OT Protection at Output Question:

[Raoul Duke]

Over the past week while playing my ‘77 Marshall 1987 I’ve lost output suddenly. Both times I shut the amp down within a second or two and wiggled cables, re-started, and everything was fine. Yesterday it did it again, so I stopped playing and began troubleshooting. What I’ve narrowed it down to is the stock/original jumpered impedance selector and/or a faulty speaker out jack. Today I’m going to replace both - but was also thinking of adding the power resistor to ground protection off the 16 ohm tap as an additional measure (along with a switched jack) to preserve the old OT. My research says 220-470r 5w wire wound should be adequate?

Two questions:

1. Would I wire the jack as usual (with a jumper across switch and sleeve) with the resistor at the impedance selector off the 16 ohm tap to ground?

2. Is this overkill? Would the switched jack be enough? My research seems fairly evenly divided - so I wanted to get some advice from the trusted folks here.

Any advice is welcomed and appreciated!
Thanks!

[martin manning]

Re 1., that is what I would do. that way any failure in the selector, jacks, cable, or player is protected. The resistor value should be at least 10x the speaker impedance for it to disappear. If it is too big it will look like an open circuit and risk damage to the OT. You could experiment with a 180R and 470R to see if the sound is affected in a negative way with the lower value. A 5W resistor is adequate, based on the assumption that the power output will be much reduced and the player will (should) realize that something is wrong in short order.

[Raoul Duke]

Great, that’s exactly what I’ll do; thank you Martin!

Just because Cliff-style jacks always trip me up, I drew it up first. This is looking at the jack with the contacts “down” and the solder lugs “up” - as they appear when looking at the jack with a Marshall chassis upside-down on the bench.

IMG_0547.jpeg

I think it’s correct, but Cliff jacks get me turned around. How’d I do?

Also - the NFB goes to the 16 ohm lug; will the safety resistor interact with that at all? My reading suggests it won’t, just wanted to double check.

Last question: can I just attach the ground side of the resistor to the speaker jack ground lug?

Thanks again!

[Roe]

jtm45s used 1k.

with the very low primary impedance of the el34 models I'd avoid low values like 180R. Also keep in mind that this resistor will be in parallel with the NFB and presence circuit (usually a 32k-105k load)

[Roe]

The resistor value should be at least 10x the speaker impedance for it to disappear.

at low and high frequencies a 16 ohms speaker can read close to 150 ohms in some cases? If so, a 150 ohm speak in parallel could get half of the power at those frequencies, something that may smoke a 5w resistor?!

Even if it works, it may affect bass response and speaker damping significantly. I would not go below 470R personally on the 16 ohms secondary

[martin manning]

Here is a mark-up on your Cliff jack drawing. They are confusing.

An open secondary might cook something in the GNFB network if played long enough. The DCR of the secondary is very low compared to the safety resistor or the speaker, so I wouldn't worry too much about that. I think you might go up to 5x or 10x the speaker impedance peak and still have adequate protection, so maybe 1k makes sense.

[martin manning]

Yes, the safest place for the safety resistor is soldered directly across the OT secondary.

[Raoul Duke]

Thanks Roe!
Your deep knowledge of all things Marshall is appreciated and always a learning experience for me.
More to ponder…

[Raoul Duke]

And thanks as well Martin!
So my drawing should have the jumper on the switched side then - switch to switch so to speak?

[martin manning]

No your drawing looks correct. Speaker is directly across OT common and the selected secondary tap, and the jumper grounds the common lead.

[Raoul Duke]

Ahh, now I get it. Those jacks always get me. I just don’t work with them enough and always feel like I’m seeing them for the first time, lol.

I truly appreciate the help guys! Thank you!

[SamRE]

Under the same premise, would a parallel pair of 50W 330Ohm resistors be an effective failsafe for the 16Ohm tap on a 75-ish Watt amp? Like in the event I don't have access to a reactive load, I could still do some live measurements? Or in this case would it be better to do a pair of 33Ohms for appropriate loading?

[pdf64]

As I see it, the point is that to avoid the protecting resistor's resistance overly affecting the speaker's frequency-impedance characteristics, its resistance should be about 3x the speaker's peak impedance.
So about 30x the nominal speaker impedance.

Ideally, the resistor's power rating should be sufficient to accommodate the amp's maximum overdriven output power into that resistance.
It depends really on how long the user is expected to take to realise there's a problem with the speaker / its connection.
Many guitarists just keep turning it up when they can't hear anything.
I suppose worst case scenario might be 5 minutes cranked until they finally admit defeat

As a guideline, the cranked power output into a resistive load 30x the nominal load might be perhaps 1/3 of the rated power output (ie at the onset of clipping into the nominal load).

[martin manning]

A while back I built a couple of full power supply and power amp simulations to look at transient voltage sag. Both resistive and reactive loads (speaker simulation) are included. The setup is to run with no signal for 5 sec or so to stabilize, then slam the PI input with enough signal to get full clean power when stabilized, which takes about 150 ms. In the case of the 100W Marshall 1987 SLP, full clean power is about 75W at 3.5% THD. Peak anode voltage for the normal 16R load is about 660V for the reactive load and 640V for a resistive load. Open circuit secondary takes it up close to 800V, which is probably still well below the transformer hi-pot test. Adding a safety resistor from 180R to 470R doesn't really make much difference, reducing the peak 10-20V. A 1k doesn't make any practical difference at all. Now I'm wondering if this idea isn't just wishful thinking.

[Helmholtz]

With a square-wave-like output the tubes are essentially switching between saturation and cutoff.
Without a load rapid switching into cutoff can generate high (kV) flyback spikes on the OT primary due to the energy stored in the (e.g. leakage) inductance.
A base load resistor across the output helps to damp the spikes by dissipating stored energy.
Another solution is to use catch-diodes on the primary.

Tim Robbins has put together a great article on OT protection:
https://dalmura.com.au/static/Output%20 ... ection.pdf