Collins 212Y - Increase filter cap value?

Hey guys,

Working on a Collins 212Y remote amplifier. Recapped, refreshed, and sounding good except for a faint 120Hz hum.

All four filter caps have been replaced with 22uf. When I put another 22uf in parallel of the first two caps (C108 and C109) it hushes up nicely.

It uses a 7Y4 rectifier (I think that's equal to a 6X5), but it's only running 182vdc at its output and 174vdc at the 2nd filter cap. These things had to run off batteries in the field.

Is there any harm in swapping the two 22uf out for 47uf?

(BTW. Nothing changes when increasing the values of the third and fourth 22uf caps. I'll leave em as is.)

Schematic.

I just observed, from cold startup, with 47uF the power indicator lamp flickering off and on a few times. Perhaps 47uF is too much for the inrush?

Two more tests and questions...

1. Dropped first filter cap back to 22uF. Increased second filter cap (after choke) to 47uf, then 69uf (+22uF), then 91uF (+22uF x2). Noise dropped with each added cap. Cold startup did not cause the pilot light to turn off and on.

2. Same setup as above, but increased first cap to 33uF. Cold startup also did not cause the pilot light to turn off and on. Noise level dropped a bit more.

Questions...

1. Is it okay to raise the second filter cap to somewhere between 68uF and 100uF? Any ill effects?

2. Is it okay to raise the first filter cap to 33uF?

That's all. I'm off to bed!

32 uf max for first cap,

after that, you can use 10,000 uf, the choke will keep 7Y4 from arcing.

Ive done a few jobs similar to that ,  there is a limit to what you can use directly after a tube rectifier (that info you will find in the spec sheet), the other thing is changing the value of the first cap will also change the basic HT voltage , the bigger the first cap the more ripple gets turned to DC . The old caps will probably not be to spec ,so I'd be expecting a bit of a jump in the HT when you go to 32uf.  I've generally found larger values of cap later in the filter  are a good thing , but as pointed out by Abbey one time  ,you are changing a time constant in the supply rail ,so this might have an effect on how transients or short term overloads are handled. and that can be part of the signature sound of these pieces.
All that being said  I generally like to bring hum in the HT line down to a point where it no longer contributes as a source of noise in the output of the equipment.

The problem here is power transformer proximity. 

Yes the power transformer is very close, but I’m actually surprised at how quiet this preamp is. Everything is smooshed together and only this little bit of 120Hz remains.

I can barely get my preamps this quiet with a toroidal on the opposite end of a 19” chassis.

Gain is so high with proximity so close.  In my experience this problem is common across all makes of portable broadcast preamps. 

6BQ6 voltage amps? Or do I need clean glasses?

PRR said:

6BQ6 voltage amps? Or do I need clean glasses?

Click to expand...

6AQ6 first and second, 6AK6 third. 

CLC filtering to output stage, adds an RC for the second stage and another RC for the first.  20mfd on each. 

Gain control between 1st and 2nd.

May have tube shields on all 3 signal tubes.  Some did, some didn't have any. 

Row of can caps between power and audio sections, some shielding there. 

Whole thing is on a chassis 7" x 6.25". 

And 85 dB of gain. This little booger packs a punch.

I'll post a sound sample and photos later this evening. I'm serious, once this filtering situation is resolved, it is way quieter than I ever expected it could be.

Thats a lot of iron packed into a small space ,
just arranging all those cans for minimal interaction must have been a bit of a juggling act ,
maybe they got lucky and some of the residual hums cancelled.

It looks like approximately 310uF on the second filter is what it takes to knock out the noise. Somebody convince me why I shouldn't do it.

hum disappear with gain all the way down? 

Okay, what the hell, I just looked at your 1955 schematic.  I've never seen that version in person.  6BQ6 is a typo, 6AQ6 in the parts list.  Totally different drawing layout, not sure what else is different if anything.  actually, that's a really horrible version of the drawing compared to the original.  I had looked up my own copy which is the 1948 version.    Do you have the hum balance pot?  Otherwise the same circuit, different input transformer.

EmRR said:

hum disappear with gain all the way down? 

Okay, what the hell, I just looked at your 1955 schematic.  I've never seen that version in person.  6BQ6 is a typo, 6AQ6 in the parts list.  Totally different drawing layout, not sure what else is different if anything.  actually, that's a really horrible version of the drawing compared to the original.  I had looked up my own copy which is the 1948 version.    Do you have the hum balance pot?  Otherwise the same circuit, different input transformer.

Click to expand...

1. Yes, the 120Hz hum was a constant level that didn't change much with gain. Noise floor would come up over it with gain increase.

2. Do you have the 1948 version handy? The version I'm working on has a different input TX from the schematic and a hum balance pot.

https://groupdiy.com/index.php?topic=60313.msg764969#msg764969

Thanks, Doug. That is indeed the correct schematic, and confirms some of the modifications I did already.

It was strapped for 50 ohms, and I found when strapped for 250 the high frequencies rolled off. So, I removed the .03uF cap and all was well.

Seems like the instructions are also to change R106 from 68k to 100k....hmmm.

This is helpful. Thanks again.

Sure thing.

AFAIK Collins shipped everything connected for 50 ohms in the late 40’s.  There were usually some circuit changes to smooth response for source Z connection.