Hello Radio Friends
The valve KT61 (Marconi)) is it equivalent to the valve EL33, (Mullard)? Retired data the book in Spanish of the author R. J. Darkness. 1.ª edition November 1956, pág.96.
Hello Júlio Branco,
The EL33 (or 6M6) it is NOT equivalent to the KT61. This data book is not serious !
A Philips or Mullard data book would never make such recommendations !
The EL33 needs a cathode resistor of 150 ohms at least, while the KT61 needs a cathode resistor of only 90 ohms.
Without changing the cathode resistor the EL33 and the KT61 may never be used to replacing each other.
As a makeshift one can use a KT61 to replace an EL33, but the performance will decrease.
An EL33 should never be used as direct replacement for the KT61. Due to the wrong cathode resistor the EL33 will be severely overloaded and will give only low performance.
Best Regards, Jacob
Hello Jacob Roschy,
THANK YOU FOR THE CLARIFICATION, WHICH YOURS IS SUGGESTION TO CHANGE THE VALVE KT61 THAT THIS CHANGE, FOR ANOTHER THAT A GOOD SUBSTITUTION?
unfortunately there is no direct replacement type for the KT61.
This is because of its exceptionally low value cathode resistor of only 90 ohms.
Any other output tubes in this power range requires a cathode resistor of higher value.
The next and best suitable common replacement tube is the EL33. To operate it properly, you need to exchange the cathode resistor from 90 to 150 ohms, at least 0.5 watt type. Normally this is the resistor between pin 8 and ground (chassis).
A further very common replacement tube is the 6V6 (-G, -GT). In this case you need to exchange the cathode resistor from 90 to 270 ohms, at least 1 watt type. Since the 6V6 needs twice the control input voltage, the EL33 is to prefer.
Hello Jacob Roschy,
Thank you for your ready answer to this theme.
I already rehearsed the radio in cause, (HIS Master's Voice Mod. 5100), I put a 6V6, substituting the resistance of 90 ohms for another of 250, maintaining the condenser that is in parallel with the same value, or be of 50 microfarads, obtaining like this a polarization of -12,5 volts. The valve began to work in good conditions, but without audio frequency. Continuing with the diagnosis of the mishap injected signs, for the previous walks, I arrived to the grill of the valve IF KTW61M and sign didn't pass, all of the tensions are corrects; now there is to substitute the same, because it is also damaged. I rehearsed it in the analyzer meets very poor. Will it be that to 6S7G it can substitute, more than it is not for rehearsal, KTW61M?
since the KTW61M has a screen voltage of only 80 V, we have to look about replacement IF tubes as close as possible.
There are only the 6U7 and the 6S7 with 100 V screen voltage, as common types with the same base connections. The may work sufficiently.
On the other side, as you told, all voltages are correct, even the screen and cathode voltages of the KTW61M (?), I have some doubts, whether the KTW61M has actually failed. Probably an IF circuit is either detuned, interrupted or short circuited ?