Newbie with RCA basket cases seeks guidance

[Michael Jinks]

I have a pair of old RCA tube amplifiers which used to drive the PA system
in a department store. Their rear plates identify them as "Type SA752LC"
(no help there from Google), and say their output is 235 watts. (More tech
details further down.) I'd like to rip out their mic, phono, and preamp
sections and update/refurb what's left into a pair of nice hefty
monoblocks for home stereo use.

Factors working against me: (1.) I have only a vague idea what I'm doing
(I know how to hold a soldering iron, and I have "Inside Tube Amps," by
Dan Torres, a book mainly aimed at people who want to hack on their guitar
amp); (2.) I haven't heard either of these amps actually work, ever;
though one of them does power on and light up its tubes, the other has an
obviously melted-out power supply transformer and would probably need to
be largely rewired from scratch, after a good cleaning; and (3) I don't
know enough about circuit design to be able to tell whether the stuff I
have here is even worth saving, or if I'd be better off looking around for
some other derelict to use as my starting point.

On the plus side, one of the units has a fully intact circuit schematic
pasted to the inside of the bottom cover plate, and from the better part
of a day spent with that and the less-filthy of the two units,
I've been able to teach myself a bit about the layout. The rectifier is a
5R4GYB. There's a pair of 0C3 voltage regulator tubes wired between the
power supply and the plates of the output tubes, but I'm not sure what
role they play. (noise filter?)

The two mics and single phono input (amped by a 7025 and a 6AU6,
respectively) drive a 12AU7, where the master volume pot connects and acts
as the grid resistor. If I'm right, all of that could be pulled out.

The output stage appears to be a (good ol') 12AT7 driving a pair of 6146a
power tubes, and it's those which mostly get my doubts up. I've found
tube suppliers which carry them, but I haven't been able to find any
examples of modern tube rigs using those tubes, and I don't know enough
about the engineering of these circuits to know how to replace them with
something else (or whether I should).

So, it seemed like I should go in search of someone experienced, and ask
what's the gut reaction to a project like this? Not worth doing at all?
Possibly worth doing depending on factors I'm unaware of? Any tips for
good sources of information on how to plan a project like this?

Thanks,
--Michael

[maxhifi]

"Type SA752LC"
> (no help there from Google), and say their output is 235 watts. (More tech
> details further down.)

This is likely the power they consume from the wall outlet, and not their
power output.

I'd like to rip out their mic, phono, and preamp
> sections and update/refurb what's left into a pair of nice hefty
> monoblocks for home stereo use.

Good idea - why not bypass it, and disconnect power, instead of rip it out,
though? (I like conservation of this old stuff for some reason!)

> I've been able to teach myself a bit about the layout. The rectifier is a
> 5R4GYB. There's a pair of 0C3 voltage regulator tubes wired between the
> power supply and the plates of the output tubes, but I'm not sure what
> role they play. (noise filter?)

These tubes probably regulate the screen grid (G2) voltage of the output
tubes.

>
> The two mics and single phono input (amped by a 7025 and a 6AU6,
> respectively) drive a 12AU7, where the master volume pot connects and acts
> as the grid resistor. If I'm right, all of that could be pulled out.

I would hesitate to pull the 12AU7, take a look and see if there's any
negative feedback, and if so, where it goes... you might also want more gain
than a single 12AT7 can do.

>
> The output stage appears to be a (good ol') 12AT7 driving a pair of 6146a
> power tubes, and it's those which mostly get my doubts up. I've found
> tube suppliers which carry them, but I haven't been able to find any
> examples of modern tube rigs using those tubes, and I don't know enough
> about the engineering of these circuits to know how to replace them with
> something else (or whether I should).

You could easily re-wire, and re-bias the output stage to replace them with
a pair of 6550's. It'll look better too without the 'top caps'. If you want
to be cheap, check out some of the larger octal TV horizontal output tubes.

>
> So, it seemed like I should go in search of someone experienced, and ask
> what's the gut reaction to a project like this?

Old RCA stuff is cool - go ahead and do it, but the missing power
transformer may end up being a problem. I would want to get two new ones, so
they matched.

Not worth doing at all?
> Possibly worth doing depending on factors I'm unaware of? Any tips for
> good sources of information on how to plan a project like this?

Read as much as you can about tube amplifiers.
The best souce I can think of, is if you have a university local with a well
established techincal library.

Oh, you will want to replace all of the capacitors, unless you want to
replace them one at a time over the next year, as they all fail during the
amp's new lease on life.

[Michael Jinks]

On Mon, 04 Sep 2006 18:51:34 -0700, Bret Ludwig wrote:

> RCA theater amps are generally considered reliable but harsh sounding.
> They have their adherents and if you can get any money out of them I
> advise you to sell them for needed funds for test equipment and parts to
> someone who may wish to restore them. If they have sentimental value,
> set them aside for now.

No sentimental value beyond the fact that I've been hauling them around in
crates for years planning to do "something" with them "someday".

Is there actually a market for these? I'd assumed that in their present
state they're all but worthless.

> The "derelicts" today are worth too much to start on, as a rule. Get
> some sheet steel, bend up a chassis (any air conditioning place will do
> the bending if you lay it out) and buy some new parts. If you are a
> guitar player guitar amps are a good start. Regen shortwave radios are
> too, although you won't learn a whole lot about how to fix real
> (superheterodyne) radios from them, they are good beginner projects.

Not a guitar player, don't know too many HAM operators, just a weekender
audio freak looking to maybe salvage something from junk and learn a
little in the process. Seeing what's involved in reverse-engineering an
old circuit (and one that's possibly out of spec from who knows how many
degraded parts), I had thought about starting from scratch, but then it
seemed like a shame to throw out eight (well, seven) apparently good
transformers. If I could recoup a few bucks for them, that changes the
picture considerably.

> Dan Torres is not a good writer nor tech. I can highly recommend some
> tube guitar amp books, I especially like Jack Darr's book which has been
> reprinted finally. Tom Mitchell's is also halfway decent.

I definitely get that impression about Torres, certainly his writing
isn't that great, and his audience isn't me either, but I wasn't aware of
Jack Darr or Tom Mitchell. Thanks for the tip.

Cheers,
--Michael

[thomas]

Michael Jinks wrote:
> On Mon, 04 Sep 2006 18:51:34 -0700, Bret Ludwig wrote:
>
> > RCA theater amps are generally considered reliable but harsh sounding.
> > They have their adherents and if you can get any money out of them I
> > advise you to sell them for needed funds for test equipment and parts to
> > someone who may wish to restore them. If they have sentimental value,
> > set them aside for now.
>
> No sentimental value beyond the fact that I've been hauling them around in
> crates for years planning to do "something" with them "someday".
>
> Is there actually a market for these? I'd assumed that in their present
> state they're all but worthless.
>
> > The "derelicts" today are worth too much to start on, as a rule. Get
> > some sheet steel, bend up a chassis (any air conditioning place will do
> > the bending if you lay it out) and buy some new parts. If you are a
> > guitar player guitar amps are a good start. Regen shortwave radios are
> > too, although you won't learn a whole lot about how to fix real
> > (superheterodyne) radios from them, they are good beginner projects.
>
> Not a guitar player, don't know too many HAM operators, just a weekender
> audio freak looking to maybe salvage something from junk and learn a
> little in the process. Seeing what's involved in reverse-engineering an
> old circuit (and one that's possibly out of spec from who knows how many
> degraded parts), I had thought about starting from scratch, but then it
> seemed like a shame to throw out eight (well, seven) apparently good
> transformers. If I could recoup a few bucks for them, that changes the
> picture considerably.
>
> > Dan Torres is not a good writer nor tech. I can highly recommend some
> > tube guitar amp books, I especially like Jack Darr's book which has been
> > reprinted finally. Tom Mitchell's is also halfway decent.
>
> I definitely get that impression about Torres, certainly his writing
> isn't that great, and his audience isn't me either, but I wasn't aware of
> Jack Darr or Tom Mitchell. Thanks for the tip.

The Jack Darr book is downloadable in pdf format, if you dig around a
bit.

[Ned Carlson]

Bret Ludwig wrote:

> Since as I recall it's a Howard Sams title I'd be surprised to find
> that to be the case. Sams renews most all their copyrights religiously.
> I know Pete Millett doesn't have it.

I don't think all Jack Darr books are Sams titles, but that
one is.


--
Ned Carlson
SW side of Chicago, USA
www.tubezone.net

[Michael Jinks]

On Tue, 05 Sep 2006 01:53:01 +0000, maxhifi wrote:

>> I'd like to rip out their mic, phono, and preamp
>> sections and update/refurb what's left into a pair of nice hefty
>> monoblocks for home stereo use.
>
> Good idea - why not bypass it, and disconnect power, instead of rip it out,
> though? (I like conservation of this old stuff for some reason!)

Well for one thing there isn't much to preserve; lots of old caps (I know
they age), couple tube sockets, and lots of wiring that gets in my way
when I try to trace connections. So getting all that out of the case
seemed like it would make my life easier, and maybe also cut out places
for noise to get into the unit.

>> The two mics and single phono input (amped by a 7025 and a 6AU6,
>> respectively) drive a 12AU7, where the master volume pot connects and
>> acts as the grid resistor. If I'm right, all of that could be pulled
>> out.
>
> I would hesitate to pull the 12AU7, take a look and see if there's any
> negative feedback, and if so, where it goes... you might also want more
> gain than a single 12AT7 can do.

I don't know how to spot negative feedback but I think keeping both tubes
may be a good call... I do see that a plate (post 1) and cathode (post 3)
of the 12AU7 each feed through a .1uF cap and into the controls of the
12AT7. The other side of the AU7 looks like the input to receive the mics
and phono after they leave their individual stages, so maybe this is the
obvious place to take in line signal.

>> So, it seemed like I should go in search of someone experienced, and
>> ask what's the gut reaction to a project like this?
>
> Old RCA stuff is cool - go ahead and do it, but the missing power
> transformer may end up being a problem. I would want to get two new
> ones, so they matched.

Looks like one of the things I need to learn is how to spec a transformer.
I have part numbers for these but I'd be shocked if that did me any good,
and figured my only hope was to have the bad one rewound, possibly with
the good one as a reference.

From the mixed opinions posted so far though, I'm wondering if it's worth
that level of commitment. Rewinding ain't cheap.

> Read as much as you can about tube amplifiers. The best souce I can
> think of, is if you have a university local with a well established
> techincal library.

I think I can come up with something...

> Oh, you will want to replace all of the capacitors, unless you want to
> replace them one at a time over the next year, as they all fail during
> the amp's new lease on life.

Yup, knew that, though I know it'll lead to more questions and possibly
some trial and error... ("says here they're both 20uF but they sure don't
look anything alike...")

This is all very helpful, thanks.

[Michael Jinks]

On Mon, 04 Sep 2006 19:34:15 -0700, Bret Ludwig wrote:

>
> thomas wrote:
>
>> The Jack Darr book is downloadable in pdf format, if you dig around a
>> bit.
>
> Since as I recall it's a Howard Sams title I'd be surprised to find
> that to be the case. Sams renews most all their copyrights religiously.
> I know Pete Millett doesn't have it.

I put "jack darr pdf" (sans quotes) into google, and here's the first hit
I got: http://www.pacificrecone.com/JackDarrBook.html

It's not clear to me whether that's the whole book or not but it does seem
to have some useful information.

[Ned Carlson]

Michael Jinks wrote:


> So, it seemed like I should go in search of someone experienced, and ask
> what's the gut reaction to a project like this? Not worth doing at all?
> Possibly worth doing depending on factors I'm unaware of? Any tips for
> good sources of information on how to plan a project like this?

How big are the output transformers?

Some PA equipment has decent size transformers, some are
merely voice quality. A pair of 6146 might be 50 to 100W,
if the transformers look like they belong on a 25W amp, they're
probably useful but won't make 20-30 Hz at what the
amp is rated at.

Decent 6146's aren't horribly hard to find, though you
may have to go to ham radio suppliers to find them.

--
Ned Carlson
SW side of Chicago, USA
www.tubezone.net

[Michael Jinks]

On Tue, 05 Sep 2006 03:11:32 +0000, Ned Carlson wrote:

> Michael Jinks wrote:
>
>
>> So, it seemed like I should go in search of someone experienced, and ask
>> what's the gut reaction to a project like this? Not worth doing at all?
>> Possibly worth doing depending on factors I'm unaware of? Any tips for
>> good sources of information on how to plan a project like this?
>
> How big are the output transformers?

Physically, 4.5"x4.5"x3.75", or thereabouts. Noticeably bigger than the
ones on my Dynaco ST70, for whatever that's worth.

Electrically, I have no idea. The lines to the plates of the power tubes
are marked 580V on the schematic, if that tells us anything.

> Decent 6146's aren't horribly hard to find, though you
> may have to go to ham radio suppliers to find them.

Yeah, my local tube warehouse of choice didn't have them, but I didn't
have to look too long on the web to find at least once place that does.
Not knowing anything about their reputation I wanted to learn more before
I tried really hard to track anything down.

[Michael Jinks]

On Mon, 04 Sep 2006 18:51:34 -0700, Bret Ludwig wrote:

> RCA theater amps are generally considered reliable but harsh sounding.
> They have their adherents and if you can get any money out of them I
> advise you to sell them for needed funds for test equipment and parts
> to someone who may wish to restore them.

On that front, suppose I decide to unload them. My kneejerk plan was to
just take some snapshots, write up a description, and give eBay a try.
But that's coming from someone who doesn't deal in parts for old gear too
often. Is there a better approach (say, should I part them out?) or a
venue where they're more likely to be spotted by someone who'd want them?

[Patrick Turner]

Michael Jinks wrote:

> I have a pair of old RCA tube amplifiers which used to drive the PA system
> in a department store. Their rear plates identify them as "Type SA752LC"
> (no help there from Google), and say their output is 235 watts. (More tech
> details further down.) I'd like to rip out their mic, phono, and preamp
> sections and update/refurb what's left into a pair of nice hefty
> monoblocks for home stereo use.

Unless there are 8 x 6L6 in the output stage the power out won't be anything
like 235W.

Maybe 100W if there are 4 x octal output tubes.

>
>
> Factors working against me: (1.) I have only a vague idea what I'm doing
> (I know how to hold a soldering iron, and I have "Inside Tube Amps," by
> Dan Torres, a book mainly aimed at people who want to hack on their guitar
> amp);

Well maybe you will electrocute yourself with ease if you only have a vague
idea,
ie, no idea.

VALVE AMPS ARE LETHAL TO THE UNSKILLED PERSON!!!

> (2.) I haven't heard either of these amps actually work, ever;
> though one of them does power on and light up its tubes, the other has an
> obviously melted-out power supply transformer and would probably need to
> be largely rewired from scratch, after a good cleaning; and (3) I don't
> know enough about circuit design to be able to tell whether the stuff I
> have here is even worth saving, or if I'd be better off looking around for
> some other derelict to use as my starting point.

Expect to replace ALL the transformers, both power and output.
If one power T is shot then the other may be close to it.

I bet that the OPTs have 100V output lines for the multiple speakers in the
store from where the
amp came, ie, the cirrect load for the amps is way higher than the normal
speaker loads of
between 4 and 8 ohms.

Your schematic should give some indication of the load needed at the output to
match the tubes.
And if you ignore the loading and connect 6 ohm speakers anyway, then expect
clouds of smoke and very poor sound and not much power.

>
>
> On the plus side, one of the units has a fully intact circuit schematic
> pasted to the inside of the bottom cover plate, and from the better part
> of a day spent with that and the less-filthy of the two units,
> I've been able to teach myself a bit about the layout. The rectifier is a
> 5R4GYB. There's a pair of 0C3 voltage regulator tubes wired between the
> power supply and the plates of the output tubes, but I'm not sure what
> role they play. (noise filter?)
>
> The two mics and single phono input (amped by a 7025 and a 6AU6,
> respectively) drive a 12AU7, where the master volume pot connects and acts
> as the grid resistor. If I'm right, all of that could be pulled out.
>
> The output stage appears to be a (good ol') 12AT7 driving a pair of 6146a
> power tubes, and it's those which mostly get my doubts up.

The 6146A isn't a great choice for an output tube but was used to get 120W
in class B.

> I've found
> tube suppliers which carry them, but I haven't been able to find any
> examples of modern tube rigs using those tubes, and I don't know enough
> about the engineering of these circuits to know how to replace them with
> something else (or whether I should).

A long way up the learning curve is where you might expect to learn something.

But I think you have a pair of boat anchors requiring far too much work
to make them into anything nice.
I have had a few old Phillips PA amps given to me and they were all useless.


> So, it seemed like I should go in search of someone experienced, and ask
> what's the gut reaction to a project like this? Not worth doing at all?
> Possibly worth doing depending on factors I'm unaware of? Any tips for
> good sources of information on how to plan a project like this?

Start reading the website at

http://www.turneraudio.com.au

Don't stop reading until you understand the content.

Build a little one triode preamp and work up, but you will have to
get a decent tool kit and include a good oscilliscope, single trace will do to
start
with and up to 5MHz.

Restoring modding these old PA amps you have is diving into the deep end for
sure
without knowing much about swimming, except instead of water there are lots of
volts.

Patrick Turner.

>
>
> Thanks,
> --Michael

[[email protected]]

Hi RATs!

It seems you may have a couple of transformers worth a good try. Maybe
not enough for a complete stereo system, but, there are others
suspected of being available to a guy with hot soldering gear and a
couple of bucks.

Just clean and measure and be ready to shout 'Eureka!' if anything
releases the magic blue smoke. We get to witness instant, meaningless,
Death at any time. Keep yourself ready.

Reality is pretty dull. Someone had an idea for an amp. You got what
was left after cost reduction and fate had had their go. Whether you
can save anything is more about your imagination than any classical
circuit critique or analysis.

Some old transformers are very good. Nobody bothered with how to build
really ****ty ones for many decades ... what seemed like Pure Genius at
some annual meeting may or may not have survived the joy and enthusiasm
of subsequent meetings, of like formed groups.

If you try and get the best possible sound, you may succeed and join us
who have found there is some useful ideas in previous generations of
technodrivel. If not, I cannot presume to judge whether you will go on
to future glory or simply fall asleep at your solder station,
unfulfilled :)

We have never executed anything like full circuit evaluation nor
listening for large context performance evaluation.

It is possible you best guess for amp and speakers will ease your
slippage into the noise blocks / Maybe not, but, we are here, let us
hear what we may.

None of man's efforts have presumed to match God's Imagination. It is
OK if your little enterprises raise us as a group on the Great Chain of
Being. Even if none of the crap you solder together ever gives you any
reason to think there is any hope for any of us human audio nerds :)

A few moments of Grand Illusion will keep us all sailing on these dark
waters of Audio Joy.
_____

Go, Man, Go!

Love is all you get.

Love is all you get.

Happy Ears!
Al

[Iain Churches]

"Michael Jinks" > wrote in message
.. .
>I have a pair of old RCA tube amplifiers which used to drive the PA system
> in a department store. Their rear plates identify them as "Type SA752LC"
> (no help there from Google), and say their output is 235 watts. (More tech
> details further down.) I'd like to rip out their mic, phono, and preamp
> sections and update/refurb what's left into a pair of nice hefty
> monoblocks for home stereo use.
>
(snip)

Hi Michael. Sorry to be a pessimist, but this doesn't sound at all
like a project for someone new to thermionic audio.

Putting aside the inherent danger of high voltages for the unwary,
you really need to do a lot of reading and have some exprience of
tube amps before attempting such a project. You need to walk
before you can run:-))

Having said that, tube audio is a fascinating and rewarding hobby.
Two good books you should read are:

Morgan Jones. Valve Amplifiers
Bruce Rozenblit: The Beginner's guide to Tube Audio Design.

Start off with a simple project, say a triode preamp, and take
it from there. It may be that your RCA PA amps have
100V line output transformers which are not suitable for driving
domestic loudspeaker systems, so replacing these and both the
mains transaformers may not really be a viable proposition.

Best regards

--
Iain
www.kolumbus.fi/iain.churches

[Ned Carlson]

Michael Jinks wrote:
>>How big are the output transformers?
>
>
> Physically, 4.5"x4.5"x3.75", or thereabouts. Noticeably bigger than the
> ones on my Dynaco ST70, for whatever that's worth.

That sounds like something worth saving. Not a Marantz but not
crap, either. RCA generally didn't put big iron in anything unless
they expected the bandwidth to be used, so they're probably
pretty decent.


>>Decent 6146's aren't horribly hard to find, though you
>>may have to go to ham radio suppliers to find them.
>
>
> Yeah, my local tube warehouse of choice didn't have them, but I didn't
> have to look too long on the web to find at least once place that does.
> Not knowing anything about their reputation I wanted to learn more before
> I tried really hard to track anything down.

Good NOS 6146 are usually a fair sight cheaper than NOS 6550's,
since the demand is less.


--
Ned Carlson
SW side of Chicago, USA
www.tubezone.net

[Chris Hornbeck]

On 7 Sep 2006 19:35:05 -0700, "Bret Ludwig" >
wrote:

> You aren't much good then are you?


Chris Hornbeck
"Man is the measure of all things. Sigh.
Happy Ears!" -Al

[robert casey]

>
> I bet that the OPTs have 100V output lines for the multiple speakers in the
> store from where the
> amp came, ie, the cirrect load for the amps is way higher than the normal
> speaker loads of
> between 4 and 8 ohms.
>
> Your schematic should give some indication of the load needed at the output to
> match the tubes.
> And if you ignore the loading and connect 6 ohm speakers anyway, then expect
> clouds of smoke and very poor sound and not much power.
>

Connecting a too low impedance load to a tube amp that has output
transformers in the plate circuit of the output stage shouldn't really
damage it. But you won't get much audio output. SS amps (most use
emitter follower related output circuits, like complementary pairs)
however require protection circuits from shorted and overloaded outputs.

What tube amps don't like is no load at all. If there is no load on the
output, the primary's inductance can allow voltage excursions to go
wildly high, causing arcing over to degrade insulation in the
transformer and maybe the tube itself too. A proper load damps this.

[Ned Carlson]

Patrick Turner wrote:

> Unless there are 8 x 6L6 in the output stage the power out won't be anything
> like 235W.

235W is the input wattage.

> Well maybe you will electrocute yourself with ease if you only have a vague
> idea,
> ie, no idea.
>
> VALVE AMPS ARE LETHAL TO THE UNSKILLED PERSON!!!

So are transistor amps. Ever see how much juice the
power supply in some of those big honkin' transistor
amps can put out?
People can and have been killed by 48 volt telephone
battery supplies.
All kinds have connections to mains voltage, which in
Oz is even more deadly than here, being roughly twice the
voltage.

> Expect to replace ALL the transformers, both power and output.
> If one power T is shot then the other may be close to it.

Huh? Why would the outputs be shot? Plenty of ancient
output transformers work just fine. If he'd dug up
a pair of Marantz 5's or Leaks, would you suggest he
replace the outputs?

> I bet that the OPTs have 100V output lines for the multiple speakers in the
> store from where the
> amp came,

The US does not have 100 volt constant-voltage speaker lines,
they're 25 or 70 volts.

ie, the cirrect load for the amps is way higher than the normal
> speaker loads of
> between 4 and 8 ohms.
>
> Your schematic should give some indication of the load needed at the output to
> match the tubes.
> And if you ignore the loading and connect 6 ohm speakers anyway, then expect
> clouds of smoke and very poor sound and not much power.

Most 8 ohm speakers have impedance dips to 6 ohms or
even below (depends on the cabinet), some have "real"
nominal impedances in the 6 ohm range. Why would
putting a 6 ohm speaker on an 8 ohm tap do anything but
possibly cause the amp to clip at a slightly lower output?
Depending on the plate load RCA chose, one might
actually get MORE power with a 6 ohm load on an 8 ohm
tap!

> I have had a few old Phillips PA amps given to me and they were all useless.

What's THAT got to do with an RCA amp?

Lastly, I have to ask one more time: are ALL you Aussies
out of your gourds? I mean, the ones that post on MTUT
(misc.transport.urban-transport) are mostly nuts, too,
this is not just a RAT thing. I mean, they're practically
talking armed rebellion over uniforms worn by Sydney
CityRail employees. Could you guys just calm down, back
off the lager, maybe do some yoga? Pancho Villa practically
looks like a modern Buddha in comparison.

If I ever go on a real long-distance vacation, I'm going
to New Zealand, Hong Kong or Singapore. I might scratch
Kiwiland off the list if I think there's too
Tolkein-based tourists there. I mean, if I just want to see
someplace that *looks* like Middle Earth, I can go to
Oaxaca, which is a lot cheaper and has better mezcal.

--
Ned Carlson
SW side of Chicago, USA
www.tubezone.net