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    the safety precautions around high voltages.

How to Fuse Circlotrone OTL ?

Sorento

Member
2008-03-12 9:59 pm
The question is not what type or rate of fuse but where to fit fuses.
Say we look at a Circlotrone OTL e.g. ATMA-Sphere 60 or similar.
What is the logical / effective / mostly harmless place for fuses other than mains input to transformers:
1) ac secondary of each of the 4 main transformer windings
2) plates of the power tubes
3) cathodes of the power tubes
4) speaker output
5) none
6) other ideas ?
Thank you ...
 
From the M60 manual:

"Fuses: The first fuse by the power cord entry is the Output section B+ and is 3
Amp Slow (1.5A slow for 235V). The second fuse is for the output filaments and
is 3 Amp Slow (1.5A for 235). The last fuse controls the driver circuit and is
0.5 Amp slow (0.25A Slow for 235 Volt). WARNING! To avoid fire hazard, always
replace fuses with same type and rating."

Using this to address your list:
1. Using fuses on the AC output of the PT winding means you need to size the fuse big enough to be able to charge the power supply caps and not blow. If you put the fuse on the output of the power supply, it only needs to be big enough not to blow from the tubes themselves drawing current. The M60 uses a single fuse on the primary side of the output stage power transformer, and while this fuse should be here, I'd probably add the two between each B+ and the plates of the output tubes to react a little more quickly.
2. This is now duplicated by the fuse between B+ and the plates.
3. Same as above.
4. Could be worth exploration.

My other idea would be to make a circlotron with an output transformer ;)
 
Perhaps appropriate to provide a link to a full schematic (all circuitry and power supplies), as secondary side protection needs to cover not only the power supplies, but the likely fault paths, and the effective resistances of all major items, and an awareness of whether a fault on one power supply (and a fuse protecting it) does not then stress some other section or cause collateral damage and take out valves and caps.

No point adding a fuse if it will never open during a fault because the fault current never sufficiently exceeds the fuse rating.
 

Sorento

Member
2008-03-12 9:59 pm
Thanks for your answers.
Yes, ATMA has fuses only on the mains primaries, and this is what puzzled me in the first place.
They claim that they can pull tubes or short the outputs w/o damage. But these are the easy ones. The real concern is a flash- over in an output, I think, in which case the speaker connects directly across one of the power supply reservoir caps.
My first idea was to put fuses in the cathodes so that they would be part of the lokal feedback. But wait, cathode resistors only stabilize current changes due to changes in tube characteristcs, not fluctuations or distortion in cathode resistors themselves. On the contrary, they aggravate them.
So the plates have it, I guess...