1972? Orange PA120 (Warning BIG PICS)

[Don Doucette]

A client of mine purchased this gorgeous ~72 Orange PA120 (OR120 for PA) from a fellow in the UK.



When the amp arrived he plugged it in and poof red plated one of the original Mullards. I know.. I know... I got a little misty when I heard about it too!

He contacted me and dropped the amp off... take a look at what I found...



3 of the 4 screen resistors are burned up. Look at the solder joint on that thicker yellow wire


Also I will be replacing this sketchy inadequate power cord...



I love this amp, can't wait to fire it up but first I must repair it, modify the power supply to reduce the voltages for use with 120v mains (originally designed for 115v mains in N. America) and then install a 5 amp fuse.

Don

[ahamay79]

Why can't you use the 115 VAC setting?

[Don Doucette]

Well when you think about it, orange runs those output tubes out on the ragged edge as far as plate voltage goes. The transformer was designed for 115v mains at the primary but our local average is up around 122v representing a 6% increase, a 6% increase at the secondary is going to severely abuse the EL34s.

I'm going to set my variac at 115v turn on the amp and measure voltages then modify the power supply so that these voltages are not exceeded at our current 120+ mains.

Don

[ahamay79]

How about a big fat dropping resistor for the B+?

[Steve Conner]

If I were you, I'd get it working first and then worry about modifying the power supply. It might work just fine on 120V. If it doesn't, some variation on RG Keen's Vintage Voltage adapter is surely the way to go. A dropping resistor in a 100 watt amp would generate a lot of heat.

All of the electrolytic caps will need replacing, and if the screen resistors are burnt, then the EL34s are probably fried too.

[pontiacpete]

I've used bucking transformers for vintage equipment such as radios and hifi's, not for vintage music equipment, but you could if you are concerned about it.

[txstrat]

I know, not the right place, but does anyone have a schematic for this amp?
I'm quite interrested in building a PA/vocals amp and haven't made a decision yet. Still looking for schematics to compare.

[guitician]

What happened to that cap? It looks like the lead broke off in shipping. Whats the redish stuff under it? Caps kept out of the heat will last longer. Thats why I like mounting them under the chassis. And 105c caps work better for tube amps.

[Gtr_tech]

Looks as though it may have vented. Since all that looks original I'd not even powered it up without checking the condition of the caps or just replacing them without hesitation. Did he ask about what maintenance had been done to the amp? Certain things you just *need* to know when buying vintage gear. If yer not sure......have it checked out *before* powering it up. Live and learn.....oh yea....looks like a 73 date code on one of the caps.

[Bruce / Mission Amps]

I've worked on a fairly large number of these and other Orange amps over the years and one of the short comings is also the bias supply filter caps... suck it up and undo the board so you can remove and replace them along with the other electrolytic caps.
Many times the bad power tubes can be traced back to a faulty bias supply and improper bias voltage due to leaky/worn out bias filter caps.

[Don Doucette]

Sorry guys been AFK for a while. After nearly two weeks it was finally this amps turn on my bench, the caps were replaced as well as a handful of other sketchy parts. The caps were victims of mechanical failure as for the "stuff" under the caps I can only assume it was a glue of some sort as it looks as though it was brushed on. The amp is currently running beautifully at 115v but at ~120v it is going to eat tubes.

I have to say that I am not too fond of the wiring in this amp, fragile and inadequate in my opinion especially after 30+ years. Pulled the board and although desoldering all connections to the board merely repositioning wires out of the way would cause them to break off their connections... What a PITA!

Thanks for all your input, the owner of the amp will have it back in his hands on Tuesday.

Have a great long weekend.

Don