Slightly OT - Converting old tube PA amps for Guitar

[forbin80]

Other than this article:

http://www.rru.com/~meo/Guitar/Amps/PA2Guitar/

does anybody know any good resources for doing this?  I have a nice 30W Bogen amp that I'm itching to convert but I'd like some more resources (preferrably w/ pics or diagrams) first.

Also has anybody done this here?  I imagine a place like this w/ so many DIY guitarists there has to be at least a few people who know how to....


Many Thanks!
Whit

[forbin80]

Another note:

If I'd be better off using parts from this amp to build a new one that's better suited for guitar, I"m up for that too.  Basically I want something that I can just sit on top of my little Peavey practice amp and run output to the speaker inside of it.

Thanks

[object88]

You'd do well to inquire around ax84.com.  They're all about the tube amp.  

[vfr800fiman]

Hi Whit.
I just did this conversion last November with a McMartin MA-20 that I purchased of from Ebay for $30. It has a nice 6L6 push-pull section that I like the sound of (Fender among others). I actually used the site you gave the link to for guidance. It came out really nice as a combo amp. I even made my own cabinet.

Anyway, all I did was rework the circuit to look like a circuit that I iked the sound of. In my case I used an AX84 schematic of the "Hi-Octane"
http://www.ax84.com/hioctane.html for the preamp section.
I used a baxandall tone stack (because the original amp had one close to that). PA amps are great because you can get a LOT of gain using all those preamp tubes!

Basically, you could use and bits and pieces of other designs that you like.
I should caution you though that you should be pretty familiar with tube amp circuitry. I actually built the "november" project there from scratch befor I tackled my PA conversion. I learned a LOT from that build.
I actually went through a few design iterations before I got something I liked the sound of.

I don't have any pix posted though. The guy's at the AX84 site could probably guide you through this project. They are very knowledgeable.
Glenn

[Somicide]

I asked something similar here, and i remember being told you may want to change the power tubes to something a little more standard.

PnL,

Jeff

[vfr800fiman]

I asked something similar here, and i remember being told you may want to change the power tubes to something a little more standard.

What is your definition of "standard"? I guess mine would be 6L6's, EL34's, EL84's, 6V6's?

What does this amp have in it now for power tubes?

Glenn

[vfr800fiman]

I forgot to add that the people at this site were a big help also:
http://www.diyaudio.com/
Look under the "tubes" section.
Glenn

[Somicide]

i doubt my old posts are still in here, as they were in the Lounge, but you may want to check anyway, or i will, i guess.  I think it was recommended I use 6l6, but im not positive.  But either those or EL34 would be good bets.

PnL,

Somicide

[forbin80]

there are 5 tubes

2 are 6EU7's, which from what I've read are like 12AX7's

then there is one in between that I think is a 7267 or something

then there are 2 more which are 7868's (I believe).

I will look when I get home.  I looked on duncan's tube pages to see what the specs were for these tubes, but I'm kind of just getting started with all this and some of it's over my head at the moment.  I kind of got turned onto this at the same time as DIY stompboxes and since I had to wait for the amp to get mailed to me (ebay) the amps took a backseat to stompboxes for a bit.  Now that I finished my first build (Orange Squeezer) I'm ready to look at this amp now.


My other question is how dangerous is it really to mess with this stuff.  From other threads I've read some people talk like it's extremely risky but I consider myself really careful when it comes to electricity.  Is there anything in particular I should know before I start?

Thanks

[Somicide]

Well, the advice, is keep your left hand in your back pocket while working with electricity, and yes, these things CAN KILL YOU.  It's strange yours didnt actually have 12ax7s, mine did.  Anyway, when i get my specs, I'll send em.

PnL,

Somicide

[smashinator]

Well, you ARE dealing with hundreds of volts...  If you get sloppy, you'll be toasty shortly.    Read everything you can find about how to work on them safely (there are lots of resources on the web) and you'll probably be ok.  I haven't heard of any amps suddenly leaping off of desks and mauling someone...  yet.

[forbin80]

Holy $#!+ I actually FOUND a 3-page thread entitled "Bogen M330A Tube PA Amp for Guitar" over at diyaudio.com.  I have searched every website I found related to DIY stuff and haven't found ANYTHING about this amp.  That thread actually has schematics posted and everything!!

Thank you so much for that link, hopefully I can follow what this guy did.  I'm tired of playing through a 20watt peavey audition, especially now that I'm making effects pedals!

As always, I appreciate all the advice

[vfr800fiman]

That's one VERY good piece of advice from Somicide, one hand only!
Also read about how to safely discharge filter capacitors.
These can store a deadly charge even when the amp is unplugged.
Just be carefull and you'll be fine. You can learn a lot by doing this type of project.

Glenn

[bigjonny]

That's one VERY good piece of advice from Somicide, one hand only!

Yes, the "one hand only" rule is so that if you DO shock yourself, you don't pass current through your heart, which is what typically is lethal.  So, don't work with one hand and the opposite foot, either.

Speaking of feet, if you have a rubber mat, do all your work with your feet on it.  Wear some good rubber sole shoes, too.

Invest in alligator clips for your DMM.  Never use two hands for your DMM.  Ensure you know how to properly discharge the capacitors before working on the amp.

[sir_modulus]

:shock:  The second time I'm the first one to post the geo link in a thread!!

http://www.geofex.com/gtramps.htm

Look and behold the wonder of R.G.!!!
Cheers,

Nish

[GreenEye]

I've got six PAs from Ebay that were either converted before I bought them or I took to a local shop to have converted.  Each one is so different.  I love 'em.  Unfortunately, I don't do the conversions myself.  You need to know what you are doing or you'll fry yourself, destroy useful parts, etc.  I'm sure the mods are relatively straight forward IF you had already built a simple tube amplifier.  Any place that works on amps can do it - some charge several hundred dollars, some do it for around $40.  Some electrical engineers or people that work on TVs will do it.  Sometimes just having them adding 1/4" ins and outs works fine - especially the Bogens.  There's a guy in California I've purchased several amps from that will convert an amp for you for like $30-$40 if you pay for the shipping both ways.  He often sells converted amps on Ebay, which are found using the search words "guitar tube PA."  I don't think he has any at this time.  PM me if you want his info.

I got lucky with my last one, and the place I went to here in Phoenix ended up being worked on by Tony Krank (of Krank Amps fame).  He bypassed the tone controls, so the only controls that work on the amp now are the pre and master volumes.  There's a million different ways they can be modded.  Some I have that were supposed to be hot rodded with several pre amp stages for gain ended up very muddy and having unpleasant distortion.  A little one with 7868 tubes has the best clean sound I've ever heard from a guitar amp.  Do some searches for these tubes if they are in your amp - they are NOT cheap to replace, and becoming more and more rare.  Also, don't forget Yellow Jacket tube converter sockets can save hours of socket switching and rebiasing if they make ones for tubes you use.

I think it would be more satisfying to build a simple amp from scratch rather than try to convert an old amp myself. Not only do you have to convert it, you need to hunt for pesky parts that might be bad, and then figure out if the buzz you're hearing is because of a bad transformer or other bad part, or was it your wiring, or the original wiring, a bad tube, etc.  Maybe some dude in the past was in there goofing things up.  Too many variables for a newb like me.  If you start with new parts, you know they should all work.

I recently got Antique Electronics' book, Building Guitar and Bass Tube Amps, Vol I, and it is an invaluable resource.  It has many more safety precautions and tips that I've ever found on the web, and it walks you through the building of like four amps, up to a Bassman!

[Somicide]

ahh, i see where our tubes differ!  I had a MX30A PA amp, whereas you had a (am I right?) M330A, yes?  That would explain my 12AX7s, yep.

PnL,

Jeff

[Regan]

If you have a schematic for an MX30A, would I be able to have a copy?
Thanks,
Regan

[Somicide]

If I had ever been so lucky to find it!  Granted, I never really tried too hard; but If I find one, I'll PM you for your email.

PnL,

Jeff

[forbin80]

Here's the thread:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=33484&perpage=10&highlight=&pagenumber=1


Maybe the Mx30A is pretty much the same schematic