I think I've seen something like that too. I'm not certain I'm completely sold on series filaments. Resistors are cheap and it only takes 2 additional ones to set a variable regulator. Suppose 1 filament failed short, then the other tube's filament gets double what it's expecting. Unlikely yes, but stranger things have happened. I worked on a tape machine that used 4 14A8 14V equivalent of the 6BM8 tubes with their filaments in series fed from a 56V primary tap eek! A pair of them were the tape bias oscillators.
A thought I had was if any one of these tubes failed, the whole unit pretty much quit working. I'm leaning towards cheap being the reason. In mass production, economics play a huge role. In DIY, cheaper is not necessarily better, better is better. I knew there had to be a reason, I was simply looking in the wrong direction. Idiot proofing I love it! That means less heat in the regulator, and an easier design. Cut sheet can be downloaded here One of their example circuits shows the input bridge floating and the output of the regulator grounded thus creating a negative supply.
Seems like if it can sit on either side of the fence, it should be fine sitting right in the middle. My thought was to ground pin 9 of the 12AU7 just like Ampex did but also ground the series connection of the 2 - 's. Scrub that idea. Either reference ground just through the 12AU7, or do a virtual center tap with a pair of ohm resistors.
What's the worst that can happen? I'm gonna do this experiment with some beater 6FQ7's from one of my proverbial buckets of used tubes vs. I won't cry if I lose a couple of them or a regulator I may just go ahead and build a complete single channel. My board is stripped completely clean so I have no excuse I didn't even notice that until you mentioned it.
I've been fedidelin with plate followers so much that this one slipped right on past me. I can see no reason other than to kill high frequencies. I'll be using a completely different input transformer so I'll omit that that resistor for the time being. Filament experiment complete. It didn't make a difference where I referenced ground, virtual tap using ohm resistors, pin 9 of the 12 AU7, or the series connection of the 6V tubes. Better judgment told me to NOT pull a 6V tube when grounded at the series point.
I even tried mixing different brands not types of 6V tubes. No troubles what so ever. Seems like the virtual tap should be the way to go, but I guess I won't know that for certain until I have an actual audio circuit under test.
Theoretically since this is DC, noise shouldn't be a problem, but the proof is in the punch. Why are you center-tapping DC?? That's one of the things on the original which made me think this was a hasty-re-work-up of some other bits and pieces. Yes, tubes of the same nominal current should work in series. Series-string TVs demanded it, and the standard heater-stuff got used for almost all types even when not specifically rated for series use.
Yes, TVs gave series a bad name, because they had a dozen very-different types monster H-outs and teeny RFs in string, all low-bid. Run 2 or 4 tubes in series you won't have a problem. Ya know Where's the sport in that? Funny thing you guys nailed it. Ampex documentation is excellent. I especially like the sketches of people with Ampex equipment.
I guess it my day job standards influencing my thinking. Funny thing is we discussed this very topic about 2 months ago except it was a Ultimately it was decided the filament circuit could be grounded anywhere, but the series point was probably easiest IIRC. Seriously, it's not all in 1 ear and out the other but I'll be the first to admit I suffer from SBD slow brain disease.
It's also funny that the drawing shows the tech measuring the performance of the mixer with a cigarette in his hand. Times have changed This thing cost a week's pay for a computer engineer of the time. The noise figure is ample for any realistic use and almost exactly what my favorite Bi-Amp mixer did 20 years later but 5dB-6dB shy of theoretical that 22K damper. The big boast seems to be the high output levels, but who needs that much? It filled a gap in the line-up of low-price for Ampex products.
You sell suitcase recorders, you sell snazzy microphones; some guy is gonna wanna use more than two mikes. There were not a lot of alternatives in ; none to fit the Ampex suitcase.
Gates probably had something, though probably not stereo. The layout seems awkward. While it may have been "thought about", IMHO it lacks elegance. And yes, those mike trannies will mush modern studio levels. Trumpet through dynamic at 4 feet as illustrated won't stink, and any reasonable far-mike orchestra will be fine, but you don't track drums through large-condenser with this.
Not just the trannies: with decent step-up in the iron the tube will be straining. Many modern condensers will be fine if the inputs are replaced with iron or 10KK. With hot mikes, shielding is not critical. Pages: [ 1 ] Go Up. SMF 2. Mobile Device Catalog Link. Yard Sale Discontinued. What's New. Board Building Parts. Amp trim Handles. Lamps Diodes. Hoffman Turret Boards.
Channel Switching. Fender Eyelet Boards. I haven't been using the line inputs I added, and they weren't really good anyways, no transformer or anything, just one side to a k pot and the same value resistor into the busses, but really not doing anything that I needed them. So I made two more channels of the ampex preamp to plug into the line inputs. Newcomb plug in transformers and one EF86 per channel, it works great and now I have 6 inputs!
I asked about doing this years ago on GDIY! Always thought it would make this mixer far more functional! How did you impliment it? What value resistors did you use?
The Ampex mixer and compressors are being replaced with a all in one unit! I also liked it in two pieces stacked. Cool stuff. New mixer sounds really great, like a whole different game.
The compressors sound very nice too. I haven't transferred the tapes so all I could do is hear it over the headphones and that's always not very good at these little clubs, but I could not believe the difference I could tell just with that! UTC A11's. Trying to figure out the big differences in the preamps. The Ampex was completely rebuilt and changed, but it did keep the original little Beyer input transformers AND the 22k resistor to the grid.
I always liked these little inputs and think they sound really cool, I kept the 22k. Maybe that's the biggest difference with the pres, also I have 50, , ohm input impedance switching, I just run at as of now, and pentode or triode switch. That is so freakin cool! Is it primarily composed of modified MX10 guts? It's got six separate channels that are bused down to a single channel. I don't really know the types of connections that it has since the one that I've used is already hooked up to the patch bay at the studio where I work.
This is a tube based piece of gear, and we often only use a single channel of it like a traditional preamplifier. It is rack mountable and will take up two spaces in a rack casing.
There are a series of knobs six of them in total for controlling the levels of each of the mic …. There are a series of knobs six of them in total for controlling the levels of each of the mic inputs, as well as a master volume knob as well. There are also switches to choose with of the inputs you want to use at any given time.
A manual would surely be hard to find for the MX We use it at the studio where I work for getting this exact type of sound. We don't use it at all really as a traditional preamplifier, and mostly just in situations that really call for a kind of garage band type of sound. When using only a single channel of this it helps to give you a better idea of what a single preamp on here sounds like, but most of the time we send a couple of signals in a once. Message Seller. About This Listing. This is an exceptional example of an Ampex MX, 4 x 2 tube type mixer.
I have used this model Ampex for 45 years, and they have withstood all technological advances during this period. It is still in demand to this day for its sonic excellence.
But you all knew this already. The attached photos photos pretty much illustrate the condition of this particular unit. Recapped, Tubes replaced as needed with premium tubes, All new Neutrik XLR connectors, All the wafer switches and pots have been cleaned and lubed, A new IEC AC connector installed, Altec case originally from their preamp in good condition, and works well with the MX Copy of the original operation manual with schematics Over the years, I have owned at least a dozen of these mixers, successfully capturing the sound of all the classical ensembles who make up my clientele.
Typically, when an MX appears on Ebay, the description crows about "that warm sound", but nothing is mentioned about how it has been maintained.
I see broken knobs and switches, rust forming on the chassis, old Cannon XLR's that are rotten, missing parts, not to mention the myriad busy fingers modifying them into something else.
In spite of all this, you're still getting that "warm" sound as advertised. Clearly, it's all a matter of how well one maintains equipment.
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