New Project RCA sa-101-b PA amp

[casssax]

I picked up a 10W single ended RCA sa-101-b PA amp on ebay recently (should arrive tomorrow).

It looks like it was used as an auctioneers amp or something similar.

It's in a wooden box with an 8" QUAM speaker and it has a 'silver bullet' looking microphone with it (Silvertone?).

I've looked online but was unable to find any information about the amp or the schematic.

Is anyone familiar with the amp or know where I could find a schematic?

Maybe if I'm lucky it will have something printed on the bottom or inside it.

Should be fun to turn it into a little combo guitar amp.

[PRR]

It has to be a basic Champ except with a non-6V6 output bottle. Shouldn't take long to work it out.

[Paul1453]

10W SE and it's not a 6L6.  Maybe its a 6GW8 or 6BM8 output tube?

Bogen might have made something similar enough to give you a rough schematic to start from.   

[DummyLoad]

looks llike a 7868 power tube.

agree champ, but with bypass caps that. an be switched in or out on all three stages.

--pete

[casssax]

It is a 7868 power tube, and the preamp tube is a 6AW8A (triode/pentode). I looks like the mic input uses the triode section and then goes to the pentode side. The Aux just goes straight to the pentode.

I did my best to draw a schematic last night. I'll post that and some pics of the insides soon.

It has a very strange rectifier circuit (two diodes but it doesn't look like it is full wave) and there doesn't seem to be any center tap on the high voltage section of the PT.

It does work though and the speaker doesn't sound bad when I plug a mp3 player into the aux input.

One question I have is what is the deal with having  '0 ohm' on the output (is it like a line out?) and from the pic above, is the speaker hooked up wrong with the positive hooked to the 8 ohm and the negative hooked up to the 0 ohm? shouldn't the negative be hooked up to the ground?

[shooter]

what is the deal with having  '0 ohm' on the output

I'll bet if you use your ohm meter, black to ground, red to 0 ohms tap, meter set for resistance, you'll find the meter agrees with that tap - 0 ohms     

Note, didn't look at the schematic, but I've seen *grounds* labeled zero ohms more than a few times.

[DummyLoad]

It is a 7868 power tube, and the preamp tube is a 6AW8A (triode/pentode). I looks like the mic input uses the triode section and then goes to the pentode side. The Aux just goes straight to the pentode.
I did my best to draw a schematic last night. I'll post that and some pics of the insides soon.

if you want a more "traditional" circuit, then rewire 6AW8 for 12A_7 socket and build tweed-like champ/princeton ckt.

if it were mine, i'd try and make the pentode/triode work as i favor pentodes for overdrive + that's a starting point that won't require much modification.

It has a very strange rectifier circuit (two diodes but it doesn't look like it is full wave) and there doesn't seem to be any center tap on the high voltage section of the PT.

it's probably a delon full wave voltage doubler - see bogen CHB100 schematic for clarification.

http://el34world.com/charts/Schematics/files/bogen/bogen_chb100.pdf

One question I have is what is the deal with having  '0 ohm' on the output (is it like a line out?) and from the pic above, is the speaker hooked up wrong with the positive hooked to the 8 ohm and the negative hooked up to the 0 ohm? shouldn't the negative be hooked up to the ground?

as already stated "0" ohm terminal is ground or "common" wire.

[PRR]

There's zero and there's ground. In guitar amps the "0" tap would be grounded. In long-wire work (schools and factories), sometimes there is reason to not ground the zero lead. Leave it floating; or when using the "25V" and "0" taps, ground the "12.5" tap for "balanced" operation.

For your use, strap "0" to "G" and take your 4, 8 or 32 ohm tap as desired. (There isn't a 16 ohm tap, but an SE amp can be mis-loaded 16 on the 32 tap with mild increase of distortion and mild reduction of power, no smoke.)

[casssax]

That delon full wave voltage doubler looks very much like what I'm seeing.
Here is the schematic I drew up.


I was thinking of re-wiring it to use a 12AX7 but I might try leaving it the way it is and just install a 1/4" jack where the mic input is now.
I could remove the aux input and add a single knob tone control in it's place or just leave it.

There doesn't seem to be any point where anything is grounded to the chassis. Everything seems to be grounded to one of the tabs on the capacitor can.   

[HotBluePlates]

There doesn't seem to be any point where anything is grounded to the chassis. Everything seems to be grounded to one of the tabs on the capacitor can.

Except the twist-lock tabs on the cap can are also a connection to the chassis, as well as being the negative terminal of the can.

I can't tell from the picture where the orange wire from the "G" speaker terminal goes, but suspect it contacts the chassis somewhere. I also can't tell if the rivets holding the RCA jack or some portion of the microphone jack contact the chassis.

Ultimately, you can build an amp in a plastic box. You don't need a metal chassis to "ground" things. But a metal chassis is also a handy shield, which can be connected to your circuit ground (and in modern work can be connected to the ground wire of the 3-wire power cord to provide some shock protection/prevention).

[casssax]

The orange wire is also connected to the ground post on the can cap.
As far as I can tell the can cap ground post doesn't touch the chassis at all. It's just connected to the insulating material that is riveted to the chassis.
The rca input is riveted but it is through insulated material

I found that there is a connection at the mic input. The original mic input connector has been replaced and they used a couple of washers rather than insulating material so there is a ground connection there. I'll find out when I swap it out with a 1/4" jack if there is another connection.

[Willabe]

As far as I can tell the can cap ground post doesn't touch the chassis at all. It's just connected to the insulating material that is riveted to the chassis.

Use your meter to check. 

[PRR]

> the twist-lock tabs on the cap can are also a connection to the chassis

Top-side shows an insulator mount.

Not clear on bottom-side, but I think the insulator is near-same color as the chassis underside.

Totally floating would be good practice. Amps like this could end up in many different, and large, systems. Just nailing amplifier common to chassis works for a guitar amp, but maybe not for a school or factory PA/intercom. Chassis ground might have to come from the large system, not the rack or shelf the amp happens to sit on.

Given that it floats, and you are not wiring a school, I would use a chassis-ground input jack.

AND a 3-wire power cord!!

It "is" a Champ. 2nd stage is pentode, more gain, but that is inside a NFB loop so won't have large effect. That NFB is taken from OT primary instead of secondary. Aside from needing very different NFB resistors, it is much the same until you get into extreme cases. Primary NFB will not null the secondary resistance so you can't get damping factor over 10. Most g-amps favor 10 to zero, only hifi-fanatics want high damping (and even that changes with fashion). There is an issue with ripple, but this amp is well-filtered so should not be a problem.

It adds a line-in and omits any tone-control. You can adapt any Champ or similar interstage scheme.