I want to thank everyone for your kind replies to this thread. My initial inquiry was meant to seek answers regarding the potential possibility of output tube breakup as an additional tone "tool" to use in my Mark III, and thanks to the many helpful replies to this thread and after speaking with Mike Bendinelli about it also I have gained what I sought when I made the initial post.
I have always appreciated and enjoyed the tones the preamp section can produce. It truly is a great amp capable of producing an incredibly wide range of tones from sparkling cleans all the way up to extreme distortion as I mentioned in a previous comment. And I now have a much better understanding of the amp overall and about how to better balance both the preamp and power sections to get the best tones out of the amp while not expecting or wishing it would do things it wasn't designed to do.
When speaking with Mike Bendinelli about it he did tell me that with the Volume all the way up and with the Master at its highest settings there should be some output tube saturation and break up and that if my amp isn't doing that there may be something wrong that needs attention. That said Mike also told me, as have others here, that the amp really wasn't designed to produce tube breakup in the power amp section and that to get it to do so requires pushing both the front end and back end of the amp at or near their respective max levels (when in Rhythm 1).
So after considering having several mods made to my amp I decided the following:
- to have a "cap job" done on the amp to replace the old electrolytic capacitors and any other caps that appear to need replacing
- to make a couple of minor capacitor or resistor changes based on Mike B's recommendation to help round out the top of the frequency range a little
- to have the bias adjusted a little so a wider range of 6L6 tubes can be used (the stock bias is very "cold" which limits the type of 6L6 tubes that can be used in the outer sockets)
- to leave the rest of the amp as it is for now and see how these changes affect the overall sound of the amp
*** Mike agreed that a change from pentode to triode in the power amp might (or might not) be to my liking. Since the green stripe is the only version of Mark III that runs in pentode and all the other prior Mark III versions as well as all other previous Mark series amps run in triode I may still consider having a pentode/triode switch installed at some point.
As a reminder, I bought this amp new over 30 years ago and have never had anything done to it other than changing tubes myself. At its current age a cap job is definitely overdue at minimum. That alone may have notable positive effects on the general sound and response of the amp, so I'm looking forward to hearing how it sounds after these relatively non-invasive updates.
After coming to a better understanding of the amp overall I have made some changes (lower settings) to the tone control settings that have improved the tone, at least to my ears. As many have stated in various threads the Bass (and Mid I have learned) can quickly muddy up the tone if set very high. I now typically set the Bass at the lowest setting where it "kicks in", not were it is first audible but where it "blooms" with the first notable jump in its effect - around 1-1.5 on my amp. I do the same with Mid but will increase or decrease it slightly as needed after setting Treble and depending on the mode in which I'm playing and sound desired. I've learned that an EQ in the effects loop (my amp doesn't have the native, Boogie graphic EQ) does a much better job at helping me shape the frequency curve and increase the mids (and lows) when desired without muddying up the sound of the amp. I set the Treble where it produces the overall sound I want without having to use any of the pull switches (pull bright, pull treble for lead mode, etc) and when my guitar tone controls are set roughly at their mid point. That allows me to have the overall sound I want but still have some range to boost the high frequencies if I want with the tone on my guitar. I know that's basic guitar tone 101, but I'm sharing as part of the overall settings I'm using. The treble setting is usually around 5-6 depending on the guitar I'm playing and whether I'm playing clean or with overdrive. That is all for Rhythm 1. In Rhythm 2 I have to run the treble higher and drop the mids to the lowest setting that is audible or even all the way down at 0. In Rhythm 1 I run the Volume at around 7-8 to produce a full, rich clean tone, and in Rhythm 2 I typically run Volume around 6-7 depending on how close to edge of breakup (or beyond the edge) I want to go and will run Volume even higher if I want more overdriven tones in R2 which is a different tone than Lead produces. I keep Presence low at 0-3 typically.
One of the biggest discoveries I have had is that I avoid ever running the Master lower than 3 (at home), no matter what other settings I'm using, and prefer it to be at 5 or higher for best "feel" and what sounds to me to be a better tone overall. Again, to my ears and the way the amp "feels" to me anything less than 3 on Master seems to be lacking overall. I'm not saying it can't be played at 1.5 or 2 and still sound ok, just that it starts to sound much better to me at 3 and above and especially at 5 and above when I'm able to run the Master that high. Having the Treble, Mid and Bass settings as low as I do helps to keep the signal sent from the preamp to the poweramp such that higher settings of Master (and Volume) are possible for a given decibel level.
I have a combination of RCA, Amperex, and Mullard vintage tubes in the preamp section, so that affects things quite a bit also. With Mesa tubes the preamp breaks up earlier and with a different, more modern sound, so my settings may not work as well for those using stock Mesa or other "hot" modern preamp tubes (or those who prefer different tones or play different types of music of course). I've tried different power tubes but have found they have a far less pronounced effect on tone in my Mark III. I do hear a difference with Mesa 6L6 STR 441 (Tung-Sol) compared to my older 420 or the current 440 models of 6L6 with the 441s being smoother at the top end of the frequency curve yet still plenty bright. They have helped to tame what was previously a fairly harsh high frequency range at times, again to my ears.
Others may find this info I'm sharing on settings completely useless or counter to your own experience or tastes. I'm only sharing in case someone else may get some benefit from it. I completely understand what sounds "good" is as unique to each of us as our fingerprints, but for those who may share similar taste or preferences in tone to mine hopefully some of this may be of use, especially to those who may be newer to Mark III amps and find getting the "right" sound elusive at times. Heck, I've had this amp for over 30 years, and I feel like I'm just now figuring out how to get the best out of it (for me). It took venturing further beyond the "recommended" settings than I had previously and in some cases very far from them. I can get a given tone out of the amp sometimes with several different combinations of settings, but those I'm sharing here have helped me achieve, for me, the best combination of all those I've tried. If I were playing stadium shows my settings would likely be different ha ha.
