• 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.

KT77 at 790 V

bea

Member
2011-01-13 10:55 pm
Hi,

i am using a few EL34 based amps where the rail voltage is 790 V (old Dynacord amps, a G-2000 with 4 output tubes and 2 Eminent 2 with 2 output tubes, delivering 150 W resp 80 W)

I am using these amps mainly as bass amps.



I would like to replace the EL34 by new tubes, and i am considering (JJ) KT77.
Is there anything known on the reliability of the KT77 in these environments?

The target amplifiers all allow for individual biasing of each tube. All screen resistors have been enlarged by 50% because it should be possible to operate the amps at their limits.
 
A couple of things....
1) KT77s were made for ultra-linear operation. If used in "pentode" mode, KT77 screen voltage "should not be over 300 volts". This is directly from G.E.C. specs. I have no idea how modern / reissue KT77s spec out, but most likely worse than the old G.E.C.s.
2) To your original question... Even if modern tubes could take 800 volts on the plates, the imminent problem is 800 volts on an octal socket. That's way too high! Remember that the pin next to the plate lead is at ground potential. In a properly functional tube amp, at full output power the plate lead will be seeing 2X the B+ voltage, so this means close to 1500-1600 volts at the output tube's socket. That's really tempting fate with an octal socket.
 
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bea

Member
2011-01-13 10:55 pm
Having had a look at the GEC specs:

A couple of things....
1) KT77s were made for ultra-linear operation. If used in "pentode" mode, KT77 screen voltage "should not be over 300 volts". This is directly from G.E.C. specs.

And a few lines below that sentence a design example of Ug2=396 V is given.
Ug2 is actually 400-420 V - the amps have been designed for the EL34, and the design limits of the EL34 are not exceeded in any way.

For the KT77, the GEC manual gives an absolute limit of 650 V for Ug2.


BTW: these 800-V designs are explicitly described in the manual for the EL34 by Telefunken. The amplifiers usually die from defects in the bias pots - and in these cases flashovers between anode and heater are not uncommon. Otherwise the old tubes last really long.

My motivation to ask is just that i am interested the different sound of the tetrodes. There are still EL34 of recent production which are known to be reliable in these amps

Another note: the technology is actually designed for general purpose PA, and the amps are suitable or nearly suitable for HiFi (the 80 or 150 W are specified at 1% THD)