Does an original Mark II draw more power from the wall than an original Mark III? Why did Dynaco use a 4A fuse in the Mark II and a 3A fuse in the Mark III? I even have an earlier Mark II schematic that shows a 5A fuse.

The power supply setup is similar between both amps, and the Mark II is rated for lower power output than the Mark III.

Kicking around a couple possibilities... Does the 5U4 rectifier filament in the Mark II draw significantly more current than the Mark III’s 5AR4 rectifier? Do EL34s in the Mark II draw more current than 6550s in the Mark III?

Anything else that might explain it?

Thanks.
Peter

Hi Peter,

The 5AR4/GZ34 filament draws 1.9 amps at 5V, while the 5U4 draws 3 amps at 5V. This 5 watt difference doesn't explain a 120W-240W fusing difference. EL34's draw 1.5 amp, 6550's draw 1.6 amps, also a small difference, but in the Mark II's favor.

The 5AR4/GZ34 warms up slowly because it has an indirectly heated cathode; the 5U4 warms up more quickly because it has a directly heated cathode. The GZ34 and the choke in the Mark III will reduce the surge currents, but I doubt that this is a big effect. Someone with power supply modeling software could check this out.

I think that it is likely that Dynaco just kept pushing the fuse value down to provide additional safety. Older equipment had no fuses at all, and the idea of fine tuning the fuse value may have entered slowly.

Dan

Dan,

Thank you for your response. Could I ask you to expound on your comment: "...120W-240W fusing difference." I’m having a little difficulty connecting the dots.

As far as fine-tuning the fuse, this seems to be a plausible explanation for Dynaco dropping the Mark II fuse from 5A to 4A. But, I have a 1960 copy of a Mark II manual containing a schematic showing a 4A fuse – three years after the introduction of the Mark III and its 3A fuse, which seems to suggest the Mark II really needed a 4A fuse.

But, solely from the perspective of the 5U4 filament drawing about one amp more than the 5AR4, would this not be sufficient to explain the 4A fuse in the Mark II vs. the 3A fuse in the Mark III?


Thanks.
Peter

Dan,

On second thought, I think I know what you might have meant: one more amp from a 5V secondary doesn't translate into one more amp needed from the 120V wall, is that correct?

If so, then the Mark II still uses a little more power than the Mark III, but not enough to account for a 4A fuse vs. a 3A fuse?

Any idea about how much extra juice would be needed from the wall to power the Mark II's 5U4?

Thanks.
Peter

Hi Peter,

To clarify, the rectifier current is at 5V, and the fuse current is at 120V. One amp at 120V corresponds to 120W, and one amp at 5V corresponds to 5W. Reflected back to the transformer primary, one amp at 5V would be 5/120 or 1/24 amp at 120V if the transformer is 100% efficient.

If the different fuses were used at the same time, I would lean to the power on surge current explanation. This could be tested or modeled if you really want to know.

Dan

Dan,

Sounds like a reasonable explanation. If I needed to, do you think I could get away with a 3A fuse in my Mark II if I use an appropriate thermistor in the primary to ease the current inrush?

Thank you.
Peter

Hi Peter,

I haven't used thermistors, but I am not sure that they will significantly change the peak current on turn on with a vacuum tube rectifier, even a 5U4. They will make a difference with a solid state rectifier. Someone more experienced with thermistors could comment. If you want to try a lower value fuse, go ahead and try it as is. Fuses are cheap, and if a 3 amp fuse blows we've learned something.

Dan

Fair enough, Dan. I move kind of slowly over here, but if I rebuild my Mark II, I'll try to remember to report back.

Many thanks.
Peter