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Discussion Starter · #1 ·
hey guys,

I was recently given an old 100 watt, 4 channel PA head and I’m wondering if I can turn it into a guitar amp? I mean, can I repurpose the parts to learn to build an amp?

Just an idea/thought...
 

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My first experiments with repurposing tube gear was using old reel-to-reel tape recorders as guitar amps. Lots of guys were repurposing Bogen PA amps, and I think still do. I personally found solid state PA amps too clean for my electric guitar purposes, probably I just didn't know how to (clears throat) rectify the problem.

I'll watch this thread.
 

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Basically, many PA amps ( as well as reel to reel tape recorder rec/repro amps) were made for the (lucky of us) popular-at-the-time Hi Z ( Z = impedance) mics. On those, no mod is necessary to use for guitar ( or bass, keyboards etc). The one thing is that some people find the preamp not to their liking tone wise. This is because they don't do high gain, but you can get some OD from them. Examples here include just about any tube Garnet or Traynor PA/Vocal amp ( not the solid state Traynors - those went both ways depending on model) . Generally any tube PA/Vocal amp made back in the day by a guitar amp maker you recognise will be in this category (Sunn, Marshall, Hiwatt/Sound City, Orange etc... not sure about Fender though; I only know of 1 model and it was solid state so I never cared to check it out). Most consumer level (or prosumer) reel to reel decks are also in this category.

The other category is amps designed for Lo Z mics. These are mostly made by PA/Paging system oriented companies (e.g. Bogen and Lafayette) and also includes professional level tape decks (e.g. Ampex). Some may have 1/4" unbalanced connectors but that is rather uncommon; mostly you will see XLR (Ampex) or a few different types of obsolete Mic (and speaker) connectors. At the bare minimum you will need to replace the connector with 1/4". After that you can plug in, but note that without further modification this will be a ( wonderfully) dirty ass amp. E.g. My Bogen Challenger Cha-33 - depending on pickups, everything past 2 on the gain knob is just more dirt (the inputs were changed to 1/4" but I left it LoZ because I actually have a few LoZ instruments... and I like the dirt when I plug in my Sonex). The speaker outputs are an old obsolete connector for 70V paging systems, which I did not bother to replace as I just use the barrier strip and a custom cable I made (2 bare wires for the barrier strip to 1/4" on the speaker side, later upgraded bare wifres to spade connectors). It would be very easy to get a 1/4" jack in there, but you would have to drill a hole (or replace one of the weird speaker connectors - not same size so a pc of sheet metal to make up the difference).

In either case ( plugging a modern pro LoZ mic into a HiZ input or a guitar into a LoZ input) the easiest/cheapest solution is to get an in line impedance transformer ( as well as the relevant adapters as those are usually XLR to 1/4" an d you may require 1/4 on both ends). There are various brands costing $40 and up (Nexxgen, Shure Audio Technica etc) but you can get generic ones for < 10 (e.g. at Supermetronic in Toronto).

You can also get a nicer transformer and install it internally or ( if LoZ input to be used for guitar) you can install a resistive pad or otherwise modify the preamp section.

Everything else is a matter of tone and preference. The most common mod, for those that dislike the tone of the HiZ mic inputs used for guitar, is to take 2 (of 4 usually, as many as 8, or as few as 2) of the mic inputs (typically driven by 1/2 a 12AX7 each) and merge them into a typical (e.g. Fender/Marshall) guitar preamp channel (which uses a whole 12AX7 in 2 stages). There are builders who specialise in this sort of thing - see Rockwell Amplification in Winnipeg (he really hates Garnet mic inputs and converts them into something British usually; that's his client base). He currently has a modded Armaco HiFi amp (Orange/Matamp preamp section) for sale.
 

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Granny nailed it. At its simplest, you can plug and play /add a quarter inch jack or you can completely rebuild depending on what sort of PA you have at your disposal. The nice thing about PA's is the multiple channels. if you wanted to get creative you could completely rebuild the preamp as a single channel amp with way more gain available. The other route would be to tweak the cathodes of different channels to have different voicings at your disposal.

I have an old AIMS 100 watt PA unit that I got for a steal and I ended up keeping it mostly stock with the exception of some cathode tweaking on one of the four channels.

Definitely lots of potential fun to be had with a decent tube PA
 

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What GG and Sammyr said !!!

I ve been picking up any and all tube PA headz that I find for dirt cheap...

Straight out. All of them are great as bass heads. But eventually will get enough experience and knowledge to tweak them like others here do... I'm just not there yet...

Hope you jump on your mod project since will also be following this thread.
 

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Discussion Starter · #10 ·
Looks like lots of great info here so far...I’m on the road for vacation so I’ll be sure to keep checking back and absorbing the info posted. I’ll update as I progress :)
 

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Looks like a security thing (no workie for me r now either). Some ISPs (or more likely employer/institutional firewalls) will not let you through. It's a redirect not a direct link to the source content; the OP of the link should go to the redirect location and post the direct link. ... oh, apparently googling that page title will get you the direct link: Converting Integrated/PA Tube Amps into Guitar Amps.

Internet etiquette: don't post redirects; link to the source always (also academic policy as regards sources). Redirects are bad for a whole bunch of reasons, including fairness to the content creator/owner.
 

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Um, have we confirmed that the PA is, in fact, a tube-based amplifier and not a solid-state unit? Not that a power amplifier directed at one purpose could never be reconfigured for another. But the guidance would certainly be different.
 

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