Another Altec 436c/SR124 p2p :p

[weiss]

I doubt they used a film cap of 470uF.  It would be too big to fit in the box.

what did they use? is it a typo? i can't find any 470u orange drop or film cap

 

[Rob Flinn]

what did they use? is it a typo? i can't find any 470u orange drop or film cap

That's because if it is 470uf it will be an elctrolytic.  But I suspect you probably mean a 470nF in which case there are lots of options.    Can you be a bit more specific about which cap your talking about ?  i.e circuit position.

 

[weiss]

That's because if it is 470uf it will be an elctrolytic.  But I suspect you probably mean a 470nF in which case there are lots of options.    Can you be a bit more specific about which cap your talking about ?  i.e circuit position.

looks bipolar to me and uF, not nf

 

[Rob Flinn]

looks bipolar to me and uF, not nf

Not really sure why that cap is there.  Looks unecessary to me and is not on any of the original diagrams.  It would also mean that the top half of V2 wouldn't be working because there would be no current flow through it & therefore no common mode rejection of the control signal.  So you would hear big thumps on the output when it compressed.  You would probably also blow the output transformer because it isn't gapped & you wouldn't have equal & opposite current flowing through it to cancel the effects of d.c runnong through it.  Maybe you should check with the person who drew it.

 

[weiss]

Not really sure why that cap is there.  Looks unecessary to me and is not on any of the original diagrams.  It would also mean that the top half of V2 wouldn't be working because there would be no current flow through it & therefore no common mode rejection of the control signal.  So you would hear big thumps on the output when it compressed.  You would probably also blow the output transformer because it isn't gapped & you wouldn't have equal & opposite current flowing through it to cancel the effects of d.c runnong through it.  Maybe you should check with the person who drew it.

oh okay. thanks for pointing that out!
here is the schematic i wanted to follow. anything else that is wrong?

 

[Rob Flinn]

Maybe you should check it before you follow it any further, in case it has other mistakes too.  Or just use a different diagram .....

 

[weiss]

Maybe you should check it before you follow it any further, in case it has other mistakes too.  Or just use a different diagram .....

I'm sorry you're right. Reading the thread again answered my question.

Layout is critical to avoid oscillation and or motorboating.  It would help to have a 470uF cap as your final to avoid this

so i guess lowering the final cap to a higher value like here would work.. ?
EDIT: i meant "increasing"

 

[Rob Flinn]

Which is the final cap, and what does lowering it to a higher value mean ?

 

[weiss]

Which is the final cap, and what does lowering it to a higher value mean ?

sorry, i meant "increasing the value"
the 10u of the psu (marked it in blue)

 

[Rob Flinn]

Not sure, but probably won't make a big difference.  There's not a great deal of current drawn in this circuit.  I have built a few of these to the original circut and never had any motorboating issues.  I never used any tag boards, just did the whole thing point to point except the PSU.  The issues I had were more about getting the thing balanced to reduce thumping when it compresses. 

But it probably wouldn't cost a great deal to make all the PSU caps 22uF & it won't do any harm.  I used a mains transformer with a 250v secondary with a full wave bridge rectifier rather than the voltage doubler of the original circuit.    There are lots of ways you can do the PSU, I just adapted it for what parts I had lying around to keep the cost down.

 

[weiss]

Not sure, but probably won't make a big difference.  There's not a great deal of current drawn in this circuit.  I have built a few of these to the original circut and never had any motorboating issues.  I never used any tag boards, just did the whole thing point to point except the PSU.  The issues I had were more about getting the thing balanced to reduce thumping when it compresses. 

But it probably would cost a great deal to make all the PSU caps 22uF & it won't do any harm.  I used a mains transformer with a 250v secondary with a full wave bridge rectifier rather than the voltage doubler of the original circuit.    There are lots of ways you can do the PSU, I just adapted it for what aprts I had lying around to keep the cost down.

Alright yes, i understand. I guess I'm just gonna build it that way, stick with the 120V + 6V secondaries and see if it works. The thing is where to find one..

 

[weiss]

6BC8 cathode current (what the GR meter reads) will be 5mA to 10mA (from tube specs shifted to lower voltage; and from the ~~33K in series with the tube). R8 diverts some of this around a lower current meter, and in the ratio of R8 to the meter's internal resistance. Since all meters today are loose-spec crap, you do NOT know R8's proper value until you have meter in hand. and the proper value will be much lower than most standard trim-pots. So don't be buying any 1% resistor here. Get a 10-pack of 100 Ohm 20% (OK, 5%) resistors, parallel the bunch (10 ohms), then with tube and meter and all running and warmed-up, snip-out resistors one by one (10r, 11r, 12.5r, 14r,...) until the meter comes on-scale and then near a top-mark. This can be very late in the build. At smoke-test and initial sound test, just throw 33r in there, wire any low-volt meter (needle is better but $3 DMM will do) across it, to see if tube current dips on large inputs (and if sound punks-out before tube current approaches 1/10th of idle current).

i am planning on using a 100uA meter by simpson, i guess that would work the same way?