Mark III 680 ohm resistor on output terminals

During the installation of the VTA board on my old Mark IIIs, I noticed the original design has a 680 ohm resistor on the speaker terminals going from the 16 ohm tap to ground. I looked at the schematic provided with the VTA driver board and saw that this resistor was not present. So, I removed the resistor.

Looking at some pics of other peoples Mark IIIs with the VTA on the web I noticed that the resistor was still in place.

So, should this resistor stay in or come out?

Thanks for your suggestions.

I'm not 100% sure but I think it was there as protection in case anyone ever powered up the amp without a load on the outputs, so it's not absolutely needed but has almost zero effect when left in place since it's nearly 100X as large as an ordinary speaker load.
Amplifiers should never be powered up without a load on the outputs.

Other than Bob, are there any real old-time amplifier builders out there? Not kit builders putting together a predesigned unit according to a specific circuit and layout...but those who bought a couple of transformers, some resistors, capacitors, tube sockets and a breadboard, and wired up an amplifier from scratch? You would remember watching an oscilloscope while trimming an amplifier's feedback loop for optimum, extended frequency response with stability, and would recognize what can happen when there isn't a load across the output transformer's secondary.

The amplifier oscillates due to phase shift at the extreme high frequencies, above the audible range. That resistor is chosen to present a load to the secondary that reduces or eliminates that oscillation so that it doesn't have the potential of burning out something.

I put the 680 ohm resistor back in. Just for safety's sake. I never power up a tube amp without attaching speakers, but you never know if something happens to a speaker wire connection that you don't notice !

Perhaps in the VTA Mark III schematic it might be a good idea to insert this resistor. Better to play it safe.

On my Mk3s,With stock frontend. I run the output in pentoiad not UL. I put the feedback on the 8ohm tap an moved the 680ohm over two the 8ohm ,sounds better to me.
I took the 680ohm out an put a Zobel in it place,10ohm 1w an a .47cap It has a good sound all so,use a good cap!

I would like to thank Bob for the site.

I subscribe to the less is more camp, so I try not to add any components in a system that truly do not have a major affect on the performance/sound.

So, as a safety mechanism only, the 680 resistor should do it's duty in case of no load and not sonically impact the low impedance outputs.

As for distortion, I am also of the idea that this is not a tell-all indicator of sound quality. So long as distortion is within reason, I will not want to go to extensive measures to try and reduce it as introducing more components or adding feedback may affect subtle qualities of music playback that can be heard (imaging, speed, localization, etc.). As they say, the best amplifier is a wire with gain. The VTA triode design already has lowered distortion compared to the original Dynaco design and the fact that I built the low gain version also results in reduced distortion, so I feel no need to aggressively purse that demon. Coming from the world of class A zero feedback amp designs, I prefer low as possible feedback. That is one reason why I chose the VTA design.

IMHO