Analysis of Altec 436/438

[maxwall]

Been reading thru these 436 posts and have yet to see any published specs on the
two required transformers needed for accurate repro of this vari mu amp.

The Sowters seem to use modern day materials to get close to the real formula, but not exact. However its possible
to accomplish a near original image of the original Iron with modern core materials and winding techniques.

4651 and 16402

I'm wondering if these can be custom wound by Cinemag since TR is decendant of the original
Altec factory.

anyone ?

I think the input was said to be a 1:2 CT  ( 600:28K ) not sure. Still would need DCR and H

Output is said to be a 6.2:1  ?:300(150,150), not sure on this.

 

[opacheco]

DaveP,...Thanks a lot for your explanations!!!...in conclusion: the R15 and P3 is a Bias polarization network for the 6AL5 taking the 210volts HT supply for the in the 6BC8.....right?.
In DC analysis this become a FEEDBACK network (that's look to me!) coming from the 6AL5( its equivalent network will be a kind of DC source changing in proportion to the output Ac voltage) this in turn will change the Q point of the 6BC8….Am I wrong or that’s true???...for another hand, the 6CB8 and the 6CG7 are working in the diferential mode…right??

I guess you are a solid state guy trying to understand tube circuits?
Its probably more simple than you think, the negative DC output from the 6AL5 adjusts the grid voltages of the 6BC8 changing its gain instantaneously with the output voltage.  It varies from 0V (maximum gain) to -10V (minimum gain).  You are right in that the 6BC8 and the 6CG7 are working in push- pull (differential mode) in order to cancel the considerable distortion that arises from operating the 6BC8 in this way.  The halves of the tubes have to be very well balanced to avoid "thumps" (low frequency noise) from this type of circuit.
all the best
Dave

Dave,
How this variation of -10volts in the 6BC8 chage its gain???....and How I can get a better balance in the halves of the tubes???.....in the real life the tubes halves never will have the same spec.!!......Someone named a balance circuit mod in another post before in order to balance the halves of tube.

Thanks a lot,
Opacheco.

 

[vinyvamos]

Hi guys,

I know I am digging up a VERY old thread but for me this is exactly on topic for my query, and may add to the thread. I am currently in the planning of a Vari-Mu Compressor and was initially going with the VK1 approach using the 12AT7 but I have also been considering building the original Altec 436 design seeing as that's the "Original". But then I found this interesting thread and DaveP has a very good point regarding the output tube.

I have been looking over plate resistances because I am trying to find an O/P trafo which is cheaper than Sowter but not as cheap as Edcor (if you get what I mean ;-)). What I don't like about the Edcor's is they only spec 70Hz-18KHz. I have looked at Carnhill but they only do 9k6:600R (VTB2290) in a gapped design which is not high enough primary impedance for either a 12AT7 or a 6CG7. There is a 15K:600R (VTB2380) but it is ungapped. Would this work or will there be too much DC current in the primary? Another option would be to use the 9K6 trafo and run a pair of paralleled 6CG7 or 12AT7...

As for the side chain, I am still digesting your breakdowns of it in this thread, but I'm getting there .

 

[opacheco]

Hi Guys,
Sad git that I am, I worked out how the threshold control works and some implications which may interest some of you with curious minds, here we go.

If we start with Altec's published figures of 0dBm, +10dBm and +16dBm on the compression chart, we get the following voltages across a 600 ohm load, 0.775, 2.45 and 4.88 Volts.

Now thanks to CJ's "take apart" http://www.groupdiy.com/index.php?topic=35658.0 we know that the turns ratio on the output TX is 6.2:1, this means that our voltages are now 4.805, 15.19 and 30.26 on the primary.

These RMS voltages appear across both 270k resistors so we must divide by 2 and multiply by 1.414 to get the following peak voltages on the cathodes of the 6AL5; 3.40, 10.74 and 21.39 Volts.  These are the voltages available by turning the threshold control through its range.  By adjusting the threshold control, the 6AL5 only conducts when the peaks exceed the set voltage, it's a very elegant system.........

DaveP,

I am taken this thread again because I got a ALTEC 436C and I am analysing the P2 can be used in a RATIO fashion in this circuit!....The point is the Vu Meter calibration will be affected and will not representative of the GR

What do you think about?

Thanks again

Opacheco