Hi Dave,
I don't think one can do better, for that driver, than a GOOD MLTL. Make it out of one inch Baltic Birch, NOT 3/4s. Use GMs designs, by all means, he KNOWS what he is doing there!!
Go on line and look at my friend Dennis' Serious Stereo MLTL offerings, out of Montana. That was done, some years ago, with GM's help and input I hear this type of design every year, last ten years, when I go to RMAF shows, for three days- each year. Last year, I heard fabulous bass out of the unit, with a GOOD 3 Watt Jj J 2A3-40 tube amp on them, and good wiring inside and to them. Jaw dropping good bass,that I can't get out of my mind!!
Adding a sub woofer, would be a huge mistake, an unnecessary expense, and a degrade, after hearing what I have !!
Lets talk Amps and ALTECS or GPAs. Most listening on a GPA 604 speaker ( or my A7 hybrid VOTT ) is done at 1 Watt or less.
So, a 6 Watt amplifier, IF ( a huge word - my Daddy used to tell me ) the amp is executed well, and the wiring , amp-to-speakers, is well done, .... six Watts will easily work. However, most amp designs and executions are unworthy of the ALTEC or GPA drivers, and don't come close to exploiting the drivers' full potentials..
I have had a 1.3 Watt Type 46 amp, on well wired A7s, in a smallish room, over the last month, and its built right. The dynamics will easily exceed any amp , solid state or tube, that is 2 Watts to 2,000 Watts. A powerful amp usually doesn't get off dead center till its pumping out ten Watts, so its sub optimal on a ALTEC / GPA. Yes, the powerful amp is always SUB OPTIMAL at normal to high listening levels, when used with our type of speakers !! Counter-intuitive, but true !! I hear it !!
What is needed is an amp that can perform, really perform, at the sub one Watt level. That is the design challenge for our system, and the thing to look for.
The answer, for the BEST possible amp topology on an ALTEC or GPA, from my direct experience, goes all the way back to 1929, Messers Loftin and White. After that, look seriously at the design schematics of Isamu Asano and Nobu Shishido, in Japan ( both 2A3 ). It was getting better than 1929, but still had mistakes. In the USA, Sound Practices Magazine, Spring 1994 issue, did a superb article by Ciro Marzio and Christiano Jelasi " A Direct Coupled Single Triode Amplifier", which is a lovely read, 'dem Italians have a sense of style.
IspraNet Audio: Direct Coupled Single Ended Amplifier
( Necessary 2017 changes : V1 becomes a JJ or EH 5U4GB. Eliminate R1 / C1. Add a CL-80, in series with FU2. Retain L2 / C3. L1 and L2 is equal and under 10 Ohms DCR. Use two Hammond 159ZAs, 6 Ohms each, which measure about 400 mHY when lightly loaded. Adjust circuit for V2 = 1/2 12AX7, V3 = Type 46 at 22 mA. or a JJ 2A3-40 at ABOUT 43 - 47 mA. - not 60. )
In the USA, we had actual amps being offered - look at the following L-W ( Loftin White ) variants, the MOTH 2A3 by Craig Uthus , the Don Garber Fi 2A3 ( only a small window of what was possible ) , and finally, the best one of all, easily, since 1989 on, the Serious Stereo 2A3.
All of these amps I mention , spanning 1929 to 2017, have the following similarities ( a) ONLY two TRIODE stages (b) Direct Coupling of the Driver stage to the Finals stage ( c ) High mu ( 70 to 100, 100 preferred ) driver tubes ( d) Typically tube rectification (e) NO negative feedback in the circuit, the EEs crutch !!.
So WHY am I mentioning AMPs in your speaker enclosure POST David?? Because, everything works TOGETHER, and is part of a chain. It is EASY to lose what is the BEST possible from an ALTEC / GPA driver, when anything, any aspect of the chain, is not right . Well, I gave you "my " best answer about an optimal enclosure choice, in my post's first sentence!!
Thought : Instead of rushing into a design, on any expert's say - so ( Bill H, etc etc ) .........can your friend come to RMAF in October, where he'd have three days to listen to a GPA 604 in a MLTL to decide for himself, with good gear and good wiring attached ?? Or, visit Montana, even bring his six Watt amp, and give a listen, before building ??. Nothing like having a direct listening experience..... to learn from - before cutting wood..
Disclosure, I am friends with Dennis Fraker of Serious Stereo, you should be too !! :-)
Jeff Medwin.....Low Ohms
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