shfa amp output xfmr protection

So reading the post about distortion on a 222 Seeburg I got to thinking. I once worked (In a past life) for a company who was a great believer in fuse links. Basicly a 2.5 inch of copper wire of determined size for a fuse. The idea was that rather than a major castrophy the copper wire would open. How about this in the cathode of the output tubes of the amp. Havn't tried it yet but I think I will.. Just find the max plate current of the output tube and determine what length and size of wire to use..
What do you think Ron? Lot easier to replace the wire than the output xfmr.
recore

Sounds like a "plan" to me--
Ron Rich

Sounds like a backyard mechanic crappy idea to me.

Better to install a 10 ohm 1/4 watt resistor in the cathode circuit.
But aren't these amplifiers using a mains fuse at all?

Matt,
Very poor fusing on the first 3 models, (2a SB) for the 5U4 tube only--no fusing on the 4th and 5th model.---
Only other fuse in the whole machine is a 5 amp at the input line cord--I don't know how Seeburg "got away with that" at all---
Ron Rich

Matt's suggestion is essentially what commercial amp manufacturers used...but the "fuse" resistor was placed in the plate and G2 circuits.

Rowe/AMI added them after the initial version of the amp in the "L" series. Typical values were: 10 ohm in plate. 100 ohms in G2. Both 1/2 watt. Shorted 7868s are common and these resistors were a big help.

Theater amps made by RCA-DuKane-Rauland-Altec used 1 watt or more. RCA placed them in the oversized caps on the output tubes (807-809 -god, those things were lethal).

Here in NYC a company named Commercial Radio and Sound Corp designed a lot of institutional sound systems for schools, hospitals from the 1940's-on. Their main design engineer spec'ed mostly RCA components and their C.E. a Mr Gerber told me that they later modded the RCA amps to use partial cathode bias.

They installed a long flat variable resistor inside the pan and set the slider to balance the tubes (4 of ea series-para). While at NYU I worked at a co. that had the contract for the Bd of Ed here and rebuilt dozens of these large amps. Output tran failures never existed in the units I came across but burned up protection resistors due to shorted finals were common. I came across a lot of the resistor biased units and just set the slider for equal value on both sets of cathodes (lazy).

Rob/NYC

Ron Rich wrote:Matt,
Very poor fusing on the first 3 models, (2a SB) for the 5U4 tube only--no fusing on the 4th and 5th model.---
Only other fuse in the whole machine is a 5 amp at the input line cord--I don't know how Seeburg "got away with that" at all---
Ron Rich

Ron,
Not knowing the details/shematic of the machines you mentioned, I'll spew my thoughts..
A 5 amp fuse at line input isn't wrong if it's got to supply electronics and lighting, and any animation motors... most tv sets today use a 4,5, or higher fuse at the line input.
Plasma sets sometimes have a 8-10 amp fuse installed!
Yes, people, those lovely huge flat-screen toys draw juice like a microwave oven - and a few hours of use add a hefty amount to your electric bill.
You can have them!

However, if JUST powering an amp chassis, and knowing a 5U4's operating conditions, I'd gather a 2A sb or 3 amp fb fuse would suffice.
Adding additional protection on output tube cathodes is a common preventive measure of audio beernuts - and really not a bad idea.
The tiny 10 ohm 1/4 watt resistor I mentioned acts as a fuse to save the tubes and transformers from excessive current.

OK maybe backyard. Their explanation was cheap, easily obtainable, and reliable,and it did not offer any real resistance to the circuit. They were used in the heater circuit of the tubes and output of direct coupled transistor circuits. It looks like a 2" piece of #36 gauge wire should do it. Probably the transformer is wound with #36 too but it has some heatsinking. So one strand of some zip cord.in fiberglass tubing should work. Total cathode current of about 90 ma. A #36 wire has current handling cap of 180 ma. so it should work
recore