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Old 28th November 2008, 02:01 PM   #1
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Join Date: Oct 2008
Question Tube PA to Guitar Amp Conversion Help

Hi everyone,

So I'm definitely no expert in amp building or electronics, but I was very motivated to have a tube guitar amp without spending much money, so I bought a Lafayette PA off ebay for like $40.

I've been using it mostly as a power amp with a podxt, so it's worked pretty well for me, but I'd really like to make it into a proper guitar amp.

That's where you all come in!

I've read some articles on converting PA amps to guitar amps, and this forum: Bogen M330A Tube PA Amp for Guitar , but some specific help with my schematics would be really good.

I really like how orange amps sound, so I was thinking about trying to copy an Orange preamp, But....

I really don't even know if my amp has negative feedback- - why is there only a wire going from the 16ohm output of the OT to the 12ax7(2)? - - Advice here??

I don't really understand what the circled numbers in the "Tone Network" section mean (I know there are some labels on the physical tone parts in the amp) -- Help?

I did replace the Output Transformer with a 40 watt Hotrod Deluxe transformer, because the original didn't work, But I don't think that affects the schematics too much (Although the new OT doesn't have a 16ohm output).

I also put a three prong power cable in, grounding it to the chassis.

I'd really appreciate any help! (Schematics are attached)

Thanks,
-Peter
Attached Files
File Type: pdf 3830 schematics2.pdf (68.5 KB, 56 views)
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Old 28th November 2008, 03:24 PM   #2
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Join Date: Mar 2008
Valadezpm,

I have been fixing, modifing, and building guitar amps for years. I started out buying these old Lafayette and Bogen PAs and they work out pretty good as a starting point.

This amp has negative feedback which is the 68K/100mmf from the 16 ohm output tap. The circled numbers on the tone control are output pin numbers on the tone module.

Now lets get to some changes you will want to make to get this into a guitar amp configuration.

1. Add a 470 ohm 2W resistor from pin 4 of each 6L6GC to the 560/2w resistor at the 435v end on the schematic.

2. Replace the two 100k resistors connected to pin 5 of the 6L6Gcs with 220k.

3. The tone control presently there is a Baxandall circuit which is great for a hi-fi style amp but not for a guitar amp. You can use the Orange circuit or the front end from any Fender blackface circuit. Since this amp already has a 12AX7 front end this lends itself to the Fender circuit as it only requires a single 12AX7 for the preamp gain, tone and volume circuits.

4. The second 12AX7 in this amp is a gain stage where the NFB is applied driving a split load phase inverter. This can be left in as it will work OK.

I hope this gives you a starting point to get going.

Guitar88
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Old 29th November 2008, 02:25 PM   #3
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Join Date: Oct 2007
I'd say the question is how does the amp sound right now for guitar use? I ask this because you never know till you give it a try but you might find that it doesn't need a whole lot of "help" in the voicing department.

The reason I say this is that back when we started our band over 20 years ago we had NO real equipment at all so we were forced to improvise. Our guitarist Myles discovered how good an old reel to reel tape recorder that my grandfather gave to me(Voice of Music by Motorola) sounded as a guitar amp! It was and still is great sounding for whatever reason using either the built in 6X9 speaker or even better a single 12 extension cabinet that I built. It has a very nice overdrive tone that's quite similar to a tweed Deluxe or similar amp with less than 10 watts of power. Later when Myles bought a couple of other little amps he still used the reel to reel for his basic sound for I know 3 years.

It's funny too after listening to some recordings but it took a LONG time for his tone to equal or surpass this setup after he switched to "real" guitar amps. For the last 15 years he's played through a '74 Marshall 50 watt top through a Peavey Classic 4x10 cabinet which is somewhat unusual but it sounds great.
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Old 6th February 2009, 10:45 AM   #4
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Hey guitar88 and Dave,

Sorry I hadn't responded in a while, but I appreciate the feedback!

I actually made the modifications that guitar88 suggested. I knew if wasn't going to change the amp's sound dramatically, but I didn't notice too much of a difference.

Speaking of what Dave said, I tried playing my amp with an EQ pedal after my podxt to roll off some the 6.9k highs, and I liked how that sounds. So... I was wondering if I could just install a presence control to replace the EQ pedal... could you give advice how to do that.

Also what are the advantages/difficulties of having Post Power Inverter type presence control?
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