PA 120 working at last

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PA 120 working at last

Postby tommie on Sat Jul 14, 2001 2:10 pm

I finally repaired my PA120 bought a few months ago.<P>Straight out of the shop for £65 (didn't even try before I bought it, cos I thought I'd just plunder it for parts) it was blasting the 100 ohm smoothing resistor when standby was flipped on - a common problem I've found. I checked out all the bulgy old smoothing caps and found one or two reading short circuit from +ve to -ve tags. <P>So I've replaced the 200mF cans with Sprague 190mF from a high spec bench power supply, the dual 32's with LCR 50+50's, and the single 32 with a 47mF. Close enough values to get running, and I had all of these bits in stock. Then I replaced the blown 100 ohm res with a 10 watt cement version.<P>I must admit that I did all this work over a few weeks, and eventually had to guess how it had been wired up in the first place, using a Lead 120 schematic for guidance. Not recommended, but it worked OK first time. <P>Sound wise it's superb for guitar. With all volume and treble on 10 and Bass rolled almost all the way off, plus a Les Paul or SG Special, the early 70's Townshend tone is nailed.<P>Haven't touched bias yet, and despite having its original sextet of Mullard EL34's (now you know why for £65 I didn't care if it worked!), they don't seem too well matched.<P>Do I use the usual cathode current biasing process, but adjust the maths to allow for the cathode resistors that are in there?
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Re: PA 120 working at last

Postby SteveG on Mon Jul 16, 2001 8:37 pm

Hi there!<P>I've just been fiddling with a PA120 (bought recently for £50, na na-na na na!)<P>Does your's have individual bias adjustments for each set of three valves? Mine looks as if this arrangement has been added by someone.<P>I thought mine was fixed and ready to go, but after using it for a week at home with no problems, I took it out to a local pub, plugged it in and it immediately started 'motorboating'. I noticed three of the valves glowing alarmingly, so switched it off, and haven't had a chance to look at it since.<P>BIG clean sound for guitar while it was working, though.<P>Steve
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Re: PA 120 working at last

Postby tommie on Tue Jul 17, 2001 10:12 am

Hi Steve,<P>Yours has the motorboating problem 'cos it was £15 cheaper. You should have found a more expensive one! <P>Mine also has twin bias pots, and this is original according to the L120 schematic in the Groove Tubes book. I'm assuming that the cathode resistors that are in there can be used to measure mV and hence bias the thing. Once I know the HT voltage and check the cathode resistor values, I can work out the maths to get the expected mV reading to produce 17 watts idling per valve. With 6 valves, I suspect that I'll have to chop and change them about to get both sides of the amp drawing equal current. As it is, one side of 3 glows when playing hard, whereas the other side only 1 glows (glow as in the grid glowing, the plates are all fine). <P>The amp sounds great for guitar, a really tight crunch on low notes, and a touch of mid rangey squawk (think Schenker, and pick the notes tightly with a thumb flesh follow through), with Bass set to about 5-10% and treble at 10. That's through a Power Brake too, with channel and master volume all the way up. Put a Tube Screamer in front and it's the George Lynch tone.
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Re: PA 120 working at last

Postby velatones on Tue Jul 17, 2001 2:16 pm

A pawn shop in town has a 50W Sound City PA head in fair condition - they said it was £200 :cool:
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Re: PA 120 working at last

Postby tommie on Tue Jul 17, 2001 5:53 pm

There used to be a 2nd hand shop on Northumberland Street (the street with the Cinema, Beatties models etc - have I got the right street name?) in Newcastle, maybe at the end of High Bridge. They once had a 1969 metal panel Super PA head, near mint, except for the logo being present but broken in two places, and only one of the little grey feet on the bottom.<P>I'd just acquired a Super Bass, exact same age, which had three grey feet and one stud from an Adidas bag on the bottom. I scrounged the grey foot off the PA head for nothing! This was back in 1988...<P>And the amp was £50.
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Re: PA 120 working at last

Postby velatones on Tue Jul 17, 2001 10:03 pm

Tommie,<P>Yup, that's the same shop :(
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Re: PA 120 working at last

Postby greengod on Wed Jul 18, 2001 9:58 am

Ian,<BR>how much would it cost today a 1969/70 Super Lead in working condition in your town? It seems that there was in the past a lot of great amps. Here in Italy too, it was easy at the end of the '80 to buy cheap great vintage amps. Now it's just another story and the prices are ****in' high. Here you can have deals only with private sellers. Forget the shops: they are here to rip you off.
"Without music, life would be a mistake" (F. Nietzsche).

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Re: PA 120 working at last

Postby tommie on Wed Jul 18, 2001 7:09 pm

Best bargains I've had are from local for sale papers (Loot type stuff) Adtrader is good for local things in UK, but now they have their website together and you can search for things, local bargains are being quickly poached by buyers from further afield. I rang about an Orange 100W slave in Glasgow for £55, and it had been promised to a geezer in London who was paying another £30 to have it couriered...<P>I did get the empty Marshall 2x12 for £35 though ;) <P>And I'm eagerly waiting for the 68/69 Super Bass that a local shop wouldn't pay £200 for, recommending the punter stick it in the paper instead!
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Re: PA 120 working at last

Postby ShaneT on Thu Jul 19, 2001 1:47 am

Marshallhead- Have you had any luck using ScotMart? Isn't that one of those local buy and sell papers?
Shane
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Re: PA 120 working at last

Postby tommie on Thu Jul 19, 2001 6:45 pm

We get Scotads which has less stuff in it that Adtrader. Stuff turns up every now and then, but you need to be quick if its tasty stuff. Although saying that a "Marshall MkII 100W Valve Amp - £200" turned out to be a 1959 SLP reissue plus a JCM900 1960B cabinet. I left it a week or two before phoning and still got em both for the £200!
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