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Tube amp fusing separate HT and Filament transformers

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tobias

Mon Dec 24 2018, 10:21AM

Registered Member #1956 Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172

Hi!
I did a Google search but it gets flooded with standby switches, maybe I am not using the appropriate search terms.

I am 'designing' a tube amp with separate H.T and Filament transformers and I need some help with figuring out the best fusing/protection scheme.

More specifically, if a filament fuse blows (on low or high side), does the presence of H.T. for a longer period on cold tubes causes any damage?

I was thinking about using triacs, or other means of shutting down HT or filament voltages in case a fuse on the other transformer fails but I would gladly skip the complication if there is no actual need for it.

If it makes any difference, I am using ELC86s, diode rectified H.T. and filament supplies.and fusing only the primary sides of the transformers.

Tobias

Sulaiman

Mon Dec 24 2018, 12:02PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

My 30+ years old Kenwood TS-520S still uses its original transmit amplifier valves (type S2001) and still gives rated power output.
For a lot of its lifetime it has been used for receive only by operation of a switch that removes filament power.
So operating valves with h.t. but no filament power caused little or no harm.

klugesmith

Mon Dec 24 2018, 04:20PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

I agree with Sulaiman when we are talking about vacuum tubes.

Gas filled tubes with hot cathodes, such as rectifiers and thyratrons, are another matter. During each power cycle (not in the sense of AC cycles), there's some HV current while filament temperature is lower than normal. The good coating on filament gets sputtered off much faster than normal. It's a big deal for hot cathode fluorescent lamps, in applications with frequent starts and stops, if the electrical preheating is skipped.

I guess that would not be a problem in power transitions caused by blowing fuses, as long as the HV current eventually stops.

Wish we could post illustrations without jumping through hoops. Want to tell a story about abusing fluorescent lamp cathodes last month.

radiotech

Wed Dec 26 2018, 11:11AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

It was customary to fuse the primary of the power transformer, and the HV secondary
in radios with ECL86 tubes.

HVDC furnished with selenium bridge rectifiers.

In some radios the heater circuit was fused too to prevent damage to printed circuits.

This was in German radios, which I repaired 60 years ago . (still mess with them today, but not for a living). Google Grundig schematics to see hookups.

Sulaiman

Wed Dec 26 2018, 03:10PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The +HT supply has inrush current due to charging of the capacitor(s),
the filament supply has inrush current due to cold filaments

and in my opinion, every fuse/fuse-holder is a potential source of failure due to contact problems, mostly due to heating/softening/oxidation.

A fuse in the +HT line between the power supply capacitors and valves would do no harm,
but for the fuse to blow, something must already have seriously failed.

You could use a time-delay / slow-blow fuse ( T or TT ) for the filaments
or transformer primary/primaries.
worth reading

radiotech

Wed Dec 26 2018, 08:41PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The use of fuses was due to changes in safety issues, in domestic class electronics.
In order to pass certain tests vulnerable parts, that could short out , (tubes capacitors) ,
were shorted and the item was plugged into power to see if the transformer burned out
in a certain way. It failed if it emitted flaming material that escaped the enclosure, and
when burned out, left a short circuit between the incoming power lines and exposed
metal parts (shock hazard).

A good answer was to use a higher class of transformer insulation, that would stop either
conditions. Very expensive for makers of cheap things to do.

The second answer was to add fuses. The same tests were passed if the item under test
opened the primary circuit on overload.

My job then was to send samples of the product to the test authority, have them try to
destroy it, and return it with their report.

In some cases, modifications were implemented, to products in our plant, to make an
entire shipment eligible for a 'special approval certificate label'. This label was attached
under my supervision, and under my license to do so. (i.e. my butt on the line).

Below is a sample of the tag that was on our returned (destroyed) samples.
This was more than 50 years ago !

Proud Mary

Fri Dec 28 2018, 08:29PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

High quality heater circuit designs often called for a centre-tapped LT winding i.e. 3.15V-0-3.15V for 6.3V heaters. In this arrangement, it was usual to connect the centre tap to Earth/Chassis via a slow-blow fuse of appropriate amperage.

Happy New Year to All!

866rectifier

Sat Dec 29 2018, 09:18PM

Registered Member #64743 Joined: Sat Sept 08 2018, 07:41PM
Location: Canada
Posts: 9

I use a primary fuse on each transformer. There is no hazard if one blows but the other supply is up, unless you have a third transformer for the bias power supply. That has the same fuse as B+ so you don't get a blown bias fuse yet full B+ present, that would roast things.

An overload on B+ is common, so I put a slow-blow fuse there on the primary.
An overload on heaters is rare, you don't really get short-circuits there. Even a heater to cathode short doesn't overload the filament power supply. The fuse mainly protects the transformer.

Some power amplifiers use a HV DC fuse to protect the output transformer, as it will probably get cooked if an output tube (or screen) shorts and puts lots of DC through the output transformer winding. More popular in European products.

Some designs instead put a LV fuse in series at the output tube's cathode, this works but adds some distortion due to the fuse resistance being non-linear on low freq. sine-waves.

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