What's this 5W resistor on my output jacks?

[jordan86]

Anyone have any idea why there is this big 5w resistors across my output jacks, and what purpose it serves? I actually have a different OT than is spec'd here, which includes 4/8/16 ohm taps. Attached a photo as well. Looks like it goes from the 8ohm positive terminal to ground?

[Auke Jolman]

This way, you can have the amp turned on without a speaker connected to it and not have to worry to damage the OT and/or outputtube.

[glass54]

Hi Jordan
When I saw the photo I thought it was a trick question  .
The resistor is a Dale CP-10 series which indicates 1 10W 5% Resistor and obviously 220R.
To the left is a more generic 5W 1k resistor in the Screen circuit.
As Auke implied, some people like to add a suitable resistor, often 220R and often  1W or 2W in case of Speaker lead or speaker failure.
10W is probably unnecessarily large (robust engineering  ) but it might have been what was in the kitty of parts.
I am a fan of the concept especially in amplifier heads, where unknown speaker boxes do get connected. It is a little bit of cheap insurance.
Kind Regards
Mirek

[tubeswell]

220R or 270R (or even 470R) in some amps, as a ‘fail-safe’ option for speaker coils going open.


But these orders of resistance create a high reflected load, so it’s not intended to run your amp permanently in this condition.


Where they are ‘low enough’, the resistance in Global NFB loops sourced from the OT secondary might be argued to fulfil the same sort of function (although the ‘lowest’ value in BF Fender circuits - 2k7 + 47R - is still high compared to 220R).

[pdf64]

220R or 270R (or even 470R) in some amps, as a ‘fail-safe’ option for speaker coils going open.
...
Where they are ‘low enough’, the resistance in Global NFB loops sourced from the OT secondary might be argued to fulfil the same sort of function (although the ‘lowest’ value in BF Fender circuits - 2k7 + 47R - is still high compared to 220R).

The Vox AC50 uses 470ohm on the 15ohm output.
The JTM45 uses 1k, OT wired at 15ohms.
Bear in mind that around bass resonance, a regular speaker’s impedance will rise to around 10x nominal and the phase response flip maybe 90 degrees.
Rather than 2k7, and excepting the Bassman AB165 and Princeton/Reverb, the BF Fender series feedback resistor generally looks to be 820ohms, giving a total resistive secondary loading of 920ohms or less.
I think it’s essential to maintain some degree of current return path for an OT secondary; without one, I’ve seen a spark at the primary when the impedance switch of an idling amp is flipped.

[d95err]

I use this in all my builds. It’s such a cheap and simple insurance.

It just takes one glitchy speaker cable or jack to kill the output transformer.

With the safety resistor in place, I happily change cabs, plug in and out attenuators etc, without turning the amp off.

[jordan86]

Thanks everyone!!! This forum is so great.

And yes, I should’ve looked more closely. It is a 10W. I saw 5w on the schematic and just assumed they matched :)

[tubeswell]

Rather than 2k7, and excepting the Bassman AB165 and Princeton/Reverb, the BF Fender series feedback resistor generally looks to be 820ohms, giving a total resistive secondary loading of 920ohms or less.

Yes I was thinking of the PR

[marshallguy]

220R or 270R (or even 470R) in some amps, as a ‘fail-safe’ option for speaker coils going open.
...
Where they are ‘low enough’, the resistance in Global NFB loops sourced from the OT secondary might be argued to fulfil the same sort of function (although the ‘lowest’ value in BF Fender circuits - 2k7 + 47R - is still high compared to 220R).

The Vox AC50 uses 470ohm on the 15ohm output.
The JTM45 uses 1k, OT wired at 15ohms.
Bear in mind that around bass resonance, a regular speaker’s impedance will rise to around 10x nominal and the phase response flip maybe 90 degrees.
Rather than 2k7, and excepting the Bassman AB165 and Princeton/Reverb, the BF Fender series feedback resistor generally looks to be 820ohms, giving a total resistive secondary loading of 920ohms or less.
I think it’s essential to maintain some degree of current return path for an OT secondary; without one, I’ve seen a spark at the primary when the impedance switch of an idling amp is flipped.

I use a 200 ohm 10w resistor across the output jacks on both my amps for obvious reasons, but I don't think I would go flipping the impedance selector switch without putting the amp back on Standby or turning off. Going between 4, 8 & 16 you have a nice open O.T. so I'd recommend placing the resistor across the jack and not changing impedance without first turning power off or placing it on Standby.

[labb]

The AX 84 SEL. One heck of a great single ended amp. I built that amp years ago. Tried all of the power tubes in it. Finally settled on the 6v6 as the best for me. Didn’t need the power of the KT88. Shame the AX 84 site shut down.

[mresistor]

The AX 84 SEL. One heck of a great single ended amp. I built that amp years ago. Tried all of the power tubes in it. Finally settled on the 6v6 as the best for me. Didn’t need the power of the KT88. Shame the AX 84 site shut down.

labb  you sure you're on the right thread?

[labb]

Sure. The AX 84 SEL is the amp he is asking the resistor question about. Take a look at the schematic. Most of the amps designed on the old AX 84 site were single ended head type and had that resistor on the output to protect the OT if the amp was played without a speaker plugged in.

[shooter]

The AX 84 SEL

always liked their SE stuff, stole from them frequently, The PA section using KT88 became "my standard" then just throw bunches of different pre's for the spices and you have a pretty versatile meal in one.  16-23W run through a 4X12 WILL keep up with deaf drummers.  Dialed right, the next night it's a 1 X 10 playing lounge music at the country club