Dynaco Durability, VTA-style

Kentley wrote:So, folks: If I were to try a dual element fuse in the ST-120, what value would be optimal? - I don't have the equipment to do Peter W's testing.

Kentley:

I believe that the OEM slow-blow fuse is 5A. Given that a 1.25A D/E works on my stock 70, I would expect that a 2.5A D/E would be effective for a 120. I do have a Scott LK150 that is similar in output to a 120, and I am using a 3A D/E on that one. Bob is wise inasmuch as he does not get every last bleeding watt from his amps, but makes a nice balance between tube life and output. So, he is not claiming the 75wpc that the vintage Scott claims.

Very helpful, Peter W. I ordered one of these - total cost inc. shipping was just over $10 --
http://www.mouser.com/ProductDetail/Eaton/MDQ-2-1-2/?qs=%2fha2pyFaduig4ygHdJV7%252bvDszP3aXkoH9BIJshvyAPg3xafCPe7gkA%3d%3d
I'll report when item is installed and functional.

Peter W. wrote:

deepee99 wrote:I have used 3-amp slow-blows in the M-125s with no issues, rather than the recommended 5. This is of course with tube rectifiers. Doubt they would survive the surge from a WS-1-type no-delay s/s rectifier.

You might be shocked (word chosen deliberately) at exactly how much a 3A slow-blow will accept before blowing. I will continue to represent that the entire concept of the wire-wound slow-blow fuse is a creation from the nether regions except in very, very, very few applications.

One thing not mentioned on this thread (I have mentioned it elsewhere) that my retired Fluke engineer friend tells me is that a single-element fuse can, upon failure, become a perfect conductor if it flashes just right and lines the glass with the vaporised  element material. Fluke meters are double-fused (not in series) for just that contingency.

deepee99 wrote:

Peter W. wrote:

deepee99 wrote:I have used 3-amp slow-blows in the M-125s with no issues, rather than the recommended 5. This is of course with tube rectifiers. Doubt they would survive the surge from a WS-1-type no-delay s/s rectifier.

You might be shocked (word chosen deliberately) at exactly how much a 3A slow-blow will accept before blowing. I will continue to represent that the entire concept of the wire-wound slow-blow fuse is a creation from the nether regions except in very, very, very few applications.

One thing not mentioned on this thread (I have mentioned it elsewhere) that my retired Fluke engineer friend tells me is that a single-element fuse can, upon failure, become a perfect conductor if it flashes just right and lines the glass with the vaporised  element material. Fluke meters are double-fused (not in series) for just that contingency.

Is'nt that the reason that some fuses are filled with sand?

So, can a dual-element fuse replace any fast-blow fuse at the same amperage rating for additional safety?[/quote]

If an device is designed for a fast-blow fuse, use ONLY a fast-blow fuse in that device. Fast-blow fuses are a subset of standard single-element fuses inasmuch as they have a much more closely engineered failure point - put another way, a much lower overcurrent tolerance.

Peter W. wrote:So, can a dual-element fuse replace any fast-blow fuse at the same amperage rating for additional safety?

If an device is designed for a fast-blow fuse, use ONLY a fast-blow fuse in that device. Fast-blow fuses are a subset of standard single-element fuses inasmuch as they have a much more closely engineered failure point - put another way, a much lower overcurrent tolerance.
[/quote]

So Peter, were I to replace my slow-blows (5a rated) with dual elements, what rating should I be looking for? Like Kentley and others, I don't have anything but a Fluke for measuring purposes.

deepee99 wrote:

Peter W. wrote:So, can a dual-element fuse replace any fast-blow fuse at the same amperage rating for additional safety?

If an device is designed for a fast-blow fuse, use ONLY a fast-blow fuse in that device. Fast-blow fuses are a subset of standard single-element fuses inasmuch as they have a much more closely engineered failure point - put another way, a much lower overcurrent tolerance.

So Peter, were I to replace my slow-blows (5a rated) with dual elements, what rating should I be looking for? Like Kentley and others, I don't have anything but a Fluke for measuring purposes.
[/quote]


OK...... as we are being rude-and-crude, we need to look at the ancillary evidence. A conventional/stock Dynaco ST70 came with a 3A fuse back in the day. I am running mine today on a 1.25A D/E fuse with (so far) no ill effects. I DO NOT play it very loudly, and for the record, I am running into an AR Athena sub/sat system ( http://picclick.co.uk/AR-Acoustic-Research-Athena-Speaker-System-Subwoofer-Satellite-371728810729.html ), like this one. The original 70 was rated at 175 watts @ 120 Volts, or 1.46 A for round numbers. Meaning a 1.5A D/E would absolutely cover it.

So, we have "nameplate" of 175 watts. OEM of 3A, and a single operating point of 1.25A. This is our ancillary evidence.

Today, any device sold in the US with a UL listing must have a nameplate rating in watts or amps. Divide that by the line voltage, if in watts gets to the amps. So as to avoid excessive failures for those who do drive their amps hard, I would simply use the closest rated D/E fuse to the actual nameplate rating, -5%/+10%. Writing for myself, the only reason I am able to live dangerously is that I do have the means to hone in more closely than most.

OEM Dynaco power-transformers are, perhaps, their weakest links. Just keep that in mind.

Peter W. wrote:

deepee99 wrote:

Peter W. wrote:So, can a dual-element fuse replace any fast-blow fuse at the same amperage rating for additional safety?

If an device is designed for a fast-blow fuse, use ONLY a fast-blow fuse in that device. Fast-blow fuses are a subset of standard single-element fuses inasmuch as they have a much more closely engineered failure point - put another way, a much lower overcurrent tolerance.

So Peter, were I to replace my slow-blows (5a rated) with dual elements, what rating should I be looking for? Like Kentley and others, I don't have anything but a Fluke for measuring purposes.

OK...... as we are being rude-and-crude, we need to look at the ancillary evidence. A conventional/stock Dynaco ST70 came with a 3A fuse back in the day.  I am running mine today on a 1.25A D/E fuse with (so far) no ill effects. I DO NOT play it very loudly, and for the record, I am running into an AR Athena sub/sat system (  http://picclick.co.uk/AR-Acoustic-Research-Athena-Speaker-System-Subwoofer-Satellite-371728810729.html  ), like this one. The original 70 was rated at 175 watts @ 120 Volts, or 1.46 A for round numbers. Meaning a 1.5A D/E would absolutely cover it.  

So, we have "nameplate" of 175 watts. OEM of 3A, and a single operating point of 1.25A.  This is our ancillary evidence.

Today, any device sold in the US with a UL listing must have a nameplate rating in watts or amps. Divide that by the line voltage, if in watts gets to the amps. So as to avoid excessive failures for those who do drive their amps hard, I would simply use the closest rated D/E fuse to the actual nameplate rating, -5%/+10%. Writing for myself, the only reason I am able to live dangerously is that I do have the means to hone in more closely than most.

OEM Dynaco power-transformers are, perhaps, their weakest links. Just keep that in mind. [/quote]
Well, here's my long and short of it: 3-amp fuses, whether off Radium Shack or of the double-type that Peter W recommends, will protect most parts of an M-125 just fine without blowing up upon start-up. Maybe this just pertains to tube rectifiers; I'm sure s/s rectification takes a far higher initial jolt.