how does this amp work? help!

[the fuhz]

hello all,
...this is my first post in the forum. I am not too familiar with vintage tube amps and I have some questions on how to hook one up. I have a 'McMartin PA-20', which is a 20 watt P.A. Amp from the late 50s or early 60s. I plan on hooking up the amp to a vintage Jenson 8ohm 12" speaker and use it as an extension unit on a Wurlizter electric piano. But, I am having trouble figuring out what all of the terminals on the back of the unit are used for.
I attached some pics of what I have to give you a better idea. Anyway, there are two terminals marked '8', and I know from testing that these are the speaker terminals, so does the '8' stand for: '8ohm'?
There are also a few other terminals marked: '70.7v' and '25v', what are these used for?
Since I am only using one speaker, I would like to bridge the output, is this even possible on this amp? Any info would be greatly appreciated thanks.

[Rob Mercure]

I'm not sure what you want to "bridge" the output of this amp with - there's only one power amp in the chassis. Or perhaps you just mean to connect a second spearker - you can put two 16 ohm in parallel or two 4 ohm in series and maintain the desired impedance (or other multiple combinations - use your Ohm's law). Anyhoo, you're correct about the "8" being the speaker impedance but I'm suprised that other low impedance options weren't offered. The 70 and 25V outputs refer to supposedly "constant voltage" lines that allowed one to transfer an electrical signal long distances - such as providing PA for a building - by using a higher voltage signal which would be less affected by line and connections resistance. Where the speakers were connected a small tranny was used to match the line voltage to the speaker impedance and desired power at that location.

The screw panel also includes a "line in" which might give you some flexibility for driving the amp with your Wurlitzer remotely - you might even be able to use a speaker signal to drive it if there is some control for the line input (if not you've probably got a distortion machine from hell <grin>).

Rob

[bob p]

some of the mono PA amps from that era have what is called a "bridging input/output" jack on the back panel. maybe this is what he's referring to.

i recently worked on a 1950s RCA PA amp that had that sort of circuit. essentially, there was an RCA phono jack (surprise!) on the back panel that connected directly to the PI grid via a shielded cable. if you had two of the same amp, you could use a shielded jumper (ie: a modern audio interconnect cable) between the jack on each amp and slave one off of the other.

it was a handy feature from a troubleshooting standpoint -- i needed to work on the output section and it was handy to have a direct hookup to feed a test signal into the PI. when i was done, i disconnected it as i couldn't see much point in having a potential noise source in a place like that.

[the fuhz]

some of the mono PA amps from that era have what is called a "bridging input/output" jack on the back panel. maybe this is what he's referring to.

exactly. and if you look closely at the photo which shows the entire back, you can see two rca outputs marked: 'bridge output' and 'program in'. Your idea of linking two amps makes sense.
When testing, I plugged the wurly into the 'program in' connection.