Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Instruments and Amps Everything that makes music, Especially including instrument amps.

Tube PA to Guitar Amp Conversion Help
Tube PA to Guitar Amp Conversion Help
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 28th November 2008, 02:01 PM   #1
valadezpm is offline valadezpm  United States
diyAudio Member
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: http://www.diyaudio.com/forums/showt...484&highlight= , 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)

Attached Files
File Type: pdf 3830 schematics2.pdf (68.5 KB, 63 views)
  Reply With Quote
Old 28th November 2008, 03:24 PM   #2
guitar88 is offline guitar88  United States
diyAudio Member
Join Date: Mar 2008

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.

  Reply With Quote
Old 29th November 2008, 02:25 PM   #3
DaveMcLain is offline DaveMcLain  United States
diyAudio Member
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.
  Reply With Quote
Old 6th February 2009, 10:45 AM   #4
valadezpm is offline valadezpm  United States
diyAudio Member
Join Date: Oct 2008
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?
  Reply With Quote


Tube PA to Guitar Amp Conversion HelpHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Hammond guitar amp conversion bluestube Instruments and Amps 11 27th July 2009 01:09 AM
Help me out with my organ to guitar amp conversion, please. David E H Instruments and Amps 3 24th February 2009 07:47 PM
Guitar amp conversion DigitalJunkie Instruments and Amps 1 7th November 2008 03:16 AM
Heathkit A7E Conversion to Guitar PRNDL Instruments and Amps 7 17th February 2007 08:21 PM
Guitar Cab Conversion givethemrope Instruments and Amps 3 16th August 2006 03:03 AM

New To Site? Need Help?

All times are GMT. The time now is 06:31 PM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio