Push-Pull Output Stage HELP! (Sta-Level)

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Here are the two schematics I found:
Schematic A: http://sujan.hallikainen.org/~harold/BroadcastHistory/uploads/GatesStaLevel.pdf
Schematic B: http://www.waltzingbear.com/Schematics/Gates/Sta_Level.htm

I posted this on tape op, and a couple of people replied but haven't
heard from them in a while. Here's the link:
http://messageboard.tapeop.com/viewtopic.php?printertopic=1&t=49274&start=0&postdays=0&postorder=asc&vote=viewresult&sid=54e8e8786676f5f79aa0f989868c990a

I decided to build my own Sta-Level with high quality parts. I almost
have the amplifier ready to go, it's just a matter of doing the
faceplate and putting in R41 and R38. I did a search and found some
talk on this group about the Sta-Level back in the late 1990's and
early 2000's. However I didn't find any specific information
regarding pot r41 in schematic B. I'm still stomped on how to
determine the power rating of the pot that should be used for the
ouput stage. Any information regarding this pot and/or pot R38 would
be most appreciated!
I'm running out of resources and don't have access to a unit to
measure the voltage/current going through those pots.

This group is awesome, lets keep it up!

Thanks for you time.
Best,

Ivan B.

[Scott Dorsey]

> wrote:
>I decided to build my own Sta-Level with high quality parts. I almost
>have the amplifier ready to go, it's just a matter of doing the
>faceplate and putting in R41 and R38. I did a search and found some
>talk on this group about the Sta-Level back in the late 1990's and
>early 2000's. However I didn't find any specific information
>regarding pot r41 in schematic B. I'm still stomped on how to
>determine the power rating of the pot that should be used for the
>ouput stage. Any information regarding this pot and/or pot R38 would
>be most appreciated!

It's a bias pot on the grid circuit, isn't it? I don't have web access
so I can't read the schematic but I recall it just being grid bias.
Use a quarter-watt linear pot. An old Type J from the junkbox is fine.
It has no current to speak of.

If it's in the cathode circuit it's apt to have some current, but if
you know the maximum current the output tubes allow from the RCA tube
handbook, you can just assume that and have plenty of safety margin.

>I'm running out of resources and don't have access to a unit to
>measure the voltage/current going through those pots.

When in doubt, use a Type J. If it gets hot, use something bigger.

I really wonder why the hell anyone would ever want to build a Sta-Level,
though. It was bad enough having to use them when they were new.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Steve King]

"Scott Dorsey" > wrote in message
...
> > wrote:
>>I decided to build my own Sta-Level with high quality parts. I almost
>>have the amplifier ready to go, it's just a matter of doing the
>>faceplate and putting in R41 and R38.
SNIP
> I really wonder why the hell anyone would ever want to build a Sta-Level,
> though. It was bad enough having to use them when they were new.
> --scott

I wonder, too. Of all the amps from that period, how did you decide to
clone that one? I think every radio station I worked at had one. At two
stations, where I had the power to do it, I pulled them out and trained the
staff to watch levels. The stations sounded better. Still had a limiter in
the chain to keep legal.

Steve King

[Mark]

On Dec 13, 11:37 am, wrote:
> Here are the two schematics I found:
> Schematic A: http://sujan.hallikainen.org/~harold/BroadcastHistory/uploads/GatesSt...
> Schematic B: http://www.waltzingbear.com/Schematics/Gates/Sta_Level.htm
>
> I posted this on tape op, and a couple of people replied but haven't
> heard from them in a while. Here's the link:http://messageboard.tapeop.com/viewtopic.php?printertopic=1&t=49274&s...
>
> I decided to build my own Sta-Level with high quality parts. I almost
> have the amplifier ready to go, it's just a matter of doing the
> faceplate and putting in R41 and R38. I did a search and found some
> talk on this group about the Sta-Level back in the late 1990's and
> early 2000's. However I didn't find any specific information
> regarding pot r41 in schematic B. I'm still stomped on how to
> determine the power rating of the pot that should be used for the
> ouput stage. Any information regarding this pot and/or pot R38 would
> be most appreciated!
> I'm running out of resources and don't have access to a unit to
> measure the voltage/current going through those pots.
>
> This group is awesome, lets keep it up!
>
> Thanks for you time.
> Best,
>
> Ivan B.

wow what an oldie...

R41 is in series with the cathodes of the push pull 6V6's. And also
in series with R23 which is 150 Ohms 10 Watts. To be safe you should
use a pot rated for 10 Watts but 5 Watts might do.
Do you have the input and output transformers and the power
transformer and filter choke?

Mark

[Richard Crowley]

"Scott Dorsey" wrote ...
> > wrote:
>>I decided to build my own Sta-Level with high quality parts. I almost
>>have the amplifier ready to go, it's just a matter of doing the
>>faceplate and putting in R41 and R38. I did a search and found some
>>talk on this group about the Sta-Level back in the late 1990's and
>>early 2000's. However I didn't find any specific information
>>regarding pot r41 in schematic B. I'm still stomped on how to
>>determine the power rating of the pot that should be used for the
>>ouput stage. Any information regarding this pot and/or pot R38 would
>>be most appreciated!
>
> It's a bias pot on the grid circuit, isn't it? I don't have web access
> so I can't read the schematic but I recall it just being grid bias.
> Use a quarter-watt linear pot. An old Type J from the junkbox is fine.
> It has no current to speak of.
>
> If it's in the cathode circuit it's apt to have some current, but if
> you know the maximum current the output tubes allow from the RCA tube
> handbook, you can just assume that and have plenty of safety margin.

It is a 100-ohm pot with each end going to the cathode of
the output 6V6es. Apparently used to tweak balance in the
differential pair. The wiper goes to R23 which is a 150-ohm
resistor rated at 10W. It would seem safe to use a 10-watt
rated pot for R41. Do they still make/sell such things?

[Mark]

On Dec 13, 12:08 pm, Mark > wrote:
> On Dec 13, 11:37 am, wrote:
>
>
>
>
>
> > Here are the two schematics I found:
> > Schematic A: http://sujan.hallikainen.org/~harold/BroadcastHistory/uploads/GatesSt...
> > Schematic B: http://www.waltzingbear.com/Schematics/Gates/Sta_Level.htm
>
> > I posted this on tape op, and a couple of people replied but haven't
> > heard from them in a while. Here's the link:http://messageboard.tapeop.com/viewtopic.php?printertopic=1&t=49274&s...
>
> > I decided to build my own Sta-Level with high quality parts. I almost
> > have the amplifier ready to go, it's just a matter of doing the
> > faceplate and putting in R41 and R38. I did a search and found some
> > talk on this group about the Sta-Level back in the late 1990's and
> > early 2000's. However I didn't find any specific information
> > regarding pot r41 in schematic B. I'm still stomped on how to
> > determine the power rating of the pot that should be used for the
> > ouput stage. Any information regarding this pot and/or pot R38 would
> > be most appreciated!
> > I'm running out of resources and don't have access to a unit to
> > measure the voltage/current going through those pots.
>
> > This group is awesome, lets keep it up!
>
> > Thanks for you time.
> > Best,
>
> > Ivan B.
>
> wow what an oldie...
>
> R41 is in series with the cathodes of the push pull 6V6's. And also
> in series with R23 which is 150 Ohms 10 Watts. To be safe you should
> use a pot rated for 10 Watts but 5 Watts might do.
> Do you have the input and output transformers and the power
> transformer and filter choke?
>
> Mark- Hide quoted text -
>
> - Show quoted text -

actually I shot from the hip and sometimes that can be wrong..

the schematic shows there is 12 volts DC on the two cathodes... from
that info it is easy to calculate that the pot dissipates just over
0.1 Watt. A 1/2 or 1 Watt pot will do fine.

[Scott Dorsey]

Richard Crowley > wrote:
>> If it's in the cathode circuit it's apt to have some current, but if
>> you know the maximum current the output tubes allow from the RCA tube
>> handbook, you can just assume that and have plenty of safety margin.
>
>It is a 100-ohm pot with each end going to the cathode of
>the output 6V6es. Apparently used to tweak balance in the
>differential pair. The wiper goes to R23 which is a 150-ohm
>resistor rated at 10W. It would seem safe to use a 10-watt
>rated pot for R41. Do they still make/sell such things?

Sure, but you're going to need a big wirewound.

If you don't mind having a slider instead of an adjustable knob, it's
a lot easier... Vitrohm no longer makes the slider types, but the
Ohmite 280 series is still around and probably available from Allied.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Paul[_11_]]

On Thu, 13 Dec 2007 09:54:13 -0800 (PST), Mark >
wrote:
----stuff deleted------
>actually I shot from the hip and sometimes that can be wrong..
>
>the schematic shows there is 12 volts DC on the two cathodes... from
>that info it is easy to calculate that the pot dissipates just over
>0.1 Watt. A 1/2 or 1 Watt pot will do fine.

The supply is shown to draw 80 mA. That makes 40 mA (max) for each
half of the push pull pair. Do the math... I squared R, 40 mA, 100
ohms, you get 0.16 watts. I agree, 1/2 to 1 watt is fine. The other
pot draws even less current and voltage, so 1/2 to 1w should be OK
there, too.
If you get gassy tubes, the current could get quite high, and the
resistors will cook. The line fuse should be specified to make sure
that heat damage from bad tubes won't cook the circuit. I wonder if
that's the reason they specified R23 to be 10 watts. Under normal
circumstances it shouln'td need to be that powerful.
The circuit requires resistors from the ends of R41 to ground. If
there is a poor contact on R41's wiper, all hell will break loose. My
guess is some value like 1000 ohms 1W from each end of R41 to ground.
This circuit has a feedback circuit that should have been from the
secondary side of the transformer, but it seems that they wanted the
secondary isolated from ground.
The compression circuitry looks a bit crude..... it is taking
advantage of what appears to be a non-linear remote-cutoff triode
pair, and using push-pull to reduce some of the distortion. It seems
like overkill when today you can use a FET as a voltage controlled
resistor in a feedback loop, and put the whole thing into a tiny box,
with a fraction of the weight, power, and distortion. Plus, todays
components will give a circuit from DC to at least a Mhz, and with
differential inputs and outputs. The biggest plus of the modern parts,
is you could breadboard the whole thing in a few hours, and have it
fully tested and designed!

-Paul

[Scott Dorsey]

Paul > wrote:
> The compression circuitry looks a bit crude..... it is taking
>advantage of what appears to be a non-linear remote-cutoff triode
>pair, and using push-pull to reduce some of the distortion. It seems
>like overkill when today you can use a FET as a voltage controlled
>resistor in a feedback loop, and put the whole thing into a tiny box,
>with a fraction of the weight, power, and distortion.

Or for that matter, you could use a remote-cutoff pentode. Or (cheesy
trick) a pentagrid converter with only one grid used.

The trick to getting low distortion out of these things is to keep
the signal level very low, which of course means noise becomes a
problem.

>Plus, todays
>components will give a circuit from DC to at least a Mhz, and with
>differential inputs and outputs. The biggest plus of the modern parts,
>is you could breadboard the whole thing in a few hours, and have it
>fully tested and designed!

I bet I could do that with pentagrid converters too, if you handed me
a prebuilt power supply and chassis.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Flaco]

Hey guys,

Thanks for all the helpfull info. I'll be able to finish this thing
off pretty soon and put it to good use. It seems like some of you
don't think much of the compressor. I think it's just a tool that I'd
like to have around when the occasion may call for it. My first
encounter with one was not an original, but one that my friend built
in the mid 50's with HA Utc transformers, stepped daven attenuators,
and a bunch of things he did to improve it.
Anyways, I'm trying to get into building my own stuff and it seemed
like a relatively simple circuit to do. So the one I'm building uses
LS Utc, Davens, and all that stuff. Maybe I can post some pictures of
my friend's 6386 and mine after it's done.
The way I look at it is somewhat like photography. Sometimes you want
a super clear 30mm hasselblad lens, and sometimes you might need a
30mm plastic lens. I think it depends on the image and what you're
trying to say. Nevertheless, I'm excited to see what it will sound
like.
I think it's fun to build old designs. That said I also plan to build
a Collins 26-U, and a Federal AM-864/U. I'll probably end up asking
more questions!

I think this is the BEST resource there is around when it comes to pro-
audio. This NG rocks!

Thanks for your time and patience : )
Best,

Ivan


On Dec 13, 7:45 pm, (Scott Dorsey) wrote:
> Paul > wrote:
> > The compression circuitry looks a bit crude..... it is taking
> >advantage of what appears to be a non-linear remote-cutoff triode
> >pair, and using push-pull to reduce some of the distortion. It seems
> >like overkill when today you can use a FET as a voltage controlled
> >resistor in a feedback loop, and put the whole thing into a tiny box,
> >with a fraction of the weight, power, and distortion.
>
> Or for that matter, you could use a remote-cutoff pentode. Or (cheesy
> trick) a pentagrid converter with only one grid used.
>
> The trick to getting low distortion out of these things is to keep
> the signal level very low, which of course means noise becomes a
> problem.
>
> >Plus, todays
> >components will give a circuit from DC to at least a Mhz, and with
> >differential inputs and outputs. The biggest plus of the modern parts,
> >is you could breadboard the whole thing in a few hours, and have it
> >fully tested and designed!
>
> I bet I could do that with pentagrid converters too, if you handed me
> a prebuilt power supply and chassis.
> --scott
> --
> "C'est un Nagra. C'est suisse, et tres, tres precis."