same transformer in reverse mode: 100K:150R.
Only practical side of this is that you can put them in separate cases....
Otherwise it looks more like a tube amp that needs a strong SS preamp. Maybe even using some specilized OP-amp...