Tweed Vibrolux 5E11 vs 5F11 Fuse Rating

[K Teacher]

These are nearly the same amp. The main differences are on the vibrato oscillator circuit.
However, the 5E11 schematic (link below), shows 3/4-amp mains fuse, and the 5F11 schematic, shows 2-amp mains fuse.

A rough calculation, gives the total power required on the PT secondary, to be 55W.
Assuming PT efficiency of 95%, and applying the standard and temperature derate coefficients, gets a required PT primary power of approx. 84W.
For 115VAC mains voltage (CA on 1955), the estimated primary current will be 84W / 115Vrms = 730mA.

So, the 5E11 3/4-amp fuse makes sense to me. But why 2-amp fuse rate for the 5F11?

Schematics here:
https://www.schematicheaven.net/fend...5e11_schem.pdf
https://www.schematicheaven.net/fend...5f11_schem.pdf


BTW, the Fender Harvard 5F10 schematic, also shows 3/4-amp mains fuse.

Any insights/ comments?
Thanks.

[mozz]

I thought most fenders with a pair of 6v6 always used 2amp SB. You have startup draw which is always more than once it warms up.

[Chuck H]

This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

And then there are potential voltage spikes from the wall where such things happen or if the amp were used off a generator at an outdoor venue, etc. It's a real pita having to replace a fuse every time the slightest thing goes a little sideways. After all, the fuse is there to prevent a catastrophe like fire. Not as much to protect the amp or it's parts. Since Fender almost universally switched to 2A fuses in their 6v6 amps, and the 5f11 is the later model I would guess that Fender decided the 2A fuse was a better choice.

[Helmholtz]

Agree with the posts above.
My 2x6V6 Super Champ specifies: Only use 1.5A slo-blo fuse.
With a non-timed fuse the only thing you know is that it won't blow below its rated current.
Otherwise the exact current it will stand for how long is unknown.
A mains fuse should always be a slo-blo type and often is rated for twice the nominal amp current.
Wondering if slo-blo fuses were available in the 50s.

[Tom Phillips]

About 1964 Fender changed the fuse specification for the Deluxe Reverb from 1A to 2A. I note that the service info did not show "Slo Blo" Just "1A" and then "2A." So for all I know they may have made the change as a reaction to nuisance fuse blowing.

[Chuck H]

Wondering if slo-blo fuses were available in the 50s.

early 60's would be the crossover years I think. The Fender amp models before the BF amps had higher fuse ratings and no indication of a time delay unit on the schematic. For example, all the aa/ab763 amps indicate "SLO-BLOW" fuse on the schematics. None of the earlier amp schematics do and they have higher fuse ratings for the same power. I'll bet time delay fuses became a commercially viable option for amps a little bofore the BF amps went into production.

[K Teacher]

Thank you all for your opinions…

So, the consensus here is, if I build a Harvard 5F10 or a Vibrolux 5E11 replica and install a 3/4-amp fuse called in the original schematic, it will blow at start-up, right?

[Chuck H]

Thank you all for your opinions…

So, the consensus here is, if I build a Harvard 5F10 or a Vibrolux 5E11 replica and install a 3/4-amp fuse called in the original schematic, it will blow at start-up, right?

No. Since Fender sold them that way I don't expect the fuse would blow right away at all. But perhaps more often with various circumstances than it should. I'd suggest a 1A Slo-Blow. Slo-Blow (time delay) are best for this application and Fender seemed to have settled on the 1A value for the 2x6v6 BF amps. By that time they would have been well aware of any problems with fuses causing their product to fail inconveniently on players.

[Helmholtz]

Here's a very comprehensive article on amp fusing by MEF member trobbins:
https://dalmura.com.au/static/Valve%20amp%20fusing.pdf

Mains side fusing from page No. 14.

[K Teacher]

Thanks again Chuck H & Helmholtz for the clarification !

[K Teacher]

F.Y.I. -- 58 Tweed Vibrolux Fuse

[loudthud]

If you look closely at the 5F11 schematic, it looks like the "2" was a later revision to the schematic. Some people like me believe that it's Leo's handwriting on these old schematics. That "2" doesn't look exactly like the others on the page.

Do both amps use the same power transformer ? It would seem like the 5F11 has slightly higher bias current even though both say -31V for bias. Also, bias wiggle amps tend to draw a little more current when the tremolo is operating. That might not have been accounted for in the earlier amps.

[K Teacher]

loudthud,

Looking at several pics posted on the net, both Tweed Vibrolux, the 5E11 and 5F11 have the NYT 5396 power transformer (mid ‘50s) and the Triad 66079 (late ‘50s).

Same goes for the Harvard 5F10, which indicates B+ of 305VDC on the schematic (w/ 5Y3 rectifier) and B+ of 340VDC on the schematic (also w/ 5Y3 rectifier) for the 5E11/ 5F11.

With the same power transformer? There is some difference in 6V6's bias, but I don't think that it's causing that much difference in the B+.

Also, noticeable is that the 5F10 mains fuse, indicated on its schematic is 3/4-amp, like the 5E11...

[g1]

Voltages are specified as +/- 20%, so that difference falls in spec. Also line voltage is not specified and may have been different.

[Tom Phillips]

...With the same power transformer? There is some difference in 6V6's bias, but I don't think that it's causing that much difference in the B+....

I have noticed a similar issue with the Fender Deluxe line. As the line matured from the brown Tolex to the silverface years I notice that the PT secondary AC voltage listed on the schematics remains essentially the same while the B+ voltage changes significantly. Actual measurements can be even more variable. As g1 mentioned, the tolerance is specified as ±20%. Therefore, the printed values should not be used with caution when evaluating the proper operation of a given amp. Sometimes, I think variations are due to PT design changes that are not always obvious. This is in addition to the ±20% tolerance which is actually quite large by modern standards.