Experimenting with Altec 755E

[Bauhausler]

All;
This is the first step or two in an experiment with Altec 755E drivers. For those of you not familiar with this driver, it's a fullrange that originated with Western Electric. Many people consider the original WE version to be the best sounding full range driver ever made. They were used in the AR1 speaker, paired with the AR woofer in a sealed cabinet. The response of the 755E, a version from the '60s, is said to be from about 40 hz to 13khz. That's nearly high enough on top to satisfy my aged ears, but an octave short on the bottom, so I won't be running these fullrange. Right now I have a single one loaded in a 1.3 cu ft speaker box that I have adapted for this purpose by blanking off the midrange and tweeter holes and fabricating an adaptor plate to fit the 755E into the woofer hole. A pic of this setup is attached. I picked this cab, which is a bit small for the application, because bothe the back and front are fully removable. That makes modifying it a breeze. The material is good 5/8" plywood with nice walnut veneer and a fairly transparent permanent grillecloth. The box that you see has been stuffed with two sheets of foam aprox 12" X 24" each. They cover the sides and bend around the back to meet in the middle. This controls resonances a bit. The box is sealed. Theoretically, this size box needs a port to get the best bottom end response from this driver. I may go that route if I think that I can get enough bottom end to run them full range, but probably not. If I'm going to use a sub anyway, why bother fooling with the alignment of the main driver. Right now I have this box with the 755E sitting on top of a Cambridge powered sub, rolled in at around 55 hz. The 755E is running full range. I also have a Hiquphon dome tweeter paralelled with the 755E, just sitting on top to boost the top end. I have it fed through a 1.8uf film cap, which rolls it in at about 11Khz. The level seems about right without an L-pad. If necessary, and if I keep these cabs for the 755Es, I will install a good tweeter permanently in the existing tweeter hole. There's another 755E coming this week by UPS so I'll have a pair running by the weekend.
My initial impression is that there's a major excess of energy in the presence region (2000 - 3500 hz). It's not quite 'shouty' but way too forward for my taste. The bass is fine as low as it goes. I'm using a preamp, my Yamaha C-70, that has a parametric tone control section. I'm lowering the presence region by about 4db and that helps a lot. It also needs a boost in the 250 hz range according to my ear, but I like a warm bass anyway. this driver doesn't seem as 'open' as my Lowther DX3s. Neither is perfect, and I'll keep experimenting to tweak up the 755E.
This project might be the excuse I need to build the GM70 20W SET amp that I've been planning. Schem scribble to follow. That's just about the right amount of power for this driver and I really like the sound of DHTs and fullrange drivers.

 

[Bauhausler]

Here's the schem, sloppily drawn, of my idea for a 20W SET amp to drive the 755Es. It's a direct coupled design based somewhat on Jack Elliano's Direct Reactance amp. Jack wound a pair of Electra-Print 80H 30ma driver chokes for me that I would use for this. The design works great but leans heavily on a large value bypass cap in the output stage to maintain the bass. It's great for a design that doesn't need to do the bottom octave because you can fool with the cathode bypass values to get the bass rolloff you want basically for free. This design should get 20W out of the GM70 with relatively low distortion because it's just 2 low mu stages direct coupled. The 10Y, with its 5K Rp driving the GM70 grid directly should swing the bejesus out of it. The gain, end-to-end, is just 1.5, but that should be plenty with my existing preamps.

 

[DanTana]

Hey Bauhausler,

I've been thinking along the same lines you are. I would love to build a SET tube amp, or at least a class A amp of some type. I've also been looking at some high-effeciency speakers too. Fostec's are very well regarded and have high effeciency. I've been checking out some horn tweeters, I am not a big fan of horns however, normally they are way to forward for my taste. I've seen some JBL Jubal tweeters that looked very interesting, but I have no idea about their effeciency. It sounds like you have a good recipe for those SET amps. The Hiquphon tweeter might not be a good match for that. I would think a good ribbon would work better. I'm interested in your design for the 20w SET amp, is that per channel or total watts? I've been also toying with the idea of the Gainclone amp as well.

 

[Billfort]

I'll be very interested in your findings Bauhausler, I'm about to start experimenting with the 755 as well. I have a pair of 755C's in house as I'm building cabinets for a friend - I'll be building something similar to the 2ft3 sealed box shown on the JELabs site, but I'll be using 3/4" Baltic birch.

We should have some fun test driving these as my buddy has a killer system with several amp (10y, 26, 45, 50, 2A3, 300B and 211 SET's) and speaker combinations to try (Altec 604E, Lowther PM2A in Medallions).

 

[Bauhausler]

Originally posted by DanTana
Hey Bauhausler,

I've been thinking along the same lines you are. I would love to build a SET tube amp, or at least a class A amp of some type. I've also been looking at some high-effeciency speakers too. Fostec's are very well regarded and have high effeciency. I've been checking out some horn tweeters, I am not a big fan of horns however, normally they are way to forward for my taste. I've seen some JBL Jubal tweeters that looked very interesting, but I have no idea about their effeciency. It sounds like you have a good recipe for those SET amps. The Hiquphon tweeter might not be a good match for that. I would think a good ribbon would work better. I'm interested in your design for the 20w SET amp, is that per channel or total watts? I've been also toying with the idea of the Gainclone amp as well.

I really like SET amps. The best sounding amplification I have heard for high efficiency speakers is a SE 45. It's just flat wonderful. It took me a while to get my Lowthers set up so I like them. Like it took 3 years. Still it's the best I've ever heard in my home and visitors volunteer praise as well. It's not for everyone but it's the right direction for me to go in. High efficiency fullranges are Different and have to be approached with a whole other set of expectations.
The Hiquphon was laying around loose, having had its flanges busted off when my intoxicated wife backed into and knocked over a pair of Sumo Opus 1 speakers tha tI had JUST finished rebuilding for my sister. I put in different tweeters and kept the busted one as a tester.
I have some JBL 077 tweets (Jubal) here but they're in my L65s, which are ready to be shipped out to a new owner. The 755Es probably only v=need help from 10K up. I have some Realistic electrostats that I'd like to try out for grins. I like the sound of an eletrostat running right. I'll probably experiment with some available tweets and just settle for something that sounds OK. I have a pair of NOS KEF T27s that were built for LS3/5As and have never been out of the box. Cost me $150 10 years ago to have those hand carried from London as spares for my 3/5's. I'll probably try those as well.
I kinda like the sound of a conventional soft fabric dome tweeter but maybe that's because I have heard so many of them I'm used to it.
I have a single gainclone breadboarded and running here. I should try that as well on the 755E but it feels like something special should be done for amplification. I tried my PP KT-90 tube amp but the McCormack DNA-1 kills it on this speaker. I don't know why unless maybe it's due for a tuneup again.
I'll bust out the SE 300B and try that but it has disappointed me lately compared to the SE 45 which unfortunately does not have the juice to run the 755Es.

 

[Bauhausler]

The GM70 is good for more than 20W output from a single tube. the max PD is 120W and the VA max is 1600V. It's a transmitting triode and you can drive the grid positive for more output if you have a low impedance driver and lots of b**ls. I'll bet you can get 35W out of it with the right circuit. That'd be Class A2, where things are quite different from A1.
The Rp is only about 1500 Ohms or so, which makes me think it would be compatible with a PP OPT made for 6550s. Hammond makes a 100W PP OPT with a Pri Z of 6K6 Ohms. I'll bet a pair of GM70s would get 100W out of that with about a 1000V supply. You'd have a heck of a time getting the 300V P-P grid drive, though. It makes me think, but I 'll first do a simple 2-stage amp like I drew.
Here's a better pic of the schem.

 

[DanTana]

I've read some great things about the Aurum Cantus G2-si tweeter, it has a very high effeciency. It won the DIY speaker shootout placing #1 and #2 in two different designs with the Seas Excel magnesium cone woofers. The Seas however, doesn't have the greatest efficiency and the G2 would probably have to be attenuated to match the Seas. PHL does make some very high effieciency 6.5" woofers that would work well with a SET. I also have been considering this tweeter as well. It has a very wide frequency range and 92db efficiency. It is suppose to be very similiar to the Scan Speak 9500 as far as sound goes.

http://www.northcreekmusic.com/Drivers/NorthD28-06S.PDF

Right now they are on sale for $139.00 a matched pair. I'm curious what you thought of those JBL 077 Prism tweeters. They look interesting, don't ever remember hearing any.

The schematic you drew looks even easy enough for someone like me to build. Have you built a prototype of it yet? A 20WPC SET would be something that I think would catch fire around here. Most I've seen barely break 7WPC, and super high efficiency speakers are needed. With 20WPC I would think even something around 90db would be efficient enough with that. I know it's a work in progress but maybe soon you could get a parts list together so I could start putting it together myself? I'm pretty dangerous with a soldering iron when it comes to SS, at least tube stuff I'd have room for my shaky hands to work in. Tube gear always was easy to maintain for me also, having had a set of Dynaco Mark III's at one time, but then I could go to the local drug store and test and buy replacements easy.

 

[Wardsweb]

Hey I have one of those scribbles.

OK, I have it in a formal schematic too. You can check the entire build on my site:
http://www.wardsweb.org/audio/dc2a3.html

 

[Bauhausler]

Originally posted by DanTana

The schematic you drew looks even easy enough for someone like me to build. Have you built a prototype of it yet? A 20WPC SET would be something that I think would catch fire around here. Most I've seen barely break 7WPC, and super high efficiency speakers are needed. With 20WPC I would think even something around 90db would be efficient enough with that. I know it's a work in progress but maybe soon you could get a parts list together so I could start putting it together myself? I'm pretty dangerous with a soldering iron when it comes to SS, at least tube stuff I'd have room for my shaky hands to work in. Tube gear always was easy to maintain for me also, having had a set of Dynaco Mark III's at one time, but then I could go to the local drug store and test and buy replacements easy.

It is sort of easy enough to build but has some serious drawbacks. The plate voltage is around 1450VDC. The plate supply is choke-input, which means if current drops below critical the supply voltage zooms to 2240VDC (1600 X 1.4). This requires very different construction and materials than conventional tube amps. I'd like everyone to have a 20W SET amp but I don't carry quite enough liability insurance to deal with your widows if you slip with a screwdriver. To be honest, the reason I'm not listening to mine right now is my uncertainty about my own shop practices. There are a lot of issues to be worked out before you throw the switch on one of these.
I have breadboarded the circuit twice and had it running under instrumentation. I tore it down each time before I got to the music stage.
Right now I'm listening to the 755E in its temporary cabinet with tweeter and subwoofer. It's driven fullrange by my McCormack DNA-1 amp. I'm tweaking the level at 10Khz and 80 hz up by about 4 dB. The 'test disc' in this case is Morphine's "Cure For Pain". I love the sound of that bari sax on a fullrange driver. It just sounds so hairy-chested. I can imagine being in some NYC dive club at 3:00 am gassed to the eyeballs on Jim Beam while this plays. Makes me want to break stuff and sleep past noon
.
I have a few projects to finish befor I get back to the GM70 power amp but I'll post a thread on it under DIY or Tubes.

 

[DanTana]

Yea, that's some serious voltage alright. You could power the whole block with that. How many amps we talking though? Low amps? I guess with that many volts it doesn't matter too much. I appreciate you sharing this with us. It sounds like a great project.

 

[Bauhausler]

Originally posted by DanTana
Yea, that's some serious voltage alright. You could power the whole block with that. How many amps we talking though? Low amps? I guess with that many volts it doesn't matter too much. I appreciate you sharing this with us. It sounds like a great project.

DAN;
It's only 100ma/side, and just a single B+ supply as drawn. It could be scaled down for lower voltages. You could get away with about 800VDC if you dropped the direct coupling and used a 5K OPT and grid bias instead of cathode bias. The power woould be closer to 12 - 15 watts I think.
I notice that the copper plate version of the GM70 has shot up to $100 each. I bought 4 X GM70 for $90 a while back. 2 were copper plates and 2 were carbon plates. I have no idea if there's a difference in sound.

 

[Thatch_Ear]

Heil dipole AMTs are great tweets for 2 ways, they work great with SET and you can buy them new or used. Isolate them and they will do the job as good or better than any ribbon or stat on the market. I have a pair crossed at 8K and another at 10K.

 

[Bauhausler]

Pair of 755E are now running

All;
Tonight I finished putting together the other 755E speaker so I can listen to the pair. This is just a single 755E in a box of about 1.2 cu ft with some foam. The box is adapted from another speaker and happened to be approximately appropriate. See pix above for the interior of that box.
In the latest configuration, I'm using these stacked on top of Cambridge powered subs crossed in at '55 hz'. The 755E is run full range and the crossover control point was chosen to approximate the rollout of the 755E in that cab, roughly. The 755E is really lacking on the top end. Pretty much done by 10K or so. Not surprising for a cone more than 6" across. I paralleled in a tweeter with a 1.8uf cap (9 Ohms @ 10 Khz). The tweets I'm using are JBL LE25 belonging to someone else that I happen to have here. the sensitivity turns out to be just about right to my ears.
Note that these ears were front and center last night to see Blanche and The Tough And Lovely at the famous Magic Stick in Detroit. Forgot my ear plugs. It will be a day or two before my auditory judgement can be fully trusted.
After a little dinking with EQ via the parametric functions on a Yamaha C-70 preamp, the sound is quite credible. I really like this a lot. I'm boosting a broad band at 300 hz by about 3 db and pulling down the 3khz presence region by a couple dbs or so. I have the subs pumped up a bit to give the bottom some socko impact but that will get backed off as the boom starts to annoy.
What surprises me is a new speaker usually needs a little time for me to get used to it but I'd happily settle down and listen to anything on these. Well, maybe not heavy Techno.
Wait a minute. Now I gotta dig out an Oakenfold track to see what that sounds like......
Right. Not enough dynamics running the 755Es fullrange. If I was to bandwidth limit the bottom end I could probably get the necessary SPLs out of the 755Es to do Techno, but why bother when there are JBL L65s in the room to do Techno?
Trilln451, my wife, says these are nice with acoustic music but not so good with rock. I'd agree, based on the current configuration. It's a lot like the Lowther DX3s in our main system, only with a little more 'richness' maybe, and a little deeper bass. Also less efficient than the Lowthers.
I gotta tell ya I'm really into this fullrange thing. I rebuilt a pair of LE8 drivers for Grumpy and I was damn impressed by those. The mids were, like, THERE. Real clarity and low distortion. The response isn't all that flat, and the power handling is problematic but I really like what happens when you take the crossover regions a couple octaves away from the critical midrange.
You listening, Ski? get those Trimlines running and give them a listen. It's special and you'll like it a lot.

 

[Bauhausler]

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