Mid 1940's PA Amp - What can I make of it?

[WebMentor]

Hi,

I've been told that this little PA amp is mid to late 40's and that it should produce about 15w.

1 x 5Y3G1 Mullard Rectifer tube
1 x 6V6GT MiniWatt Power tube
1 x 6V6GT Mullard Power tube
1 x 6C6 MiniWatt PreAmp tube
1 x 6J7G RadioTron PreAmp tube
1 x 6C6 JAN Nation Union USA PreAmp tube

I found this amp in an old factory that I purchased in Australia a few years ago and saved it from being trashed - no reason I just liked it and thought that one day I might tinker with it to see what it is.

That time has come as my 14 year old son has been playing guitar for a year through a Roland Cube 60 and has now discovered the joys of tone from tube amps hence I'd like to rebuild the amp as a father son project.

I have experience in electronics (but not tube) so this will be an exciting learning curve.

Any ideas of what it could become would be appreciated

I hope you find this early amp interesting - just PM me if you would like high resolution images of the circuit.

Cheers,
Nigel

[centervolume]

I know there are tube amp guys that do this kind of work as a side project.

here's an example of a serious-minded effort

http://geofex.com/Article_Folders/old2new.htm

I just googled "converting old PA head to guitar amp" and got 6 or so links

At least, I contacted one myself when thinking of converting a couple of similar pieces I have (stromberg-carlson and RCA). But I never sent it away as the items are heavy and I wasn't that committed to the project. I think the dude wanted 2-300 dollars for the work. It's interesting and I'd like to know what you find out. Sorry I couldn't be more help

pl

[52 Bill]

Wow! What a great amp.

It looks like it would make for a great platform for a Fender tweed Deluxe circuit.

Have you tried to power it up yet?

[J Martin]

The 6J7 makes a great sounding input tube. You may have to triode strap the 6C6 tubes to build a decent phase inverter. Draw youself a schematic if you are up to it. Get rid of all the wax/paper caps and the old electrolytics. It just may come back to life. I keep sheets of stainless steel and aluminum in my shop to make new jack plates for projects like this.

[6267]

Remember to make sure speakers are attached anytime you power it up–those old transformers are great for guitar and you won't want to replace them. Those big jacks on the back are probably speaker connections, but you can hardwire a speaker to the appropriate wires in the meantime. I also may think the circuit might be great as is! The mic input will be very high gain, good guitar dirty channel (maybe different coupling caps). The music channel will probably have a little less gain. There seems to be plenty of room for another turret or tag strip if you needed one. With many old amps, all it takes is a little tweaking as opposed to a total redesign. It may be more fun to decide on component values and do a little soldering (maybe change one of the input channels to something different) than do all of the appropriate load lines, PS design, etc. to make an old fender amp work with what you have.

[Brad1]

Remember to make sure speakers are attached anytime you power it up–those old transformers are great for guitar and you won't want to replace them. Those big jacks on the back are probably speaker connections, but you can hardwire a speaker to the appropriate wires in the meantime. I also may think the circuit might be great as is! The mic input will be very high gain, good guitar dirty channel (maybe different coupling caps). The music channel will probably have a little less gain.

That looks like an AC wall plate on the back...but why is it not connected to the power transformer, but to the other one? (Follow the green wires).

If those tube sockets on the back are the output jacks, where are any input connectors?

The "Music" knob looks like a tone pot, with "Microphone" and "Pickup" volume pots.

Is it possible that the two tube sockets are the inputs...one 'Mic" one "Pickup", and the poor choice of an AC wall outlet-looking plate is the speaker out? And why are both wires green? Is it even possible that the thing is putting out some voltage through that wall plate to connect to something else? Possibly like a Bogen with the 70V out for use in a large facility?

Also appears to have no power switch?

Looks like a few connectors and switches, etc., to consider, as well as possibly a grounded cord?

Be careful until you find out.

Brad1

[Brad1]

Any replies to confirm or dispel my observations...before he tries to connect something to the wrong thing? I searched the Internet for that amp and came up empty. Looks like some weird things happening there?

Plus...I'm just curious, now.

Brad1

[Regis]

Looks to me like the AC jack thing is the speaker out and the Amphenol plugs are the inputs.

Cool amp, it's in a lot better shape than many newer amps I've seen.

I'll bet if you replaced the caps and maybe any suspect resistors it would sound pretty good. Then you could mod to taste, the tone stack and so on.

Oh, and a new AC cord too.

I have a couple of Bogens that use the Amphenol jacks for speaker outs. I made adapters to be able to use 1/4 inch jacks.

[WebMentor]

Thanks to all who have offered input.

Brads observations are 100% correct.

Here's what I have found out from other sources.
Being a PA amp for a large factory it has a 70 or 100v line out (the AC socket more info here http://ozvalveamps.elands.com/optrans.htm) and the tube sockets are for inputs.

I've asked several credible resources about this amp with the most informative reply coming from Roly Roper http://ozvalveamps.elands.com/

Here's what Roly had to say about the amp.
*******************************************************
Hi Nigel - Oh boy. Another seriously old stager of the general PA/factory PA class. As to date, I'd place it in the 1950's.

Nice pix. The 5Y3 reccy with the white crown has air in it.

It's always an interesting question with amps of this vintage to refurbish or leave alone as exhibits.

The can electros, waxed paper caps, and some of the resistors at the very least will need replacing, and if any of the 6C6 pre's are duff these might be the decider - if you can't get replacements then leave it alone and put it in a glass case. Otherwise you're in for an interesting restoration project. It's normal with these to leave the can electros in place for "look", and disconnect them and wire in modern ones underchassis.

The resistor markings here are body-end-dot type, and will be round number value sequence rather than the later 12th-root-of-2 "prefered value", so you will find 250k rather than 270k, 500k rather than 470k, and so on. The octal sockets on the rear are for connecting turntable, tuner, and/or a "control unit"
(preamp, source selector, and controls) that drew power from the amp.

It's difficult to tell without doing some tests, but the output is likely high impedance 70 or 100 volt "voltage line" rather than 8 or 16 ohms. See; http://ozvalveamps.elands.com/optrans.htm

Again, if the resistance of the output winding is more than a few ohms it will be line output and I'd leave it as original as an exhibit.

Since this looks *very* original, and I can only see half a dozen more modern components in it, I'd be inclined to ask Philips themselves or your State Science museum if they are interested in it.
Seriously. Apart from cleaning and finding some matching 'long' chicken-head knobs, I'd leave it as is.

If you want a 6V6 guitar amp I honestly think you would be much better starting again from scratch with something like the AVA100 design.

Hope this helps, and thanks again for the neat pix - it will get its place in the AVA "virtual museum".

Cheers, Roly http://ozvalveamps.elands.com/
*******************************************************

So now I have the problem of what to do with it.
Should I rebuild it as a guitar amp or leave it as a museum piece?

It's probably a good idea to pass on some important safety tips to someone who has just found a vintage amp.

Rule No1: Don't power it up - Replace the power lead first.
It's a sure bet that the old rubber insulation in the power lead will have gone hard and brittle. I showed my son this important lesson as when I touched the insulation it just fell off leaving the conductor exposed and shorted.

Rule No2: After replacing the power lead - Don't power it up.
Just as the rubber will have dried out so will have all the electrolytic capacitors. If they have dried out they will offer little resistance to current and probably explode if power is applied. It's best to replace all the capacitors or seek the help of a experience person to see if they can be reformed.

Rule No3: After replacing the capacitors - Don't power it up - unless you have some experience. There are some seriously lethal high voltages just waiting to zap inquiring finger. If you don't know what you are doing ask for help and stay alive.

[6267]

I wouldn't glass case it even if it is 70v line. I would follow all of the recommendations, recap, check resistor and cap values (this will be tedious, you could just recap and power it up on a non-conductive surface using a ten foot pole and eye protection ). I'd pull all the tubes the first time. Find a way to ground (securely) one lead of a DMM. Set it to the highest AC volts setyting, and with one hand in your pocket, check the volts between the metal chassis and ground (no tubes). This should be 0 or damn close to it, like 0.024. As long as this is zero, switch the DMM to the highest DC volts setting, and check the chassis to ground. This is generally between 2 and 0 volts, I wouldn't expect to see much more than 6 accounting for variations in ground etc. If the chassis is free of voltage, unplug it, relax, and come back later. There is hope! Now you are going to want to go to www.tubedata.org and look up the tubes in the amp. You will need a new 5y3, and install it in the amp. Turn it on with the aforementioned non-conductive pole, (a cheap all plastic pen will work ok, as long as it's dry). Check the chassis voltageUsing the pinouts from the datasheets, and again using one hand, you might try verifying that voltages are present where they belong. Everything will measure too high, but what you are checking is that the power supply is not short etc. Heater voltages will sometimes require measuring between two pins in a socket. For this, turn the amp off, insert the probes, set the meter, and turn the amp back on for best safety. They will often measure very high with no tubes in the amp, but not more than a few volts higher (i.e., 7-8 for a 6.3v heater). If there is B+ and heater voltage to all tubes, you may be very close. Replace all the tubes (perferably tested on a tester first, some old electronics stores have these still, thankfully). Using an old power cord , wire a 470ohm (this should work without any input, for either 8 ohm or 70v for a short period of time) 5-10w resistor across the end without the pplug. Put this in the speaker jack. Using the non-conductive pen, flip the amp on, but be ready to flip it off after anything strange happens. If the tubes just light up and nothing sparks, arcs or catches fire, you may be in luck. Turn it off and check the voltages on the chassis again for safety. www.radioremembered.org/outimp.htm will explain how to test the transformer after it has been disconnected. If the voltage ratio is a 500-2k:1, you probably have an 8-ohm output. If it's a lot less, it's probably a 70v transformer. Some transformers have other output wires, so you could conceivably find a different tap inside the amp for 4,8,16 ohms–I have a 60s PA that does this. If there is only a 70v tap, use an appropriate edcore, hammond, or other output transformer in its place (it'll wire up similarly and not require much more than soldering/bolting). Some of the Fender replacement parts would work here and are available for less than $50 USD. Good luck, I think it's worth turning into a cool project!
/phew

[capnjuan]

Hi WebMentor; I'd suggest putting it in your attic for your next father / son project or, if you son gets interested, his next project. Just because it has the feel of 'found money' isn't a justification for recycling it dressed up as quality time and avoiding an expense for a more suitable candidate.

There are lots of off-brand, beater, or less well-regarded but newer amp models to rebuild, re-hab, or refurbish; dozens of US and Canadian made tube amps that, with less intensive effort, will yield a satisfactory outcome. Some of the encouragement you're getting here is from techs who know they can do it or have done it; not the same if your skill set doesn't match theirs. I assure you that, were you two to get in a corner, you'll not easily solve the problem via Post & Reply over the web ... with a badly out-of-date amp. You might have a chance with a Silvertone 1471 / 1482 / Univox whatever, where people can see your schematic and research your issue.

If the objectives are spending some time with your son with the hope of sparking some interest, a project that offers a simpler-to-achieve outcome provides less risk of disappointment. If your PA project is over your head, then you'll always feel like you have to be two steps ahead of him to avoid a catastrophic outcome. Keep it simple; save the PA, maybe he gets interested, maybe not but, IMO, possibly too much of a challenge for both of you.


Been there, done that ... I have the t-shirt. Good luck!
John