Portrait of a Tube

By Jac van de Walle. Last Update:

EL50 / 4654

NOTE: 4654 is NOT an EL50 replacement, but 4654 is limited to 600V Plate

EL34 is a tube capable to deliver maximum 100 Watt from a push pull pair, at 800 Volts, but only some very good tube brands like TFK actually can do 800V. Besides, from what I know, actually only TFK can be loaded at 800V, and even for those it is not recommended, and it is hardly ever done with historical equipment. So yes, 100 Watt from a PP pair is possible, but better reduce that to a number which gives more lifetime.

As a rule of thumb, a Push-Pull stage will give maximum 2x the plate power of the tubes as output power (that means both tubes together). So if both tubes are 25 Watt types, you get maximum 100 Watt output power. And yes, the tube like EL34 can do that.

But.. 800V over an octal socket is no good idea. It gives leakage, inside the glass part of the tube base. Glass may develop small leakage current when hot. If this happens, the heat from the leakage itself will warm up the spot further, and the tube can self destroy. Besides, 800V for a Barium Cathode is very high, and only possible when you have EXTREMELY tight control over the vacuum. And EL34 cannot have that tight control, no matter how many getters. The plates get simply too hot, and the vacuum at 800V cannot be maintained like this for a long time.

So EL34 is used at lower voltage normally. HOWEVER, at reduced voltage, output power goes down a lot, and ends up somewhere around 50 Watt for 400...500 Volts plate voltage. So the efficiency of the tubes go down, you develop heat, and not output power.

The solution is called EL50 / 4654
NOTE: 4654 is an EL50, but 4654 is limited to 600V Plate. That still gives 64 Watt.

 

PUSH PULL, 64 Watt Output Power.

Basic circuit, to show what is the limit for this tube. Though it is recommended to take more practical values from the Philips datasheet (See below).

Application Note #1

Application Note #2

Application Note #3

Application Note #4

Recommended transformers:

• LL1620-PP. Order number 218-202-49. Wire for 5k primary. Secondary 4-8-16 Ohms
• LL1682-PP. Order number 218-105-69. 5.5k to 4Ohms
• LL1663-PP. Order number 218-104-74. 5k to 8Ohms

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SINGLE ENDED, 9.2 Watt Output Power at only 275/250 Volt Supply Voltage!

 

Recommeded SE transformers:

• LUNDAHL LL1623-90mA. Order Number: 218-110-72
• LUNDAHL LL1623-AM- 90mA. Order Number: 218-123-96 (Amorph core)
  

This tube uses a top cap for the plate connection, allowing higher plate voltage. Still... 800V is a lot. There is a downgraded version, that will work at 600V maximum, it is the 4654. From this 4654 some wonderful big coke bottle tubes can be found. Sockets I have hundreds of NOS still. China sockets are around, but these shoudld NOT ne used, and the Chinese do not comply with the dimension tolerance. These can have bad contacts, which bend too easy, or even fall out of the socket.

The 4654 is only 18 Watt plate dissipation per tube, but now look at 400Volts the push pull stage will give 48 Watt at 2,5% distortion. So that is 2.5% at full power, no feedback. Note the tubes run at 18 Watt dissipation (each) for this, which is less than EL34. And still 48 Watt. EL34 at 400 Volts will give 45 Watt at 6% distortion. Clearly 4654 is better!