The max spec for 6L6, 6L6G, 6L6GA, and possibly 6L6GB is 360 Volts although I have seen this value violated by many guitar amp makers. Depending upon whose spec for the 6L6GC you look at the max plate voltage spec is 450, 500, or 550 volts. I am currently looking at a Tung Sol data sheet from 1960 that states the maximum plate voltage to be 450 volts in triode mode, and 500 volts in pentode mode. The typical operation section shows two tubes in AB1 push pull with 450 volts on the plate and 400 volts on the screen making 55 watts. Many of the 50's and 60's vintage guitar amps (including Fender) ran the 6L6GC's at this operating point. Weak tubes didn't last long. Fender also ran 6V6's at almost 400 volts which IS way over the line.
I have two SimpleSE amps one is set up in the typical fashion with 370 volts of B+, cathode bias and 3.6 Kohm transformers. I don't really like the sound as much as the second amp which runs 430 to 460 volts of B+ and 5K ohm transformers. I find that the combination of more voltage, and a higher load impedance makes for a more dynamic amplifier. It also provides lower distortion, and greater damping factor at a slightly reduced power output. 5K is not the typical "audiophile approved" load but as someone pointed out "I did it my way". It also allows me to swap between the different tubes without any of them being taxed too hard in the load department. With a 560 ohm cathode resistor the EL34 will dissipate about 24 watts, the 6L6GC about 28 and the KT88 about 33 watts. I often parallel another resistor across the cathode resistor to bring the current up near 100 Ma when using Electro Harmonix KT88's. It just seems to tighten up the bass a little bit and the EH tubes do not seem to mind.
These are just my observations and opinions which may have no bearing on someone else with different musical tastes, speakers, or ears!
We seem to be having two threads here - one based on the hypothetical "best" irrespective of power etc, and the other on the best pentodes and power tubes - loads of good stuff here and good advice from people in the know.
Could be useful if the original poster could steer us in the right direction here, since there's a lot of difference between a DHT and a KT88! Maybe a word on the power that needs to be delivered, type of speakers, size of room etc. would be a useful start
Andy
Could be useful if the original poster could steer us in the right direction here, since there's a lot of difference between a DHT and a KT88! Maybe a word on the power that needs to be delivered, type of speakers, size of room etc. would be a useful start
Andy
I was having this discussion with a radio ham friend.
We made the observation that many of the best sounding valves were triode pass regulators (300b and 6080-from my own experience), also many of the really old valves which came before really high frequency radio.
We decided that often the triode strapped pentodes/tetrodes were disappointing in comparison to these lowly regulator valves. The conclusion that we came to was that the common factor that seems to tie these excellent sounding tubes together was that they were mostly designed for relatively low frequency duty.
The pentodes/tetrodes are all primarily radio valves pressed into service in audio. We came to the conclusion that probably the reason these valves sounded poorer was because they were exhibiting minute amounts of oscillation due to poor detailing. The case would be made worse by such things as inadequately terminated top caps (KT88, 807, etc) and the audiophile fetish for having these sensitive valves out on show. Termination and shielding are prime considerations for radio circuit designers - and they understand a lot of things that the audio community simply don't take the time to learn.
Just our thoughts for what its worth.
Shoog
We made the observation that many of the best sounding valves were triode pass regulators (300b and 6080-from my own experience), also many of the really old valves which came before really high frequency radio.
We decided that often the triode strapped pentodes/tetrodes were disappointing in comparison to these lowly regulator valves. The conclusion that we came to was that the common factor that seems to tie these excellent sounding tubes together was that they were mostly designed for relatively low frequency duty.
The pentodes/tetrodes are all primarily radio valves pressed into service in audio. We came to the conclusion that probably the reason these valves sounded poorer was because they were exhibiting minute amounts of oscillation due to poor detailing. The case would be made worse by such things as inadequately terminated top caps (KT88, 807, etc) and the audiophile fetish for having these sensitive valves out on show. Termination and shielding are prime considerations for radio circuit designers - and they understand a lot of things that the audio community simply don't take the time to learn.
Just our thoughts for what its worth.
Shoog
The 6080 is electrically identical to the 6AS7 which has already earned a decent reputation in the audio world. The 7236 and 5998 are similar with a higher Mu and Gm. My experience is that they are easier to drive because of this. They sound good too. The 6336A is similar to the 6080 except for its massive graphite plates that can dissipate twice the power of the 6080. The 6528 is similar to the 6336A except for higher Mu and Gm. I find all of these very capable audio tubes.
I agree with the observation that the low Rp has a lot to do with the improved sound quality. I also believe that the massive cathode structure with its high peak current capability leads to the improved dynamics that these tubes can deliver. Of course this also leads to a low Rp.
Some sweep tubes also have monster cathodes, and when triode connected have a low Rp. These sound good too.
I agree with the observation that the low Rp has a lot to do with the improved sound quality. I also believe that the massive cathode structure with its high peak current capability leads to the improved dynamics that these tubes can deliver. Of course this also leads to a low Rp.
Some sweep tubes also have monster cathodes, and when triode connected have a low Rp. These sound good too.
Tubes designed for RF can perform with adequate stability for audio duty, given sensible layout and judicious use of stoppers. It's not true to say that all pentodes/tetrodes were designed for RF duty; that applies for 807 and 6146, but not for KT88 or EL84/EL34. [Note: I think you made a typo, too - the KT88 doesn't have a top cap, as I'm sure you know.]
ray_moth said:
It's not a question of "designed for". After all, the 807 and 6L6 are the same. The 1624 was unquestionably designed for Class B/C, RF operation as it draws less than 100mA @ Vgk= 0 and puts out less than 10W in Class A. However, its Class AB2 spec looks pretty good: THD= 2.0%. 6BQ6s were designed for horizontal deflection duty, but sound like 6V6s on steroids. Find a good loadline, and it'll probably work just fine as an audio VT.
Also, I've seen "audio" types such as 6BQ5s, 6AQ5s, 50C5s used as RF amps down through 10M. Work just great for that.
Sorry I was thinking of the TT21 of which I have a pair (a KT88 with top cap and definately designed for transmitter duty - and since the KT88 is internally identical its a questionable point as to the purpose of a KT88).
I put the idea of instability forward as the result of our musings rather than as a proven idea. However I would guess that it is a factor in a lot of disappointing DIY implementations out there. I think the statement about RP is probably the dominant factor overall though.
I am going to be building a differential amp with 807's soon and will be including ferite beads on the top caps. It is a specifically mentioned point on the STC datasheet that the top cap needs terminating otherwise instability "Will" be an issue.
Shoog
Best sounding output tubes
IMO, I prefer to use 12BH7 PP, it gives 1.5W output, something like 45, have a clean and warm sound. Triode, much superior than tetrode or pentode, beam power only provides larger power without a nice warm sound. Output transformer is another factor for music reproduction.
IMO, I prefer to use 12BH7 PP, it gives 1.5W output, something like 45, have a clean and warm sound. Triode, much superior than tetrode or pentode, beam power only provides larger power without a nice warm sound. Output transformer is another factor for music reproduction.
I think it depends not only on the type of tube but also on the way it's implemented. The sound from beam tetrodes and pentodes used in their native mode, to maximize output power and give that famous 'pentode slam', is sometimes described as 'analytical', 'dry', 'strident', 'authoritative', even 'harsh', owing to the presence of high odd-order harmonic distortion.
Alternatively, these same tubes can be coaxed to produce a much warmer, more pleasant tone, especially if connected in distributed load (ultralinear or cathode feedback) configuration or strapped as triodes. In triode mode in particular, they can produce all the warmth you'd want, although some will argue that it's still not as good as a real triode.
The point I'm trying to make, really, is that beam tetrodes/pentodes don't have to sound raucous, but they can certainly be made to do so.
Any tubes may be used well to impress audiophiles. Microscopes may be used instead of hammers and work well. Everything depends on how you value end results VS materials you use to achieve them. Some DIYers take certain fashion (or lovely) tubes and use them to achieve end results; some DIYers use tubes that give end results easily according to technologies, tools, and knowledge, they have in possession. From project to project DIYers get more and more skills. i dare to say that the one word that describe all that skills is "Optimization". Trying many ways to skin the cat they find more and more optimal one. They are learning physics, mechanics, chemistry, in combination of previously mentioned they learn available materials and their properties; they discover again and again what new quality may be produced if to combine materials; they learn how people used such materials to implement solutions: they study achievements, mistakes... The farther, the more ("the more I know the more I understand that I know nothing"), DIYers probably don't know that they study ergonomics, and lot of other disciplines that are studied in high schools; they do not work in order to have diplomas, they work in order to get end results
It is great!
What I love in DIY?
I love share of information, of ideas, of findings.
Lot of things we studied in TIASUR now are rediscovered; people cry" Eureka!" and share what they found.
It it amazing!!!
Hold on fellows; continue to torture tubes; continue to make mistakes; continue to experiment and find best optimal designs!
You may take a magic eye and use it as a plain triode; you may take a fat tube designed for destructive military transmitters and use it for a peaceful audio reproduction; you are free to experiment: if somebody tells you that it is out of fashion you may ignore and go on...
You are free. You are DIYer. So take your tubes and make what you want. It is easy!
It is great!
What I love in DIY?
I love share of information, of ideas, of findings.
Lot of things we studied in TIASUR now are rediscovered; people cry" Eureka!" and share what they found.
It it amazing!!!
Hold on fellows; continue to torture tubes; continue to make mistakes; continue to experiment and find best optimal designs!
You may take a magic eye and use it as a plain triode; you may take a fat tube designed for destructive military transmitters and use it for a peaceful audio reproduction; you are free to experiment: if somebody tells you that it is out of fashion you may ignore and go on...
You are free. You are DIYer. So take your tubes and make what you want. It is easy!
