GU50 is an improved version of the German military tubes (actually for the German air force-Luftwaffe!) LS50, and EL152 after the war (World War 2) improved commercial version of LS50. All these tubes have very similar operational data, but GU50 better tolerated higher voltage on screen (G2~350V). Parallel to the LS50, in the company TELEFUNKEN, was developed RL12P50 (with same data) for the needs of the German Army-Wehrmacht, which is distinguished by a different sockets and the glass envelope.
I am also using the exact schematic for phase splitting with 6j9p but with collectors connected to the anodes of 6J9p and the sound is a lot better in my opinion. However, it is a bit trivial to adjust for the correct screen voltage and anode current and voltage. Moreover a 100 ohm trimmer between the cathodes is beneficial for adjusting the balance. You can try this in spice too and see what happens.
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Can you please comment on how good are these OPTs at say 20-30Hz at near full power?
It depends on the used circuit.
The primary inductance of TGL40 is relatively low and therefore the load of the output tubes increases at low frequencies.
This raises THD at 20...40 Hz.
The phenomenon is not caused by core saturation, only because of low inductive reactance. TGL40 can handle more than 100 W.
If the amplifier has sufficiently low output impedance (due to NFB etc.) the problem is mainly avoided.
If the amplifier has high output impedance like pentode connected output stage without any NFB, then frequency response will drop essentially below 100 Hz and THD is increased.
The other subject is how some increased THD at 20...40 Hz can be heard. I can not hear anything abnormal.
The primary inductance of TGL40 is relatively low and therefore the load of the output tubes increases at low frequencies.
This raises THD at 20...40 Hz.
The phenomenon is not caused by core saturation, only because of low inductive reactance. TGL40 can handle more than 100 W.
If the amplifier has sufficiently low output impedance (due to NFB etc.) the problem is mainly avoided.
If the amplifier has high output impedance like pentode connected output stage without any NFB, then frequency response will drop essentially below 100 Hz and THD is increased.
The other subject is how some increased THD at 20...40 Hz can be heard. I can not hear anything abnormal.
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Looks like GU50 is a good candidate for Bogen MO200 amp which uses 8 of 8417 tubes for output. at B+ voltage of 680V and screen of 340V which gives me enough drop to have regulated 280V screen supply. There are 2 sets of 6.3V filament supply, Combining them will give me 12.6 with a center tap (watch the phase),
The one on the left is trying to emulate the Duchess of Cambridge. I have to say it's very stylish with that subtle tilt of the hat. A good look!
I tested the amp when smelled like a hot iron. It was still playing nice...
I powered it off, allowed to cool down, then powered on and measured bias. All tubes were fine. Looks like one had some extra gasses that were absorbed by a getter when it run off. The amp plays nice, no problems, all tubes are fine!
I powered it off, allowed to cool down, then powered on and measured bias. All tubes were fine. Looks like one had some extra gasses that were absorbed by a getter when it run off. The amp plays nice, no problems, all tubes are fine!
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