el34 instead of 6550

[Ahmad_tbp]

hi all
can i use a pair of el34 instead the pair of 6550 in this circuit ?
http://www.drtube.com/schematics/marshall/1986mk2jp.gif

 

[ray_moth]

I would say yes, provided that the voltages are not too high and the OPT gives a suitable load for EL34 in pentode PP, Class AB1. B+ at point A should be about 425v to 450v, the screens at point B should be at about 400v and load should be 3.4k plate-to-plate. This will yield about 50w RMS output

 

[audiohead]

Hi there:
The o/p xfr pri impedance, really does not matter, use anything from 3.4k to 6.6 Kohm.

Set /adj the bias (Idle current) for EL-34 anywhere from -39 to -43V.

You need to add a resistor (10 ohm /2w) on each EL-34 cathode (to gnd) to be able to minitor and set the idle current, by measuring the DC voltage drop across them.

 

[Ahmad_tbp]

ok thank u so much men 🙂

 

[ray_moth]

The o/p xfr pri impedance, really does not matter, use anything from 3.4k to 6.6 Kohm.

I beg to differ. In pentode mode, the p-p load is very critical. 6.6k would be OK if you were using cathode bias or if your topology were UL but for fixed bias pentode AB1, 3.4k is recommended. 🙂

 

[audiohead]

Hi Ray:
Kindly enlighten us as what parameter(s) you think are effected by Z pri?


regards,
art

 

[Ahmad_tbp]

 

[ray_moth]

Pentode mode is far more "'picky" about the optimum load impedance, for maximum power and minimum distortion, than either triode or ultalinear. In pentode mode, distortion increases rapidly either side of the optimum load. If you can't get the load impedance exact, it's generally better to err on the side of lower impedance, rather than higher.

I don't know exactly what other parameters are affected by the load (Z pri as you call it). The figures I've seen for EL34 push-pull are those in Mullard data sheets.