JTM 45 negative feedback with Mercury 'Radiospares Deluxe' OT

[Chuck H]

Tacking onto the end of this thread... Here's a switch that I believe should work for impedance switching the RS or Hammond 16XX secondaries without buying a Honeywell product for a hundred bucks. It's an automotive or appliance switch so it's probably tougher than the plastic shaft implies. I eyeballed this a few years ago and tried to keep it in mind for future projects. Needless to say I forgot it until now So I went to Mouser and looked it up. For the application I'd prefer a shorting switch, but it's not a perfect world. Anyone see a problem with this switch for this purpose?

https://www.mouser.com/ProductDetail...hJ%252bGEu4%3d

[Helmholtz]

https://www.mouser.com/ProductDetail...hJ%252bGEu4%3d

A current rating of 2.5A is not sufficient for P>25W at 4 Ohms.

[J M Fahey]

Well, yes and no, depending on actual use.

In general switch rating is the maximum current they can *interrupt* safely, because when passing current is interrupted usually an arc is created and that is a major event for contacts, from surface pitting to plain melting or soldering them together in an extreme case.

Now if you switch ON/OFF without passing current at that moment, then it´s much easier.

Add to that : switching AC is way easier than DC (because AC is self interrupting all the time)

In a nutshell: I have used 2A rated switches as "fake Standby" in SS 100W@4r amplifiers (5A RMS) since forever with no issues whatsoever , a couple times into 200@2 ohm (10A RMS) just a "Saturday night emergency" , customers never ever bring it back on Monday for the proper repair ... and I don´t recall having problems.

To be precise: now and then a switch fails,the usual percentage, and years after assembly.

Now if you switch *under load* yes, the official rating must be respected.

And if you read the full datasheet, there are different ratings for the very same switch; a lower one for DC (which often also lowers acceptable voltage as low as 32V) and a higher one for AC, and accepting 125V or 250 AC .

Add to that that a power or standby switch is activated either way at least once or twice every session, while an impedance switch may be used a few times in years ... or even never.

[pdf64]

I think that the ‘switching’ rating, the ‘carry’ rating looks to be 5A.

[Chuck H]

I was looking at the carry rating indeed. There have been a ton of amps made with standby switches that had lower switching specs than this switch and a carry rating that barely met spec. For a long time Ceriatone used those Alpha switches with the open phenolic deck, you all know the ones. They used a the 4P3T switch rated at 300mA (no spec on whether that's switching or carry) and used two pole functions wired in parallel. Quite a few people didn't like the idea but I haven't read about any failures.

I'll be happy to play the Guinea pig whenever I get around to building another amp with either of those transformers. No idea when. Until then this is my contribution to the matter. People have complained about no switch suitable for the Hammond 16XX transformers forever. At least not one that's priced like a guitar amp component. Get into the $60 to $100 range and there are plenty. Now Hammond offers them with the "easy wire secondary". Just add "A" to the end of the part number. I have no idea if they "sound" the same. From what I've read here the goofy secondaries were considered a necessary evil for performance. So I'm sure there's got to be a compromise for the "easy wire". Whether it makes a difference for a guitar amps I can't say. But since I'm familiar with the standard hard wire secondary and now I have a switch I don't need to worry about it

EDIT: I just considered something else too... Because switching impedance for the 16XX requires managing connections for two secondary windings and a tap on one of them. No one switch pole will ever see the full load. The load will be shared between the poles to different degrees for each impedance selection. So that's at least a little stress relief.

[mikepukmel]

I tried drawing out the connections from the attachment in post #6 from Chuck H. The polarity of the 8 Ohm example is reverse compared to the other two. The series arrangement for 16 Ohm is not ideal.

Try this: Always ground F and take the feedback from A and use a feedback resistor for a 4 Ohm tap.

For 4 Ohm: Connect A to C (Output). Connect B to E. Connect D to F (ground).
For 8 Ohm: Connect A to B to D. Connect C to E (Output). F is ground.
For 16 Ohm: Connect A to B. Connect C to D. E is output, F is ground.

Below is a schematic for using two DPDT switches. Both switches need to be in the correct positions for it to work. Warning: If one of the switches is in the wrong position, you could damage the OT.
[ATTACH=CONFIG]48956[/ATTACH]

Ah thanks, i remember this one now. Awesome.