Building Altec preamp with power supply

[bram9206]

Hi guys,

I'm currently doing my research for building a stereo Altec 428A preamp with an external power supply. Right now I'm drawing some layouts and I got a couple of questions:

- here's the schematic of the original P522B power supply: http://www.waltzingbear.com/Schematics/Altec/Altec_P522B.JPG
does anyone know what happens there with pin 7 of the two 6X4 rectifier tubes? Is really connected to the filament heaters?? Can anyone tell me why?

- anyone any suggestion for a power transformer? I would need a universal primary that takes 230V. The thing is, in the original schematic there are two seperate filament wirings. One that goes to the two 6X4 rectifier tubes and one that goes to all heaters in the preamp tubes. Does anyone know why this is seperated in the first place? And do I really need seperate wires like this or can I use just one pair and go from the 6X4 heaters to the preamp tube heaters like one would do in for example a Pultec EQP-1A?
Also, I would like a PT that also gives me 5VCT. for a standard pilot light.

- Are the 6,3VAC heater wires really rated at 4,9A in the original schematic? Can't really see it in the schematic... Would 2A be good as well?
- And any idea at what ampère the plate wires are rated? Voltage should be 275-0-275, I think...

- And beside all the power supply stuff: does anyone know, is the Altec 428A really a line amp? And the 428B a microphone preamp? Since I'm building a mic pre, it might be better to choose the "right" one. I also know there are 458 and 459 preamps. Does anyone know the difference between these Altec preamps and what would be the best to clone as for a microphone preamp (that also gives me enough gain for ribbon mics)?


Thanks a lot guy!

Previous builds: Pultec EQP-1A: http://groupdiy.com/index.php?topic=54468.0

 

[emrr]

Show us the 428A schematic, I've never seen it.  It is a mic pre, the 428B is the revision.  The 429 is clearly a power amp.

Do you have the right output transformers?  You won't build either one of them without it.  Tertiary winding.  DaveP did the 458, but had to use two input transformers wired together to get the right impedances to meet gain. 

Filament current for the number of preamps powered in the console.  Add up the tube filament requirements for what you are building.  Plate load: same scenario, but here they give you the max current rating. 

Pin 7: This is how rectifiers work in many cases.  Look at the 5V winding of amps with 5V rectifiers. 

 

[bram9206]

I have already checked the web for the 428A schematic, but I really can't find it... Here is the schematic of the 428B preamp, which they say is very similar to the 428A:

http://www.waltzingbear.com/Schematics/Altec/Altec_428B.htm

 

[ruffrecords]

Hi guys,

I'm currently doing my research for building a stereo Altec 428A preamp with an external power supply. Right now I'm drawing some layouts and I got a couple of questions:

- here's the schematic of the original P522B power supply: http://www.waltzingbear.com/Schematics/Altec/Altec_P522B.JPG
does anyone know what happens there with pin 7 of the two 6X4 rectifier tubes? Is really connected to the filament heaters?? Can anyone tell me why?

The tube can only withstand a certain amount of voltage between the cathode and the heater. If the heater was unconnected it would float and there would be no telling what the potential difference between the two would be. Connecting the cathode to the heaters ensures it is a small known value.

Cheers

Ian

 

[emrr]

Pin 7 is the rectified voltage feed.  The associated 6V winding is only for the rectifiers.  C1/R1 is the beginning of the B+ string.  This makes it act like any other rectifier lacking separate heater and cathode, insuring no voltage differential.  Look at any schematic with a 5U4. 

The 428A uses different tubes, though is reputed to be very similar to the 428B.    I've never seen a drawing for it, or the 429A. 

 

[bram9206]

So I definitely need two seperate pairs of 6,3VAC? Is that what you're saying, or can I share the 6X4 heaters with the preamp tubes?

Then, any suggestion for a power transformer?

 

[bram9206]

Here is my first effort for the layout of the Altec P522B power supply. Keep in mind it's not finished yet, since I still have a couple of questions:

- according to the schematic pin 7 of the 6X4 tubes is attached to the heaters. Can I just bridge pin 4 and pin 7 of the 6X4 tubes?
- still need to draw the heaters to the preamp tubes, but I was wondering, can't I use the same heater wires as for the 6X4's? Or do I need a seperate pair for the preamp tubes? Just can't find a power transformer with two seperate pairs of wires for the heaters...
- what about the central tap?

And a side question: how can I power up two pilot lights? I'd like to have one for the (seperate) power supply and one for the preamp rack. Can I hook up a pilot light in series? Or...?

Thanks a lot for all the help. Here's the layout (high res in attachement) and the original schematic:

 

[DaveP]

Bram,

While you are in the research stage, why not take this approach?

The 12AY7's are rated at 1.5W each, assuming you use 300V B+ then each tube can only consume 5mA max.  Sensibly you may like to run less than the maximum rating as these are expensive tubes to change too regularly.  The pre-amp stages will run less than 5mA in any case.

All this adds up to a B+ supply of about 10mA per channel so 20mA in total.  In this case, why would you want to use an Altec design that can provide 140mA from two 6X4's???

Better to just use a bog standard single 6X4 in a simple conventional power supply that you can find on any amp.

If you work from facts like this it takes all the doubts and mumbo jumbo out of the equation.

best
DaveP

 

[bram9206]

Thanks for the info Dave. Concerning the preamps: I'm not going to build them, I'm just going to buy them (already found them actually) and build a (external - if that makes sense) power supply for it, rack them in a nice 2U rack with some extra features like a pad, a gain knob, etc... So the main idea is to build a power supply to power up two Altec 428A preamps.

So what you are trying to say is that the original Altec P522B power supply is way too powerful to power up only two preamps? Then what would you propose? Work with a single 6X4 instead of two? And then how do I do that? And does that mean it also solves my problem that I can't find a power transformer with two seperate 6,3V windings?
The best would be that I only need one 6,3V winding, is that possible? I did it in my Pultec, but that is not a preamp of course, and uses, apart from the 6X4, different tubes... http://groupdiy.com/index.php?topic=54468.0

 

[DaveP]

A single 6X4 will do the job.

I would keep the 6.3V winding (hopefully centred tapped) for the tubes, then use a separate small TX for the Rectifier tube.
If there is any leakage in the rectifier then it won't affect the audio tubes.  Put a 100 ohm pot across the 6.3V with wiper down to earth for a CT if there is not one.

As you are fairly new to this then its better to do it in stages, so just connect the tubes up without the B+ to start with.  Put a big resistor in series to make sure you don't over voltage the heaters, then gradually use smaller resistors until you get down to 6.3V.  Use ohms law all the time to get your head around what's going on here.  Same thing when sorting out the B+, this avoids smoke!!!

I have a spare copy of the schematic in front of me, then systematically wire a line in then over write the line on the plan with a red pen until its all in.  This avoids costly mistakes.

Download the tube data from Franks site to check whether you need plate resistors, its all necessary research, GE data sheets are clearest.  Self discipline is required for this work, first master yourself, then the circuit 8)
best
DaveP