All the feedback has had me doing a lot of thread reading, thinking and trying to distill the heaps of information.
My conclusion at this point: I might be attempting to build the world's most inefficient guitar amplifier. A Pentodes rather than all triodes in the pre-amp section and perhaps Pentodes wired as triodes in the Output section and a very inefficient speaker.
We currently run this amp on a 2X12 cab. Speakers are stolen from Johnson Mirage JT50. If I would say they are some type of JBL clones. I have no idea what the SPL rating is on the speakers, but we like the way they sound and they have become our reference cab.
All speaker efficiency numbers I have seen published are standardized as XXdb SPL 1W/1M. Most guitar speakers have efficiency ratings in excess of 95dB. It seems difficult to source a low efficiency guitar speaker, let alone one that sounds good. I realize it is unrealistic to get output tubes designed for clean output up to 45W to provide a pleasant distortion while operating at an output of less than 1 watt and reducing speaker efficiency is probably one of the most efficient ways to quiet an amp regardless of the size and power. I will keep my eyes peeled for an inefficient speaker. (thinking of that, I do have an old field coil speaker floating about somewhere that might be an interesting addition to the mix)
I also realize I am not the first to pursuit the Holy Grail of guitar amps and many, more capable than I have come up empty, and broke when it comes to getting an amp to pleasantly distort at room volume levels and still be useable to gig. Most amps that achieve distortion at room volume levels end up becoming one trick ponies incapable of much else lacking versatility. It is a high bar.
The amp that I feel best represents what I am trying to achieve right now is the PRS MT15. My son has played on it, and it is amazingly versatile, touch sensitive, and dynamic. I find it particularly interesting as it is billed as a 15w amp but it is 6L6 based. I understand it actually has a power output of roughly 24W. It runs at a reduced plate voltage of roughly 300V from what I can gather.
It appears most all of the desired distortion is in the preamp and PI stages (6 12AX7's) and they left the 6L6 tubes to do what they do best which is clean amplification. There is also an attenuator switch to cut it to "7W".
So my thought process here is to develop all of the "tone" in the preamp stages, and not need to push the Output tubes into the stratosphere to obtain heavenly sweetness. Once again, my knowledge and experience is limited and I draw more from a sound reinforcement and hi-fi background than as a guitar player.
One way of making the 6L6 output tubes super clean at low volume, and reduce max power output is to run them in triode mode. For a guitar amp manufacturer, this would be a costly and counter intuitive approach. However, if you are just looking for the output section to serve as minimal sound reinforcement and are developing all the "tone" in the pre-amp section, is it not possible that this could be a good thing? It is something I wish to explore but am not decided on. I am fine with pentode mode output and some form of master volume but I don't expect to push the output tubes into distortion at low volumes, rather just serve as volume control.
I understand that "tone goodness" of rock and roll comes from Pentodes as PRR has pointed out. So If I want rock and roll goodness, I need to employ pentode distortion somewhere in the circuit. I understand the the 12AX7/6SL7 is one of the most efficient tubes for obtaining high gain with the fewest number of sockets. I understand that many experienced builders and players say that a pentode in the pre-amp section creates a superior tone to triodes, but it is less efficient in terms of sockets when it comes to gain stages and driving tone controls and works best with some sort of cathode follower.
I have heard many players say octal tubes sound better than noval. Since I have this octal amp at my disposal, and it has reasonable voltages to work with, I would like to adapt it for my purposes, hoping it might offer a sonic edge over 12AX7 variants.
How about this. V1= 5879 with volume control and bass cut > V2 6SL7, with vol control between cascaded triodes followed by > V3 5879 whcih will hopefully at this point be able to be pushed into pentode distortion, which would be followed by a simple volume control and drive the >V4 6SL7 PI. Treble control would be post PI as a treble cut. As I see it, this would provide at least 4 gain stages before the PI. I will redo schematic today to try and reflect my current thoughts.
I believe the transformer on this old organ amp is capable of handling the additional small tubes, will likely convert the pilot light to an LED and save the needed current there.
Topic: Adding VVR and Pentode preamp stage (Read 2965 times)