Non-CT 660 sidechain input TX?

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Dmichel123

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I've got a transformer I am thinking of trying in my Fairchild-ish limiter build. It does not have a CT on the secondary. Will it work?
 
You can approximate a CT connection by adding a pair of resistors from each phase of a transformer winding and connecting them together 'in the middle' to form a 'psuedo' CT. 

In the case of a fc style side chain signal amp, the input traffo secondary carries a negative dc bias voltage, used to 'partial' setup the operating conditions of the gain-reduction tubes  ....  not much current

I would use reasonably high values to 'approximate the CT' something like 50K to 100K.

On the other hand, for an elaborate build like a fc type, it could be worth while investing in a more suitable traffo  ...  having 2 windings with a 'join' CT sometimes indicates that a good balance is achieved with respect to the CT, which can be important when using in 'balanced' or 'differential' circuits (like fc)

Good luck with it
 
Oops - you're right - I corrected that above.

Same principle though ... psuedo CT with a pair of resistors  .. and CT to ground.
 
alexc said:
Oops - you're right - I corrected that above.

Same principle though ... psuedo CT with a pair of resistors  .. and CT to ground.

The sidechain input TX secondary goes straight to a dual pot,  which goes to ground. So, it basically is already setup as a psuedo-CT. Right?
 
Yes that's true, generally speaking - but personally, I would still do the 2 resistor thing.

It's easy enough to try a few scenarios  ...  I did just that to find my 'best' loading for the grids for  my transformer choice for side chain input , utc a-25
 
alexc said:
Yes that's true, generally speaking - but personally, I would still do the 2 resistor thing.

It's easy enough to try a few scenarios  ...  I did just that to find my 'best' loading for the grids for  my transformer choice for side chain input , utc a-25

Gotcha. I was considering using a 50k linear pot in place of the two 24k "taper-steering" resistors as a "balance adjustment" for the sidechain amp.
 
Not quite sure exactly how you mean, but one could add a small trim in series with one the psuedo CT arms  to offset it a little, for balance reasons.

I'm not sure how useful that might be - there can be a little unbalance there  for sure ....  the signal phases probably won't exactly matched for voltage ...  and gain of the ax7 for sure can be a little different  ..  and finally, the 'bal attenuatator' can be different again, depending on the type (potentiometers or switched resistors etc).

I like to use those inexpensive 2deck 24pos rotary switches from eby in this role .. you can get them already soldered up with good resistors like Dale for log or lin in some common values .. my own ones were a little less 'well assembled and the law was a little 'lumpy' too.

Well worth the investment in a solid unit  ...  otherwise dual potentiometers are mostly fine.

The upshot I think is that the balance is going the net of all that ...  perhaps you can trim that up a bit ..  certainly if you use dual-deck-potentiometers  ...  but it will vary for different attenuation levels.

And, after all that, the sidechain signal is mashed about a lot anyway ...  with the dc bias and so on ...

Still, if I was going for precision as best as possible, I would consider it.

Good luck with it  ...  it's almost time for me to have another go at it  :)

I'm still exploring mine [adaption of the fc660] , thru 'measurement' as well as in normal usage. I still can't get over how 'clean' [low distortion] the signal amp is, with the limiting action turned off.

Super low THD considering the strong levels flowing, in my collection of 4x frame grid pentodes in triode mode per phase,  - near idle, its nearly a pair of 6L6s worth ...  I think mine was 45mA per phase at the signal amp output traffo primary.
 
Yea, I built up the signal amp first.... Used a HUGE input TX from an RCA transmitter and OT from an HP oscillator. Big, clean sound for sure!!
 
My own choice of signal amp output traffo was a very affordable compromise  .. an Edcor CXPP 8K/600 10Wrms model.

In no way is it 'ideal' but it really does perform very well - my many GR tubes drops the output impedance of the signal amp considerably ..  so the low primary impedance is not as bad as one might expect.

I'm hitting it pretty hard at idle  ... in terms of current ...  I think I went for  B+275V  regulated there ....  I did work thru a lot of bias conditions for the nest of GR tubes  ....  I just found it was best performing really spanking the thing.

I think my next one should have an 'idle' switch ..  to reduce the current when not in use  ;)

I too have found a huge and impressive signal output traffo for the next one  .. an old 'broadcast radio.

Keep on truckin ...
 

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