Modifying my PA head

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Modifying my PA head

Postby Down&Out on Thu Oct 01, 2015 9:05 pm

Hi folks!

I own a 1972 Laney PA60 like this:

Image

I've been reading up on this forum that people have had work done to PA heads to make them more 'guitar friendly'. Whilst i can get a reasonable driven tone out of this head it isnt quite there. It sounds a bit flat, like you'd expect a PA head to sound, after all, acoustic transparency was the probable intended goal.

I am presuming this is a plexi-esque circuit, so i'd like to get it sounding closer to the late 60s plexi heads. Its full of good things like mustard caps and partridge transformers, so its worth having the work done. Im going to have a company do the work rather than risk death by doing it myself, I'm a competent DIYer but im not running the risk with tubes! (unless of course, the work involved is as fantastically easy as changing components)

In short, can anyone tell me what I would need to change to reach my intended goal?

Regards.
Down&Out
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Re: Modifying my PA head

Postby Racing on Fri Oct 02, 2015 10:15 am

In my opinion you need to turn to a tech.
A competent one at that.

Getting that thing there to sound like a supergroup or a non boosted Klipp is the least of your problems. The main reason you need to seek a competent tech out is the powertube setup IMO.
Them amps carry the good stuff Partridge transformers but see..that presents an issue as they will hand you voltages that soar to high with current mains voltage rendering that the powertubes will take a beating you won´t believe.

What IMO needs to be done is to lower the voltage for what´s know as the powertube screens. Their so called g2 points.

There´s various ways of handling this but one thing´s for sure. If you leave it unattended expect to spend half a fortune on old real high quality NOS tubes,as they´re the only ones that´ll take the beating.

Resetting preamp values to make it become a true supergroup clone for instance is the least of you problems.
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Re: Modifying my PA head

Postby Down&Out on Fri Oct 02, 2015 6:13 pm

Racing wrote:In my opinion you need to turn to a tech.
A competent one at that.

Getting that thing there to sound like a supergroup or a non boosted Klipp is the least of your problems. The main reason you need to seek a competent tech out is the powertube setup IMO.
Them amps carry the good stuff Partridge transformers but see..that presents an issue as they will hand you voltages that soar to high with current mains voltage rendering that the powertubes will take a beating you won´t believe.

What IMO needs to be done is to lower the voltage for what´s know as the powertube screens. Their so called g2 points.

There´s various ways of handling this but one thing´s for sure. If you leave it unattended expect to spend half a fortune on old real high quality NOS tubes,as they´re the only ones that´ll take the beating.

Resetting preamp values to make it become a true supergroup clone for instance is the least of you problems.


I was presuming it to be a challenge which i why i said i'd get a company on the case, http://www.surreyamps.co.uk to be specific.

I'm not quite sure what you're getting at, are you saying that i'd have to get the powertube screen voltages lowered on top of preamp value changes? This is fine, I need to know these sorts of specifics to get a quote and to know what work is required.
Down&Out
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Re: Modifying my PA head

Postby ParadiseSyndrome on Sat Oct 03, 2015 10:11 am

the issue with the power section is that you often find with amps from this era c1970 the voltage on the output valves is 550V+, where replacement EL34 valves manufactured today are usually rated up to around 450V max. I'm not familiar with Laney but think i read somewhere they can go up past 600V!

So your amp tech would first test the voltages and is then likely to recommend further modification work - for example lowering the voltage and/or installing larger screen-grid resistors. In extreme cases they could recommend changing the power-transformer or modding the amp to use KT88 valves instead which can handle the higher voltage better. Another thing to consider, assuming your amp has not been refurbished in the past - all the electrolytic capacitors (power filter caps & bias supply caps) are now over 40 years old and it's usually best practice to replace them after 20-30yrs.

..anyway back to your original question - sounding less transparent and more like a 60's plexi!

I've done a couple of PA to Guitar amp conversions in the past and usually its good to change the preamp circuit so there's an extra gain stage, and perhaps make a few small changes to component values in the preamp to get 'more distortion'. Plexi amps don't have a master volume and get their tone from turning the amp up loud! If you intend to play at lower volumes at home maybe it would be best to have a master-volume and swap out the preamp for a circuit like a marshall JMP/2203 or similar...

hope this helps - and please be careful if you choose to modify your amp, get professional help as these old amps have dangerous voltages
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Re: Modifying my PA head

Postby Mr. Foxen on Sat Oct 03, 2015 10:55 pm

Modern Shuguang EL34B deal fine with vintage Laney voltages with 1k grid resistors. Its pretty minor changes to make these work fine for guitar.
New music: https://warriorpope.bandcamp.com/album/anchorite
Gear tinkering blog: http://ampstack.wordpress.com/
Made a Facebook page for my ampguts stuff. Feel free to come post amp guts and rig shots: https://www.facebook.com/pages/Ampstack/351966881575839
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Re: Modifying my PA head

Postby Mr. Foxen on Sat Oct 03, 2015 10:56 pm

Modern Shuguang EL34B deal fine with vintage Laney voltages with 1k grid resistors in a properly serviced amp. Its pretty minor changes to make these work fine for guitar.
New music: https://warriorpope.bandcamp.com/album/anchorite
Gear tinkering blog: http://ampstack.wordpress.com/
Made a Facebook page for my ampguts stuff. Feel free to come post amp guts and rig shots: https://www.facebook.com/pages/Ampstack/351966881575839
Mr. Foxen
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